Detection and attribution od recent climate change: A status report

This paper addresses the question of where we now stand with respect to det ection and attribution of an anthropogenic climate signal. Our ability to e stimate natural climate variability, against which claims of anthropogenic signal detection must be made, is reviewed. The current situation suggests control runs of global climate models may give the best estimates of natura l variability on a global basis, estimates that appear to be accurate to wi thin a factor of 2 or 3 at multidecadal timescales used in detection work. Present uncertainties in both observations and model-simulated anthropogeni c signals in near-surface air temperature are estimated. The uncertainty in model simulated signals is, in places, as large as the signal to be detect ed. Two different, but complementary, approaches to detection and attributi on are discussed in the context of these uncertainties. Applying one of the detection strategies, it is found that the change in ne ar-surface, June through August air temperature field over the last 50 year s is generally different at a significance level of 5% from that expected f rom model-based estimates of natural variability. Greenhouse gases alone ca nnot explain the observed change. Two of four climate models forced by gree nhouse gases and direct sulfate aerosols produce results consistent with th e current climate change observations, while the consistency of the other t wo depends on which model's anthropogenic fingerprints are used. A recent i ntegration with additional anthropogenic forcings (the indirect effects of sulfate aerosols and tropospheric ozone) and more complete tropospheric che mistry produced results whose signal amplitude and pattern were consistent with current observations, provided the model's fingerprint is used and det ection carried out over only the last 30 years of annually averaged data. T his single integration currently cannot be corroborated and provides no opp ortunity to estimate the uncertainties inherent in the results, uncertainti es that are thought to be large and poorly known. These results illustrate the current large uncertainty in the magnitude and spatial pattern of the d irect and indirect sulfate forcing and climate response. They also show det ection statements depend on model-specific fingerprints, time period, and s easonal character of the signal, dependencies that have not been well explo red. Most, but not all, results suggest that recent changes in global climate in ferred from surface air temperature are likely not due solely to natural ca uses. At present it is not possible to make a very confident statement abou t the relative contributions of specific natural and anthropogenic forcings to observed climate change. One of the main reasons is that fully realisti c simulations of climate change due to the combined effects of all anthropo genic and natural forcings mechanisms have yet to be computed. A List of re commendations for reducing some of the uncertainties that currently hamper detection and attribution studies is presented.