Study sheds light on natural carbon cycle dynamics
An international team of scientists has reconstructed the Last Ice Age's marine and terrestrial productivity and carbon stocks by combining isotope data that are relevant to both global quantities and models. The study, published in the journal Nature, is funded in part by the MOTIF ('Models and observations to test climate feedbacks') project, which clinched more than EUR 181 000 under the 'Energy, environment and sustainable development' (EESD) Thematic programme of the EU's Fifth Framework Programme (FP5).
Researchers led by the Laboratoire des Sciences du Climat et l'Environnement in France say atmospheric carbon dioxide (CO2) is one of the most significant greenhouse gases. Global warming is bring triggered by the growing amount of CO2 in the atmosphere. The researchers point out that in past times, during the transition between an ice age and a warm period, atmospheric CO2 concentrations changed by some 100 parts per million (ppm) — from an ice age value of 180 ppm to about 280 ppm during warm periods.
By using direct measurements of atmospheric CO2 trapped in air bubbles in the depth of Antarctica's ice sheets, it is possible to reconstruct these changes in the atmospheric carbon stock. But scientists have found it hard to explain what triggers these 100 ppm changes in atmospheric CO2 concentrations between glacial and interglacial climate states. It is also difficult to estimate the marine and terrestrial carbon stocks.
In this study, the scientists combined measurements of isotopes of atmospheric oxygen (18O) and carbon (13C) in marine sediments and ice cores with results from dynamic global vegetation models.
'The difference between glacial and pre-industrial carbon stored in the terrestrial biosphere is only about 330 petagrams of carbon, which is much smaller than previously thought,' says Dr Marko Scholze from the School of Earth Sciences at the University of Bristol in the United Kingdom. 'The uptake of carbon by vegetation and soil, that is the terrestrial productivity during the ice age, was only about 40 petagrams of carbon per year and thus much smaller: roughly one third of present-day terrestrial productivity and roughly half of pre-industrial productivity.'
The results of their study suggest that the cycling of carbon in the terrestrial biosphere — what is essentially the time between uptake by photosynthesis and release by decomposition of dead plant material — must have been much smaller than in the current, warmer climate. According to the researchers, there must have been a bigger size of non-decomposable carbon on land during the Last Glacial Maximum (the period when ice sheets were at their maximum extension, between 26 500 and 19 000 years ago).
'This inert carbon should have been buried in the permanently frozen soils and large amounts of peat of the northern tundra regions,' the researchers conclude. The study's results will help boost our understanding of natural carbon cycle dynamics.