surface, in the free atmosphere and in the oceans. Evidence of the effect of external influences, both anthropogenic and natural, on the climate system has continued to accumulate since the TAR." Other findings of the IPCC Fourth Assessment Report include: "It is extremely unlikely (<5%) that the global pattern of warming during the past half century can be explained without external forcing (i.e., it is inconsistent with being the result of internal variability), and very unlikely that it is due to known natural external causes alone. The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling." "From new estimates of the combined anthropogenic forcing due to greenhouse gases, aerosols, and land surface changes, it is extremely likely (>95%) that human activities have exerted a substantial net warming influence on climate since 1750." "It is virtually certain that anthropogenic aerosols produce a net negative radiative forcing (cooling influence) with a greater magnitude in the Northern Hemisphere than in the Southern Hemisphere." Over the past five decades there has been a global warming of approximately 0.65 °C (1.17 °F) at the Earth's surface (see historical temperature record). Among the possible factors that could produce changes in global mean temperature are internal variability of the climate system, external forcing, an increase in concentration of greenhouse gases, or any combination of these. Current studies indicate that the increase in greenhouse gases, most notably , is mostly responsible for the observed warming. Evidence for this conclusion includes: Estimates of internal variability from climate models, and reconstructions of past temperatures, indicate that the warming is unlikely to be entirely natural. Climate models forced by natural factors and increased greenhouse gases and aerosols reproduce the observed global temperature changes; those forced by natural factors alone do not. "Fingerprint" methods (see below) indicate that the pattern of change is closer to that expected from greenhouse gas-forced change than from natural change. The plateau in warming from the 1940s to 1960s can be attributed largely to sulphate aerosol cooling. Details on attribution Recent scientific assessments find that most of the warming of the Earth's surface over the past 50 years has been caused by human activities (see also the section on scientific literature and opinion). This conclusion rests on multiple lines of evidence. Like the warming "signal" that has gradually emerged from the