CLIMATE FORCINGS IN THE INDUSTRIAL ERA

The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change. Anthropogenic gr eenhouse gases (GHGs), which are well measured, cause a strong positiv e (warming) forcing. But other, poorly measured, anthropogenic forcing s, especially changes of atmospheric aerosols, clouds, and land-use pa tterns, cause a negative forcing that tends to offset greenhouse warmi ng. One consequence of this partial balance is that the natural forcin g due to solar irradiance changes may play a larger role in long-term climate change than inferred from comparison with GHGs alone. Current trends in GHG climate forcings are smaller than in popular ''business as usual'' or 1% per year CO2 growth scenarios. The summary implicatio n is a paradigm change for long-term climate projections: uncertaintie s in climate forcings have supplanted global climate sensitivity as th e predominant issue.