Not only do floods threaten – the increase in tropical rainfall as a result of climate change could further exacerbate global warming, researchers report. Accordingly, more rain in the tropics increases the release of carbon dioxide by soil microbes. This is the result of investigations of sediment cores, which reflect the soil respiration rates in the catchment area of the Ganges and Brahmaputra rivers in the past 18,000 years.
It is getting warmer and the climate systems are out of balance: The greatest environmental problem of our time was primarily the release of the greenhouse gas carbon dioxide from the combustion of fossil fuels. As is well known, mankind is currently struggling to limit this anthropogenic warming. It is now clear that the measures must also prevent other problematic processes from starting. Because the changes caused by warming can in turn contribute to a worsening of the situation, as some studies have already shown.
Feedback effects threaten
Up to now, the fearful look has mainly focused on the thawing permafrost soils of the north. When heated, the strong greenhouse gas methane rises from these ancient biomass deposits. So-called ground breathing also increases there. Microbes decompose the deposited organic materials and release further carbon dioxide into the atmosphere. The process in the north is therefore a positive feedback in the context of climate change. Christopher Hein’s researchers at the Virginia Institute of Marine Science at Gloucester Point have now investigated a potentially similar effect in the tropics. In contrast to the far north, this feedback process is linked to the increased rainfall, which according to climate models will now occur more and more frequently in this area of the world.
The researchers’ results are based on investigations of sediment cores from the sea floor in the Bay of Bengal. This is where the largest river in Asia flows into the sea: the combined water masses of the Ganges and Brahmaputra rivers. They created the world’s largest delta through sediment deposits. The two rivers transport more than a billion tons of sediment from their vast catchment areas to the Gulf of Bengal each year.
As the researchers explain, the environmental history of the Ganges-Brahmaputra area has emerged in the layers of the drill cores over the past 18,000 years. In addition to rainfall and runoff rates, these deposits also allow conclusions to be drawn about how breathable the soils in the region have been over time: With the help of certain plant biomarkers of the organic remains in the sediments, it is possible to determine how long the biomass in the soil before it is broken down was stored. In other words, it shows how old she was before she got into the river system.
High rainfall – lots of soil breathing
“Based on this information, we found that the shift towards a warmer and wetter climate in the catchment area of the Ganges and Brahmaputra rivers has increased soil respiration rates and reduced carbon stocks in the soil over the past 18,000 years,” says Hein. As he and his colleagues explain, the analysis results showed a clear link between runoff rates and soil age: they were able to associate wetter periods with younger, rapidly breathing soils. In contrast, drier times were associated with older soils, which can store carbon for longer periods.
As the researchers explain, the wetter epochs were shaped by the strength of the Indian summer monsoon – the main period of precipitation in India, the Himalayas and south-central Asia. The extent was considerable: In the 2600 years after the end of the last ice age, when the Indian summer monsoon intensified particularly intensely, the rate of soil respiration and carbon turnover almost doubled in the region, the researchers report.
In this context, Hein emphasizes: “Even small changes in the amount of carbon stored in the earth’s soil can have a significant impact on the development of the atmospheric CO2 concentration and thus on the global climate. Because the floors are a primary global reservoir of this element ”. So far, the problematic potential of Arctic permafrost has been in the foreground. “We are now showing a similar climate feedback effect in the tropics,” says Hein. “There is reason to be concerned that increased soil breathing due to more precipitation could further increase the CO2 concentration in our atmosphere there,” said the scientist.
Source: Virginia Institute of Marine Science, technical article: Nature, doi: 10.1038 / s41586-020-2233-9