Published online by Cambridge University Press: 17 December 2010
In Paris on 2 February 2007, while this book was at its page-proof stage prior to press, the IPCC launched the first part of its Fourth Assessment Report. The announced summary report was itself yet to be checked editorially, but its core message was clear: it gave a more assured statement, compared to the previous assessments, that human activity was in all probability affecting the global climate. Its summary statements included the following.
The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming, with a radiative forcing of + 1.6 [+ 0.6 to + 2.4] W m− 2.
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level.
At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones.
Anthropogenic warming and sea level rise would continue for centuries due to the timescales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilized.
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