Temperature rises linked to CO2 emissions from rock
Earth’s surface temperature is influenced by atmospheric concentrations of gases such as CO2, which contribute to the so-called greenhouse effect. Since the industrial revolution, CO2 concentrations have increased dramatically, due largely to the combustion of fossil fuels. Tackling these emissions is critical to keeping temperature rises in check. To measure our progress, scientists need to accurately measure CO2 emissions, and identify sources of CO2 leakage. This means taking into account not just man-made emissions, but natural sources of CO2 leakage as well.
Natural carbon cycle
The ROC-CO2 project was launched to address a gap in our understanding of the natural carbon cycle. Scientists know that when rocks break down, CO2 can be both released and absorbed, due in part to the oxidation of organic carbon contained within. This is a process known as chemical weathering. To date however, scientists have been unable to accurately measure or fully understand the mechanisms of this process. To get a better handle on chemical weathering, the ROC-CO2 project, which was supported by the European Research Council, developed pioneering new analytical methods.
Measuring CO2 emissions
“Measuring CO2 release from rock is very difficult,” explains ROC-CO2 project coordinator Robert Hilton from the University of Oxford in the United Kingdom. “We know however that when rock decomposes, a range of elements end up in nearby rivers and lakes.” One of these elements is rhenium, which Hilton found could act as a proxy measurement for carbon. By taking river samples and analysing rhenium content, Hilton and his team were able to build up a picture of rock breakdown in a given area, and measure how fast this process is taking place. Secondly, Hilton and his team sought to measure levels of CO2 emissions and absorption from rock weathering directly. “We attached CO2 sensors to rocks at our site in France and watched how rocks ‘breathed’,” says Hilton. “This was a lot harder than it sounds, because we had to be sure that we were not measuring CO2 emissions from the atmosphere, or from plant roots.” For this, the team took gas samples and measured the radiocarbon levels. Given that the half-life of radiocarbon isotopes is thousands of years, a negative reading meant the team could be sure that the CO2 came from a non-living source, i.e. rock.
Climate impact
Over a couple of years, the project was able to conclude that the amount of CO2 released from weathering rocks actually increased with temperature. In winter, CO2 emissions from the test site went down; in summer they went up. “This suggests that more CO2 will be released as the climate warms,” adds Hilton. “If this is happening beyond our test site, then some aspects of how we think about the carbon cycle will have to change.” For Hilton, the project findings have opened up new avenues of research. “We don’t get to choose when or where the natural carbon cycle will leak CO2 into the atmosphere,” he says. For example, there are sedimentary rocks under permafrost. “Will these start to leak CO2 if the permafrost melts? This is something we need to find out.” ROC-CO2’s methods are now being adopted by other researchers around the world. Their findings will help to fill in the picture of how CO2 fluxes from decomposing rocks will likely impact climate, from a timescale ranging from one season to thousands of years.
Keywords
ROC-CO2, atmospheric, CO2, gases, Earth, carbon, emissions, climate, sedimentary
