(97h) Global Climate Change: What Is Certain and What Is Uncertain

That the increase of greenhouse gases over the last century has led to a warming of the Earth has been established unequivocally. The climate sensitivity of the Earth, expressed as the global temperature increase that would result from a doubling of the atmospheric level of CO2 from pre-industrial times, is still uncertain The role of atmospheric particles, or aerosols, which are both directly emitted and formed in the atmosphere by conversion of gases, stands as the single largest uncertainty in predicting future climate. Aerosols affect the Earth's energy balance by scattering and absorbing radiation and by altering the properties of clouds. We will review the effect of this uncertainty on the climate sensitivity of the Earth.