Abstract

The net greenhouse gas “radiative forcing” of climate is now at the highest level for at least 2 million years. The atmospheric concentrations of the main greenhouse gases carbon dioxide, methane and nitrous oxide have increased in the past centuries well beyond their natural levels and many other greenhouse gases have appeared only in recent years. The evidence for these changes will be the focus of this presentation. It is found from observational networks around the world over past decades and is extended back in time using measurements of air naturally enclosed in polar ice. The causes of the changes are inferred from the concentration of each gas over time and location and from its isotopic composition which retains a signature of the gas source and of its atmospheric loss processes. The concentration records are used together with estimates of changes in natural and anthropogenic aerosols, solar radiation and land surface in climate models to compare model output with observed or proxy climate indices. With this information it is possible to better understand the main drivers of the earth system, to predict future climate under various emissions scenarios and to make more informed decisions regarding mitigation.