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Gaia and Climate Change

SCHUMACHER COLLEGE
An International Centre for Ecological Studies

Holistic Science MSc 2001/2003, module:
Global Level of Emergence

Gaia and Climate Change
by Neil Meikleham

Introduction

There is little doubt that human activities are directly influencing the chemical composition of the planet through our agricultural and industrial practises. The changes that are confronting us include the continuing destruction of the tropical forests, global warming, ozone depletion, acid rain, degradation by pollution, destruction of diversity, etc. This impact has the potential to set in motion unpredictable changes with wide ranging implications for both humans and the other species we share the planet with. In order to fully understand the complex issues of global environmental change that challenge society today we need to look at how these changes are seen in the context of the Earth as a system; identifying the complex linkages and feedback processes that exist among its components. Hutton, often referred to as the father of Geology, considered the Earth to be a “super organism” and that “it’s proper study would be physiology” – the science of the function of living things. He drew parallels between the circulation of blood and the circulation of nutrient elements of the Earth in which sunlight distils water from the oceans so that it “may later fall as rain and refresh the land”. However the developing Earth sciences were divided into disciplines such as chemistry, geology, biology, etc. with the result that any study of the Earth by one of these disciplines has tended to focus on the particularities of that discipline. This has led to miscommunication and an inadequate understanding of a planet that is best considered as a whole system. Furthermore the dominant position of the earth sciences has tended to see life as a passenger on a planet controlled by the inexorable forces of physics and chemistry, adapting as best as possible in the face of changing energy output from the Sun, collisions of the Earth by meteorites, orbital variations, continental drift, etc. It was this that led James Lovelock, with the help of Lynn Margulis, to propose a view of the Earth that would incorporate all its elements. This theory, which he called Gaia, would see the “Earth as a system where the evolution of the organisms is tightly coupled to the evolution of their environment. Self-regulation of climate and chemical composition are emergent properties of the system.” The Gaia theory challenges the traditional earth sciences by suggesting that life has a profound influence on the evolution of the planet by serving as an active control system leading to the stabilisation of global temperature and chemical composition. This self-regulation is seen to be an emergent property of the tight coupling between life and the abiotic environment involving systems, many of which have taken thousands if not millions of years to evolve. The study of this is the science of the Gaia theory. The main focus of this essay will be to explore current and possible climate feedback mechanisms, and in the light of Gaian thinking suggest how these might influence climate change as it is forced by anthropological greenhouse gas emissions.

The climate is known to be subject to a natural variability. Recent analysis of the cores of undisturbed sediments from the ocean floor and ice from Antarctica and Greenland support the theory that the Earth alternates between ice ages and interglacials. It has been suggested that this rhythm is brought about by slight changes in the Earth’s orbit around the sun, however, these changes are not thought to be strong enough to explain the rapidity and magnitude of the switch from glacial to interglacial and back again. The small change in solar flux must somehow be amplified, possibly through various kinds of feedbacks. In particular the measurement of oxygen isotope ratios in ice cores can give an indication of the air temperature at the time the ice was formed. Using this technique a continuous record of temperature over 160 000 years has been measured from samples from Vostok, Antarctica. This can be compared to the carbon dioxide concentration in atmospheric gas pockets trapped in the same core. It is found that the carbon dioxide content of the atmosphere through the time the ice was laid follows closely the temperature. For example in interglacial periods the carbon dioxide content was high, averaging about 280 ppm, and in glacial times it is low, averaging 210 ppm and falling as low as 180 ppm. Figure 1 below shows this strong correlation between the carbon dioxide levels in the atmosphere and the temperature. From this data the interglacials appear to last about 10 000 years and as we have just experienced about 10 000 years of the current interglacial, many scientists believe that the Earth is due another ice age. However with the emission of anthropogenic greenhouse gases the Earth is heading towards an increase in average global temperatures unprecedented in human history.

Figure 1: Vostok ice analysis for the past 160 000 years

It is in the uncertainties associated with the feedbacks that amplify orbital variations that we find the heart of most scientists concerns. Will the Earth’s systems tend to incorporate negative feedbacks in an attempt to dampen and minimise the additional greenhouse gases or will they tend to positive feedback resulting in the so-called “runaway greenhouse effect?”

The Nature of the Greenhouse Effect.

Greenhouse gases are gases that have the property of retaining infrared radiation in the atmosphere, so warming the Earth’s surface and lower atmosphere. In general these gases consist of molecules with three or more atoms with a frequency of absorbtion of infrared radiation matching the frequency of internal motion of these molecules. For this to happen the molecule must have a dipole moment, a difference in position between its centre of positive charge and negative charge, during the vibration caused by the absorbtion of infrared light. The centres of charge coincide in free atoms and homonuclear diatomic molecules thus species such as Ar, O2, N2, etc. do not absorb infrared light. Heteronuclear diatomic molecules such as CO and NO have little impact as greenhouse gases as they absorb in a frequency that lies outside the thermal infrared region. Greenhouse gases exist naturally in the atmosphere and are cycled through many natural pathways. In fact without the presence of carbon dioxide it is believed the temperature at the Earth’s surface would be 33° C lower than it is today. Human activities, however, substantially add to the quantities of these gases in the atmosphere, principally by the burning of fossil fuels and the deforestation of tropical forests now and temperate forests in the past. This anthropogenic or enhanced greenhouse effect is normally distinguished from the “natural” greenhouse effect. Some important anthropogenic sources of greenhouse gases are listed in the table below:

Table 1: Common greenhouse gases, origins, rate of build-up in the atmosphere and contribution to global warming.

Gas

Principle Sources

Concentration

Current rate of annual increase

Contribution to global warming (%)

Carbon Dioxide (CO2)

Fossil fuel burning (c. 77%)

Deforestation (c. 23%)

353 ppmv

0.5%

55

Methane (CH4)

Rice paddies, Enteric fermentation, gas leakage

1.72ppmv

0.9%

15

Nitrous oxide (N20)

Biomass burning, Fertiliser use, fossil-fuel combustion

310 ppbv

0.8%

6

CFC’s and related gases

Various industrial uses

289 pptv CFC-11

484pptv
(CFC-12)

4%

24

Leggett, J., “The Nature of the Greenhouse Threat” in Global Warming: The Greenpeace Report, Oxford UP: Oxford, 1990, p 15.
Leggett, J., “The Nature of the Greenhouse Threat” in Global Warming: The Greenpeace Report, Oxford UP: Oxford, 1990, p 17.

According to the Intergovernmental Panel on Climate Change (IPCC) the atmospheric concentrations of carbon dioxide, methane and nitrous oxide have grown significantly by about 30%, 145% and 15% respectively since the beginning of the industrial revolution. If the growth in carbon dioxide emissions is maintained at current levels (1994 for this report) then atmospheric levels of this gas will reach about 500 ppmv by the end of the 21st century. This is nearly double the pre-industrial concentration of 280ppmv. The rise in carbon dioxide levels can be clearly seen in figure 2 for the period 1958 to 2000. The oscillation is due to seasonal cycles of photosynthesis and respiration and represents the short time-scale for the turnover of carbon.

Figure 2: Concentration of atmospheric CO2 measured at
monthly intervals (Hawaii.)

The global mean surface air temperature has increased by between 0.3 and 0.6° C since the late 19th century. This is dramatically illustrated in the figure on the covering page. Global sea level has risen between 10 and 25cm over the past 100 years. IPCC models predict an increase of global mean surface temperature relative to 1990 of about 2 to 3.5° C by 2100 as well as projecting a rise in sea level of 50cm from the present to 2100. These estimates are put into context when they are compared to the approximate temperature increase since the last ice age 18 000 years ago. This took place at a much slower pace of only about 1 or 2° C per 1 000 years. It is this acceleration in global warming; a rate of change tens of times faster than the average rate of change that is the scientific concern rather than the actual temperatures reached. Leggett describes this concern as follows: “we are heading for rates of temperature-rise unprecedented in human history; the geological record screams a warning to us of just how unprecedented, and of how stressed the natural environment will become if that happens”. How this global warning will affect the planet with respect to rain fall patterns, drought, growing seasons, sea level, etc is controversial but it is widely accepted by the scientific community that there will be dramatic repercussions of some sort.

Climate Regulation and the Carbon Cycle — Short and Long-term

Carbon is found throughout the environment and is at the heart of climate regulatory mechanisms. It is found in the atmosphere as methane (10Gt) and carbon dioxide (760Gt); in the ocean as dissolved carbon dioxide (740Gt), oceanic carbonate ion (1300Gt) and oceanic bicarbonate ion (37 000Gt), on the land in biomass (610Gt) and fossil fuels (4200Gt), organic carbon in soils and sediments (1600Gt), organic carbon in sedimentary rocks (10 000 000Gt), and finally in limestone in sedimentary rocks (40 000000Gt). Carbon is central to the workings of Gaia and any change to its composition, especially in the atmosphere, will have an impact on climate regulatory mechanisms. Since 1860 humans have added about 175Gt of carbon to the atmosphere through fossil fuel burning and concerted deforestation. Assuming the carbon cycle was in a steady state before humans started influencing these concentrations the question arises: how and to what extend will this influence the steady state of the carbon cycle and therefore what are the climatic implications of such a change? The models we have of climate systems are very limited in their predictions due to the complexity of the interactions between all the components of the Earth. This is further hampered by our lack of understanding of the regulatory mechanisms which could either minimise any changes through negative feedback or reinforce a runaway situation through positive feedbacks. The nature of these feedbacks is complex and it is in the next section that I will explore a number of them and their possible implications.

Photosynthesis is the major mechanism in which carbon is transferred from its oxidised state of carbon dioxide in the atmosphere to its reduced state as organic carbon, with the subsequent release of gaseous oxygen:

The growth of plants through photosynthesis directly influences the composition of the atmosphere, clearly seen by the presence of oxygen levels of ~21% and the low concentration of carbon dioxide. The organic carbon is found as plant tissue (biomass), dead plant mass (detritus), and soil organic matter. In fact the creation and development of soil is directly due to the presence of the biota. The overall storage of carbon is determined by the balance between primary production on the one hand and respiration and decomposition on the other hand. Respiration, both by plants and other living organisms, is simply the reverse of equation (1) above:

Decomposition by micro-fauna, bacteria and fungi leads to the release of organic carbon in the form of carbon dioxide under aerobic conditions and methane under anaerobic conditions. There is also an accumulation of organic compounds called humus and these compose the bulk of soil organic matter. The structures of these compounds are varied but most are based on numerous aromatic rings with phenolic and/or organic acid functional groups. These compounds are very resistant to biodegradation and although most of the carbon in the soil is rapidly turned over in 10’s of years, a small percentage in the form of humus begins to accumulate at depth with a turnover in the 100’s or even 1000’s of years. When soils are cultivated their soil organic matter declines, typically 20 — 30% in the first few decades of cultivation. Currently about 10% of the world’s soils are under cultivation, losses of organic soil carbon can be expected to have contributed to the overall increase of carbon dioxide in the atmosphere. This is further exacerbated by the high levels of erosion world-wide which in mobilising the soil carbon can lead to its premature release to the atmosphere.

Deforestation leads to the release of large amounts of biomass carbon into the atmosphere where it combines directly to form carbon dioxide. It also leads to significant release of methane and nitrous oxide. According to Myers tropical deforestation has contributed up to 30% of the build-up of carbon dioxide in the global atmosphere, and with the methane and nitrous oxide emitted contributes about 20% of the current global warming effect. A moratorium on deforestation with immediate large-scale reforestation programs could in principle minimise the effects of this greenhouse gas build-up. However todate no such programs have been started with a wide range of political, economic and social factors against implementation. There is a common proposal today that considers forestation programs as not only a carbon sink for carbon dioxide created by deforestation but also as a potential sink for carbon dioxide created by burning fossil fuels. These proposals do not, however, recognise that the residence time for carbon in trees is many magnitudes lower than that of carbon locked in the fossil fuels. It is this that could cause further problems in the future.

Terrestrial ecosystems are thought to play an important role in determining regional and global climate. When we cut back the forests or change existing ecosystems to accommodate human agricultural needs we diminish the ability of these systems to regulate their own chemistry and hence the climate they ultimately influence. For example destruction of the tropical rainforest can lead to warmer and drier conditions. The trees themselves evaporate large quantities of water through their leaves. This rising vapour condenses to form clouds, which in due course falls as rain. This relationship between the rain and the trees is one of a positive feedback to support the growth of these trees. Secondly the clouds, which have a high albedo, can create a negative feedback helping to cool the forest area. Both of these feedbacks can be seen in the following diagram:

Traditional environmental science identifies the wet and cloudy tropics as an environmental state independent of the trees, with the trees adapting to this state. Gaian science, however, recognises that the trees and the rain are intimately connected and that they can not exist independently. This effect is clearly seen in the case of the Harrapan of Western Pakistan. This region was once a self-sustaining forest ecosystem with an adequate rainfall. After more than half of the forest was cleared for cattle and goats the remaining forest was unable to sustain itself and soon decayed leaving behind a semi-desert. This is as a very real scenario which can be applied to the rapid deforestation of the tropics and is of concern. It is possible that there will be a line crossed whereby the climate relationship between the trees and the environment starts to breaks down. It is also possible that this relationship will show hysterisis, or in other words if the climate arising between the trees and the environment changes due to tree loss it might not be possible to recreate the original climate by replanting trees at a later date.

Concerns are not just restricted to the tropics. Historically the extent and nature of the boreal forests was thought to be determined by the climate, much like the traditional view of the tropical forests above. However results by Bonan et al indicate that the locations of the boreal forest and of correlated climate indices are the outcome of coupled dynamical interactions in which the geographical distribution of the boreal forests affects climate and visa versa. In particular they suggest that the presence of the trees in the northern latitudes moderates the high albedo of the snow/ice leading to warmer winter temperatures. The decrease in snow-covered land surface albedo caused by the northward migration of boreal forests into tundra in response to climate warming may produce further warming initiating a long-term irreversible positive feedback. This ice/snow albedo effect is a feedback mechanism in it own right: as the Earth warms the ice and snow in polar regions will melt decreasing the albedo of the Earth. The darker surface will absorb more solar radiation creating a positive feedback.

Turning our attention to the oceans it has been found that as much as half of the Earth’s photosynthesis may occur in the sea. This is largely a result of phytoplankton in the surface layer. Most of this marine organic carbon production is consumed by zooplankton and bacterioplankton, about 80 —90%, and the remainder sinks below the euphotic zone to the deep ocean. About 98% of this deep ocean carbon is further degraded by bacterial respiration with only small quantities reaching the sediments. Significant degradation continues in the sediments, mostly under anaerobic conditions. The permanent burial of reduced carbon in the sediments is generally much less than one percent, however, in the absence of this biological pump atmospheric carbon dioxide concentration in the atmosphere could be much higher than current values — possibly even as high as 470 ppm, and certainly it has contributed to the release of oxygen to the atmosphere. A number of organisms, such as the coccolithophores, are responsible for depositing large amounts of carbon onto the ocean floor in the form of calcium carbonate. This will be discussed now as part of the long term carbon cycle.

When looking at these regulatory mechanisms it is important to consider the changing flux of the sun’s luminosity over the history of the planet. Current astrophysical theory suggests that this flux has increased by 25 to 30% since the first living cells appeared on the Earth about 4 billion years ago. With less solar energy the early planet would have been much colder and possibly ice covered. The fossil evidence, however, suggests that the Earth had surface temperatures not very different from today – certainly not much bellow freezing or much above 35° C. A solution to the early sun paradox is found in the proposal of a supergreenhouse: it suggests that the Earth’s early volcanic activity was greater than it is now resulting in large quantities of carbon dioxide being emitted. This would result in a greenhouse effect that warmed the planet. However as the sun’s flux increased it would become necessary for a stabilising or negative feedback mechanism to establish itself. This would then maintain the Earth’s temperature within the observed limits by removing appropriate amounts of carbon dioxide. The Urey cycle is a suggested planetary carbon dioxide — climate control mechanism; it involves the weathering of calcium silicate on land by atmospheric CO2 releasing bicarbonate, calcium ions and silicic acid:

Runoff brings these dissolved chemicals to the ocean. Calcium carbonate and silica deposit on the ocean floor and stoichiometrically one half of carbon dioxide escapes into the atmosphere:

Subsequently subduction brings the calcium carbonate and silica into the deep Earth to reform calcium silicate and carbon dioxide. The calcium silicate surfaces up in time and is ready to continue the cycle, while volcanoes blow carbon dioxide into the atmosphere.

All the chemicals are recycled continuously and the system settles into a steady state, although this state is likely to deviate over time. This cycle consists of two processes, which operate on entirely different time scales. Whereas weathering and sedimentation equilibrate within a few thousand years, the processes in the deep earth work on time scales of millions of years.

600 million years ago without any strong intervention by organisms vast amounts of calcium carbonate precipitated spontaneously out of the sea. The spontaneous incrustations formed a hazard for life in the ocean, however, the biota in the course of evolution developed a range of potent chemical inhibitors of the spontaneous precipitation of carbonates. The open ocean has become supersaturated and precipitation of limestone has become biologically regulated at present. Marin et al observed that the same anticalcifying mucus that protects the soft tissues from incrustation helps in organising skeleton formation by keeping crystallisation in check. Thus, in the course of evolution the production of limestone in the open sea, and therefore a key part of the carbon cycle was brought under a strict biological control mechanism. As with calcium carbonate, it is believed that silica simply precipitated from the seawater 600 million years ago without the help of organisms. Once again these spontaneous incrustations would have proved to be a problem for organisms that lived in the ocean. In the course of evolution organisms such as the sponges, radiolarians, and more recently diatoms, were able to use the silica in the production of their finely sculptured skeletons. Spontaneous precipitation no longer occurs as the ocean water is now strongly undersaturated.

Carbon dioxide also dissolves directly into the ocean; the quantity is a function of the concentration of carbon dioxide in the overlying atmosphere. It enters the deeper waters through the downward flux of cold water in the polar latitudes. As carbon dioxide dissolves in water it dissociates to form bicarbonate:

Due to the higher solubility of carbon dioxide at lower temperatures there is an increase in the dissolution of the calcium carbonate, produced by marine algae, with increasing depth. The reaction is as follows:

Carbonate dissolution is complete below the carbonate compensation depth and this normally occurs between 4200 — 4500m. This means that carbonate sediments are only found in shallow ocean basins. The solubility of carbon dioxide is inversely proportional to temperature consequently as the ocean temperature rises the solubility of carbon dioxide decreases. In the case of a warming planet due to increasing greenhouse gases we can expect a decrease in the overall uptake of carbon dioxide by the oceans. There are major uncertainties in the scale of this effect but it is likely that it will result in a positive feedback.

A climate regulating mechanism based on the chemistry of the Urey cycle was proposed by Walker, Hays and Kasting, known as the WHAK mechanism. This mechanism suggests that a warmer Earth would increase weathering of the silicate rocks due to both an increase in the rate of chemical reactions as well as an increase in rainfall through evaporation and precipitation. This increased weathering slowly removes carbon dioxide from the atmosphere matching the increase in solar flux, ultimately allowing the climate to regulate. A negative feedback diagram would look as follows:

This mechanism, however, does not account for the removal of enough carbon dioxide, with the result that the planet would have a global average temperature 10° C hotter than today. From a Gaian perspective this mechanism is inadequate as it does not consider the role of living organisms. Lovelock and Watson suggest that the presence of organisms in soils greatly enhances the rate of rock weathering. This is done by pumping carbon dioxide from the atmosphere into the soils and directly into close contact to the silicate rocks. The soil partial pressure of carbon dioxide (pCO2) could be greater than 30 times that of the soil without the presence of life. In addition to elevating the pCO2 in the soil, the biota accelerates chemical weathering by mechanical fracturing of the rock, release of organic acids, stabilisation of the soil to act as a sponge for water thus creating a medium for acid attack, role of earthworms, insects, burrowing animals, etc., and finally the huge increase in surface area of roots, bacteria and fungi. This enhancement of weathering called the biotic enhancement factor (B) varies from 10 to 1000 times that of weathering without life, and if taken into consideration predicts appropriate atmospheric carbon dioxide levels. A negative feedback mechanism for climate regulation that includes life can now be proposed:

A further example of how much the biota can influence the weathering of silicate rocks was found when Berner modelled the carbon cycle to make a rough estimate of the concentration of atmospheric carbon dioxide during the past 600 million years. In the period from 600 to 450 million years ago the concentration of atmospheric carbon dioxide was 15—20 times higher than today. Then, between 450 and 300 million years ago, there was a sudden decrease until the present level is reached. Berner shows that the origin and evolution of the vascular plants could have been crucial to this drop because it is in this period that extensive forests begin to cover the continents and the plants with their deep roots enormously enhance the weathering rate of calcium silicate.

A change in cloud cover brought about by global warming and the radiative consequences of this is probably the biggest uncertainty in predicting the magnitude of this warming. Clouds through changes in such characters as their amount, altitude, and water content, can act as both positive and negative feedbacks to global warming. Generally the overall effect is for clouds to cool the planet by reflecting solar radiation, however, if in a warming world more clouds form at a higher and colder altitude, or if the water content changes, then some models predict they could act as a positive feedback to the greenhouse effect. Furthermore an increase in global temperature will increase the concentration of water vapour in the atmosphere due to evaporation. As water vapour is itself a potent greenhouse gas its increased presence will have a positive feedback on global warming. This has been empirically demonstrated with satellite-based measurements for the Earth Radiation Budget Experiment (ERBE).

A warming planet would further complicate matters as circulation of water in the oceans is complex and driven by the climate. As the ocean-surface temperature rises, the thermacline may become stable and resistant to vertical mixing between the surface and the deeper waters. Phytoplankton growth is dependent on receiving nutrients from the deeper waters and if this changes it could influence the negative dimethyl sulphide and cloud condensation nuclei feedback mechanism described next.

Studies in the North Atlantic have shown that dimethyl sulphide (DMS) production by phytoplankton in the oceans becomes oxidised in the atmosphere to form a sulphate and a methane sulphonate (MSA) aerosol which can act as cloud condensation nuclei (CCN):

This non-sea-salt sulphate (NSS-SO42- ) aerosol is found throughout the marine-atmosphere boundary layer and is the principal CCN in the marine atmosphere. It has been suggested that a Gaian climate negative feedback mechanism could be operating as a result of this. As the temperature rises there is an increase in algal growth, more DMS is produced, more clouds are formed, the albedo of the earth increases and the temperature decreases:

However this mechanism is limited in its ability to control the climate. According to models by Lovelock and Kump: in glacial conditions these marine ecosystems can provide a negative feedback on temperature. However as the temperature rises to present-day levels algae begin to lose their strong climate influence and as these global mean temperatures rise above 20° C, marine ecosystems have the potential to go into positive feedback, amplifying any further increase of temperature. This positive feedback may have occurred in the past as is suggested by some ice core data.

Anthropogenic sulphur emissions have increased as a result of burning fossil fuels. These aerosols can act in the same way as the DMS produced by the phytoplankton thus influencing the amount of cloud formation in such a way as to cool the Earth. It is possible that this anthropogenic negative forcing could have moderated the greenhouse effect to date, although this is difficult to quantify. This sulphur aerosol as method of controlling future warming is, however, not advised as it could increase the acidification of the biosphere thus risking damaging the integrity of natural reservoirs of carbon dioxide.

Conclusions

The Earth’s climate is changing, it changed in the past and will certainly change in the future. This in itself is not worrying but what is worrying is the impact humans are having on a complex system we have very little understanding of with implications that are potentially far reaching. Lovelock and Kump argue that if “the burning of fossil fuel were continued long enough, we might arrive at a runaway greenhouse world due to a failure in climate regulation.” In this essay I have shown how this failure could arise through regulatory feedback systems susceptible to the forcing brought about by rising carbon dioxide levels. Both plant and ocean uptake of carbon dioxide is likely to increase with increasing concentrations of carbon dioxide in the atmosphere. These sinks, however, are in a non-steady-state condition because the increase of atmospheric concentrations of carbon dioxide is greater than the uptake processes can handle. Furthermore the buffering effect of the plants and ocean is subject to complex positive feedbacks as a result of an increase in average global temperature. The silicate-carbonate cycle has the potential to buffer atmospheric concentrations of carbon dioxide, however, this will not have any significant effect in periods under 100 000 years. This is an example of the many different timescales that carbon cycles through, and with this the recognition that to be successful in reducing the greenhouse effect requires a focus on short term carbon-cycles over long-term carbon-cycles. Another clear theme running throughout this essay has been the recognition of the tight coupling between life and the abiotic environment and the suggestion that it is not possible to separate life out of any climate regulatory mechanism. Whether it is in the biological pumping of carbon dioxide, the enhancement of silicate rock weathering or controlling carbonate and silica precipitation, the presence of life is strongly linked to the concentration of carbon dioxide in the atmosphere and in fact its presence appears to be regulatory. Any study of climate change needs to take this into consideration. Finally the evidence of runaway positive feedbacks on planet warming as discussed throughout the essay could take the planet’s climate into a new steady-state. One where it might become impossible, through hysterises effects, to address global warming with technical fixes such as reforestation. This is a serious risk and one that I believe has not been sufficiently recognised.

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