Uncertainty in climate models.

Thomas M. Smith et al . ("how accurate are climate simulations?", Perspectives, 19 April, p. 483) suggest that today's climate models simulate the climate history of Earth over the past 150 years "within the observed uncertainty of the observations." In comparing model results with trends in sea surface temperature in several ocean basins, they estimate the uncertainty in model output arising from the inherent chaotic variability of the climate system from the spread of three separate simulations of a single climate model "forced with the same greenhouse gases and sulfate aerosols" but initiated with different conditions. They conclude that the variability in modeled temperature trends arising from the nonlinear dynamics of the climate system is small relative to the uncertainty in observations. However, as the model studies used only a single set of forcings, the conclusions neglect the major source of uncertainty in model simulations of temperature trends, that arising from uncertainty in the forcing.