Timeline of the five known great glaciations, shown in blue. The periods in between depict greenhouse conditions.
Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: Greenhouse and Icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur as alternate phases within an Icehouse period, and tend to last less than 1 million years. There are five known Icehouse periods in Earth's climate history; known as the Huronian, Cryogenian, Andean-Saharan, Late Paleozoic, and Late Cenozoic glaciations. The main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide (CO 2), changes in the Earth's orbit, long term changes in the solar constant, and oceanic and orogenic changes due to tectonic plate dynamics.
Greenhouse and Icehouse periods have played key roles in the evolution
of life on earth by directly and indirectly forcing biotic adaptation
and turnover at various spatial scales across time.
Greenhouse Earth
Overview of greenhouse Earth
A "greenhouse Earth" is a period in which there are no continental glaciers whatsoever on the planet. Additionally, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. The Earth has been in a greenhouse state for about 85% of its history.
This state should not be confused with a hypothetical hothouse earth, which is an irreversible tipping point corresponding to the ongoing runaway greenhouse effect on Venus. The IPCC
states that "a 'runaway greenhouse effect'—analogous to [that of]
Venus—appears to have virtually no chance of being induced by anthropogenic activities."
Causes of greenhouse Earth
There
are several theories as to how a greenhouse Earth can come about.
Geologic climate proxies indicate that there is a strong correlation
between a greenhouse state and high CO2 levels. However, it is important to recognize that high CO2
levels are interpreted as an indicator of Earth's climate rather than
an independent driver. Instead other phenomena have likely played a key
role in influencing global climate by altering oceanic and atmospheric
currents and increasing the net amount of solar radiation absorbed by Earth's atmosphere.
Such phenomena may include but are not limited to: 1. Tectonic shifts
that result in the release of greenhouse gases (such as CO2 and CH4) via volcanic activity, 2. An increase in the solar constant that increase the net amount of solar energy absorbed into the Earth's atmosphere, and 3. Changes in Earth obliquity and eccentricity that increase the net amount of solar radiation absorbed into Earth's atmosphere.
Icehouse Earth
Overview of Icehouse Earth
The Earth is in an Icehouse state when ice sheets are present in both poles simultaneously. Climatic proxies indicate that greenhouse gas concentrations tend to lower when the Earth is in this state. Similarly, global temperatures are also lower under Icehouse conditions.
In this climatic state Earth fluctuates between glacial and
interglacial periods where the size and distribution of continental ice
sheets fluctuate dramatically.
The fluctuation of these ice sheets result in changes in regional
climatic conditions that affect the range and distribution of many
terrestrial and oceanic species.
These glacial and interglacial periods tend to alternate in accordance
with solar and climatic oscillation until Earth eventually returns to a
Greenhouse state.
Earth is currently in an Icehouse state known as the Quaternary Ice Age that began approximately 2.58 million years ago. However, an ice sheet has been present on the antarctic continent for approximately 34 million years. At this time, Earth is in a clement interglacial period that started approximately 11.8 kya. Earth will likely phase into another interglacial period such as the Eemian,
which occurred between 130 and 115 kya, during which evidence of
forests in North Cape, Norway as well as hippopotamus in the Rhine and
Thames rivers can be observed.
The Earth is expected to continue to transition between glacial and
interglacial periods until the cessation of the Quaternary Ice Age where
it will enter another Greenhouse state.
Causes of Icehouse Earth
It is well-established that there is strong correlation between low CO 2 levels and an Icehouse state. However, this does not mean that decreasing atmospheric levels CO 2 is a primary driver of a transition to the Icehouse state. Rather, it may be an indicator of other solar, geologic, and atmospheric processes at work.
Potential drivers of previous Icehouse States include the
movement of the tectonic plates and the opening and closing of oceanic
gateways.
These seem to play a crucial part in driving Earth into an Icehouse
state as such tectonic shifts result in the transportation of cool deep
water circulations to the ocean surface that assist in ice sheet
development at the poles. Examples of this oceanic current shifts as a result of tectonic plate dynamics include the opening of the Tasmanian gateway 36.5 million years ago that separated Australia and Antarctica as well as the opening of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica - both of which are believed to have allowed for the development of the Antarctic Ice sheet. The closing of the Isthmus of Panama and the Indonesian seaway approximately 3 to 4 million years ago may also be a contributor to the Earth's current icehouse state. One proposed driver of the Ordovician Ice Age was the evolution of land plants. Under this paradigm, the rapid increase in photosynthetic biomass gradually removed CO 2 from the atmosphere and replaced it with increasing levels of O2 inducing overall climate cooling. One proposed driver of the Quaternary Ice age is the collision of the Indian subcontinent with Eurasia- forming the Himalayas and the Tibetan Plateau.
Under this paradigm the resulting continental uplift revealed massive
quantities of unweathered silicate rock (CaSiO3) which reacts with CO 2
which then produces CaCO3 (lime) and SiO2 (silica). The CaCO3 is
eventually transported to the ocean and taken up by plankton which then
die and sink to the bottom of the ocean- effectively removing CO 2 from the atmosphere.
Glacials and interglacials
Within icehouse states, there are "glacial" and "interglacial"
periods that cause ice sheets to build up or retreat. The causes for
these glacial and interglacial periods are mainly variations in the
movement of the earth around the Sun. The astronomical components, discovered by the Serbian geophysicist Milutin Milanković and now known as Milankovitch cycles, include the axial tilt of the Earth, the orbital eccentricity (or shape of the orbit) and the precession (or wobble) of the Earth's rotation.
The tilt of the axis tends to fluctuate between 21.5° to 24.5° and back
every 41,000 years on the vertical axis. This change actually affects
the seasonality upon the earth, since more or less solar radiation
hits certain areas of the planet more often on a higher tilt, while
less of a tilt would create a more even set of seasons worldwide. These
changes can be seen in ice cores, which also contain information showing
that during glacial times (at the maximum extension of the ice sheets),
the atmosphere had lower levels of carbon dioxide. This may be caused
by the increase or redistribution of the acid/base balance with bicarbonate and carbonateions that deals with alkalinity. During an icehouse, only 20% of the time is spent in interglacial, or warmer times. Model simulations suggest that the current interglacial climate state will continue for at least another 100,000 years, due to CO 2 emissions - including complete deglaciation of the Northern Hemisphere.
Snowball earth
A "snowball earth"
is the complete opposite of greenhouse Earth, in which the earth's
surface is completely frozen over; however, a snowball earth technically
does not have continental ice sheets like during the icehouse state.
"The Great Infra-Cambrian Ice Age" has been claimed to be the host of such a world, and in 1964, the scientist W. Brian Harland brought forth his discovery of indications of glaciers in low latitudes (Harland and Rudwick). This became a problem for Harland because of the thought of the "Runaway Snowball Paradox" (a kind of Snowball effect)
that, once the earth enters the route of becoming a snowball earth, it
would never be able to leave that state. However, in 1992 Joseph Kirschvink [de]
brought up a solution to the paradox. Since the continents at this time
were huddled at the low and mid-latitudes, there was less ocean water
available to absorb the higher amount solar energy hitting the tropics,
and at the same time, increased rainfall due to more land mass exposed
to higher solar energy might have caused chemical weathering (removing
CO2 from atmosphere). Both these conditions might have caused a substantial drop in CO2
atmospheric levels resulting in cooling temperatures, increasing ice
albedo (ice reflectivity of incoming solar radiation), further
increasing global cooling (a positive feedback). This might have been
the mechanism of entering Snowball Earth state. Kirschvink explained
that the way to get out of Snowball Earth state could be connected again
to carbon dioxide. A possible explanation is that during Snowball
Earth, volcanic activity would not halt, accumulating atmospheric CO2. At the same time, global ice cover would prevent chemical weathering (in particular hydrolysis), responsible for removal of CO2 from the atmosphere. CO2 was therefore accumulating in the atmosphere. Once the atmosphere accumulation of CO2
would reach a threshold, temperature would rise enough for ice sheets
to start melting. This would in turn reduce ice albedo effect which
would in turn further reduce ice cover, exiting Snowball Earth state. At
the end of Snowball Earth, before reinstating the equilibrium
"thermostat" between volcanic activity and the by then slowly resuming
chemical weathering, CO2 in the atmosphere had accumulated
enough to cause temperatures to peak to as much as 60° Celsius, before
eventually settling down. Around the same geologic period of Snowball
Earth (debated if caused by Snowball Earth or being the cause of
Snowball Earth) the Great Oxygenation Event (GOE) was occurring. The
event known as the Cambrian Explosion
followed, which produced the beginnings of populous bi-lateral
organisms and a greater diversity and mobility in all multicellular
life. However some biologists claim that a complete snowball Earth could not have happened since photosynthetic life would not have survived underneath many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica. Most scientists
today believe that a "hard" Snowball Earth, one completely covered by
ice, is probably impossible. However, a "slushball earth", with points
of opening near the equator, is possible.
Recent studies may have again complicated the idea of a snowball
earth. In October 2011, a team of French researchers announced that the
carbon dioxide during the last speculated "snowball earth" may have been
lower than originally stated, which provides a challenge in finding out
how Earth was able to get out of its state and if it were a snowball or
slushball.
Transitions
Causes
The Eocene, which occurred between 53 and 49 million years ago, was the Earth's warmest temperature period for 100 million years. However, this "super-greenhouse" eventually became an icehouse by the late Eocene. It is believed that the decline of CO2 caused this change, though it is possible that positive feedbacks contributed to the cooling.
The best record we have for a transition from an icehouse to greenhouse period where that plant life existed during the Permianperiod
that occurred around 300 million years ago. 40 million years ago, a
major transition took place, causing the Earth to change from a moist,
icy planet where rainforests covered the tropics, into a hot, dry and windy location where little could survive. Professor Isabel P. Montañez of University of California, Davis,
who has researched this time period, found the climate to be "highly
unstable" and "marked by dips and rises in carbon dioxide".
Impacts
The
Eocene-Oligocene transition, the latest transition, occurred
approximately 34 million years ago, resulting in a rapid global
temperature decrease, the glaciation of Antarctica and a series of
biotic extinction events. The most dramatic species turnover event
associated with this time period is the Grande Coupure, a period which saw the replacement of European tree-dwelling and leaf-eating mammal species by migratory species from Asia.
Research
The science of paleoclimatology attempts to understand the history of greenhouse and icehouse conditions over geological time. Through the study of ice cores, dendrochronology, ocean and lakesediments (varve), palynology, (paleobotany), isotope analysis (such as Radiometric dating and stable isotope analysis), and other climate proxies, scientists can create models of Earth's past energy budgets and resulting climate. One study has shown that atmospheric carbon dioxide levels during the Permian age rocked back and forth between 250 parts per million (which is close to present-day levels) up to 2,000 parts per million. Studies on lake sediments suggest that the "Hothouse" or "super-Greenhouse" Eocene was in a "permanent El Nino state" after the 10 °C warming of the deep ocean and high latitude surface temperatures shut down the Pacific Ocean's El Nino-Southern Oscillation. A theory was suggested for the Paleocene–Eocene Thermal Maximum on the sudden decrease of the carbon isotopic composition of the global inorganic carbon pool by 2.5 parts per million. A hypothesis posed for this drop of isotopes was the increase of methane hydrates, the trigger for which remains a mystery. This increase of methane in the atmosphere,
which happens to be a potent, but short-lived, greenhouse gas,
increased the global temperatures by 6 °C with the assistance of the
less potent carbon dioxide.
List of Icehouse and Greenhouse Periods
A greenhouse period ran from 4.6 to 2.4 billion years ago.
Huronian Glaciation – an icehouse period that ran from 2.4 billion years ago to 2.1 billion years ago
A greenhouse period ran from 2.1 billion to 720 million years ago.
Cryogenian – an icehouse period that ran from 720 to 635 million years ago, at times the entire Earth was frozen over
A greenhouse period ran from 635 million years ago to 450 million years ago.
A greenhouse period ran from 260 million years ago to 33.9 million years ago
Late Cenozoic Ice Age – the current icehouse period which began 33.9 million years ago
Modern conditions
Currently, the Earth is in an icehouse climate state. About 34 million years ago, ice sheets began to form in Antarctica; the ice sheets in the Arctic did not start forming until 2 million years ago.
Some processes that may have led to our current icehouse may be
connected to the development of the Himalayan Mountains and the opening
of the Drake Passage between South America
and Antarctica but climate model simulations suggest that the early
opening of the Drake Passage played only a limited role, while the later
constriction of the Tethys and Central American Seaways is more
important in explaining the observed Cenozoic cooling.
Scientists have been attempting to compare the past transitions between
icehouse and greenhouse, and vice versa, to understand where our planet
is now heading.
Without the human influence on the greenhouse gas concentration, the Earth would be heading toward a glacial period. Predicted changes in orbital forcing suggest that in absence of human-made global warming, the next glacial period would begin at least 50,000 years from now, but due to the ongoing anthropogenic greenhouse gas emissions, the Earth is heading towards a greenhouse Earth period.
Permanent ice is actually a rare phenomenon in the history of the
Earth, occurring only in coincidence with the icehouse effect, which has
affected about 20% of Earth's history.
According to the paradigm, "transgressing one or more planetary
boundaries may be deleterious or even catastrophic due to the risk of
crossing thresholds that will trigger non-linear, abrupt environmental
change within continental-scale to planetary-scale systems."
The Earth system process boundaries mark the safe zone for the planet
to the extent that they are not crossed. As of 2009, two boundaries have
already been crossed, while others are in imminent danger of being
crossed.
History of the framework
In 2009, a group of Earth System and environmental scientists led by Johan Rockström from the Stockholm Resilience Centre and Will Steffen from the Australian National University collaborated with 26 leading academics, including Nobel laureate Paul Crutzen, Goddard Institute for Space Studies climate scientist James Hansen and the German Chancellor's chief climate adviser Hans Joachim Schellnhuber
and identified nine "planetary life support systems" essential for
human survival, attempting to quantify how far seven of these systems
had been pushed already. They estimated how much further humans can go
before planetary habitability is threatened.
Estimates indicated that three of these boundaries—climate change, biodiversity
loss, and the biogeochemical flow boundary—appear to have been crossed.
The boundaries were "rough, first estimates only, surrounded by large
uncertainties and knowledge gaps" which interact in complex ways that
are not yet well understood. Boundaries were defined to help define a
"safe space for human development", which was an improvement on
approaches aiming at minimizing human impacts on the planet. The 2009 report was presented to the General Assembly of the Club of Rome in Amsterdam. An edited summary of the report was published as the featured article in a special 2009 edition of Nature
alongside invited critical commentary from leading academics like Nobel laureate Mario J. Molina and biologist Cristián Samper.
In 2015, a second paper was published in Science to update the Planetary Boundaries concept including regional boundaries and findings were presented at the World Economic Forum in Davos, January 2015. The 2009 publications in Nature and Ecology and Society, along with the 2015 Science article, have been cited
more than 7,000 times from 2009 to 2019, showing that the planetary
boundaries framework has been influential in subsequent scholarly work.
The 2015 paper emphasized the intersection of the nine boundaries and
placed them in a hierarchy of importance, with climate change and
biodiversity as boundaries of core importance.
A 2018 study, co-authored by Rockström, calls into question the
international agreement to limit warming to 2 degrees above
pre-industrial temperatures set forth in the Paris Agreement. The scientists raise the possibility that even if greenhouse gas emissions are substantially reduced to limit warming to 2 degrees, that might be the "threshold" at which self-reinforcing climate feedbacks add additional warming until the climate system stabilizes in a hothouse climate state.
This would make parts of the world uninhabitable, raise sea levels by
up to 60 metres (200 ft), and raise temperatures by 4–5 °C (7.2–9.0 °F)
to levels that are higher than any interglacial
period in the past 1.2 million years. Rockström notes that whether this
would occur "is one of the most existential questions in science."
Study author Katherine Richardson stresses, "We note that the Earth has
never in its history had a quasi-stable state that is around 2 °C warmer
than the preindustrial and suggest that there is substantial risk that
the system, itself, will ‘want’ to continue warming because of all of
these other processes – even if we stop emissions. This implies not only
reducing emissions but much more.”
Background
The planet Earth is a finite system, which means it has limits
The idea
The
idea that our planet has limits, including the burden placed upon it by
human activities, has been around for some time. In 1972, The Limits to Growth was published. It presented a model in which five variables: world population, industrialization, pollution, food production, and resources depletion, are examined, and considered to grow exponentially, whereas the ability of technology to increase resources availability is only linear. Subsequently, the report was widely dismissed, particularly by economists and businessmen, and it has often been claimed that history has proved the projections to be incorrect. In 2008, Graham Turner from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) published "A comparison of The Limits to Growth with thirty years of reality".
Turner found that the observed historical data from 1970 to 2000
closely matches the simulated results of the "standard run" limits of
growth model for almost all the outputs reported. "The comparison is
well within uncertainty bounds of nearly all the data in terms of both
magnitude and the trends over time."
Turner also examined a number of reports, particularly by economists,
which over the years have purported to discredit the limits-to-growth
model. Turner says these reports are flawed, and reflect
misunderstandings about the model.
In 2010, Nørgård, Peet and Ragnarsdóttir called the book a "pioneering
report", and said that it "has withstood the test of time and, indeed,
has only become more relevant."
With few exceptions,
economics as a discipline has been dominated by a perception of living
in an unlimited world, where resource and pollution problems in one area
were solved by moving resources or people to other parts. The very hint
of any global limitation as suggested in the report The Limits to Growth
was met with disbelief and rejection by businesses and most economists.
However, this conclusion was mostly based on false premises.
Of a different kind is the approach made by James Lovelock. In the 1970s he and microbiologistLynn Margulis presented the Gaia theory or hypothesis, that states that all organisms and their inorganic surroundings on Earth are integrated into a single self-regulating system.
The system has the ability to react to perturbations or deviations,
much like a living organism adjusts its regulation mechanisms to
accommodate environmental changes such as temperature (homeostasis).
Nevertheless, this capacity has limits. For instance, when a living
organism is subjected to a temperature that is lower or higher than its
living range, it can perish because its regulating mechanism cannot make
the necessary adjustments. Similarly the Earth may not be able to react
to large deviations in critical parameters. In his book The Revenge of Gaia, he affirms that the destruction of rainforests and biodiversity, compounded with the increase of greenhouse gases made by humans, is producing global warming.
From Holocene to Anthropocene
Our planet’s ability
to provide an accommodating environment for humanity is being challenged
by our own activities. The environment—our life-support system—is
changing rapidly from the stable Holocene state of the last 12,000
years, during which we developed agriculture, villages, cities, and
contemporary civilizations, to an unknown future state of significantly
different conditions.
The Holocene began about 10,000 years ago. It is the current interglacial
period, and it has proven to be a relatively stable environment of the
Earth. There have been natural environmental fluctuations during the
Holocene, but the key atmospheric and biogeochemical parameters have
been relatively stable. This stability and resilience has allowed agriculture to develop and complex societies to thrive. According to Rockström et al.,
we "have now become so dependent on those investments for our way of
life, and how we have organized society, technologies, and economies
around them, that we must take the range within which Earth System
processes varied in the Holocene as a scientific reference point for a
desirable planetary state."
Since the industrial revolution, according to Paul Crutzen, Will Steffen and others, the planet has entered a new epoch, the Anthropocene. In the Anthropocene, humans have become the main agents of not only change to the Earth System but also the driver of Earth System rupture,
disruption of the Earth System's ability to be resilient and recover
from that change. There have been well publicized scientific warnings
about risks in the areas of climate change and stratospheric ozone. However, other biophysical Earth System processes are also important and have limits which are being exceeded.
For example, since the advent of the Anthropocene, the rate at which
species are being extinguished has increased over 100 times, and humans are now the driving force altering global river flows as well as water vapor flows from the land surface.
Continuing pressure on the Earth System from human activities raises
the possibility that further pressure could be destabilizing, and
precipitate sudden or irreversible responses by the Earth System,
shunting it towards a variation or mode that is dangerous to life
including to human society, for example a Hothouse Earth
mode. According to Rockström et al., "Up to 30% of all mammal, bird,
and amphibian species will be threatened with extinction this century."
It is difficult to restore a 'safe operating space' for humanity that
is described by the planetary boundary concept, because the predominant
paradigms of social and economic development are largely indifferent to
the looming possibilities of large scale environmental disasters
triggered by humans.
Legal boundaries can help keep human activities in check, but are only
as effective as the political will to make and enforce them.
Thresholds and boundaries
The threshold, or tipping point, is the value at which a very small increment for the control variable (like CO2)
triggers a larger, possibly catastrophic, change in the response
variable (global warming) through feedbacks in the natural Earth System
itself.
The threshold points are difficult to locate, because the Earth
System is very complex. Instead of defining the threshold value, the
study establishes a range, and the threshold is supposed to lie inside
it. The lower end of that range is defined as the boundary.
Therefore, it defines a 'safe operating space', in the sense that as
long as we are below the boundary, we are below the threshold value. If
the boundary is crossed, we enter into a danger zone.
The proposed framework lays the groundwork for shifting approach to
governance and management, away from the essentially sectoral analyses
of limits to growth aimed at minimizing negative externalities,
toward the estimation of the safe space for human development.
Planetary boundaries define, as it were, the boundaries of the
"planetary playing field" for humanity if major human-induced environmental change on a global scale is to be avoided
Transgressing one or more planetary boundaries may be highly
damaging or even catastrophic, due to the risk of crossing thresholds
that trigger non-linear, abrupt environmental change within continental-
to planetary-scale systems. The 2009 study identified nine planetary
boundaries and, drawing on current scientific understanding, the
researchers proposed quantifications for seven of them. These seven are
climate change (CO2 concentration in the atmosphere < 350 ppm and/or a maximum change of +1 W/m2 in radiative forcing); ocean acidification (mean surface seawater saturation state with respect to aragonite ≥ 80% of pre-industrial levels); stratospheric ozone (less than 5% reduction in total atmospheric O3 from a pre-industrial level of 290 Dobson Units); biogeochemicalnitrogen (N) cycle (limit industrial and agricultural fixation of N2 to 35 Tg N/yr) and phosphorus (P) cycle (annual P inflow to oceans not to exceed 10 times the natural background weathering of P); global freshwater use (< 4000 km3/yr
of consumptive use of runoff resources); land system change (< 15%
of the ice-free land surface under cropland); and the rate at which
biological diversity is lost (annual rate of < 10 extinctions per
million species). The two additional planetary boundaries for which the
group had not yet been able to determine a global boundary level are chemical pollution and atmospheric aerosol loading.
Subsequent work on planetary boundaries begins to relate these thresholds at the regional scale.
Science indicates that we are transgressing planetary boundaries that
have kept civilization safe for the past 10,000 years. Evidence is
growing that human pressures are starting to overwhelm the Earth’s
buffering capacity. Humans are now the most significant driver of global change, propelling the planet into a new geological epoch, the Anthropocene.
We can no longer exclude the possibility that our collective actions
will trigger tipping points, risking abrupt and irreversible
consequences for human communities and ecological systems.
Christopher Field, director of the Carnegie Institution's Department of Global Ecology, is impressed: "This kind of work is critically important. Overall, this is an impressive attempt to define a safety zone." But the conservation biologist Stuart Pimm
is not impressed: "I don’t think this is in any way a useful way of
thinking about things... The notion of a single boundary is just devoid
of serious content. In what way is an extinction rate 10 times the
background rate acceptable?" and the environmental policy analyst Bill Clark
thinks: "Tipping points in the earth system are dense, unpredictable...
and unlikely to be avoidable through early warning indicators. It
follows that... 'safe operating spaces' and 'planetary boundaries' are
thus highly suspect and potentially the new 'opiates'."
The biogeochemist William Schlesinger
queries whether thresholds are a good idea for pollutions at all. He
thinks waiting until we near some suggested limit will just permit us to
continue to a point where it is too late. "Management based on
thresholds, although attractive in its simplicity, allows pernicious,
slow and diffuse degradation to persist nearly indefinitely."
The hydrologist David Molden thinks planetary boundaries are a
welcome new approach in the 'limits to growth' debate. "As a scientific
organizing principle, the concept has many strengths ... the numbers are
important because they provide targets for policymakers, giving a clear
indication of the magnitude and direction of change. They also provide
benchmarks and direction for science. As we improve our understanding of
Earth processes and complex inter-relationships, these benchmarks can
and will be updated ... we now have a tool we can use to help us think
more deeply—and urgently—about planetary limits and the critical actions
we have to take."
The ocean chemist
Peter Brewer queries whether it is "truly useful to create a list of
environmental limits without serious plans for how they may be achieved
... they may become just another stick to beat citizens with. Disruption
of the global nitrogen cycle is one clear example: it is likely that a
large fraction of people on Earth would not be alive today without the
artificial production of fertilizer. How can such ethical and economic
issues be matched with a simple call to set limits? ... food is not
optional."
The environment advisor Steve Bass says the "description of
planetary boundaries is a sound idea. We need to know how to live within
the unusually stable conditions of our present Holocene period and not
do anything that causes irreversible environmental change ... Their
paper has profound implications for future governance systems, offering
some of the 'wiring' needed to link governance of national and global
economies with governance of the environment and natural resources. The
planetary boundaries concept should enable policymakers to understand
more clearly that, like human rights and representative government,
environmental change knows no borders."
The climate change policy advisor Adele Morris
thinks that price-based policies are also needed to avoid political and
economic thresholds. "Staying within a 'safe operating space' will
require staying within all the relevant boundaries, including the
electorate’s willingness to pay."
In summary, the
planetary boundary concept is a very important one, and its proposal
should now be followed by discussions of the connections between the
various boundaries and of their association with other concepts such as
the 'limits to growth'.
Importantly, this novel concept highlights the risk of reaching
thresholds or tipping points for non-linear or abrupt changes in
Earth-system processes. As such, it can help society to reach the
agreements required for dealing effectively with existing global
environmental threats, such as climate change.
In 2011, at their second meeting, the High-level Panel on Global
Sustainability of the United Nations had incorporated the concept of
planetary boundaries into their framework, stating that their goal was:
"To eradicate poverty and reduce inequality, make growth inclusive, and
production and consumption more sustainable while combating climate
change and respecting the range of other planetary boundaries."
Elsewhere in their proceedings, panel members have expressed
reservations about the political effectiveness of using the concept of
"planetary boundaries": "Planetary boundaries are still an evolving
concept that should be used with caution [...] The planetary boundaries
question can be divisive as it can be perceived as a tool of the "North"
to tell the "South" not to follow the resource intensive and
environmentally destructive development pathway that rich countries took
themselves... This language is unacceptable to most of the developing
countries as they fear that an emphasis on boundaries would place
unacceptable brakes on poor countries."
However, the concept is routinely used in the proceedings of the United Nations, and in the UN Daily News. For example, the UNEP Executive Director Achim Steiner states that the challenge of agriculture is to "feed a growing global population without pushing humanity's footprint beyond planetary boundaries." The United Nations Environment Programme (UNEP) Yearbook 2010 also repeated Rockström's message, conceptually linking it with ecosystem management and environmental governance indicators.
The planetary boundaries concept is also used in proceedings by the European Commission, and was referred to in the European Environment Agency synthesis report The European environment – state and outlook 2010.
In their 2012 report entitled "Resilient People, Resilient
Planet: A future worth choosing", The High-level Panel on Global
Sustainability called for bold global efforts, "including launching a
major global scientific initiative, to strengthen the interface between
science and policy. We must define, through science, what scientists
refer to as "planetary boundaries", "environmental thresholds" and
"tipping points"."
Development studies scholars have been critical of aspects of the framework and constraints that its adoption could place on the Global South.
Proposals to conserve a certain proportion of Earth's remaining forests
can be seen as rewarding the countries such as those in Europe that
have already economically benefitted from exhausting their forests and
converting land for agriculture. In contrast, countries that have yet to
industrialize are asked to make sacrifices for global environmental
damage they may have had little role in creating.
Climate change
The
black line shows the atmospheric carbon dioxide concentration for the
period 1880–2008. Red bars show temperatures above and blue bars show
temperatures below the average temperature. Year-to-year temperature
fluctuations are due to natural processes, such as the effects of El Niño, La Niña, and the eruption of large volcanoes.
Radiative forcing is a measure of the difference between the incoming radiation energy
and the outgoing radiation energy acting across the boundary of the
earth. Positive radiative forcing results in warming. From the start of
the industrial revolution in 1750 to 2005, the increase in atmospheric
carbon dioxide has led to a positive radiative forcing, averaging about
1.66 W/m².
The climate scientist Myles Allen
thinks setting "a limit on long-term atmospheric carbon dioxide
concentrations merely distracts from the much more immediate challenge
of limiting warming to 2 °C." He says the concentration of carbon
dioxide is not a control variable we can "meaningfully claim to
control", and he questions whether keeping carbon dioxide levels below
350 ppm will avoid more than 2 °C of warming.
Adele Morris, policy director, Climate and Energy Economics Project, Brookings Institution,
makes a criticism from the economical-political point of view. She puts
emphasis in choosing policies that minimize costs and preserve
consensus. She favors a system of green-house gas emissions tax, and emissions trading, as ways to prevent global warming. She thinks that too-ambitious objectives, like the boundary limit on CO2, may discourage such actions.
Biodiversity loss
According to the biologist Cristián Samper,
a "boundary that expresses the probability of families of species
disappearing over time would better reflect our potential impacts on the
future of life on Earth."
The biodiversity boundary has also been criticized for framing
biodiversity solely in terms of the extinction rate. The global
extinction rate has been highly variable over the Earth's history, and
thus using it as the only biodiversity variable can be of limited
usefulness.
Since the industrial revolution, the Earth's nitrogen cycle has been disturbed even more than the carbon cycle.
"Human activities now convert more nitrogen from the atmosphere into
reactive forms than all of the Earth´s terrestrial processes combined.
Much of this new reactive nitrogen pollutes waterways and coastal zones,
is emitted back to the atmosphere in changed forms, or accumulates in
the terrestrial biosphere."
Only a small part of the fertilizers applied in agriculture is used by
plants. Most of the nitrogen and phosphorus ends up in rivers, lakes and
the sea, where excess amounts stress aquatic ecosystems. For example,
fertilizer which discharges from rivers into the Gulf of Mexico has
damaged shrimp fisheries because of hypoxia.
The biogeochemist William Schlesinger
thinks waiting until we near some suggested limit for nitrogen
deposition and other pollutions will just permit us to continue to a
point where it is too late. He says the boundary suggested for
phosphorus is not sustainable, and would exhaust the known phosphorus
reserves in less than 200 years.
Phosphorus
Peak phosphorus is a concept to describe the point in time at which the maximum global phosphorus
production rate is reached. Phosphorus is a scarce finite resource on
earth and means of production other than mining are unavailable because
of its non-gaseous environmental cycle.
According to some researchers, Earth's phosphorus reserves are expected
to be completely depleted in 50–100 years and peak phosphorus to be
reached in approximately 2030.
Ocean acidification
Estimated
change in sea surface pH from the pre-industrial period (1700s) to the
present day (1990s). Δ pH is in standard pH units.
Surface ocean acidity
has increased thirty percent since the industrial revolution. About one
quarter of the additional carbon dioxide generated by humans is
dissolved in the oceans, where it forms carbonic acid. This acidity inhibits the ability of corals, shellfish and plankton to build shells and skeletons. Knock-on effects could have serious consequences for fish stocks.
This boundary is clearly interconnected with the climate change
boundaries, since the concentration of carbon dioxide in the atmosphere
is also the underlying control variable for the ocean acidification
boundary.
The ocean chemist Peter Brewer thinks "ocean acidification has
impacts other than simple changes in pH, and these may need boundaries
too."
Land use
Europe land use map. Human land uses include arable farmland (yellow) and pasture (light green)
Across the planet, forests, wetlands and other vegetation types are being converted to agricultural and other land uses, impacting freshwater, carbon and other cycles, and reducing biodiversity.
The environment advisor Steve Bass says research tells us that
"the sustainability of land use depends less on percentages and more on
other factors. For example, the environmental impact of 15 per cent
coverage by intensively farmed cropland in large blocks will be
significantly different from that of 15 per cent of land farmed in more
sustainable ways, integrated into the landscape. The boundary of 15 per
cent land-use change is, in practice, a premature policy guideline that
dilutes the authors' overall scientific proposition. Instead, the
authors might want to consider a limit on soil degradation or soil loss. This would be a more valid and useful indicator of the state of terrestrial health."
Human pressures on global freshwater systems are having dramatic effects. The freshwater cycle is another boundary significantly affected by climate change. Freshwater resources, such as lakes and aquifers, are usually renewable resources which naturally recharge (the term fossil water is sometimes used to describe aquifers which don't recharge). Overexploitation
occurs if a water resource is mined or extracted at a rate that exceeds
the recharge rate. Recharge usually comes from area streams, rivers and
lakes. Forests enhance the recharge of aquifers in some locales,
although generally forests are a major source of aquifer depletion. Depleted aquifers can become polluted with contaminants such as nitrates,
or permanently damaged through subsidence or through saline intrusion
from the ocean. This turns much of the world's underground water and
lakes into finite resources with peak usage debates similar to oil. Though Hubbert's original analysis did not apply to renewable resources, their overexploitation can result in a Hubbert-like peak. A modified Hubbert curve applies to any resource that can be harvested faster than it can be replaced.
The hydrologist David Molden says "a global limit on water
consumption is necessary, but the suggested planetary boundary of 4,000
cubic kilometres per year is too generous."
Ozone depletion
During 21–30 September 2006 the average area of the Antarctic ozone hole was the largest ever observed
The stratospheric ozone layer protectively filters ultraviolet radiation (UV) from the Sun, which would otherwise damage biological systems. The actions taken after the Montreal Protocol appeared to be keeping the planet within a safe boundary. However, in 2011, according to a paper published in Nature, the boundary was unexpectedly pushed in the Arctic;
"... the fraction of the Arctic vortex in March with total ozone less
than 275 Dobson units (DU) is typically near zero, but reached nearly
45%".
The Nobel laureate in chemistry, Mario Molina, says "five per cent is a reasonable limit for acceptable ozone depletion, but it doesn't represent a tipping point".
Atmospheric aerosols
Smog over southern China and Vietnam
Aerosol particles in the atmosphere impact the health of humans and influence monsoon and global atmospheric circulation
systems. Some aerosols produce clouds which cool the Earth by
reflecting sunlight back to space, while others, like soot, produce thin
clouds in the upper stratosphere which behave like a greenhouse,
warming the Earth. On balance, anthropogenic aerosols probably produce a
net negative radiative forcing (cooling influence).
Worldwide each year, aerosol particles result in about 800,000
premature deaths. Aerosol loading is sufficiently important to be
included among the planetary boundaries, but it is not yet clear whether
an appropriate safe threshold measure can be identified.
Chemical pollution
Some chemicals, such as persistent organic pollutants, heavy metals and radionuclides, have potentially irreversible additive and synergic effects on biological organisms, reducing fertility and resulting in permanent genetic damage.
Sublethal uptakes are drastically reducing marine bird and mammal
populations. This boundary seems important, although it is hard to
quantify.
A Bayesian emulator for persistent organic pollutants has been
developed which can potentially be used to quantify the boundaries for
chemical pollution.
To date, critical exposure levels of polychlorinated biphenyls (PCBs)
above which mass mortality events of marine mammals are likely to occur,
have been proposed as a chemical pollution planetary boundary.
Interaction among boundaries
A planetary boundary may interact in a manner that changes the safe operating level of other boundaries. Rockström et al.
2009 did not analyze such interactions, but they suggested that many of
these interactions will reduce rather than expand the proposed boundary
levels.
For example, the land use
boundary could shift downward if the freshwater boundary is breached,
causing lands to become arid and unavailable for agriculture. At a
regional level, water resources may decline in Asia if deforestation continues in the Amazon.
Such considerations suggest the need for "extreme caution in
approaching or transgressing any individual planetary boundaries."
Another example has to do with coral reefs and marine ecosystems. In 2009, De'Ath, Lough & Fabricius (2009) showed that, since 1990, calcification in the reefs of the Great Barrier
that they examined decreased at a rate unprecedented over the last 400
years (14% in less than 20 years). Their evidence suggests that the
increasing temperature stress and the declining ocean saturation state
of aragonite is making it difficult for reef corals to deposit calcium carbonate. Bellwood & others (2004) explored how multiple stressors, such as increased nutrient loads and fishing pressure, move corals into less desirable ecosystem states. Guinotte & Fabry (2008)
showed that ocean acidification will significantly change the
distribution and abundance of a whole range of marine life, particularly
species "that build skeletons, shells, and tests of biogenic calcium
carbonate. "Increasing temperatures, surface UV radiation levels and ocean acidity all stress marine biota,
and the combination of these stresses may well cause perturbations in
the abundance and diversity of marine biological systems that go well
beyond the effects of a single stressor acting alone."
Subsequent developments
The concept of
planetary boundaries challenges the belief that resources are either
limitless or infinitely substitutable. It threatens the
business-as-usual approach to economic growth. The fact that reference
to planetary boundaries was excluded from the [ Rio+20
] conference statement is a counterintuitive sign that the concept is
being taken very seriously and has indeed gained enough traction to be
threatening to the status quo. Had planetary boundaries remained in the
statement, the most credible interpretation is that they would join a
growing list of nice-sounding goals that are included but never achieved
in the end. Planetary boundaries will not go away. The intrinsic limits
to the amount of resources and environmental services that humanity can
extract safely from the Earth System cannot be eliminated by wishful
thinking, denial, or omission from official sustainable development
conference statements. It is simply the nature of the planet we inhabit.
The Doughnut with indicators to what extent the ecological ceilings are overshot and social foundations are not met yet.
In 2012 Kate Raworth from Oxfam noted the Rockstrom concept does not take human population growth into account.
She suggested social boundaries should be incorporated into the
planetary boundary structure, such as jobs, education, food, access to
water, health services and energy and to accommodate an environmentally
safe space compatible with poverty eradication
and "rights for all". Within planetary limits and an equitable social
foundation lies a doughnut shaped area which is the area where there is a
"safe and just space for humanity to thrive in".
An empirical application of the doughnut model by O'Neill et al.
showed that so far across 150 countries not a single country satisfies
its citizens' basic needs while maintaining a globally sustainable level
of resource use.
National environmental footprints
Several studies assessed environmental footprints of nations based on planetary boundaries: for Sweden, Switzerland, the Netherlands, the European Union as well as for the world’s most important economies. While the metrics and allocation approaches applied varied, there is a
converging outcome that resource use of wealthier nations – if
extrapolated to world population – is not compatible with planetary
boundaries.
Proposed new or expanded boundaries
In 2012, Steven Running suggested a tenth boundary, the annual net global primary production of all terrestrial plants, as an easily determinable measure integrating many variables that will give "a clear signal about the health of ecosystems".
In 2017, Nash et al. argued that marine systems are underrepresented in the framework. Their proposed remedy was to include the seabed
as a component of the earth surface change boundary. They also wrote
that the framework should account for "changes in vertical mixing and ocean circulation patterns".
Not yet endorsed by United Nations
The United Nations secretary general Ban Ki-moon
endorsed the concept of planetary boundaries on 16 March 2012, when he
presented the key points of the report of his High Level Panel on Global
Sustainability to an informal plenary of the UN General Assembly.
Ban stated: "The Panel’s vision is to eradicate poverty and reduce
inequality, to make growth inclusive and production and consumption more
sustainable, while combating climate change and respecting a range of
other planetary boundaries." The concept was incorporated into the so-called "zero draft" of the outcome of the United Nations Conference on Sustainable Development to be convened in Rio de Janeiro 20–22 June 2012.
However, the use of the concept was subsequently withdrawn from the
text of the conference, "partly due to concerns from some poorer
countries that its adoption could lead to the sidelining of poverty
reduction and economic development. It is also, say observers, because
the idea is simply too new to be officially adopted, and needed to be
challenged, weathered and chewed over to test its robustness before
standing a chance of being internationally accepted at UN negotiations."
The planetary boundary framework was updated in 2015.
It was suggested that three of the boundaries (including climate
change) might push the Earth system into a new state if crossed; these
also strongly influence the remaining boundaries. In the paper, the
framework is developed to make it more applicable at the regional scale.
Boundaries related to agriculture and food consumption
Visualization of the planetary boundaries related to agriculture and nutrition
Human activities related to agriculture and nutrition globally
contribute to the transgression of four out of nine planetary
boundaries. Surplus nutrient flows (N, P) into aquatic and terrestrial
ecosystems are of highest importance, followed by excessive land-system
change and biodiversity loss. Whereas in the case of biodiversity loss, P
cycle and land-system change, the transgression is in the zone of
uncertainty—indicating an increasing risk (yellow circle in the figure),
the N boundary related to agriculture is more than 200%
transgressed—indicating a high risk (red marked circle in the figure).
Here, nutrition includes food processing and trade as well as food
consumption (preparation of food in households and gastronomy).
Consumption-related environmental impacts are not quantified at the
global level for the planetary boundaries of freshwater use, atmospheric
aerosol loading (air pollution) and stratospheric ozone depletion.
