The effect of human activity on radiative forcing of climate change: a review of recent developments

Human activity has perturbed the Earth's energy balance by altering the pro perties of the atmosphere and the surface. This perturbation is of a size t hat would be expected to lead to significant changes in climate. in recent years, an increasing number of possible human-related climate change mechan isms have begun to be quantified. This paper reviews developments in radiat ive forcing that have occurred since the second assessment report of the In tergovernmental Panel on Climate Change (IPCC), and proposes modifications to the values of global-mean radiative forcings since pre-industrial times given by IPCC. The forcing mechanisms which are considered here include tho se due to changes in concentrations of well-mixed greenhouse gases, troposp heric and stratospheric ozone, aerosols composed of sulphate, soot, organic s and mineral dust (including their direct and indirect effects), and surfa ce albedo. For many of these mechanisms, the size, spatial pattern and, for some, even the sign of their effect remain uncertain. Studies which have a ttributed observed climate change to human activity have considered only a subset of these mechanisms; their conclusions may not prove to be robust wh en a broader set is included. (C) 1999 Elsevier Science B.V. All rights res erved.