Evolutionary psychiatry

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https://en.wikipedia.org/wiki/Evolutionary_psychiatry 

Evolutionary psychiatry, also known as Darwinian psychiatry, is a theoretical approach to psychiatry that aims to explain psychiatric disorders in evolutionary terms. As a branch of the field of evolutionary medicine, it is distinct from the medical practice of psychiatry, in its emphasis on providing scientific explanations rather than treatments for mental disorder.

This often concerns questions of ultimate causation. For example, psychiatric genetics may discover genes associated with mental disorders, but evolutionary psychiatry asks why those genes persist in the population. Other core questions in evolutionary psychiatry are why heritable mental disorders are so common, how to distinguish mental function and dysfunction, and whether certain forms of suffering conveyed an adaptive advantage.

Disorders commonly considered are depression, anxiety, schizophrenia, autism, eating disorders, and others. Key explanatory concepts are of evolutionary mismatch (when modern environments cause mental health conditions) and the fact that evolution is guided by reproductive success rather than health or wellbeing. Rather than providing an alternative account of the cause of mental disorder, evolutionary psychiatry seeks to integrate findings from traditional schools of psychology and psychiatry such as social psychology, behaviourism, biological psychiatry and psychoanalysis into a holistic account related to evolutionary biology. In this sense, it aims to meet the criteria of a Kuhnian paradigm shift.

Though heavily influenced by evolutionary psychology, as Abed and St John-Smith noted in 2016, "Unlike evolutionary psychology, which is a vibrant and thriving sub-discipline of academic psychology with a strong and well-funded research program, evolutionary psychiatry remains the interest of a small number of psychiatrists who are thinly scattered across the world." It has gained increasing institutional recognition in recent years, including the formation of an evolutionary psychiatry special interest group within the Royal College of Psychiatrists and the Section on Evolutionary Psychiatry within the World Psychiatric Association, and has gained traction with the publication of texts aimed at the popular audience such as Good Reasons for Bad Feelings: Insight from the Frontier of Evolutionary Psychiatry by Randolph Nesse.

History

The pursuit of evolutionary psychiatry in its modern form can be traced to the late 20th century. A landmark text was George Williams and Randolph Nesse's Why We Get Sick: The New Science of Darwinian Medicine (which could also be considered as marking the beginning of evolutionary medicine), the publication of Evolutionary Psychiatry: A New Beginning by John Price and Anthony Stevens and others. However, the questions which evolutionary psychiatry concerns itself with have a longer history, for instance being recognised by Julian Huxley and Ernst Mayr in an early paper considering possible evolutionary explanations for what has become known as the 'schizophrenia paradox'.

Concepts applied by modern evolutionary psychiatry to explain mental disorder are also much older than the field, in many cases. Psychological suffering as an inevitable, and sometimes useful, part of human existence has been long-recognised, and the idea of divine madness pervades ancient societies and religions. Cesare Lombroso, a pioneering psychiatrist, began utilising evolutionary theory to explain mental disorder as early as 1864, proposing that insanity was the price of genius, as human brains had not evolved with the capacity to become hyper-intelligent and creative and yet remain sane. Darwin applied evolutionary theory to explain psychological traits and emotions, and recognised the usefulness of studying mental disorders in pursuit of understanding natural psychological function. Freud was heavily influenced by Darwinian theory, and towards the end of his life recommended psychoanalysts should study evolutionary theory. Bowlby's attachment theory was developed in explicit reference to evolutionary theory.

Evolutionary Psychiatry Special Interest Group (EPSIG)

In 2016 the Evolutionary Psychiatry Special Interest Group (EPSIG) was set up in the Royal College of Psychiatrists, UK by Riadh Abed and Paul St-John Smith. It is now the largest global institution for connecting psychiatrists and researchers interested in evolutionary psychiatry with over 1700 members. It has run several seminars and meetings dedicated to evolutionary psychiatry, hosting lectures by prominent academics such as Simon Baron-Cohen and Robin Dunbar. All of the meetings are available on the EPSIGUK YouTube channel. EPSIG also publishes regular newsletters, organising conferences, conducting interviews and hosting special essays related to evolutionary psychiatry (for which there is not yet a dedicated academic journal). As Riadh Abed, (previous chair) stated in a newsletter "Our aims are both big and radical: they are for evolution to be accepted as the overarching framework for psychiatry and for evolution to take center stage in our understanding of mental health and mental disorder."

Abed and St-John Smith edited a 2022 volume Evolutionary Psychiatry: Current Perspectives on Evolution and Mental Health, co-published by the Royal College of Psychiatrists and Cambridge University Press, marking the most extensive publication in the field to date, and forming the basis for the first podcast dedicated to evolutionary psychiatry, the 'Evolving Psychiatry' podcast.

Psychological function and dysfunction

Mental disorders are often defined by 'dysfunction' in psychiatric manuals such as the DSM, without a precise definition of what constitutes dysfunction, allowing any mental state deemed socially unacceptable (such as homosexuality) to be considered dysfunctional, and thus a mental disorder.

Evolutionary theory is uniquely placed to be able to distinguish biological function from dysfunction by evolutionary processes. Unlike the objects and processes of physics and chemistry, which cannot strictly be said to be functioning nor dysfunctioning, biological systems are the products of evolution by natural selection, and so their 'function' and 'dysfunction' can be related to that evolutionary process. The concept of evolutionary function is tied to the reproductive success brought about by phenotypes which caused genes to be propagated. Eyes evolved to see – the function of the eyes is to see – so dysfunctional eyes are those that cannot see. This sense of function is defined by the evolutionary history of eyesight providing reproductive success, not current cultural opinions of normality and abnormality on which common conceptions of health and disorder often depend. Jerome Wakefield's influential 'Harmful Dysfunction' definition of disorder utilises evolutionarily selected effects to ground the concept of 'dysfunction' in the objective process of evolution. Wakefield proposes that mental disorder must be both harmful, in a value-defined sense, and dysfunctional, in an evolutionary sense.

This grounding of dysfunction in an objective historical process is important in the context of psychiatry's history of labelling socially undesirable mental states and traits as 'disorders', such as female masturbation and homosexuality. Current diagnostic manuals are decided by consensus. For example, in 1973 the APA called a vote to reconsider homosexuality's status as a mental disorder. By a 58% majority, it was struck off. The category of borderline personality disorder was created upon the basis of a single paper and consensus between about a dozen psychiatrists. In 2014 psychiatrists voted on the features of a new disorder, internet gaming disorder. The reliance on votes and expert consensus rather than objective evidence or biomarkers is a longstanding criticism of psychiatry that evolutionary psychiatry can avoid by adopting the evolutionary definition of dysfunction.

Evolutionary causation and Tinbergen's four questions

The research questions and concerns of evolutionary medicine and psychiatry can be distinguished from normal biomedicine and biological-psychiatry research as asking ultimate instead of proximate questions. This ultimate-proximate distinction was introduced by Ernst Mayr to identify different levels of causational explanation: proximate explanations refer to mechanistic biological processes (e.g. genes, ontogenetic development, hormones, neurological structure and function) whilst ultimate explanations ask about the evolutionary process of natural selection which led to these biological structures and processes functioning as observed. This could be conceived of as proximate explanations are 'how' questions whilst ultimate explanations are 'why' questions.

Niko Tinbergen further deconstructed this ultimate-proximate distinction into his 'four questions'. These questions of mechanism, ontogeny, function and phylogeny can be asked of any single trait or disorder (often behavioural, although not necessarily) to identify the different questions of causation which are simultaneously relevant.

Proximate questions can be separated into questions of mechanism, which concerns how the trait works, the structure and process of its biological mechanism, and questions of ontogeny or individual development which concerns how the trait develops in an individual.

Ultimate questions can be either of or evolutionary function or adaptive value, which concerns how the trait influenced fitness throughout evolutionary history; and questions of phylogeny or evolution, which concern the history of a trait down the phylogenetic tree.

To take the example of depression, we can ask about proximate mechanisms (e.g. neurotransmitter properties), ontogenetic development (e.g. neurological development over an individual's lifespan), adaptive function (e.g. low mood system) and phylogeny (e.g. apparent low mood in reaction to social defeat in primates).

Key explanatory concepts in evolutionary psychiatry

Mental disorder results from many different environmental and genetic causes, with various complex neurological correlates – but evolutionary medicine recognises several general principles which allow vulnerability to disorder. Adapted from Nesse (2019), Stearns (2016) and Gluckman (2016).

Reproductive success over health

Natural selection acts on reproductive fitness, not biological states which are what may be considered healthy; healthy states are only selected if they also have positive effects on reproductive success. This is used in evolutionary medicine to explain aging and diseases of senescence: diseases which appear past reproductive age have minimal effect on fecundity. Psychological suffering and various cognitive states which may seem unhealthy or disorderly may equally be products of evolutionary processes if they increased reproductive success. Evidence of this may be seen in disorders associated with substantial apparent dysfunction, yet average levels of fertility.

Mismatch

Evolutionary mismatch occurs when evolved traits become maladaptive due to changes in the environment. This is a common factor causing evolutionary change (e.g. in the peppered moth) and is relevant to medicine when the mismatched traits cause problems affecting health. Psychiatric conditions may in some cases be evolved states which we are misinterpreting as disorders because they no longer fit our social expectations; or they may be mental states or traits which would manifest healthily in ancestral environments, but become pathological due to some feature of modern environments. Evidence of mismatch is most prominent when comparing traditional-living humans to modern-living humans or when new environmental factors arise which clearly cause disease (e.g. the availability of cheap, high calorie foods causing obesity).

Defences

Psychological responses such as fear and panic are adaptive in many situations, especially of imminent danger, and seen in multiple species. Certain mental disorders may result from such responses, either as a maladaptive overactivation of the response, or as an adaptive process which is specifically tuned to over-activate because the fitness cost of the response is outweighed by the fitness benefit – called the smoke detector principle. The fact that such experiences are highly distressing, debilitating and inappropriate leads to their diagnosis as mental disorders.

Mutation-selection balance

Natural selection acts upon genetic mutations, which are present in every generation, removing those which reduce fitness and increasing the prevalence of those which improve fitness. Mutations are also more likely to reduce fitness than improve it. Biological traits with a large mutational target size, such as brains, where over 80% of the genome is expressed, are especially likely to be suspect to harmful mutations which negatively affect cognitive function, which are then removed by natural selection. Such mutations are often associated with intellectual disability, certain cases of autism, schizophrenia, and many more disorders. The fact that de-novo mutations cause such disorders in a few cases has been used to argue that the other cases are caused by as-yet undiscovered disease processes, although the presence of heterogeneity within disorder categories and the lack of discovered pathology despite significant work in neuroscience and genetics is evidence against that view.

Evolutionary explanations for specific disorders

Schizophrenia

Main article: Evolution of schizophrenia

Schizophrenia is primarily characterized by psychosis (hallucinations and delusions) and symptoms of cognitive debilitation such as erratic speech, lost interest in normal activities and disordered thinking. It is the most extreme condition of the schizophrenia or psychosis spectrum, which includes schizotypy and other psychotic disorders, arguably extending to unusual experiences such as perceiving ghosts or believing in magic which are common in the population.

Schizophrenia is a heritable condition, prevalent in slightly less than 1% of the population, with negative effects of fecundity, especially in men. Because of this, it was perhaps the first psychiatric condition explicitly raised as specifically requiring an evolutionary explanation, in the so-called 'schizophrenia paradox' (now more generally known as the paradox of common, harmful, heritable mental disorders). To explain schizophrenia's persistence various evolutionary hypotheses have been made.

Hypotheses of schizophrenia as a true dysfunction are plentiful. It has been hypothesised that schizophrenia is a dysfunctional byproduct of human evolution for language and brain hemisphere lateralization, or a dysfunction of the social brain, or related to theory-of-mind. Other theories have referred to the possibility it is caused by mutation-selection balance. However, the expected rare and de novo mutations have only been found in a small proportion of cases. Many alleles predisposing to schizophrenia are common in the population, making adaptive hypotheses plausible, as has been noted since the mid 20th century.

Hypotheses explaining schizophrenia as resulting from adaptation vary widely. Early theorists proposed it conveyed improvements to the immune system or illness recovery or facilitates group-splitting. Inspired by the longstanding cultural ideas of madness as related to genius, Nettle proposed that schizotypy could be related to creative success, which added to mating success, and that the positive effects of schizotypal traits might be an explanation for why these traits persist. However, the measured fecundity benefit of such traits has been found to not outweigh the cost of schizophrenia via inclusive fitness (although this may be due to selection bias).

The shamanism hypothesis of schizophrenia states that in traditional societies the experience of psychosis facilitated the induction of shamans (magico-religious practitioners such as medicine men, diviners, witch doctors, exorcists and mediums). Shamanism is a common feature of human societies, with certain individuals deemed to have a particular connection to the supernatural world which gives them the ability to perform magic, especially healing. This in particular is used explain the common religious and grandiose content of psychotic experiences and the belief in supernatural powers, which may have been believed rather than disbelieved in traditional societies. The onset of schizophrenia also closely resembles shamanic initiations, which often feature hallucinations, delusions and incoherent speech. Possible links between shamanism and insanity have been recognised for many decades by anthropologists (e.g. "...mentally ill people are often regarded as holy in primitive societies" and "Feeblemindedness is treated with scorn in Niue today, but insanity still calls forth respect") but the most recent iteration of the theory is by Joseph Polimeni, who argues that shamans facilitate group functioning, and so psychosis evolved as a result of group selection. Critics have argued that the trance states and self-control exhibited by shamans are unlike the characteristics of schizophrenia.

Attention Deficit Hyperactivity Disorder (ADHD)

Attention-Deficit/Hyperactivity Disorder explores the origins of mental disorders by considering how certain traits may have been advantageous in ancestral environments. Attention-Deficit/Hyperactivity Disorder (ADHD) is examined within an Evolutionary psychiatry framework understands why behaviors associated with the disorder persist in the human population.

One hypothesis suggests that traits like hyperactivity, impulsivity, and novelty-seeking were beneficial for hunter-gatherer societies. These characteristics could enhance survival by promoting exploration, quick decision-making, and adaptability in changing environments. The "hunter versus farmer" theory posits that while such traits were advantageous for nomadic hunters, they became less suitable with the advent of sedentary agricultural societies, leading to conflicts with modern social structures and expectations.

Proponents of this view argue that ADHD behaviors are not inherently pathological but are mismatches between ancient adaptive traits and contemporary environments. This argument fits into larger idea of neurodiversity and encourages a reevaluation of ADHD, promoting understanding and adaptation rather than solely focusing on symptom management. Some psychiatrists have even begun to cater to evolutionary framework of ADHD.

Critics caution that evolutionary explanations may oversimplify the complex interplay of genetic, environmental, and neurodevelopmental factors that contribute to ADHD. They emphasize the importance of evidence-based approaches in diagnosis and treatment, noting that while evolutionary theories provide interesting insights, they should not replace established scientific methodologies. Though the enticing idea that ADHD was once advantageous is catching up in media.

Autism spectrum disorder (ASD)

See also: Digital media use and mental health § Autism

Autism spectrum disorder is characterized by difficulties with social interaction and communication, and restricted and repetitive behavior. In developed countries, about 2.8% of children (1 in 36) are diagnosed with ASD as of 2023, up from 0.7% in 2000 in the United States. as of 2017, up from 0.7% in 2000 in the United States. It is diagnosed four-to-five times more often in males than females.

Autism differs widely between individuals (it is highly heterogenous) with different causes for different individuals. Some cases are caused by deleterious mutations or prenatal and neonatal trauma, for which no adaptive explanation is required. These cases are often associated with intellectual disability. Estimates range that between 5–20% of the autism spectrum can be explained by these dysfunctional processes, especially of genetics. However, other cases of autism are eligible for adaptive explanations. The fact that multiple explanations for autism exist causes conflict within the autism community, especially between proponents of the neurodiversity perspective and family members caring for individuals with ASD that have severe disabilities.

The idea of autism as conveying cognitive strengths has become steadily more popular since the film Rain Man and the recent growth of the neurodiversity and autism rights movements, although recognition of unusual autistic ability be found even in the early writings of Hans Asperger who called his autistic patients 'little professors'. It has been suggested by autistics such as Temple Grandin that autistic hunter-gatherer ancestors were important figures in the community, especially for their inventive capacity:

'Who do you think made the first stone spear? (...) That wasn't the yakkity yaks sitting around the campfire. It was some Asperger sitting in the back of a cave figuring out how to chip rocks into spearheads. Without some autistic traits you wouldn't even have a recording device to record this conversation on."

Leading autism researcher Simon Baron-Cohen has proposed that autism is an extreme systemising cognitive type, on an empathising-systemising spectrum which all people fall onto, somewhat related to the things-people dimension of interests. He recognised the exceptional talent of many autistic people in some area of non-human knowledge or skill. In his book, "The Pattern Seekers: how autism drives human invention", he proposes a theory of human inventiveness that places autistic individuals as having extreme versions of these inventing (or systemising) traits.

Marco del Giudice has suggested autistic-like traits in their non-pathological form contribute to a male-typical strategy geared toward high parental investment, low-mating effort, and long-term resource allocation. He has also related this to a slow life history strategy. This is based on the fact that autistics show lower interest in short-term mating, higher partner-specific investment, and stronger commitment to long-term romantic relations.

Bernard Crespi has suggested that autism is a disorder of high intelligence, noting that autism commonly involves enhanced, but imbalanced, components of intelligence. This hypothesis is supported by evidence showing that autism and high IQ share a diverse set of convergent correlates, including large brain size, fast brain growth, increased sensory and visual-spatial abilities, enhanced synaptic functions, increased attentional focus, high socioeconomic status, more deliberative decision-making, profession and occupational interests in engineering and physical sciences, and high levels of positive assortative mating. Recent evolutionary selection pressures for high intelligence in humans have therefore conveyed autism risk.

Antisocial personality disorder

Antisocial personality disorder (sometimes known as sociopathy or psychopathy), is characterised by deceitfulness, lack of empathy and guilt, impulsiveness, and antisocial behaviour. The prevalence of psychopathy in the general population is estimated to be around 1%, and 20% in prison populations with higher rates in North America than Europe. Psychopathy, narcissism and Machiavellianism are considered to be part of the Dark Triad, traits that are generally characterised by selfishness and low agreeableness.

Various evolutionary hypotheses have been proposed to explain psychopathy and the Dark Triad. Within an ancestral context, high self-interest and low levels of empathy could function as a short-term mating strategy. There is evidence that Dark Triad traits are positively correlated with the number of sexual partners, more unrestrictive sociosexuality and preference for short-term mates.

Glenn et al. stated two theories on how selection might allow for psychopathic traits. The first is as a fast life-history strategy, associated with less focusing on the future, high risk taking and short-term mating. The second is mutation-selection balance, with many common alleles of small effect selected against, which, when accumulated, can result in psychopathic behaviour, without any significant disruption of reproductive fitness.

Mealey's influential account states that psychopaths are designed for social deception and evolved to pursue manipulative life strategies or cheating strategies, (reflected in cheater-cooperation models of game theory). Cheating strategies are stable at low frequencies in the population, but will be detected and punished at higher frequencies. This frequency-dependent strategy would explain the prevalence of psychopathic traits in the population.

Mealey makes four statements about psychopathy:

1. There is a genetic predisposition for psychopathy, which is normally distributed in all populations.
2. A few individuals will be deemed "morally insane" in any culture, due to selection filling in this small and frequency-dependent niche.
3. Depending on environmental conditions, individuals who are less extreme on the continuum will pursue a similar cheating strategy.
4. An underlying genetic continuum of psychopathy is present in all of us, becoming apparent when antisocial strategies are more profitable in certain conditions.

         Mealey also explains the higher male prevalence and predisposing environmental factors (low physical attractiveness, age, health, physical attractiveness, intelligence, socioeconomic status, and social skills) as signals that a cheating strategy is preferable, hence why these factors are associated with psychopathic traits.

Depression

Main article: Evolutionary approaches to depression

Major depressive disorder (MDD) is characterized by at least two weeks of persistent low mood. It is accompanied by a wide variety of negative feelings such as low self-esteem, loss of interest in normally enjoyable activities and low energy. There are multiple possible evolutionary explanations for the occurrence of depression and low mood in humans. Many different hypotheses are not mutually exclusive. It has been suggested that different life events and other disease processes are responsible for different forms of depression with subtypes related to infection, long-term stress, loneliness, traumatic experience, hierarchy conflict, grief, romantic rejection, postpartum events, the season, chemicals, somatic diseases and starvation. Individualising treatment based on causational subtypes is suggested as lending direction in treatments. Other hypotheses include:

Social-oriented hypotheses

The social competition hypothesis (similarly to the social rank theory) interprets depression as an emotion of submission, an involuntary strategy to create a subjective sense of incapacity. Feelings of powerlessness or helplessness cause this incapacity, inhibiting aggression towards higher-ranked people and signalling submission. Low mood encourages acceptance of a loss in rank and promotes yielding. John Price endorsed this theory, noting that chickens who lose a fight withdraw from social engagement and act submissively, reducing further attacks by chickens higher in the hierarchy and avoiding being wounded or even killed.

Similar to the social competition hypothesis, the 'social risk hypothesis' states that depression prevents people from engaging in social interactions that might lead to them being ostracised. This hypothesis is inspired by risk-sensitive foraging. It suggests that people in successful social relationships can tolerate higher levels of social risk-taking, while on the other hand, people with low social standing cannot. The theory suggests that the low mood which accompanies MDD exists in order to reduce potential risk taking and encourages isolation in those individuals.

Psychic pain hypotheses

Depression is common in people who are pursuing unreachable goals and depression might be a manifestation, similarly to the social competition hypothesis, of a failure to yield. Low mood increases an organism's ability to cope with the adaptive challenges characteristic of unpropitious situations. Pessimism and lack of motivation may give a fitness advantage by inhibiting certain actions. When current life plans are not working, the distress and lack of motivation that characterize depression may motivate planning and reassessment or escape, even by suicide. Feelings of sadness and discouragement may be a useful stimulus to consider ways of changing the situation, by disengagement of motivation from an unreachable goal. In nature, it would make sense to decrease motivation in situations where taking action would be futile and therefore a waste of resources. Therefore, low mood in those situations would help the individual to preserve energy. This hypothesis is inspired by the marginal value theorem.

The 'analytical rumination' hypothesis is a refinement of the psychic pain hypothesis. It suggests that depressive symptoms are triggered by complex problems and an inability to find the correct course of action. This theory describes how this could lead to a loss of interest in virtually all activities in order to benefit the individual to single-mindedly focus on the problem at hand.

Cry-for-help and bargaining hypotheses

Depression, deliberate self-harm and suicide may be reactions to life circumstances that encourage others to provide resources and help to the depressed or suicidal individual. Group members, and especially family members, have a vested interest in keeping the depressed individual alive and changing their circumstances in such a way as to make them a functioning member of society again. It may be the case that certain life choices (e.g. marrying somebody who your parents dislike) may become possible only when depressed or suicidal behaviour is observed by the family or social group. This could explain various precipitating factors for depression. However, some research has found that signs of depression only lead to a short-term increase of care by family members, after which they tend to withdraw.

Eating disorders

Evolutionary perspectives exist on Anorexia nervosa (henceforth 'anorexia') and Bulimia nervosa (henceforth 'bulimia'). Anorexia is characterized by restriction of food intake, bulimia by cycles of binging (excessive eating) and purging (forced removal of the food). Both are associated with fear of weight gain, body image disturbance, and physical attractiveness concerns.

The Sexual Competition Hypothesis relates eating disorders to body shape and physical appearance as of adaptive function in human females (who are highly over-represented in eating disorders): eating disorders are supposed to increase female attractiveness. Some evidence from non-clinical and clinical populations support this hypothesis. They apply the framework of life-history theory, proposing anorexia as a slow life history strategy whilst bulimia is a fast strategy. Both studies had their limitations and it was further mentioned that the deep structures of eating disorders may not be reflected by their current classifications.

An alternative account comes from Nesse. Recognising that many anorexia patients are neither actively chasing men nor particularly interested in sex, and that eating disorders became more common in the second half of the 20th century, he argues eating disorders are new problems with no redeeming features. They are caused by increasingly high concerns about appearance linked with the possibility in modern societies to compare someone's appearance to thousands of others instantly. Glorification of unrealistic body types in media, as well as increased availability of sex, may contribute to this. He does, however, acknowledge that intra-sexual competition is a driving force of anorexia and bulimia in undergraduate women.

Obesity is not an eating disorder in any classification system, though it is established that overweight and obesity in particular is connected with various diseases, and an evolutionary perspective can explain the tendency towards overeating. The human body has evolved to cope with the environments of scarcity, selecting for beneficial adaptations of hunger and eating. Fat storage allows preparation for future food shortages. In a case of mismatch, modern environments have cheap, readily available food, and very few times of scarcity. Kardum et al. also elaborated the differences in nutrient composition in modern and ancestral societies to illustrate the challenge modern diet imposes on the not-yet adapted human body and genotype.

Anxiety

Anxiety is a feeling of worry, unpleasantness and dread towards possible future events and exists to protect us from dangers. In the US, anxiety disorders are the most common mental illness, with around 29% of adults expected to have any anxiety disorder in their lifetime. Women are disproportionately affected.

Evolutionary perspectives on anxiety disorders generally consider the adaptive function of the emotion of normal anxiety, and reasons this adaptive system may manifest in the various types of anxiety disorder.

A key evolutionary explanation for anxiety disorder is the Smoke Detector Principle. It is often preferable to overactivate anxiety in dangerous situations, in the same way a smoke detector is designed to overactivate. Randolph Nesse writes:

"You are thirsty on the ancient African savanna and a watering hole is just ahead, but you hear a noise in the grass. It could be a lion, or it might just be a monkey. Should you flee? It depends on the costs. Assume that fleeing in panic costs 100 calories. Not fleeing costs nothing if it is only a monkey, but if the noise was made by a lion, the cost is 100'000 calories – about how much energy a lion would get from having you for lunch!"

Next to normal anxiety there are multiple types of anxiety disorders which are all characterised by excessive fear and anxiety. These disorders include: specific phobias (e.g., agoraphobia), generalized anxiety disorder, social anxiety disorder, separation anxiety disorder, panic disorder, and selective mutism.

Treatment

Evolutionary psychiatry has so far primarily concentrated on scientific explanations for mental disorders rather than developing novel treatment approaches. However, there are various consequences of taking an evolutionary perspective on mental disorder for treatment decisions, at an individual and public health level, which make evolutionary psychiatry an important field of future research and application.

Evolutionary explanations for disorders which reframe them as mismatched or otherwise costly adaptations may be taken to imply that treatment is unnecessary – but this is not the view of evolutionary psychiatrists – and is the same mistake made by those who believe evolutionary biology means endorsing eugenics, a version of the naturalistic fallacy – that what is natural (in this case, evolved) is good. Many medical interventions are 'unnatural' in this sense (e.g. contraception and anaesthetic). The explanations of evolutionary psychiatry have no inherent value in directing treatment. Randolph Nesse writes:

"On learning that low mood can be useful, some people conclude that it therefore should not be treated. This mistake is like the one that arose when anethesia was first invented: some doctors refused to use it, even during surgery, because, they said, pain is normal. We must not let new understanding of the utility of low mood interfere with our efforts to relieve mental pain."

Although evolutionary explanations may not affect the necessity for treatment, they can be directive or supportive of treatment, or make current treatment strategies more effective. Proposed benefits of taking an evolutionary perspective on mental disorders have largely come from integrating evolutionary explanations into psychotherapy. Bailey and Gilbert write:

"The evolutionary approach helps to answer three fundamental questions about humanity that go to the heart of professional helping and clinical practice: First, what and who are we as human beings – that is, what is human nature or species 'normality'?; second, how and why do humans develop and/or behave in less than optimal ways – that is, what can evolution tell us about the causes of suffering and psychopathology?; and, third, what can professional helpers and psychotherapists do to ameliorate or even 'cure' the suffering of heart and mind?"

It has been suggested that patients are encouraged and destigmatised by hearing evolutionary explanations for their conditions, with positive effects during cognitive behavioural therapy – integration of knowledge of behavioural genetics, neuroscience and evolutionary psychiatry into psychotherapy has been called 'Informed Cognitive Therapy' by Mike Abrams. Abrams also proposes that recognising the inherited and somewhat immutable nature of certain traits (such as psychopathy and autism) implies that therapists should not try and alter the traits characteristics, but instead provide advice on how to best utilise these cognitive types within the context of modern society. This aligns with the aims and claims of the neurodiversity movement.

Evolutionary explanations for mental disorders, especially of mismatch, have connotations for public health measures and organisational psychology. Disorders which are consequences of novel environments may be rectified or prevented by implementing social structures which better replicate ancestral environments. For example, postpartum depression may be more likely in modern environments where single parents are given sole responsibility in raising a child, which is highly unusual in the context of an evolutionary history of alloparenting and communal care. Reversing this mismatch, social services supporting new mothers in parenting may prevent postpartum depression (see Evolutionary approaches to postpartum depression). Education and employment environments which are particularly likely to cause mental disorders may also be altered to better suit natural human psychological capacities.

Methodology and Evidence Synthesis

A recent contribution to the field is the DCIDE framework, developed to improve the systematic evaluation of evolutionary hypotheses in psychiatry by structuring evidence synthesis across disciplines (e.g. genetic, neuroscientific, epidemiological). The framework aims to mitigate issues of speculative narratives by requiring transparent linkage of proximate mechanisms to evolutionary function and acknowledging alternative hypotheses.

Its adoption may help evolutionary psychiatry align more closely with standards of evidence used in biological psychiatry and evolutionary medicine.

Special Report on Climate Change and Land

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Special_Report_on_Climate_Change_and_Land 

 

Intergovernmental Panel on Climate Change

The United Nations' Intergovernmental Panel on Climate Change's (IPCC) Special Report on Climate Change and Land (SRCCL), also known as the "Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems", is a landmark study from 2019 by 107 experts from 52 countries. The SRCCL provides a comprehensive overview of the entire land-climate system for the first time and decided to enlist land as a "critical resource". The IPCC's 50th session (IPCC-50) formally adopted the SRCCL's Summary for policymakers (SPM) and approved the underlying report. The SPM and the full text of Special Report on Climate Change and Land—in an unedited form—were released on 8 August 2019. The report is over 1,300 pages long and includes the work of 107 experts from 52 countries.

The report is the second of three Special Reports in the current Sixth Assessment Report (AR6) cycle which began in 2015 and will be completed in 2022. The first was Special Report on Global Warming of 1.5 °C, and the third is the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) released on 25 September 2019. The AR6 cycle is considered by the IPCC to be their most ambitious since the panel was formed in 1988.

Previous IPCC assessments cycles and special reports

Since it was established in 1988, the IPCC provides the governments of their 195 member countries, with periodic comprehensive summaries of scientific assessments of the drivers of climate change, including current impact and potential risks. The reports also put forward possible responses in terms of adaptation and mitigation. The IPCC has published five Assessment Reports, including the 1990 IPCC First Assessment Report (FAR), the 1995 IPCC Second Assessment Report also known as Climate Change 1995, the 2001 Third Assessment Report, the IPCC Fourth Assessment Report (AR4) also known as Climate Change 2007, and the Fifth Assessment Report. Each report also includes the "full scientific and technical assessment of climate change, generally in three volumes, one for each of the Working Groups of the IPCC, together with their Summaries for Policymakers, and a Synthesis Report."

SRCCL is part of a series of Special Reports and Assessments in the sixth assessment report (AR6) cycle which began in 2015—following the election of a new Bureau—and concludes in 2022. The IPCC considers the current assessment cycle to be their most ambitious in the 30-year history of the panel. So far, during this cycle, along with the SRCCL, IPCC has published the Special Report on Global Warming of 1.5 °C in October 2018, and the May 2019 Refinement. The third in the series, the Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC), is upcoming. As well, in March 2018, the IPCC held a conference Archived 2019-08-09 at the Wayback Machine in preparation for a special report on cities and climate change in the seventh assessment cycle.

Since its inception, the IPCC had also published over a dozen Special Reports which are included in the ARs. The Special report also includes a Summary for policymakers providing an "assessment of a specific issue and generally follow the same structure as a volume of an Assessment Report and Methodology Reports which are "materials that provide practical guidelines for the preparation of greenhouse gas inventories."

Sections

SRCCL summary for policymakers (SPM)

"Land provides the principal basis for human livelihoods and well-being including the supply of food, freshwater and multiple other ecosystem services, as well as biodiversity. Human use directly affects more than 70%...of the global, icefree land surface... Land also plays an important role in the climate system."

— SRCCL summary for policymakers (SPM)

There are four sections in the Summary for Policymakers (SPM). The first deals with "people, land and climate in a warming world"; the second covers "adaptation and mitigation response options"; the third focuses on "enabling response options" and the fourth considers "action in the near-term". In the first section, the SPM says that human-induced land degradation—with increased consumption and population growth causing more land use for "food, feed, fibre, timber and energy"—has negatively affected ice-free land area globally. Approximately "25–30% of total food produced is lost or wasted" while "821 million people are undernourished" and "2 billion adults now being overweight or obese." In part two, the SPM says that "increased food productivity, dietary choices and food losses and waste reduction, can reduce demand for land conversion." In part 3, suggested response options that will also help eradicate poverty, include "improving access to markets, securing land tenure, factoring environmental costs into food, making payments for ecosystem services, and enhancing local and community collective action". The SPM says that, "Many sustainable land management technologies and practices are profitable within three to 10 years (medium confidence). While they can require upfront investment, actions to ensure sustainable land management can improve crop yields and the economic value of pasture. Land restoration and rehabilitation measures improve livelihood systems and provide both short-term positive economic returns and longer-term benefits in terms of climate change adaptation and mitigation, biodiversity and enhanced ecosystem functions and services."

Chapters

The SRCCL consists of seven chapters, Chapter 1: Framing and Context, Chapter 2: Land-Climate Interactions, Chapter 3: Desertification, Chapter 4: Land Degradation, Chapter 5: Food Security, Chapter 5 Supplementary Material, Chapter 6: Interlinkages between desertification, land degradation, food security and GHG fluxes: Synergies, trade-offs and Integrated Response Options, and Chapter 7: Risk management and decision making in relation to sustainable development. Like all IPCC reports, the SRCCL includes a "Summary for Policymakers".

Chapter 1: Framing and context

..."Neither our individual or societal identities, nor the world's economy would exist without the multiple resources, services and livelihood systems provided by land ecosystems and biodiversity."

— Chapter 1

In the executive summary of Chapter 1, the authors described the land area on earth as "finite" saying that sustainable land use is "fundamental for human well-being."

One of the authors of "Framing and Context", Karlsruhe Institute of Technology's Almut Arneth, told Deutsche Presse-Agentur (DPA) journalists that "The surface of the earth is limited, the population is growing, and more acreage is needed for food and for fibres that can be used for clothing."

Chapter 2: Land-climate interactions

According to the 8 August Carbon Brief in-depth article on the SRCCL, Chapter 2 provides data on livestock methane emissions, about 66% is agricultural methane" and "about 33% of global methane emissions" come from livestock.

Chapter 3: Desertification

In Chapter 3, the authors said that while climate change is exacerbating desertification, there are technologies and innovations including those based on indigenous and local knowledge (ILK), that are available and that could, if put into practice in local regions, "avoid, reduce and reverse desertification, simultaneously contributing to climate change mitigation and adaptation."

Chapter 4: Land degradation

"Unravelling the impacts of climate-related land degradation on poverty and livelihoods is highly challenging. This complexity is due to the interplay of multiple social, political, cultural, and economic factors, such as markets, technology, inequality, population growth, each of which interact and shape the ways in which social-ecological systems respond."

— Chapter 4

While the impact of land degradation on peoples' livelihoods globally is already evident, "particularly those living in vulnerable and poverty-stricken regions", the authors of chapter four do not make a clear connection between climate change and land degradation because of the multiple factors at play which include inequality, population growth, technology, and markets.

Chapter 5: Food security

"The report highlights that climate change is affecting all four pillars of food security: availability (yield and production), access (prices and ability to obtain food), utilization (nutrition and cooking), and stability (disruptions to availability)."

— IPCC press release. 8 August 2019

Climate change has had a negative impact on vegetation at the same time that we are experiencing population growth. Ice-free land is needed to maintain food security. The IPCC report authors cautioned against "converting land" to bioenergy in which energy is produced by burning vegetation instead of burning fossil fuels, and advised countries to "set limits on the amount of land" used for energy crops—biofuels and afforestation. Land use in this way means there is less soil to grow much-needed food crops. In the original draft of the summary for policymakers the warning was even more pronounced, but it was considerably watered down due to the intervention of a group of governments led by Brazil, United States, United Kingdom and Sweden.

According to Chapter 5, with extreme weather events, rising temperatures increase, and either not enough or too much rain, there is more food insecurity. In some cases, yields of maize and wheat, for example, have increased in higher altitudes while yields of the same crops have declined in regions with lower altitudes. They also discuss "climate change-induced displacement and migration" in "eight countries in Asia, Africa and Latin America." They said that in countries such as, Guatemala, Honduras, and Nicaragua where 30% of the agriculture is dependent on rainfall, food security is undermined. They cited studies that show that migration from Mexico and Central America, fluctuates in "response to climate variability". Furthermore, in these countries, the "food system is heavily dependent on maize and bean production and long-term climate change and variability significantly affect the productivity of these crops and the livelihoods of smallholder farmers."

Chapter 6: Interlinkages

"IPCC does not recommend people's diets. What we've pointed out on the basis of the scientific evidence is that there are certain diets that have a lower carbon footprint — Jim Skea."

— Jim Skea. SRCCL Facebook 8 August 2019

Chapter 6, "Interlinkages between desertification, land degradation, food security and GHG fluxes: Synergies, trade-offs and Integrated Response Options", offers pathways of mitigating the effects of global climate change on land use, such as reduced deforestation and agricultural diversification. In addition, Chapter 6 also says that a shift in consumer behaviour towards a more plant-based diet with less protein from livestock, such as cattle, "sheep, buffalo and goats" would result in lower emissions. One of the three lead authors for Chapter 6, the University of Aberdeen's environmental scientist, Pete Smith, who teaches plant and soil science, said that as growing conditions deteriorate, there will be a "massive pressure for migration." Ruminant livestock not only produce a lot of methane, a powerful greenhouse gas, but the "deforestation in critical forest systems" is caused by need for grazing land in countries such as Brazil. Smith clarified that, "We're not telling people to stop eating meat. In some places people have no other choice. But it's obvious that in the West we're eating far too much."

Rutgers University's Department of Human Ecology at the School of Environmental and Biological Sciences's associate professor, Pamela McElwee, who is one of Chapter 6's co-authors, said that "dietary change"—"particularly in developed countries of the West"—such as reducing "excess" consumption of "high-greenhouse-gas-emissions diets" high in lamb and cattle consumption is a "win-win" because it addresses both climate crisis and health problems at the same time. McElwee said that food security in a warming world is a major concern as there is potential for "food crises developing "on several continents at once," as quoted in the New York Times. Climate change can be a multiplier effect on existing problems like "rate of soil loss and land degradation" which heightens the risk of "severe food shortages." While there are a lot of actions available to combat these problems, McElwee said that "what some of these solutions do require is attention, financial support, enabling environments." The science on which the report is based shows that "increasing the productivity of land, wasting less food" and shifting diets "away from cattle and other types of meat" lowers the carbon footprint. According to an 8 August 2019 PBS NewsHour report, about 15% of current emissions could be cut by mid-century if "people change their diets, reducing red meat and increasing plant-based foods, such as fruits, vegetables and seeds." Rosenzweig added, it "would also make people more healthy."

Chapter six discusses material substitution as it relates to sustainable long-term forest management, with more production of harvested wood products (HWPs) which have a lower carbon footprint. HWP that are used to as a substitute for metal, plastic, or concrete— which are more emissions-intensive—would also result in removing carbon dioxide (CO₂) from the atmosphere. In Canada, for example, Margot Hurlbert said that forest industry could increase the percentage of solid wood products that it harvests and decrease the amount of pulp and paper products.

Chapter 7: Risk management and decision making in relation to sustainable development

Risk management, which "reduces vulnerabilities in land and the food system" and increases resilience in communities, can include making dietary changes and growing a "variety of crops", which can prevent soil degradation. It also includes sharing risks, "reducing inequalities, and improving incomes." Other "ways to adapt to the negative effects of climate change, include ensuring that there is "equitable access to food" particularly in those regions where "land cannot provide adequate food".

University of Regina's Margot Hurlbert, who is also Canada Research Chair in climate change, energy and sustainability policy, is co-ordinating co-author of "Risk management and decision making in relation to sustainable development", said that, "...there's lots of opportunities to make these changes, and it's not too late." This includes using natural shade, through agroforestry, for example, to grow crops, and—in tropical climates—using biochar as a fertilizer. Hurlbert said that countries, like Canada, can "wield influence through its own use of trade conditions and policies" to "ensure imported food is grown without damaging landscapes and widening deserts overseas". Consumers, as well as organizations and governments, can use sustainability certifications when sourcing wood products and food, for example.

Main statements

"Land is where we live. Land is under growing human pressure. Land is a part of the solution. But land can't do it all."

— IPCC. 9 August 2019. Twitter

This report provided a comprehensive overview of the entire land-climate system for the first time and addressed land itself as a "critical resource".

In a 9 August United Nations video, Valerie Masson-Delmotte cited the newly released report, saying that there are currently over 500 million people who live in areas negatively affected by climate change on land transformed by land degradation or desertification. Many are forced to migrate. The international group of 107 authors urged all nations to adopt sustainable land use in order to "limit greenhouse gas emissions before it is too late" and to work together to build long-term food security systems to support farmers with programs that will help build their resilience and help them engage in the market.

IPCC vice-chair and ANU Climate Institute director Mark Howden, who is one of the authors of the SRCCL, said that "the land sector is currently contributing to climate change" and better land management "would have multiple economic, environmental and health benefits" and would deliver "win-wins for farmers, communities, governments and biodiversity but also helps address climate change."

Contributors

The scientific leadership of the SRCCL included 107 experts from 52 countries as coordinating lead authors and lead authors in the three IPCC Working Groups as well as the Task Force on National Greenhouse Gas Inventories with support from the Working Group III Technical Support Unit. Fifty three percent of the authors of the report are from developing countries and forty percent of the coordinating lead authors are women.

Reactions

The Economist stated that the report "fires another warning shot about the state of the planet and the way people are transforming virtually every corner of every continent. Human activities affect roughly three-quarters of Earth's ice-free land, with huge consequences for the climate." The BBC published a series of articles related to the publication of the report. Roger Harrabin, a BBC environment analyst, said that the report would "become the most authoritative report yet on the way we use and abuse the land." An 8 August 2019 article by New York Times (NYT) highlighted the facts that the SRCCL was "prepared by more than 100 experts from 52 countries" and that it found that a "half-billion people already live in places turning into desert, and soil is being lost between 10 and 100 times faster than it is forming." The NYT also pointed to the fivefold increase in migrants at the United States/Mexico border from El Salvador, Guatemala and Honduras "coinciding with a dry period that left many with not enough food." Some scientists "suggested it bears the signal of climate change." An article in The Atlantic was titled, "This Land Is the Only Land There Is". The article said that the SRCCL is a milestone because it says that ice-free land itself, which humans need for growing food, is "scarce and precious". The article pointed to some climate scientists and researchers having, prior to SRCCL, sometimes portrayed land as a "limitless" cleansing "global sponge", in the sense that replanting sufficient numbers of trees would significantly reduce the net CO₂ burden. The Washington Post called SRCCL a "landmark study".

Deforestation and climate change

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Deforestation_and_climate_change 

Deforestation in the tropics – given as the annual average between 2010 and 2014 – was responsible for 2.6 billion tonnes of CO₂ per year. That was 6.5% of global CO₂ emissions.

Deforestation is a primary contributor to climate change, and climate change affects the health of forests. Land use change, especially in the form of deforestation, is the second largest source of carbon dioxide emissions from human activities, after the burning of fossil fuels. Greenhouse gases are emitted from deforestation during the burning of forest biomass and decomposition of remaining plant material and soil carbon. Global models and national greenhouse gas inventories give similar results for deforestation emissions. As of 2019, deforestation is responsible for about 11% of global greenhouse gas emissions. Carbon emissions from tropical deforestation are accelerating.

When forests grow they are a carbon sink and therefore have potential to mitigate the effects of climate change. Some of the effects of climate change, such as more wildfires, invasive species, and extreme weather events can lead to more forest loss. The relationship between deforestation and climate change is one of a positive (amplifying) climate feedback. The more trees that are removed, the greater the effect of climate change which then results in the loss of more trees.

Forests cover 31% of the land area on Earth. Every year, 75,700 square kilometers (18.7 million acres) of the forest is lost. There was a 12% increase in the loss of primary tropical forests from 2019 to 2020.

Deforestation has many causes, among them agricultural clearcutting, livestock grazing, logging for timber, and wildfires.

Causes of deforestation

Forest area net change rate per country in 2020

Causes due to climate change

The rate of global tree cover loss has approximately doubled since 2001, to an annual loss approaching an area the size of Italy.

Amazon slash-and-burn agriculture, Colombia

Globally, wildfires and deforestation have reduced forests' net absorption of greenhouse gases, reducing their effectiveness at mitigating climate change. Global warming increases forest fires that release more greenhouse gases, creating a feedback loop that causes more warming.

 

Over recent decades, "forest disturbance" (damage) by fire has increased in most of the planet's forest zones. The increase in area, frequency, and severity of forest fires creates a positive feedback that increases global warming.

Another cause of deforestation is due to the effects of climate change: More wildfires, insect outbreaks, invasive species, and more frequent extreme weather events (such as storms) are factors that increase deforestation.

A study suggests that "tropical, arid and temperate forests are experiencing a significant decline in resilience, probably related to increased water limitations and climate variability" which may shift ecosystems towards critical transitions and ecosystem collapses. By contrast, "boreal forests show divergent local patterns with an average increasing trend in resilience, probably benefiting from warming and CO₂ fertilization, which may outweigh the adverse effects of climate change". It has been proposed that a loss of resilience in forests "can be detected from the increased temporal autocorrelation (TAC) in the state of the system, reflecting a decline in recovery rates due to the critical slowing down (CSD) of system processes that occur at thresholds".

23% of tree cover losses result from wildfires and climate change increase their frequency and power. The rising temperatures cause massive wildfires especially in the Boreal forests. One possible effect is the change of the forest composition. Deforestation can also cause forests to become more fire prone through mechanisms such as logging.

Effects of deforestation on climate change

Biophysical mechanisms by which forests influence climate

Irreversible deforestation would result in a permanent rise in the global surface temperature. Moreover, it suggests that standing tropical forests help cool the average global temperature by more than 1 °C or 1.8 °F. Deforestation of tropical forests may risk triggering tipping points in the climate system and of forest ecosystem collapse which would also have effects on climate change.

Several studies since the early 1990s have shown that large-scale deforestation north of 50°N leads to overall net global cooling while tropical deforestation produces substantial warming. Carbon-centric metrics are inadequate because biophysical mechanisms other than CO₂ impacts are important, especially the much higher albedo of bare high-latitude ground vis-à-vis intact forest.

Deforestation, particularly in large swaths of the Amazon, where nearly 20% of the rainforest has been clear cut, has climactic effects and effects on water sources as well as on the soil. Moreover, the type of land usage after deforestation also produces varied results. When deforested land is converted to pasture land for livestock grazing it has a greater effect on the ecosystem than forest to cropland conversions. Other effect of deforestation in the Amazon rainforest is seen through the greater amount of carbon dioxide emission. The Amazon rainforest absorbs one-fourth of the carbon dioxide emissions on Earth, however, the amount of CO₂ absorbed today decreases by 30% than it was in the 1990s due to deforestation.

Modeling studies have concluded that there are two crucial moments that can lead to devastating effects in the Amazon rainforest which are increase in temperature by 4 °C or 7.2 °F and deforestation reaching a level of 40%.

Forest fires

Main article: Wildfire

Statistics have shown a direct correlation between forest fires and deforestation. Statistics regarding the Brazilian Amazon area during the early 2000s have shown that fires and the air pollution that accompanies these fires mirror the patterns of deforestation and "high deforestation rates led to frequent fires".

The Amazon rainforest has experienced fires inside the forest as well as wildfires on its outer edges. Wetlands have faced an increase in forest fires as well. Due to the change in temperature, the climate around forests have become warm and dry, conditions that allow forest fires to occur.

Under unmitigated climate change, by the end of the century, 21% of the Amazon would be vulnerable to post‐fire grass invasion. In 3% of the Amazon, fire return intervals are already shorter than the time required for grass exclusion by canopy recovery, implying a high risk of irreversible shifts to a fire‐maintained degraded forest grassy state. The south‐eastern region of the Amazon is currently at highest risk of irreversible degradation.

According to a study in tropical peatland forest of Borneo, deforestation also contributes to the increase in fire risk.

Carbon sequestration through forestry

Further information: Carbon dioxide removal

Forests are an important part of the global carbon cycle because trees and plants absorb carbon dioxide through photosynthesis. Therefore, they play an important role in climate change mitigation. By removing the greenhouse gas CO₂ from the air, forests function as terrestrial carbon sinks, meaning they store large amounts of carbon in the form of biomass, encompassing roots, stems, branches, and leaves. By doing so, forests sequester approximately 25% of human carbon emissions annually, playing a critical role in Earth's climate. Throughout their lifespan, trees continue to sequester carbon, storing atmospheric CO₂ long-term. Sustainable forest management, afforestation, reforestation are therefore important contributions to climate change mitigation.

An important consideration in such efforts is that forests can turn from sinks to carbon sources. In 2019 forests took up a third less carbon than they did in the 1990s, due to higher temperatures, droughts and deforestation. National-scale forest inventory data also shows trends from 1999 to 2020 that some forests were already approaching climate thresholds shifting them from carbon sinks to carbon sources. The typical tropical forest may become a carbon source by the 2060s.

Researchers have found that, in terms of environmental services, it is better to avoid deforestation than to allow for deforestation to subsequently reforest, as the latter leads to irreversible effects in terms of biodiversity loss and soil degradation. Furthermore, the probability that legacy carbon will be released from soil is higher in a younger boreal forest. In particular, boreal forests have been noted to support the growth of Armillaria (honey fungus), which is a root pathogen that breaks down compounds necessary for wood integrity, increasing the likelihood of carbon release. Global greenhouse gas emissions caused by damage to tropical rainforests may have been substantially underestimated until around 2019. Additionally, the effects of afforestation and reforestation will be farther in the future than keeping existing forests intact. It takes much longer − several decades − for the benefits for global warming to manifest to the same carbon sequestration benefits from mature trees in tropical forests and hence from limiting deforestation. Therefore, scientists consider "the protection and recovery of carbon-rich and long-lived ecosystems, especially natural forests" to be "the major climate solution".

The planting of trees on marginal crop and pasture lands helps to incorporate carbon from atmospheric CO
₂ into biomass. For this carbon sequestration process to succeed the carbon must not return to the atmosphere from biomass burning or rotting when the trees die. Several species of Ficus such as Ficus wakefieldii have been observed to sequester atmospheric CO₂ as calcium oxalate in the presence of oxalotrophic bacteria and fungi, which catabolize the oxalate, which produces calcium carbonate. The calcium carbonate is precipitated throughout the tree, which also alkalinizes the surrounding soil. These species are current candidates for carbon sequestration in agroforestry. This Calcium-oxalate fixation process was first observed in the Iroko tree, which can sequester up to a ton of calcium carbonate in the soil over its lifespan. Also Cacti, such as the Saguaro, transfer carbon from the biological cycle to the geological cycle by forming the mineral calcium carbonate.

Earth offers enough room to plant an additional 0.9 billion ha of tree canopy cover, although this estimate has been criticized, and the true area that has a net cooling effect on the climate when accounting for biophysical feedbacks like albedo is 20-80% lower. Planting and protecting these trees would sequester 205 billion tons of carbon if the trees survive future climate stress to reach maturity. To put this number into perspective, this is about 20 years of current global carbon emissions (as of 2019). This level of sequestration would represent about 25% of the atmosphere's carbon pool in 2019.

Life expectancy of forests varies throughout the world, influenced by tree species, site conditions, and natural disturbance patterns. In some forests, carbon may be stored for centuries, while in other forests, carbon is released with frequent stand replacing fires. Forests that are harvested prior to stand replacing events allow for the retention of carbon in manufactured forest products such as lumber. However, only a portion of the carbon removed from logged forests ends up as durable goods and buildings. The remainder ends up as sawmill by-products such as pulp, paper, and pallets. If all new construction globally utilized 90% wood products, largely via adoption of mass timber in low rise construction, this could sequester 700 million net tons of carbon per year. This is in addition to the elimination of carbon emissions from the displaced construction material such as steel or concrete, which are carbon-intense to produce.

A meta-analysis found that mixed species plantations would increase carbon storage alongside other benefits of diversifying planted forests.

Although a bamboo forest stores less total carbon than a mature forest of trees, a bamboo plantation sequesters carbon at a much faster rate than a mature forest or a tree plantation. Therefore, the farming of bamboo timber may have significant carbon sequestration potential.

The Food and Agriculture Organization (FAO) reported that: "The total carbon stock in forests decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020". In Canada's boreal forests as much as 80% of the total carbon is stored in the soils as dead organic matter.

The IPCC Sixth Assessment Report says: "Secondary forest regrowth and restoration of degraded forests and non-forest ecosystems can play a large role in carbon sequestration (high confidence) with high resilience to disturbances and additional benefits such as enhanced biodiversity."

Impacts on temperature are affected by the location of the forest. For example, reforestation in boreal or subarctic regions has less impact on climate. This is because it substitutes a high-albedo, snow-dominated region with a lower-albedo forest canopy. By contrast, tropical reforestation projects lead to a positive change such as the formation of clouds. These clouds then reflect the sunlight, lowering temperatures.

Planting trees in tropical climates with wet seasons has another advantage. In such a setting, trees grow more quickly (fixing more carbon) because they can grow year-round. Trees in tropical climates have, on average, larger, brighter, and more abundant leaves than non-tropical climates. A study of the girth of 70,000 trees across Africa has shown that tropical forests fix more carbon dioxide pollution than previously realized. The research suggested almost one-fifth of fossil fuel emissions are absorbed by forests across Africa, Amazonia and Asia. Simon Lewis stated, "Tropical forest trees are absorbing about 18% of the carbon dioxide added to the atmosphere each year from burning fossil fuels, substantially buffering the rate of change."

Concerns with forestry projects

This section is an excerpt from Carbon offsets and credits § Concerns with forestry projects.[edit]

Forestry projects have faced increasing criticism over their integrity as offset or credit programs. A number of news stories from 2021 to 2023 criticized nature-based carbon offsets, the REDD+ program, and certification organizations. In one case it was estimated that around 90% of rainforest offset credits of the Verified Carbon Standard are likely to be "phantom credits".

Tree planting projects in particular have been problematic. Critics point to a number of concerns. Trees reach maturity over a course of many decades. It is difficult to guarantee how long the forest will last. It may suffer clearing, burning, or mismanagement. Some tree-planting projects introduce fast-growing invasive species. These end up damaging native forests and reducing biodiversity. In response, some certification standards such as the Climate Community and Biodiversity Standard require multiple species plantings. Tree planting in high latitude forests may have a net warming effect on the Earth's climate because tree cover absorbs sunlight thus creating a warming effect that balances out their absorption of carbon dioxide. Tree-planting projects can also cause conflicts with local communities and Indigenous people if the project displaces or otherwise curtails their use of forest resources.

Changes in rainfall

Further information: Flying river

As a consequence of reduced evapotranspiration, precipitation is also reduced. This implies having a hotter and drier climate, and a longer dry season. This change in climate has drastic ecological and global impacts including increases in severity and frequency of fires, and disruption in the pollination process that will likely spread beyond the area of deforestation.

According to a study published in 2023, tropical deforestation has led to a significant decrease in the amount of observed precipitation. By the year 2100, researchers anticipate that deforestation in the Congo will diminish regional precipitation levels by up to 8-10%.

Decreasing albedo

Deforestation changes the landscape and reflectivity of earth's surface, i.e. decreasing Albedo. This results in an increase in the absorption of light energy from the sun in the form of heat, enhancing global warming.

Policies and programs to reduce deforestation

Main article: Deforestation § Control

Deforestation in Bolivia

Reducing emissions from deforestation and forest degradation in developing countries

Examples of issues addressed by REDD+ in tropical forest landscapes: large-scale deforestation visible from space (top), conversion of carbon-rich peat forests for agriculture such as oil-palm (left), and community-based efforts to reduce forest degradation by creating income incentives for conservation (right).

REDD+ is a voluntary climate change mitigation framework developed under the United Nations Framework Convention on Climate Change (UNFCCC). It aims to encourage developing countries to reduce greenhouse gas emissions from deforestation and forest degradation, and to promote conservation, sustainable forest management, and enhancement of forest carbon stocks through financial incentives and policy support. The acronym is commonly expanded as "reducing emissions from deforestation and forest degradation in developing countries", and the "+" denotes the additional forest conservation and enhancement activities included in the UNFCCC scope.

UNFCCC decisions describe REDD+ as a phased approach, beginning with "readiness" activities (planning, capacity-building and institutional development), moving to implementation of national policies and measures, and evolving toward results-based actions that are fully measured, reported and verified. Countries undertaking REDD+ are expected to develop a national strategy or action plan, establish a forest reference (emission) level (FREL/FRL) as a benchmark for assessing performance, and build a national forest monitoring system to support monitoring, reporting and verification (MRV). UNFCCC decisions also include social and environmental safeguards (often referred to as the Cancún safeguards) and require countries seeking results-based payments to provide information on how safeguards are addressed and respected.

REDD+ is supported through a mix of multilateral and bilateral channels and can receive results-based finance when reported results meet UNFCCC methodological and transparency requirements under the Warsaw Framework on REDD-plus. A 2024 multi-country impact evaluation reported modest average forest outcomes and limited average welfare effects, with impacts not always sustained over time. Reviews and methodological assessments highlight uncertainties in baselines (reference levels), additionality, leakage, non-permanence and measurement capacity, especially for forest degradation and carbon pools that are harder to quantify. Criticisms and controversies also focus on governance and equity issues, including land tenure and carbon rights, benefit sharing, and the participation and consent of Indigenous peoples and local communities, alongside broader debates over the role of forest offsets in climate policy.

REDD+ remains an active part of the UNFCCC and Paris Agreement architecture. Most of the core UNFCCC decisions that define REDD+ were adopted between 2010 and 2015, including the Warsaw Framework on REDD-plus (2013). Countries continue to report REDD+ results through technical annexes to developing-country reports, including under the Paris Agreement's enhanced transparency framework via technical annexes to biennial transparency reports (BTRs). Results-based finance also continues through multilateral channels such as the Green Climate Fund's REDD+ results-based payments window.

The Bali Action Plan

Scioto grove reforestation area

The Bali Action Plan was developed in December 2007 in Bali, Indonesia. It is a direct result of the Kyoto Protocol of December 1997. One of the key elements of The Bali Action Plan involves a concerted effort by the member countries of the Kyoto Protocol to enact and create policy approaches that incentivize emissions reduction caused by deforestation and forest degradation in the developing world. It emphasized the importance of sustainable forest management and conservation practices in mitigating climate change. This coupled with the increased attention to carbon emission stocks as a way to provide additional resource flows to the developing countries.

Trillion Tree Campaign

Main article: Trillion Tree Campaign

Afforestation at Kanakakunnu

The Billion Tree Campaign was launched in 2006 by the United Nations Environment Programme (UNEP) as a response to the challenges of climate change, as well as to a wider array of sustainability challenges, from water supply to biodiversity loss. Its initial target was the planting of one billion trees in 2007. Only one year later in 2008, the campaign's objective was raised to 7 billion trees—a target to be met by the climate change conference that was held in Copenhagen, Denmark in December 2009. Three months before the conference, the 7 billion planted trees mark had been surpassed. In December 2011, after more than 12 billion trees had been planted, UNEP formally handed management of the program over to the not-for-profit Plant-for-the-Planet initiative, based in Munich, Germany.

The Amazon Fund (Brazil)

Four-year plan to reduce in deforestation in the Amazon

The Amazon Fund (in Portuguese: Fundo Amazônia) is an initiative created by the Brazilian Government and managed by the National Bank for Economic and Social Development (BNDES). It was established on 1 August 2008, with the aim of attracting donations for non-reimbursable investments in actions for the prevention, monitoring, and combat of deforestation, and for the promotion of conservation and sustainable use of the Amazon rainforest. Additionally, the fund supports the development of monitoring and control systems for deforestation in the rest of Brazil and in other tropical countries.

The fund is used in various areas, including the management of public forests and protected areas, control, monitoring and environmental enforcement, sustainable forest management, economic activities developed from the sustainable use of the forest, ecological and economic zoning, land planning and regularization, conservation and sustainable use of biodiversity, and the recovery of deforested areas. The projects supported by the fund must be aligned with applicable public policies and the guidelines and criteria, in addition to demonstrating their direct or indirect contribution to the reduction of deforestation and forest degradation. The actions foreseen in the projects must be coherent with the proposed objective, with the budget and with the schedule of its implementation. Eligibility for accessing the Amazon Fund is determined based on compliance with several plans and criteria, including the PPCDAm (Action Plan for Prevention and Control of Deforestation in the Legal Amazon Region), ENREDD+ (National Strategy for REDD+), state plans for preventing and combating deforestation, and BNDES Operational Policies. Projects eligible for funding should directly or indirectly contribute to reducing deforestation in the Amazon. Various types of entities can submit projects for funding, including public administration bodies, NGOs, private companies, cooperatives, and research institutions.

Until 2018, the fund received R$ 3.4 billion in donations, with the majority coming from Norway, followed by Germany and Petrobras. Since 2023, several countries announced contributions to the fund or interest in contributing, including Germany, Norway, the United States, the United Kingdom, Switzerland, Denmark, France, Spain, Japan and others.