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ANNUAL SYMPOSIUM
Dangerous Climate Change: Is it inevitable?
Friday, 9 May 2008

The carbon cycle at the climate crossroads
by Dr Michael Raupach

Michael Raupach has a PhD in micrometeorology from Flinders University in South Australia. After a postdoctoral position at the University of Edinburgh, he joined CSIRO in 1978. His interests include land-air interactions, micrometeorology, the fluid mechanics of turbulent flows, particle transport and soil erosion by wind, global and continental carbon and water cycles, and carbon-climate-human interactions. He is a co-chair of the Global Carbon Project of the Earth System Science Partnership and a Fellow of the Australian Academy of Technological Sciences and Engineering.

The carbon cycle interacts with other parts of the Earth system, principally global climate and human societies, in two broad ways.

1. Biophysical feedbacks: Land and oceans together take up more than half the carbon dioxide (CO₂) put into the atmosphere each year by human activities, mainly fossil fuel combustion. This is a strong negative (stabilising) feedback on anthropogenic perturbation of the Earth system, but it is slowly weakening. In the presence of significant natural interannual variability in atmospheric CO₂, mostly related to El-Niño and to volcanic activity, there is a detectable increasing trend in CO₂ airborne fraction (the fraction of anthropogenic CO₂ emissions staying in the atmosphere) over the last 50 years. Several land and ocean processes are responsible, and these are likely to continue in the future.

2. Human actions: Anthropogenic CO₂ emissions are accelerating, in response to changes in three drivers: population (tending to increase emissions), per capita income (likewise increasing emissions), and the carbon intensity of the global economy (tending to decrease emissions, but at a weakening rate). Together, these trends represent an uncontrolled positive feedback on atmospheric CO₂. While the climate consequences are broadly understood, human response is still unknown. The Earth system is at a crossroads.