J. A. Kettleborough

Origins and estimates of uncertainty in predictions of twenty-first century temperature rise

Predictions of temperature rise over the twenty-first century are necessarily uncertain, both because the sensitivity of the climate system to changing atmospheric greenhouse-gas concentrations, as well as the rate of ocean heat uptake, is poorly quantified and because future influences on climate—of anthropogenic as well as natural origin—are difficult to predict. Past observations have been used to help constrain the range of uncertainties in future warming rates, but under the assumption of a particular scenario of future emissions. Here we investigate the relative importance of the uncertainty in climate response to a particular emissions scenario versus the uncertainty caused by the differences between future emissions scenarios for our estimates of future change. We present probabilistic forecasts of global-mean temperatures for four representative scenarios for future emissions, obtained with a comprehensive climate model. We find that, in the absence of policies to mitigate climate change, global-mean temperature rise is insensitive to the differences in the emissions scenarios over the next four decades. We also show that in the future, as the signal of climate change emerges further, the predictions will become better constrained.

Estimates of Uncertainty in Predictions of Global Mean Surface Temperature

A method for estimating uncertainty in future climate change is discussed in detail and applied to predictions of global mean temperature change. The method uses optimal fingerprinting to make estimates of uncertainty in model simulations of twentieth-century warming. These estimates are then projected forward in time using a linear, compact relationship between twentieth-century warming and twenty-firstcentury warming. This relationship is established from a large ensemble of energy balance models. By varying the energy balance model parameters an estimate is made of the error associated with using the linear relationship in forecasts of twentieth-century global mean temperature. Including this error has very little impact on the forecasts. There is a 50% chance that the global mean temperature change between 1995 and 2035 will be greater than 1.5 K for the Special Report on Emissions Scenarios (SRES) A1FI scenario. Under SRES B2 the same threshold is not exceeded until 2055. These results should be relatively robust to model developments for a given radiative forcing history.

Observed climate change constrains the likelihood of extreme future global warming

If cooling due to present-day levels of atmospheric aerosol is suppressing global temperatures, future reductions in aerosols emissions would allow the full greenhouse gas induced warming to be realised. The many uncertainties in aerosol physics and chemistry mean that a large range of present-day aerosol cooling is possible which could imply a large climate sensitivity, extremely large future warming and the increased risk of catastrophic consequences. Despite large uncertainties in aerosol physics and chemistry, observed spatial and temporal patterns of past temperature change allow quantitative assessment of the strength of present-day aerosol cooling. Such observational constraints provide a probabilistic framework in which to assess the likelihood of extremely large warming if a very large suppression of global warming by aerosols were to be removed. The likelihoods of future warming extents are calculated assuming four scenarios of future anthropogenic emissions. While such results are still subject to uncertainty, they indicate that future warming by the end of the 21st century is likely to be between the extremes implied by very strong or very weak present-day aerosol cooling. It is very likely that present-day aerosol cooling is suppressing a major portion of current greenhouse warming.