Since the 1990 IPCC Report, considerable progress has been made in attempts to distinguish between natural and anthropogenic influences on climate. This progress has been achieved by including effects of sulphate aerosols in addition to greenhouse gases, thus leading to more realistic estimates of human induced radiative forcing. While the negative forcing is focused in particular regions and subcontinental areas, it can have continental to hemispheric scale effects on climate patterns. Locally, the aerosol forcing can be large enough to more than offset the positive forcing due to greenhouse gases. In contrast to the long lived greenhouse gases, anthropogenic aerosols are very short lived in the atmosphere, hence their radiative forcing adjusts rapidly to increases or decreases in emissions.
Further study is needed to refine empirical estimates of climate sensitivity to trace-gas forcing.
Abrupt climate changes of the past also affected atmospheric trace-gas concentrations, through shifts in carbon storage by the oceans and terrestrial biosphere. The processes behind these past linkages must be understood if reliable prediction of future forcing magnitude and climatic response are to be made. High in the atmosphere, in a region called the stratosphere, naturally-occurring ozone is formed when intense sunlight causes oxygen molecules to break up and reform as unstable ozone molecules. Ozone is also found in the lower atmosphere, a region called the troposphere, but normally at very low concentrations that are not harmful. A small amount of tropospheric ozone is transferred down from the ozone formed naturally in the stratosphere.