Are We Overestimating Our Global Carbon Budget?

According to the latest research, the carbon sink on land is unfortunately starting to show signs of trouble.  Instead of providing a brake on human emissions, the land carbon sink could instead soon be giving our emissions a boost.  (The ocean sink appears to be relatively safe for now, although there is a price to pay: the consequence of the process of carbon dioxide dissolving into seawater, ocean acidification, has been called climate change’s evil twin with its own, non-climate related consequences for marine life.)

We often hear climate-change deniers referring to carbon dioxide as “plant food”.  They are right:  when the amount of carbon dioxide in the air increases, it does act as fertilizer for plants.  In fact, this is the reason we have seen a land sink for carbon over the past two hundred years:  the carbon dioxide we emitted made plants grow faster.  But just as humans do not live on bread alone, neither do plants thrive solely on carbon dioxide.  Plants also need water, the right climatic conditions and, crucially, a supply of other ingredients from the soil.  As every suburbanite knows, a front lawn requires more than just air and water for it to look its best.  It also needs fertilizer, especially an extra supply of nitrogen and phosphorous.  In nature, the supply of these nutrients is limited.

A recent study in the journal Nature Geoscience by National Center for Atmospheric Research scientist Will Wieder and colleagues looked at how the land carbon sink would perform once realistic natural supplies of nitrogen and phosphorous were taken into account.  When looking at the worst-case human emissions pathway over the rest of this century, they found that instead of the land biosphere acting as a large net sink – as the IPCC studies calculate – the response of a nutrient-constrained system would be a net source of carbon.  The difference between the two estimates is 280 billion tonnes of carbon, which is roughly equivalent to 25 years of human emissions at current emission rates.

There are huge uncertainties in studies of this type and much more research needs to be done.  Nevertheless, it is a concern that the carbon-cycle response to human-caused climate change may turn out to be worse than even the IPCC’s latest report predicts.

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