Global warming causes multiple feedback loops – including parts of the tropics, it seems.

As wet regions become wetter, this is likely to fast-track the release of carbon dioxide (CO2) from the soil, in turn intensifying the warming of the planet, according to a study published in the journal Nature. 

More heat produces more evaporation, which is expected to trigger more intense rainfall in tropical areas, prompting an international team led by Christopher Hein from the Virginia Institute of Marine Science, US, to investigate how this impacts upon the Earth’s carbon sink.

Drilling down through 18,000 years of soil in the Ganges-Brahmaputra basin in Bangladesh, they compared radiocarbon levels of the sediment with organic carbon from plants to determine soil ages through the millennia.

They found a strong correlation between higher runoff rates, reflecting wetter epochs, and younger soils that respire more quickly and thus store less carbon.

To give some context, soils breathe – like humans, animals and plants – as microbes release CO2 from decomposing organic matter and respiration of plant roots.

Several factors increase the rate of soil respiration, including moisture, which increases microbial activity. 

Hein and colleagues found that soil turnover nearly doubled in the 2600 years following the end of the last Ice Age, as India’s summer monsoons – the primary source of rain across India, the Himalayas and south-central Asia – became stronger.

The authors previously reported a three-fold increase in the region’s annual rainfall over this period and confirmed increased monsoonal strength by several lines of independent evidence, including analysis of oxygen-isotope ratios from Chinese cave deposits and skeletons of open-ocean phytoplankton.

The soil’s accelerated contribution to climate change is not trivial.

“[S]mall changes in the amount of carbon stored in soils can… play an outsized role in modulating atmospheric CO2 concentrations and, therefore, global climate,” says Hein, “as soils are a primary global reservoir of this element.”

To put that in perspective, the Earth’s atmosphere currently contains 416 parts CO2 per million, equating to about 750 billion tonnes – and our soils hold 3500 billion tonnes, more than four times as much.

Spiralling effects of global warming have already been established in the Arctic, where thawing permafrost soils spur microorganisms to break down organic matter, estimated to release up to 0.6 billion tonnes of carbon to the atmosphere each year. 

“We’ve now found a similar climate feedback in the tropics,” says Hein, “and are concerned that enhanced soil respiration with greater precipitation – itself a response to climate change – will further increase concentrations of CO2 in our atmosphere.”