The World Carbon Cycle in the Last Ice Age
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. It is one of the most important cycles of the earth and allows for carbon to be recycled and reused throughout the biosphere and all of its organisms. Has it always been the same? A reconstruction of plant productivity and the amount of carbon stored in the ocean and terrestrial biosphere at the last ice age has just been published in Nature Geoscience. The research by an international team of scientists greatly increases our understanding of natural carbon cycle dynamics.
The last ice age refers to the most recent colder period that peaked at the Last Glacial Maximum approximately 20,000 years ago, in which extensive ice sheets lay over large parts of the North American and Eurasian continents.
The carbon cycle is now usually thought of as including the following major reservoirs of carbon interconnected by pathways of exchange:
The terrestrial biosphere
The sediments including fossil fuels
The Earth's interior (magma)
The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth, but the deep ocean part of this pool does not rapidly exchange with the atmosphere.
Atmospheric carbon dioxide (CO2) is one of the most important greenhouse gases and the increase of its abundance in the atmosphere by fossil fuel burning is a cause of future global warming. 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.
Scientists can reconstruct these changes in the atmospheric carbon stock using direct measurements of atmospheric CO2 trapped in air bubbles in the depth of Antarctica's ice sheets. However, explaining the cause of these 100ppm changes in atmospheric CO2 concentrations between glacial and interglacial climate states — as well as estimating the carbon stored on land and in the ocean — is far more difficult.
The researchers, led by Dr Philippe Ciais of the Laboratoire des Sciences du Climat et l'Environnement near Paris, ingeniously 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 latter being driven by estimates of glacial climate using climate models.
Dr Marko Scholze of the University of Bristol’s School of Earth Sciences, co-author on the paper said: "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. 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."
From these results, the authors conclude that the cycling of carbon in the terrestrial biosphere — that is, 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. Furthermore there must have been a much larger size of non-decomposable carbon on land during the Last Glacial Maximum (the period in the Earth’s history when ice sheets were at their maximum extension, between 26,500 and 19,000 years ago).
The authors suggest that this inert carbon could have been buried in the permanently frozen soils and large amounts of peat of the northern tundra regions.
For further information: http://www.bristol.ac.uk/news/2011/8055.html