Nitrogen fixation

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Nitrogen_fixation

Nitrogen fixation is a chemical process by which molecular dinitrogen (N
₂) is converted into ammonia (NH
₃). It occurs both biologically and abiologically in chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases. These enzyme complexes are encoded by the Nif genes (or Nif homologs) and contain iron, often with a second metal (usually molybdenum, but sometimes vanadium).

Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses, and aquatic ferns such as Azolla. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. It occurs naturally in the air by means of NO_x production by lightning.

Fixed nitrogen is essential to life on Earth. Organic compounds such as DNA and proteins contain nitrogen. Industrial nitrogen fixation underpins the manufacture of all nitrogenous industrial products, which include fertilizers, pharmaceuticals, textiles, dyes and explosives.

History

Schematic representation of the nitrogen cycle. Abiotic nitrogen fixation has been omitted.

Biological nitrogen fixation was discovered by Jean-Baptiste Boussingault in 1838. Later, in 1880, the process by which it happens was discovered by German agronomist Hermann Hellriegel and Hermann Wilfarth [de] and was fully described by Dutch microbiologist Martinus Beijerinck.

"The protracted investigations of the relation of plants to the acquisition of nitrogen begun by de Saussure, Ville, Lawes, Gilbert and others, and culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887."

"Experiments by Bossingault in 1855 and Pugh, Gilbert & Lawes in 1887 had shown that nitrogen did not enter the plant directly. The discovery of the role of nitrogen-fixing bacteria by Herman Hellriegel and Herman Wilfarth in 1886–1888 would open a new era of soil science."

In 1901, Beijerinck showed that Azotobacter chroococcum was able to fix atmospheric nitrogen. This was the first known species of the Azotobacter genus, so-named by him. It is also the first known diazotroph, species that use diatomic nitrogen as a step in the complete nitrogen cycle.

Biological

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a nitrogenase enzyme. The overall reaction for BNF is:

N₂ + 16ATP + 16H₂O + 8e⁻ + 8H⁺ → 2NH₃ +H₂ + 16ADP + 16P_i

The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of H
₂. The conversion of N
₂ into ammonia occurs at a metal cluster called FeMoco, an abbreviation for the iron-molybdenum cofactor. The mechanism proceeds via a series of protonation and reduction steps wherein the FeMoco active site hydrogenates the N
₂ substrate. In free-living diazotrophs, nitrogenase-generated ammonia is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway. The microbial nif genes required for nitrogen fixation are widely distributed in diverse environments.

Nitrogenases are rapidly degraded by oxygen. For this reason, many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as leghemoglobin.

Importance of nitrogen

Atmospheric nitrogen cannot be metabolized by most organisms, because its triple covalent bond is very strong. Most take up fixed nitrogen from various sources. For every 100 atoms of carbon, roughly 2 to 20 atoms of nitrogen are assimilated. The atomic ratio of carbon (C) : nitrogen (N) : phosphorus (P) observed on average in planktonic biomass was originally described by Alfred Redfield, who determined the stoichiometric relationship between C:N:P atoms, The Redfield Ratio, to be 106:16:1.

Nitrogenase

Main article: Nitrogenase

The protein complex nitrogenase is responsible for catalyzing the reduction of nitrogen gas (N₂) to ammonia (NH₃). In cyanobacteria, this enzyme system is housed in a specialized cell called the heterocyst. The production of the nitrogenase complex is genetically regulated, and the activity of the protein complex is dependent on ambient oxygen concentrations, and intra- and extracellular concentrations of ammonia and oxidized nitrogen species (nitrate and nitrite). Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit N_Fix, specifically when intracellular concentrations of 2-oxoglutarate (2-OG) exceed a critical threshold. The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.

Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). Three iron-dependent proteins are known: molybdenum-dependent, vanadium-dependent, and iron-only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase is most common. The different types of nitrogenase can be determined by the specific iron protein component. Nitrogenase is highly conserved. Gene expression through DNA sequencing can distinguish which protein complex is present in the microorganism and potentially being expressed. Most frequently, the nifH gene is used to identify the presence of molybdenum-dependent nitrogenase, followed by closely related nitrogenase reductases (component II) vnfH and anfH representing vanadium-dependent and iron-only nitrogenase, respectively. In studying the ecology and evolution of nitrogen-fixing bacteria, the nifH gene is the biomarker most widely used. nifH has two similar genes anfH and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.

Evolution of nitrogenase

Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga), although there is some isotopic support for nitrogenase evolution as early as around 3.2 Ga. Nitrogenase appears to have evolved from maturase-like proteins, although the function of the preceding protein is currently unknown.

Nitrogenase has three different forms (Nif, Anf, and Vnf) that correspond with the metal found in the active site of the protein (molybdenum, iron, and vanadium respectively). Marine metal abundances over Earth's geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common. Currently, there is no conclusive agreement on which form of nitrogenase arose first.

Microorganisms

Main article: Diazotroph

Diazotrophs are widespread within domain Bacteria including cyanobacteria (e.g. the highly significant Trichodesmium and Cyanothece), green sulfur bacteria, purple sulfur bacteria, Azotobacteraceae, rhizobia and Frankia. Several obligately anaerobic bacteria fix nitrogen including many (but not all) Clostridium spp. Some archaea such as Methanosarcina acetivorans also fix nitrogen, and several other methanogenic taxa, are significant contributors to nitrogen fixation in oxygen-deficient soils.

Cyanobacteria, commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as nitrate, nitrite, ammonium, urea, or some amino acids. Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the Archean eon. Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine. Indeed, the amount of nitrogen fixed in the ocean is at least as much as that on land. The colonial marine cyanobacterium Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen. Species of nitrogen-fixing cyanobacteria in fresh waters include: Aphanizomenon and Dolichospermum (previously Anabaena). Such species have specialized cells called heterocytes, in which nitrogen fixation occurs via the nitrogenase enzyme.

Algae

One type of organelle, originating from cyanobacterial endosymbionts called UCYN-A2, can turn nitrogen gas into a biologically available form. This nitroplast was discovered in algae, particularly in the marine algae Braarudosphaera bigelowii.

Diatoms in the family Rhopalodiaceae also possess cyanobacterial endosymbionts called spheroid bodies or diazoplasts. These endosymbionts have lost photosynthetic properties, but have kept the ability to perform nitrogen fixation, allowing these diatoms to fix atmospheric nitrogen. Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera Hemiaulus, Rhizosolenia and Chaetoceros.

Root nodule symbioses

Main article: Root nodule

Legume family

Nodules are visible on this broad bean root

Plants that contribute to nitrogen fixation include those of the legume family—Fabaceae— with taxa such as kudzu, clover, soybean, alfalfa, lupin, peanut and rooibos. They contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the soil. The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional farming practices, fields are rotated through various types of crops, which usually include one consisting mainly or entirely of clover.

Fixation efficiency in soil is dependent on many factors, including the legume and air and soil conditions. For example, nitrogen fixation by red clover can range from 50 to 200 lb/acre (56 to 224 kg/ha).

Non-leguminous

A sectioned alder tree root nodule

The ability to fix nitrogen in nodules is present in actinorhizal plants such as alder and bayberry, with the help of Frankia bacteria. They are found in 25 genera in the orders Cucurbitales, Fagales and Rosales, which together with the Fabales form a nitrogen-fixing clade of eurosids. The ability to fix nitrogen is not universally present in these families. For example, of 122 Rosaceae genera, only four fix nitrogen. Fabales were the first lineage to branch off this nitrogen-fixing clade; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the most recent common ancestors of all these plants, but only evolved to full function in some of them.

In addition, Trema (Parasponia), a tropical genus in the family Cannabaceae, is unusually able to interact with rhizobia and form nitrogen-fixing nodules.

Non-legumious nodulating plants

Family	Genera	Species
Betulaceae	Alnus (alders)	Most or all species
Boraginaceae	Phacelia	Phacelia tanacetifolia
Cannabaceae	Trema (Parasponia)	Trema orientale Trema lamarckiana
Casuarinaceae	Allocasuarina Casuarina Ceuthostoma Gymnostoma
Coriariaceae	Coriaria	Coriaria arborea Coriaria myrtifolia
Datiscaceae	Datisca
Elaeagnaceae	Elaeagnus (silverberries) Hippophae (sea-buckthorns) Shepherdia (buffaloberries)
Myricaceae	Comptonia (sweetfern) Myrica (bayberries)
Posidoniaceae	Posidonia (seagrass)
Rhamnaceae	Ceanothus Colletia Discaria Kentrothamnus Retanilla Talguenea Trevoa
Rosaceae	Cercocarpus (mountain mahoganies) Chamaebatia (mountain miseries) Dryas Purshia/Cowania (bitterbrushes/cliffroses)

Other plant symbionts

Some other plants live in association with a cyanobiont (cyanobacteria such as Nostoc) which fix nitrogen for them:

• Some lichens such as Lobaria and Peltigera
• Mosquito fern (Azolla species)
• Cycads
• Gunnera
• Blasia (liverwort)
• Hornworts

Some symbiotic relationships involving agriculturally-important plants are:

• Sugarcane and unclear endophytes
• Foxtail millet and Azospirillum brasilense
• Kallar grass and Azoarcus sp. strain BH72
• Rice and Herbaspirillum seropedicae
• Wheat and Klebsiella pneumoniae
• Maize landrace 'Sierra Mixe' / 'olotón' and various Bacteroidota and Pseudomonadota

Industrial processes

Historical

A method for nitrogen fixation was first described by Henry Cavendish in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the Birkeland–Eyde process of 1903. The fixation of nitrogen by lightning is a very similar natural occurring process.

The possibility that atmospheric nitrogen reacts with certain chemicals was first observed by M. Desfosses, a pharmacist from Besançon, in 1828. He observed that mixtures of alkali metal oxides and carbon react with nitrogen at high temperatures. With the use of barium carbonate as starting material, the first commercial process became available in the 1860s, developed by Margueritte and Sourdeval. The resulting barium cyanide reacts with steam, yielding ammonia. In 1898 Frank and Caro developed what is known as the Frank–Caro process to fix nitrogen in the form of calcium cyanamide. The process was eclipsed by the Haber process, which was discovered in 1909.

Haber process

Main article: Haber process

Equipment for a study of nitrogen fixation by alpha rays (Fixed Nitrogen Research Laboratory, 1926)

The dominant industrial method for producing ammonia is the Haber process also known as the Haber-Bosch process in 1909. Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial ecosystem. Ammonia is a required precursor to fertilizers, explosives, and other products. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), which are routine conditions for industrial catalysis. This process uses natural gas as a hydrogen source and air as a nitrogen source. The ammonia product has resulted in an intensification of nitrogen fertilizer globally and is credited with supporting the expansion of the human population from around 2 billion in the early 20th century to roughly 8 billion people now.

Homogeneous catalysis

Main article: Abiological nitrogen fixation

Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of lowering energy requirements. However, such research has thus far failed to approach the efficiency and ease of the Haber process. Many compounds react with atmospheric nitrogen to give dinitrogen complexes. The first dinitrogen complex to be reported was Ru(NH
₃)
₅(N
₂)²⁺.^[77] Some soluble complexes do catalyze nitrogen fixation.

Lightning

Lightning heats the air around it in a high-temperature plasma, breaking the bonds of N
₂, starting the formation of nitrous acid (HNO
₂).

Nitrogen can be fixed by lightning converting nitrogen gas (N
₂) and oxygen gas (O
₂) in the atmosphere into nitrogen oxides (NO_x). The N
₂ molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms. Lightning produces enough energy and heat to break this bond allowing nitrogen atoms to react with oxygen, forming NO_x. These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form nitrogen dioxide (NO
₂), which in turn reacts with water to produce nitrous acid (HNO
₂) or nitric acid (HNO
₃). When these acids seep into the soil, they produce nitrate (NO₃⁻), which is of use to plants.

Causes of gender incongruence

From Wikipedia, the free encyclopedia

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

Gender incongruence is the state of having a gender identity that does not correspond to one's sex assigned at birth. This is experienced by people who identify as transgender or transsexual, and often results in gender dysphoria. The causes of gender incongruence have been studied for decades.

Transgender brain studies, especially those on lesbian trans women, and those on gay trans men, are limited, as they include only a small number of tested individuals. Twin studies indicate that genes play a role in gender incongruence, although the precise genes involved are not known or well understood.

Environmental factors, such as prenatal hormone exposure, have also been investigated but are difficult to test.

Genetics

Gender identity is genetically heritable, but no convincing candidate genes are known. Gender incongruence has been associated with certain alleles relevant to steroidogenesis.

In 2013, a twin study combined a survey of pairs of twins where one or both had undergone, or had plans and medical approval to undergo, gender transition, with a literature review of published reports of transgender twins. The study found that one third of identical twin pairs in the sample were both transgender: 13 of 39 (33%) monozygotic or identical pairs of assigned males and 8 of 35 (22.8%) pairs of assigned females. Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 (2.6%) pairs where both twins were trans. The significant percentage of identical twin pairs in which both twins are trans and the virtual absence of dizygotic twins (raised in the same family at the same time) in which both were trans would provide evidence that transgender identity is significantly influenced by genetics if both sets were raised in different families.

A 2018 review of family and twin studies found that there was "significant and consistent evidence" for gender identity being genetically heritable.

Prenatal hormonal environment

Sex hormones in the prenatal environment differentiate the male and female brain. One hypothesis proposes that transgender individuals may have been exposed to atypical levels of sex hormones during later stages of fetal development, leading to brain structures atypical of their sex assigned at birth.

In people with XX chromosomes, congenital adrenal hyperplasia (CAH) results in heightened exposure to prenatal androgens, resulting in masculinization of the genitalia. Individuals with CAH are typically subjected to medical interventions including prenatal hormone treatment and postnatal genital reconstructive surgeries. Such treatments are sometimes criticized by intersex rights organizations as non-consensual, invasive, and unnecessary interventions. Individuals with CAH are usually assigned female and tend to develop similar cognitive abilities to the typical females, including spatial ability, verbal ability, language lateralization, handedness and aggression. Research has shown that people with CAH and XX chromosomes will be more likely to experience same-sex attraction, and at least 5.2% of these individuals develop serious gender dysphoria.

In males with 5-alpha-reductase deficiency, conversion of testosterone to dihydrotestosterone is disrupted, decreasing the masculinization of genitalia. Individuals with this condition are typically assigned female and raised as girls due to their feminine appearance at a young age. However, more than half of males with this condition raised as females come to identify as male later in life. Scientists speculate that the definition of masculine characteristics during puberty and the increased social status afforded to men are two possible motivations for a female-to-male transition.

Brain structure

Transgender brain studies, especially those on lesbian trans women, and those on gay trans men, are limited, as they include only a small number of tested individuals.

Several studies have found a correlation between gender identity and brain structure. A first-of-its-kind study by Zhou et al. (1995) found that in the bed nucleus of the stria terminalis (BSTc), a region of the brain known for sex and anxiety responses (and which is affected by prenatal androgens), cadavers of six trans women had female-normal BSTc size, similar to the study's cadavers of cisgender women. While the trans women had undergone hormone therapy, and all but one had undergone sex reassignment surgery, this was accounted for by including cadavers of cisgender men and cisgender women as controls who, for a variety of medical reasons, had experienced hormone reversal. The controls still had sizes typical for their sex, and thus no relationship to post-natal hormone levels (nor to sexual orientation) was found. Other post-mortem studies also found brain differences between cisgender and transgender individuals.

In 2002, a follow-up study by Chung et al. found that significant sexual dimorphism in BSTc did not establish until adulthood. Chung et al. theorized that changes in fetal hormone levels produce changes in BSTc synaptic density, neuronal activity, or neurochemical content which later lead to size and neuron count changes in BSTc, or alternatively, that the size of BSTc is affected by the generation of a gender identity inconsistent with one's assigned sex.

In the textbook Adult Psychopathology and Diagnosis, 7th edition, Lawrence and Zucker suggested that the BSTc may not be a valid biomarker for gender incongruence, as differences in size could be caused by gender-affirming hormone therapy or paraphilias, and might not occur in homosexual transsexuals.

In a review of the evidence in 2006, Gooren considered the earlier research as supporting the concept of gender incongruence as a "sexual differentiation disorder" of the sexually dimorphic brain. Dick Swaab (2004) concurred.

In 2008, Garcia-Falgueras & Swaab discovered that the interstitial nucleus of the anterior hypothalamus (INAH-3), part of the hypothalamic uncinate nucleus, had properties similar to the BSTc with respect to sexual dimorphism and gender incongruence, likewise in line with the trans individuals’ declared genders and likewise regardless of if hormonal transition had occurred or not.

A 2009 MRI study by Luders et al. found that among 24 trans women not treated with hormone therapy, regional gray matter concentrations were more similar to those of cisgender men than of cisgender women, but there was a significantly greater volume of gray matter in the right putamen compared to cisgender men. Like earlier studies, researchers concluded that transgender identity was associated with a distinct cerebral pattern. MRI scanning allows easier study of larger brain structures, but independent nuclei are not visible due to lack of contrast between different neurological tissue types, hence other studies on e.g. BSTc were done by dissecting brains post-mortem.

Rametti et al. (2011) studied 18 trans men who had not undergone hormone therapy using diffusion tensor imaging (DTI), an MRI technique which allows visualizing white matter, the structure of which is sexually dimorphic. Rametti et al. discovered that the trans men's white matter, compared to 19 cisgender lesbians, showed higher fractional anisotropy values in posterior part of the right SLF, the forceps minor and corticospinal tract". Compared to 24 cisgender males, they showed only lower FA values in the corticospinal tract. The white matter patterns in trans men were found to be shifted in the direction of cis men.

A 2011 review published in Frontiers in Neuroendocrinology found that "Female INAH3 and BSTc have been found in MtF transsexual persons. The only female-to-male (FtM) transsexual person available to us for study so far had a BSTc and INAH3 with clear male characteristics. (...) These sex reversals were found not to be influenced by circulating hormone levels in adulthood, and seem thus to have arisen during development" and that "All observations that support the neurobiological theory about the origin of transsexuality, i.e. that it is the sizes, the neuron numbers, and the functions and connectivity of brain structures, not the sex of their sexual organs, birth certificates or passports, that match their gender identities".

In 2012 and 2016 studies by Taziaux et al. reported that MtF subjects had infundibular nuclei similar to those of cis women.

A 2015 review reported that two studies found a pattern of white matter microstructure differences away from a transgender person's birth sex, and toward their desired sex. In one of these studies, sexual orientation had no effect on the diffusivity measured.

A 2016 review reported that, for heterosexual trans people, hormone treatment may have large effects on the brain, and that cortical thickness, which is generally thicker in cisgender women's brains than in cisgender men's brains, may also be thicker in trans women's brains, but is present in a different location to cisgender women's brains. It also stated that for both trans women and trans men, "cross-sex hormone treatment affects the gross morphology as well as the white matter microstructure of the brain. Changes are to be expected when hormones reach the brain in pharmacological doses. Consequently, one cannot take hormone-treated transsexual brain patterns as evidence of the transsexual brain phenotype because the treatment alters brain morphology and obscures the pre-treatment brain pattern."

A 2019 review in Neuropsychopharmacology found that among transgender individuals meeting diagnostic criteria for gender dysphoria, "cortical thickness, gray matter volume, white matter microstructure, structural connectivity, and corpus callosum shape have been found to be more similar to cisgender control subjects of the same preferred gender compared with those of the same natal sex."

A 2021 review of brain studies published in the Archives of Sexual Behavior found that "although the majority of neuroanatomical, neurophysiological, and neurometabolic features" in transgender people "resemble those of their natal sex rather than those of their experienced gender", for trans women they found feminine and demasculinized traits, and vice versa for trans men. They stated that due to limitations and conflicting results in the studies that had been done, they could not draw general conclusions or identify-specific features that consistently differed between cisgender and transgender people. The review also found differences when comparing cisgender homosexual and heterosexual people, with the same limitations applying.

Heterosexual vs. homosexual trans women

A 2016 review reported that early-onset heterosexual transgender women have a brain structure similar to cisgender women's and unlike cisgender men's, but that they have their own brain phenotype.^[2] It also reported that lesbian trans women differ from both cisgender female and male controls in non-dimorphic brain areas.

The available research indicates that the brain structure of heterosexual trans women with early-onset gender dysphoria is closer to that of cisgender women than that of cisgender men. It also reports that lesbian trans women differ from both cisgender female and male controls in non-dimorphic brain areas. Cortical thickness, which is generally thicker in cisgender women's brains than in cisgender men's brains, may also be thicker in trans women's brains, but is present in a different location to cisgender women's brains. For trans men, research indicates that those with early-onset gender dysphoria and who are heterosexual have brains that generally correspond to their assigned sex, but that they have their own phenotype with respect to cortical thickness, subcortical structures, and white matter microstructure, especially in the right hemisphere. Hormone therapy can also affect transgender people's brain structure; estrogen can cause transgender women's brains to become closer to those of cisgender women, and morphological changes observed in the brains of trans men might be due to the anabolic effects of testosterone.

MRI taken on lesbian trans women have likewise shown differences in the brain from non-trans people, though in ways not directly related to sexual dimorphism.

Heterosexual trans men

Fewer brain structure studies have been performed on transgender men than on transgender women. A 2016 review reported that the brain structure of early-onset heterosexual trans men generally corresponds to their assigned sex, but that they have their own phenotype with respect to cortical thickness, subcortical structures, and white matter microstructure, especially in the right hemisphere. Morphological increments observed in the brains of trans men might be due to the anabolic effects of testosterone.

Onset

According to the DSM-5, gender dysphoria in those assigned male at birth tends to follow one of two broad trajectories: early-onset or late-onset. Early-onset gender dysphoria is behaviorally visible in childhood. Sometimes, gender dysphoria may stop for a while in this group, and they may identify as gay or homosexual for a period of time, followed by recurrence of gender dysphoria. This group is usually androphilic in adulthood. Late-onset gender dysphoria does not include visible signs in early childhood, but some report having had wishes to be the opposite sex in childhood that they did not report to others. Trans women who experience late-onset gender dysphoria are more likely be attracted to women and may identify as lesbians or bisexual. It is common for people assigned male at birth who have late-onset gender dysphoria to experience sexual excitement from cross-dressing. In those assigned female at birth, early-onset gender dysphoria is the most common course. This group is usually sexually attracted to women. Trans men who experience late-onset gender dysphoria will usually be sexually attracted to men and may identify as gay. In general, onset of symptoms may begin at any time after an individual reaches the age of two or three.

Blanchard's typology

Further information: Blanchard's transsexualism typology

In the 1980s and 1990s, sexologist Ray Blanchard developed a taxonomy of male-to-female transsexualism built upon the work of his colleague Kurt Freund, which argues that trans women have one of two primary causes of gender dysphoria. Blanchard theorized that "homosexual transsexuals" (a taxonomic category referring to trans women attracted to men) are attracted to men and develop gender dysphoria typically during childhood, and characterizes them as displaying overt and obvious femininity since childhood; he characterizes "non-homosexual transsexuals" (trans women who are sexually attracted to women) as developing gender dysphoria primarily due to autogynephilia (sexual arousal by the thought or image of themselves as a woman), and as attracted to women, attracted to both women and men (Blanchard calls this "pseudo-bisexuality", believing attraction to males to be not genuine, but part of the performance of an autogynephilic sexual fantasy), or asexual.

Blanchard's theory has received support from J. Michael Bailey, Anne Lawrence, and James Cantor. Blanchard argued that there are significant differences between the two groups, including sexuality, age of transition, ethnicity, IQ, fetishism, and quality of adjustment.

Blanchard's typology has been criticized in papers from Veale, Nuttbrock, Moser, and others who argue that it is poorly representative of trans women and non-instructive, and that the experiments behind it are poorly controlled and/or contradicted by other data. Charles Moser conducted a survey of 29 cisgender women in the healthcare field based on Blanchard's methods for identifying autogynephilia, found that 93% of respondents qualified as autogynephiles based on their own responses. Anne Lawrence criticized the methodology of Mosers survey.

Blanchard proposed that "homosexual transsexuals", but not "autogynephilic transsexuals", would have feminized brain structure, stating: "if there is any neuroanatomic intersexuality, it is in the homosexual group". James Cantor has argued that MRI studies of transgender women offer support for Blanchard's prediction. A 2016 review of transgender brain structure states: "Cantor seems to be right. Nonhomosexual MtFs present differences with heterosexual males in structures that are not sexually dimorphic (Savic & Arver, 2011), while homosexual MtFs (as well as homosexual FtMs) show differences with respect to male and female controls in a series of brain fascicles". The review notes that only one study has compared gynephilic and androphilic transgender women, and that "more independent studies on nonhomosexual MtFs are needed".

Climate system

From Wikipedia, the free encyclopedia

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

The five components of the climate system all interact. They are the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere.

Earth's climate system is a complex system with five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). Climate is the statistical characterization of the climate system. It represents the average weather, typically over a period of 30 years, and is determined by a combination of processes, such as ocean currents and wind patterns. Circulation in the atmosphere and oceans transports heat from the tropical regions to regions that receive less energy from the Sun. Solar radiation is the main driving force for this circulation. The water cycle also moves energy throughout the climate system. In addition, certain chemical elements are constantly moving between the components of the climate system. Two examples for these biochemical cycles are the carbon and nitrogen cycles.

The climate system can change due to internal variability and external forcings. These external forcings can be natural, such as variations in solar intensity and volcanic eruptions, or caused by humans. Accumulation of greenhouse gases in the atmosphere, mainly being emitted by people burning fossil fuels, is causing climate change. Human activity also releases cooling aerosols, but their net effect is far less than that of greenhouse gases. Changes can be amplified by feedback processes in the different climate system components.

Components

The atmosphere envelops the earth and extends hundreds of kilometres from the surface. It consists mostly of inert nitrogen (78%), oxygen (21%) and argon (0.9%). Some trace gases in the atmosphere, such as water vapour and carbon dioxide, are the gases most important for the workings of the climate system, as they are greenhouse gases which allow visible light from the Sun to penetrate to the surface, but block some of the infrared radiation the Earth's surface emits to balance the Sun's radiation. This causes surface temperatures to rise.

The hydrological cycle is the movement of water through the climate system. Not only does the hydrological cycle determine patterns of precipitation, it also has an influence on the movement of energy throughout the climate system.

The hydrosphere proper contains all the liquid water on Earth, with most of it contained in the world's oceans. The ocean covers 71% of Earth's surface to an average depth of nearly 4 kilometres (2.5 miles), and ocean heat content is much larger than the heat held by the atmosphere. It contains seawater with a salt content of about 3.5% on average, but this varies spatially. Brackish water is found in estuaries and some lakes, and most freshwater, 2.5% of all water, is held in ice and snow.

The cryosphere contains all parts of the climate system where water is solid. This includes sea ice, ice sheets, permafrost and snow cover. Because there is more land in the Northern Hemisphere compared to the Southern Hemisphere, a larger part of that hemisphere is covered in snow. Both hemispheres have about the same amount of sea ice. Most frozen water is contained in the ice sheets on Greenland and Antarctica, which average about 2 kilometres (1.2 miles) in height. These ice sheets slowly flow towards their margins.

The Earth's crust, specifically mountains and valleys, shapes global wind patterns: vast mountain ranges form a barrier to winds and impact where and how much it rains. Land closer to open ocean has a more moderate climate than land farther from the ocean. For the purpose of modelling the climate, the land is often considered static as it changes very slowly compared to the other elements that make up the climate system. The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate.

Lastly, the biosphere also interacts with the rest of the climate system. Vegetation is often darker or lighter than the soil beneath, so that more or less of the Sun's heat gets trapped in areas with vegetation. Vegetation is good at trapping water, which is then taken up by its roots. Without vegetation, this water would have run off to the closest rivers or other water bodies. Water taken up by plants instead evaporates, contributing to the hydrological cycle. Precipitation and temperature influences the distribution of different vegetation zones. Carbon assimilation from seawater by the growth of small phytoplankton is almost as much as land plants from the atmosphere. While humans are technically part of the biosphere, they are often treated as a separate components of Earth's climate system, the anthroposphere, because of human's large impact on the planet.

Flows of energy, water and elements

Earth's atmospheric circulation is driven by the energy imbalance between the equator and the poles. It is further influenced by the rotation of Earth around its own axis.

Energy and general circulation

The climate system receives energy from the Sun, and to a far lesser extent from the Earth's core, as well as tidal energy from the Moon. The Earth gives off energy to outer space in two forms: it directly reflects a part of the radiation of the Sun and it emits infra-red radiation as black-body radiation. The balance of incoming and outgoing energy, and the passage of the energy through the climate system, determines Earth's energy budget. When the total of incoming energy is greater than the outgoing energy, Earth's Energy Imbalance is positive and the climate system is warming. If more energy goes out, the energy imbalance is negative and Earth experiences cooling.

More energy reaches the tropics than the polar regions and the subsequent temperature difference drives the global circulation of the atmosphere and oceans. Air rises when it warms, flows polewards and sinks again when it cools, returning to the equator. Due to the conservation of angular momentum, the Earth's rotation diverts the air to the right in the Northern Hemisphere and to the left in the Southern hemisphere, thus forming distinct atmospheric cells. Monsoons, seasonal changes in wind and precipitation that occur mostly in the tropics, form due to the fact that land masses heat up more easily than the ocean. The temperature difference induces a pressure difference between land and ocean, driving a steady wind.

Ocean water that has more salt has a higher density and differences in density play an important role in ocean circulation. The thermohaline circulation transports heat from the tropics to the polar regions. Ocean circulation is further driven by the interaction with wind. The salt component also influences the freezing point temperature. Vertical movements can bring up colder water to the surface in a process called upwelling, which cools down the air above.

Hydrological cycle

The hydrological cycle or water cycle describes how it is constantly moved between the surface of the Earth and the atmosphere. Plants evapotranspirate and sunlight evaporates water from oceans and other water bodies, leaving behind salt and other minerals. The evaporated freshwater later rains back onto the surface. Precipitation and evaporation are not evenly distributed across the globe, with some regions such as the tropics having more rainfall than evaporation, and others having more evaporation than rainfall. The evaporation of water requires substantial quantities of energy, whereas a lot of heat is released during condensation. This latent heat is the primary source of energy in the atmosphere.

Biogeochemical cycles

Carbon is constantly transported between the different elements of the climate system: fixed by living creatures and transported through the ocean and atmosphere.

Chemical elements, vital for life, are constantly cycled through the different components of the climate system. The carbon cycle is directly important for climate as it determines the concentrations of two important greenhouse gases in the atmosphere: CO₂ and methane. In the fast part of the carbon cycle, plants take up carbon dioxide from the atmosphere using photosynthesis; this is later re-emitted by the breathing of living creatures. As part of the slow carbon cycle, volcanoes release CO₂ by degassing, releasing carbon dioxide from the Earth's crust and mantle. As CO₂ in the atmosphere makes rain a bit acidic, this rain can slowly dissolve some rocks, a process known as weathering. The minerals that are released in this way, transported to the sea, are used by living creatures whose remains can form sedimentary rocks, bringing the carbon back to the lithosphere.

The nitrogen cycle describes the flow of active nitrogen. As atmospheric nitrogen is inert, micro-organisms first have to convert this to an active nitrogen compound in a process called fixing nitrogen, before it can be used as a building block in the biosphere. Human activities play an important role in both carbon and nitrogen cycles: the burning of fossil fuels has displaced carbon from the lithosphere to the atmosphere, and the use of fertilizers has vastly increased the amount of available fixed nitrogen.

Changes within the climate system

Main article: Climate change

Climate is constantly varying, on timescales that range from seasons to the lifetime of the Earth. Changes caused by the system's own components and dynamics are called internal climate variability. The system can also experience external forcing from phenomena outside of the system (e.g. a change in Earth's orbit). Longer changes, usually defined as changes that persist for at least 30 years, are referred to as climate changes, although this phrase usually refers to the current global climate change. When the climate changes, the effects may build on each other, cascading through the other parts of the system in a series of climate feedbacks (e.g. albedo changes), producing many different effects (e.g. sea level rise).

Internal variability

See also: Climate variability and change

Difference between normal December sea surface temperature [°C] and temperatures during the strong El Niño of 1997. El Niño typically brings wetter weather to Mexico and the United States.

Components of the climate system vary continuously, even without external pushes (external forcing). One example in the atmosphere is the North Atlantic Oscillation (NAO), which operates as an atmospheric pressure see-saw. The Portuguese Azores typically have high pressure, whereas there is often lower pressure over Iceland. The difference in pressure oscillates and this affects weather patterns across the North Atlantic region up to central Eurasia. For instance, the weather in Greenland and Canada is cold and dry during a positive NAO. Different phases of the North Atlantic oscillation can be sustained for multiple decades.

The ocean and atmosphere can also work together to spontaneously generate internal climate variability that can persist for years to decades at a time. Examples of this type of variability include the El Niño–Southern Oscillation, the Pacific decadal oscillation, and the Atlantic Multidecadal Oscillation. These variations can affect global average surface temperature by redistributing heat between the deep ocean and the atmosphere; but also by altering the cloud, water vapour or sea ice distribution, which can affect the total energy budget of the earth.

The oceanic aspects of these oscillations can generate variability on centennial timescales due to the ocean having hundreds of times more mass than the atmosphere, and therefore larger heat capacity and thermal inertia. For example, alterations to ocean processes such as thermohaline circulation play a key role in redistributing heat in the world's oceans. Understanding internal variability helped scientists to attribute recent climate change to greenhouse gases.

External climate forcing

See also: Climate variability and change § External climate forcing

On long timescales, the climate is determined mainly by how much energy is in the system and where it goes. When the Earth's energy budget changes, the climate follows. A change in the energy budget is called a forcing. When the change is caused by something outside of the five components of the climate system, it is called an external forcing. Volcanoes, for example, result from deep processes within the earth that are not considered part of the climate system. Human actions, off-planet changes, such as solar variation and incoming asteroids, are also external to the climate system's five components.

The primary value to quantify and compare climate forcings is radiative forcing.

Incoming sunlight

The Sun is the predominant source of energy input to the Earth and drives atmospheric circulation. The amount of energy coming from the Sun varies on shorter time scales, including the 11-year solar cycle and longer-term time scales. While the solar cycle is too small to directly warm and cool Earth's surface, it does influence a higher layer of the atmosphere directly, the stratosphere, which may have an effect on the atmosphere near the surface.

Slight variations in the Earth's motion can cause large changes in the seasonal distribution of sunlight reaching the Earth's surface and how it is distributed across the globe, although not to the global and yearly average sunlight. The three types of kinematic change are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Together these produce Milankovitch cycles, which affect climate and are notable for their correlation to glacial and interglacial periods.

Greenhouse gases

Main article: Greenhouse effect

Greenhouse gases trap heat in the lower part of the atmosphere by absorbing longwave radiation. In the Earth's past, many processes contributed to variations in greenhouse gas concentrations. Currently, emissions by humans are the cause of increasing concentrations of some greenhouse gases, such as CO₂, methane and N₂O. The dominant contributor to the greenhouse effect is water vapour (~50%), with clouds (~25%) and CO₂ (~20%) also playing an important role. When concentrations of long-lived greenhouse gases such as CO₂ are increased, temperature and water vapour increase. Accordingly, water vapour and clouds are not seen as external forcings but as feedback.

The weathering of carbonates and silicates removes carbon from the atmosphere.

Aerosols

Liquid and solid particles in the atmosphere, collectively named aerosols, have diverse effects on the climate. Some primarily scatter sunlight, cooling the planet, while others absorb sunlight and warm the atmosphere. Indirect effects include the fact that aerosols can act as cloud condensation nuclei, stimulating cloud formation. Natural sources of aerosols include sea spray, mineral dust, meteorites and volcanoes. Still, humans also contribute as a human activity, such as the combustion of biomass or fossil fuels, releases aerosols into the atmosphere. Aerosols counteract some of the warming effects of emitted greenhouse gases until they fall back to the surface in a few years or less.

In atmospheric temperature from 1979 to 2010, determined by MSU NASA satellites, effects appear from aerosols released by major volcanic eruptions (El Chichón and Pinatubo). El Niño is a separate event from ocean variability.

Although volcanoes are technically part of the lithosphere, which is part of the climate system, volcanism is defined as an external forcing agent. On average, there are only several volcanic eruptions per century that influence Earth's climate for longer than a year by ejecting tons of SO₂ into the stratosphere. The sulfur dioxide is chemically converted into aerosols that cause cooling by blocking a fraction of sunlight to the Earth's surface. Small eruptions affect the atmosphere only subtly.

Land use and cover change

Changes in land cover, such as change of water cover (e.g. rising sea level, drying up of lakes and outburst floods) or deforestation, particularly through human use of the land, can affect the climate. The reflectivity of the area can change, causing the region to capture more or less sunlight. In addition, vegetation interacts with the hydrological cycle, so precipitation is also affected. Landscape fires release greenhouse gases into the atmosphere and release black carbon, which darkens snow, making it easier to melt.

Responses and feedbacks

Some effects of global warming can either enhance (positive feedbacks) or inhibit (negative feedbacks) warming. Observations and modeling studies indicate that there is a net positive feedback to Earth's current global warming.

Main article: Climate change feedbacks

The different elements of the climate system respond to external forcing in different ways. One important difference between the components is the speed at which they react to a forcing. The atmosphere typically responds within a couple of hours to weeks, while the deep ocean and ice sheets take centuries to millennia to reach a new equilibrium.

The initial response of a component to an external forcing can be damped by negative feedbacks and enhanced by positive feedbacks. For example, a significant decrease of solar intensity would quickly lead to a temperature decrease on Earth, which would then allow ice and snow cover to expand. The extra snow and ice has a higher albedo or reflectivity, and therefore reflects more of the Sun's radiation back into space before it can be absorbed by the climate system as a whole; this in turn causes the Earth to cool down further.