Soils don´t need warming to add another positive climate feedback

Soils climate feedback: CO2, methane, nitrous oxideWe recently reported on a possible negative carbon feedback of forest soils in higher latitudes: when such soils warm, nutrient availability may increase, as would (therefore) biomass production and CO2 uptake.

But not all climate feedbacks operate through temperature. It now turns out microbes react directly to CO2 – by producing more methane and nitrous oxide, other greenhouse gases and respectively 23 and 300 times as potent as CO2.

A team of German, American and Dutch scientists specialised in soil science and bioclimatology, have conducted a meta-analysis of published research in soil CO2 responses, including 49 different experiments in forests, grasslands, wetlands and cropland. Their results were today published in Nature.

More CO2 > more N2O and methane

It turned out for all different soil types an increase of atmospheric CO2 led to an increase of soil nitrous oxide production, through anaerobic microbe activity. In wetlands and for instance rice paddles, the microbes also released extra methane when confronted with elevated CO2.

The researchers also conclude this positive climate feedback picks up with increases in plant growth [another possible negative climate feedback in higher latitude forests] – because that also creates more dead biomass and favourable anaerobic conditions for the greenhouse gas producing microbes.

To quantify matters: according to the Nature publication this removes only part of the beneficial greenhous gas effect of having extra biomass production. The researchers write: “because enhanced greenhouse-gas emissions add to the radiative forcing of terrestrial ecosystems, these emissions are expected to negate at least 16.6 per cent of the climate change mitigation potential previously predicted from an increase in the terrestrial carbon sink under increased atmospheric CO2 concentrations.”

So for now we can indeed still conclude plants and forests are ‘good for the climate’ – most forests. The capacity of land ecosystems to slow climate warming may have been overestimated though, the researchers state.

CCD climate feedback

It is not the Earth’s only climate feedback that runs through CO2 levels directly – ignoring temperature as a factor. And quite likely it’s not the biggest either. The other one we have in mind happens far from our visible reach, deep in the oceans, and -unfortunately- this one too will act as a positive feedback, further increasing CO2 and climate change. It is the rise of the carbon compensation depth (CCD) as a direct consequence of ocean acidification.

As the ocean water becomes more acidic, the CCD moves upwards, meaning bigger stretches of the deep sea will no longer be able to pile up the carbonate skeletons of dead plankton, sequestering the carbon. What’s more: old carbonate sediments at large areas of ocean floor may actually dissolve, releasing the stored CO2 back to the water [further increasing acidification] and the atmosphere.

Climate control runs through CO2

The one thing good about the CCD feedback is it happens relatively slowly. Therefore, also with timescales in mind, the researchers could be right to focus our attention on (exposing sources of) the faster-acting greenhouse gases, like methane and nitrous oxide – as these gases may deserve priority in climate policies.

It would be bad news though if the new study implies we would actually need to learn how to control CO2 first – in order to be able to control other greenhouse gases. It seems we can end with NASA’s earlier conclusion: temperature is all about CO2.

© Rolf Schuttenhelm |

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