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Abstract
The response of the carbon cycle to climate change, including carbon fluxes, is now shown to be the second largest source of uncertainty in projections of temperature. A simplified climate model using temperature records and historical estimates of CO2 concentrations demonstrates that considering these two factors together reduces uncertainty further than treating them as individual parameters. The future behaviour of the carbon cycle is a major contributor to uncertainty in temperature projections for the twenty-first century1,2. Using a simplified climate model3, we show that, for a given emission scenario, it is the second most important contributor to this uncertainty after climate sensitivity, followed by aerosol impacts. Historical measurements of carbon dioxide concentrations4 have been used along with global temperature observations5 to help reduce this uncertainty. This results in an increased probability of exceeding a 2 °C global–mean temperature increase by 2100 while reducing the probability of surpassing a 6 °C threshold for non-mitigation scenarios such as the Special Report on Emissions Scenarios A1B and A1FI scenarios6, as compared with projections from the Fourth Assessment Report7 of the Intergovernmental Panel on Climate Change. Climate sensitivity, the response of the carbon cycle and aerosol effects remain highly uncertain but historical observations of temperature and carbon dioxide imply a trade–off between them so that temperature projections are more certain than they would be considering each factor in isolation. As well as pointing out the promise from the formal use of observational constraints in climate projection, this also highlights the need for an holistic view of uncertainty.Cite this article
Bodman, R., Rayner, P. & Karoly, D. Uncertainty in temperature projections reduced using carbon cycle and climate observations. Nature Clim Change 3, 725–729 (2013). https://doi.org/10.1038/nclimate1903 