Evidence of the effect of external infl uences on the climate system has continued to accumulate since the Third Assessment Report (TAR). The evidence now available is substantially stronger and is based on analyses of widespread temperature increases throughout the climate system and changes in other climate variables.

Human-induced warming of the climate system is widespread. Anthropogenic warming of the climate system can
be detected in temperature observations taken at the surface, in the troposphere and in the oceans. Multi-signal detection and attribution analyses, which quantify the contributions of different natural and anthropogenic forcings to observed changes, show that greenhouse gas forcing alone during the past half century would likely have resulted in greater than the observed warming if there had not been an offsetting cooling effect from aerosol and other forcings.

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