Rising temperatures lead to ‘unexpectedly’ rapid carbon release from soils

Carbon cycle [credit: NASA]

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These researchers like many others assume what they ought to be trying to determine, i.e. whether humans contribute anything to planetary warming, and if so how much compared to the natural carbon cycle. But having found that carbon dioxide from soils rises naturally when the temperature goes up, they jump to the conclusion that this will mean climate model calculations of warming have to be ratcheted up to match. It doesn’t seem to occur to them that supposed but unproven anthropogenic causes of warming might need to be dialled down instead. In the ‘related stories’ footnote to the article, one of the links is: Microbes in warm soils released more carbon than those in cooler soils. Could it be that marine microbes behave in a similar way, and thus natural ocean CO2 release (outgassing) might also be significantly underestimated, again calling climate model predictions into question?
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How sensitively does organic carbon stored in soils react to changes in temperature and humidity? This question is central to a new study now published in Nature Communications, says Phys.org.

The work involved researchers from MARUM—Center for Marine Environmental Sciences at the University of Bremen and from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven.

Globally, soils store more than twice as much carbon as the atmosphere. Therefore, carbon uptake and release by soils constitutes a strong regulator of atmospheric concentrations of the greenhouse gas carbon dioxide (CO2).

In the context of the ongoing anthropogenic climate change, it is thus [Talkshop comment – no evidence given for the assertion] important to better understand the sensitivity of soil carbon, which is directly related to the release of CO2 from soils, under a changing climate, such as rising temperatures and/or variations in the hydrological cycle.

Studies have already highlighted the importance of permafrost regions, where rising temperatures lead to the release of carbon from previously frozen soils. However, large amounts of organic carbon are also stored in soils in subtropical and tropical regions. In these regions, it was previously unclear what the main factor for a change in the carbon turnover rate was.

“Microbes that break down organic matter are generally more active under warm and humid conditions, so the carbon content in tropical soils responds very quickly to climatic changes. Some studies report a main influence of changing hydroclimatic conditions, while in others temperature plays the main role,” explains first author Dr. Vera Meyer from MARUM.

Deposits provide a glimpse into the past
To shed more light on these large-scale processes, Meyer and her colleagues chose a rather unconventional approach. Instead of studying soils, they analyzed the age of land-derived organic matter that was transported from soils from the Nile to the Mediterranean and deposited near the river mouth. The Nile transports material from a huge catchment area in the subtropical to tropical regions of north-east Africa to the eastern Mediterranean.

The samples for the study come from a coastal marine sediment core in which age evidence of many thousands of years has been deposited. Such sediment cores therefore allow a much longer look back into times in Earth’s history when the climate was significantly different from today and changed considerably.

“The age of the organic material delivered by the Nile essentially depends on two factors: how long it was in the soils, and how long it took to be transported in the river. The advantage of our approach is that long time scales can be investigated, in this case the last 18,000 years since the last ice age,” says Dr. Enno Schefuß, also from MARUM.

The results surprised the researchers and showed something unexpected: The ages of the land carbon changed only slightly with changes in precipitation and the associated changes in runoff, but reacted strongly to changes in temperature.

In addition, the change in ages due to the temperature increase after the last ice age was significantly greater than expected. This means that the post-glacial warming drastically accelerated the decomposition of organic matter by microorganisms in soils and caused a much stronger outgassing of CO2 from (sub-)tropical soils than predicted by carbon cycle models.

Co-author Dr. Peter Köhler from AWI Bremerhaven says, “The fact that the models underestimate carbon release from soils so strongly shows us that we need to revise the sensitivity of soil carbon in our models.”

Full article here.
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Image: Carbon cycle [credit: NASA]