August 2, 2012

Earth’s Carbon Sink Still Strong and Growing

Filed under: Adaptation, Plants

As is widespread and common knowledge, higher atmospheric carbon dioxide (CO2) levels are beneficial to plants making them grow faster and larger, in addition to enhancing them in virtually every other way. For an immersion in the subject of plants and carbon dioxide, check out the website PlantsNeedCO2.org and revel in the good news concerning higher atmospheric CO2 levels.

This growth enhancement has led to the earth’s plants taking an increasing amount of CO2 from the atmosphere and turning it into biomass where carbon is stored for days to hundreds of years (this mechanism accounts for a significant portion of the earth’s land-based carbon sink). It seems the more CO2 we pump into the atmosphere, the more CO2 that plants take out to enhance their growth.

The oceans also take carbon dioxide out of the atmosphere and can store it for long periods of time (thousands of years). And it appears that this ocean carbon sink is also expanding as we emit more CO2 into the atmosphere.

Together, the land and ocean carbon sinks have been pretty much keeping up with the increasing anthropogenic carbon dioxide emissions. Consequently, the percentage of CO2 injected into the atmosphere from human activities that remains in the atmosphere has remained pretty much constant for the last 50 years—according to just-published research in the journal Nature—despite ever increasing anthropogenic CO2 emissions.

Figure 1, taken from the new paper by Ballantyne et al., shows the three main components of the CO2 budget. The top panel shows that the annual amount of CO2 that is staying in the atmosphere is slowly increasing. The middle panel shows the annual amount of CO2 that is emitted into the atmosphere each year from human activities (fossil fuel use and land use changes). Notice that the amount that remains in the atmosphere (top panel) is only about 45% of the amount that is emitted by humans (middle panel). This means that the other 55% is being taken up by land and ocean sinks. This increasing sink is shown in the bottom panel of Figure 1—note that the greater the negative value the greater the carbon sink.


Figure 1. (Top) Annual accumulation of CO2 in the atmosphere. (Middle) Annual CO2 emissions from anthropogenic activities. (Bottom) Net CO2 update by land an ocean sinks (from Ballantyne et al., 2012).

If all of this seems a bit familiar, it is because we reported on a similar finding published about a year and a half ago in the journal Geophysical Research Letters by Wolfgang Knorr. But instead of just focusing on the past 50 years like Ballantyne et al. did, Knorr extended his analysis back 150 years, and concluded that the airborne fraction of carbon dioxide had remained constant over that longer period as well.

Such results run counter to modeling studies which continue to suggest that the earth’s carbon sinks should be declining and that consequently, a growing percentage of anthropogenic CO2 emissions should be remaining in the atmosphere—acting of course to further enhance global warming. But the observations show that this just ain’t so.

Recall what Knorr had to say in a press release accompanying his article:

New data show that the balance between the airborne and the absorbed fraction of carbon dioxide has stayed approximately constant since 1850, despite emissions of carbon dioxide having risen from about 2 billion tons a year in 1850 to 35 billion tons a year now.

This suggests that terrestrial ecosystems and the oceans have a much greater capacity to absorb CO2 than had been previously expected.

The results run contrary to a significant body of recent research which expects that the capacity of terrestrial ecosystems and the oceans to absorb CO2 should start to diminish as CO2 emissions increase, letting greenhouse gas levels skyrocket.

And now Ballantyne et al. add:

From a global mass balance perspective, net uptake of atmospheric CO2 has continued to increase during the past 50 yr and seems to remain strong. Although present predictions indicate diminished [carbon] uptake by the land and oceans in the coming century, with potentially serious consequences for the global climate, as of 2010 there is no empirical evidence that [carbon] uptake has started to diminish on the global scale. [emphasis added –eds.]

Both Knorr and Ballantyne et al. call for more research into just why observations show one thing and models project another (a characteristic that seemingly is ubiquitous in climate change research).

What all of this means is that despite the pessimistic prognostications, the earth’s natural systems continue to mitigate the impacts of human fossil fuel utilization—a utilization which has vastly improved both our standard of living and our life expectancy. The earth is quite an extraordinary place!

References:

Ballantyne, A. P., 2012. Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years. Nature, 488, 70-72, do:10.1038/nature11299

Knorr, W., 2009. Is the airborne fraction of anthropogenic CO2 emissions increasing? Geophysical Research Letters, 36, L21710, doi:10.1029/2009GL040613.




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