Our Earth system has three active reservoirs for storing the element carbon -- the air, the land, and the sea.  From minute to minute, from millennium to millennium, our Earth system naturally cycles carbon from one reservoir to another, and thus provides a vital service to all life on Earth.  Yet we humans are now hastening the natural transfer rate of carbon to the air, mainly by burning fossil fuels, and this alters the relative distribution of carbon among Earth’s carbon reservoirs.  Fossil fuel combustion, and cement making, currently equate to more than 75% of human-caused carbon emissions; since 1751 this emission vector has filled our Earth’s atmosphere with over 315 billion metric tons of carbon emissions.  Changes in patterns of land use, mainly forest destruction and agricultural conversion, are accountable for the balance of the human-caused carbon emissions to our planetary atmosphere.

Our Earth system’s active carbon reservoirs vary in size.  The air reservoir is the smallest, with our atmosphere presently containing 597+ billion metric tons of carbon.  The sea reservoir is the largest, with the ocean presently containing 3803+ billion metric tons of carbon.  The land reservoir, middle in size, presently contains 2450+ billion metric tons of carbon.  Beyond our Earth system’s three interdependent biological carbon reservoirs, there is the “inert” fossil carbon reservoir of entombed ancient plant matter; this fossil carbon reservoir, buried in geologic history, holds 3700+ billion metric tons of carbon.  Of crucial importance, when we humans burn fossil fuels we release previously inert fossil carbon into the active carbon pool; this bloats the size of the active carbon pool, and it will take millennia for the Earth system to re-bury that carbon.  For perspective, consider this each time you visit a gas station, every 3.8 litres of gas sold at that station required near 90 metric tons of ancient plant matter as precursor material: we ritually splurge particles of condensed death into the air around us.

The good news is that the climate change impact of human-caused carbon emissions to the sky is currently lowered by the land and the sea -- at present only about 44% of the carbon emissions we humans are responsible for each year stay in the atmosphere to exacerbate global climate change -- terrestrial ecosystems and the ocean “magically” absorb the remainder.  This happens mostly through plant photosynthesis and the chemical equilibration of the ocean with the atmosphere above. In 2005, for example, terrestrial and marine carbon exchanges absorbed more than four billion metric tons of the nearly eight billion metric tons of carbon emissions that we humans were responsible for emitting into the atmosphere that year.  To utilize the terminology of the biogeochemistry profession, the land and the sea are serving as “sinks” for the removal of carbon from the sky.  Carbon sinks are carbon reservoirs increasing in size; the opposite of carbon sources.  To appreciate the relative scale of the land and sea sinks, note that from 1980 to 1999 the land sink absorbed 15+ billion metric tons of airborne human-derived carbon pollution while the ocean sink soaked-up 37+ billion metric tons.

Given the fact that carbon emission reductions are now valorized within a booming international commodity market set-up to help ease global climate change, our natural planetary carbon sinks should well be recognized for the climate equilibrium balancing service that they provide for free -- provision of like service via mitigation measures would cost trillions of dollars.  In essence, we humans are benefiting much from the ability of our Earth system to mop-up the pollution we are generating.  Thankfully too, the ability of the Earth system sinks to absorb our ‘anthropogenic’ carbon emissions is projected to increase proportionally with the amount of anthropogenic emissions that we humans emit into the atmosphere in the decades ahead.  This means that the percentage of anthropogenic carbon absorbed by Earth system sinks will stay about the same relative to the amount anthropogenic carbon going into the atmosphere, even though we humans are on a path to emit more and more emissions.  The reason that our Earth system’s active carbon sinks can together absorb growing amounts of anthropogenic carbon emissions, and continue to keep more than half of all the carbon pollution emitted by humans in check, has to do with land-use change and plant physiology.

At present, the increasing concentration of carbon in our atmosphere is stimulating plant growth.   Carbon, oxidized into the form of carbon dioxide, is after all plant food.  More carbon dioxide in our Earth’s atmosphere, given current circumstances, means that plants on Earth can grow faster, bigger, and more water-efficient.  This is the so-called “CO2 fertilization” effect.  The CO2 fertilization effect, combined with potentially strong CO2 drawdown due to land-use change over time, is a reason that Earth’s carbon sinks are now able to increase their total CO2 sequestration from the air in-step with human pollution.  If not for this carbon dioxide increase – plant growth rate correlation the ongoing CO2 accumulation in our atmosphere would be faster.  Yet this rate correlation, that now allows us humans to pay less than half the full climate impact price for our CO2 emissions, is a free-ride that is destined not to last.  The problem is that although plants absorb CO2 during photosynthesis, plants also release CO2 back into the air when plant matter breaks down the sugars they have made.  This process is called respiration, and respiration increases in response to rising temperatures.  With global warming now happening worldwide, because of the build-up of heat-trapping CO2 molecules in the sky, overall respiration by plants will eventually outpace CO2 fertilization.  This has potentially harsh consequences.  Some carbon sinks for anthropogenic emissions are now on course to “saturate”  -- which means that the net carbon sink uptake will plateau, more CO2 will stay in the sky, and climate change will amplify.

Unless we humans make significant reductions to the total amount of CO2 emissions we are releasing into the atmosphere, carbon sink saturation, driven by global warming, will cause Earth’s terrestrial and marine ecosystems to become steadily less effective at removing carbon from the atmosphere.  The decline of the land sink, described in essence above, will indeed be due to increases in warming sensitive plant respiration, yet the decline will also be due to biomass combustion by fires -- and regional-scale biomass-combusting fires are escalating in frequency, extent and duration because of climate change.  The decline of the ocean carbon sink, induced by accelerating anthropogenic carbon emissions, is projected to happen through a combination of carbon-climate feedbacks – yet certainly the ongoing warming of seawater temperatures worldwide will lead to a weakening of the ocean’s capacity to absorb carbon from the atmosphere.  Warmer water holds less dissolved gas than colder water, so the ocean will not be able to store as much anthropogenic carbon.  Another process that will influence the ocean carbon sink in the decades ahead is the ongoing chemical equilibration of the sea with the rising CO2 level in the air, which will gradually “acidify” seawater worldwide and thus impair the ability of the ocean carbon sink to sequester carbon.

The phenomenon of Earth system carbon sink saturation is a reality that concerns all people -- this is not hypothetical: sink saturation is already happening now in both marine and land ecosystems.  The risk here, moreover, is not simply that more and more of Earth’s ecosystems will begin to falter at sequestering carbon, and thus a rising fraction of anthropogenic carbon dioxide emissions will stay airborne and exacerbate climate change.  The risk is more serious than even that.  By monkeying with the chemistry of our atmosphere, we humans may not only cause the loss of natural carbon sinks on Earth, we may inadvertently trigger the transformation of natural carbon sinks into relentless engines of planetary climate change.  To cite the words of an eminent scholar at the University of Cambridge:
“the danger is that global warming may become self-sustaining, if it has not done so already”.

The land and sea continuously exchange very large fluxes of carbon with the atmosphere.  Yearly worldwide photosynthesis by land ecosystems, for instance, fixes about 19 times more carbon than humans emit via fossil fuels usage.  Respiration by terrestrial ecosystems sends most of that carbon back into the air, with a small portion being sequestered in land carbon sinks.  The Earth system is thus potentially susceptible to relatively small changes in natural carbon fluctuations causing a large impact on the presence of residual carbon dioxide sinks.  Terrestrial ecosystems could become net carbon dioxide sources on a grand scale if respiration starts to outpace photosynthesis.  This eventuality, if it were to occur, could turbo-charge global warming and prompt climate changes that are virtually irreversible on timescales meaningful to human civilization.  Unfortunately, given current rates of anthropogenic carbon dioxide emissions to the sky, it is probably inevitable that Earth’s terrestrial biosphere will go through a net “sink-to-source” transition during this century.  Lowering the rate of anthropogenic carbon dioxide emissions to the sky is the only way we humans can avoid triggering a global “sink-to-source” transition that could fatefully cause climate change to become self-sustaining.

Given that terrestrial ecosystems respire carbon into the atmosphere at a faster rate in response to rising temperatures, and that humans are fuelling global warming via carbon dioxide emissions, the saturation of the net terrestrial carbon sink and its transformation into a carbon source now seems destined to happen -- the question thus arises ‘when precisely will the sink-to-source switch occur?’  An answer to this question may be found via the utilization of high-tech computer simulations, called Ocean-Atmosphere General Circulation Models, which account for biogeochemical mechanisms in their coding methodology.  Recently, eleven such models performed simulations for the 1850 to 2100 time horizon.  There was unanimous agreement amongst the models that (a) future climate change will harm the efficiency of our Earth’s carbon sinks, and (b) the fraction of total anthropogenic CO2 emissions that remain airborne will increase during the 21st Century.  Moreover, the possibility of a sink-to-source switch, leading to additional global warming of up to 1.5°C, is evident through an inter-comparison of the models.  The precise year that the switch will happen this century is set by different models at different times -- yet even modelling teams that did not find a switch in their results agree that a switch is plausible, even though the precise timing and impacts of the switch are now uncertain.

Although there is precedent for computer modeling runs to find the Earth system carbon sink-to-source switch before 2050, the Intergovernmental Panel on Climate Change has high confidence that the switch will happen after the mid-century point.  High confidence in IPCC language means that they rate the possibility to be about an 80% chance.  This 2007 finding of the IPCC may be the most important new conclusion made by the IPCC since their prior report to the world community in 2001.  Earth System scientists anticipate the carbon sink-to-source switch mostly because they expect the widespread die-off of rainforest ecosystems because of human-induced global warming -- this will vaporize a colossal tropical carbon pool, catalyze the sink-to-switch, and hasten more climate chaos.

The die-off of temperature-stressed jungles, combined with widespread tropical soil decomposition by heat-activated microbes, will result in the disappearance of rainforest ecosystems that oxygenate much of our Earth.  By 2050 temperature increases and related reductions in soil moisture content are projected to lead to dry savanna vegetation replacing much of the Amazon jungle.  Already now the impact of warming temperatures is beginning to trump the effect that carbon dioxide fertilization has on rainforests, prompt CO2 sink saturation, and lead to reductions of up to 50% in rainforest growth.  The world’s tropical soils, which respire carbon to the sky per a microbial oxidation rate that amplifies with warming temperatures, are now regarded to be highly vulnerable.  Scientists judge that, over the course of this century, heat-stressed tropical soils could be the principal vector for a net global soil carbon dioxide pulse that could deliver an added 400 billion metric tons of carbon dioxide into our Earth system's atmospheric CO2 reservoir; for comparison, such a pulse would be approximately 60 times the total amount of CO2 emissions attributed to human burning of fossil fuels in the year 2000.

The decreasing capacity of rainforest ecosystems to sequester carbon, and the looming rise in the rate at which rainforest ecosystems emit carbon to the sky, are very serious matters.  Both of these processes are capable of notably altering the overall rate of carbon accumulation in the atmosphere.  Thus both processes have the potential to hasten the land biosphere’s ongoing transition from being a weak carbon sink to being a formidable carbon source.  Unfortunately, short of reducing our CO2 emissions, we humans will not prevent rainforest ecosystems from spiking the atmospheric carbon concentration and accelerating the land biosphere's unyielding carbon sink-to-source transformation.

The loss of global rainforests will, of course, have implications for humans that go beyond perilous climatological impacts.  Our planet’s bountiful heritage of flora and fauna biodiversity is threatened by rainforest loss too.  Yet the consequences of losing 70% of the Amazon rainforest are regrettably not something enough people actually care about.  The human-coordinated elimination of the orangutan is just one example amid many of how readily we humans can turn into the thoughtless blunt clubs of extinction.  So it seems that encouraging care for the environment is not the best way to stem the flow of harmful greenhouse gas pollution that is being emitted into our filling atmosphere.  Many of us in North America would gladly trade the last frog for cheaper gas prices.  We live in a fools’ paradise.  The damage we are doing to our climate system cannot be prevented by calls for morality.  Our parochial culture, structured by cultivated fear, is set-up to systematically attack any emergent sense of universal morality.  Sadly, there seems to be only one way that we North Americans can reduce our greenhouse gas pollution by the 80%, or more, which is necessary in order to sustain our enjoyment of a stable climate system.  We must galvanize our efforts to lower emissions premised on the view that climate change is a < threat > to the physical integrity of our human families and our human stuff.