In Paris today, Working Group I of the Intergovernmental Panel on Climate Change (IPCC) released the first section of its contribution to the IPCC's Fourth Assessment Report: a 20-page Summary for Policymakers. The report represents the combined judgment of scientists from a hundred nations. Anthropogenic global warming may no longer be presented as "controversial" by any but the disingenuous.
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... Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely [> 90%] due to the observed increase in anthropogenic greenhouse gas concentrations.
The best estimates of continued average global warming over the 21st century range from 1.4 to 4.0 degrees Celsius. All models predict the worst warming by far will occur in the Arctic--3 to 8 degrees Celsius--and under several scenarios late-summer ice vanishes entirely from the Arctic ocean by the end of the century! The estimates of sea level rise range from 18 to 59 centimeters... but the panel notes these estimates that do not include recent studies of rapid, dynamic ice flow. Consensus on rapid ice change does not yet exist (wait until the 5th report, perhaps?) but could only make the situation more dire. Furthermore, the weather will grow more violent worldwide.
It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent...it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation...
Perhaps the bleakest part of the review is that a great deal of damage has already been done. Unless we can actually pull carbon out of the atmosphere (never mind merely restricting emissions!) we are already doomed to a thousand years of global warming.
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... Both past and future anthropogenic carbon dioxide emissions will continue to contribute to warming and sea level rise for more than a millennium, due to the timescales required for removal of this gas from the atmosphere.
There is a sliver of good news though. Some of the nightmare scenarios we have feared are pretty much ruled out. The Atlantic current (the thermal "conveyor belt") will slow but not stop, and the Antarctic ice sheet is not expected to collapse.
[I]t is very likely that the meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21st century... It is very unlikely that the MOC will undergo a large abrupt transition during the 21st century... Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting
If we commit ourselves to stabilizing the atmosphere, we will not avoid all global warming, but we will avoid disaster. This is a graph summarizing the five major scenarios summarized in the IPCC report. Two decades of inaction has cost us much already. The debate is over. Now is the time to act.