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 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.
As predicted, human actions have “more likely than not” caused increases in intensity of tropical cyclones (hurricanes and typhoons). That means that expert opinion places the likelihood of that above 50% and below 2/3. The future trend is considered “likely,” which means between 2/3 and 90% likelihood.
A decrease in unusually cold days and an increase in unusually hot ones is virtually certain (greater than 99%), and the likelihood that humans have already caused such changes is likely.
The report also increased the confidence in paleoclimate estimates (including the data surrounding the “hockey-stick”):
Paleoclimate information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 metres of sea level rise.
The previous report only went back 1000 years.
Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. This is an advance since the TAR’s conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations”. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.
This statement is stronger than previous estimates. It is worth noting that few, if any, of the statements in this report are weaker than previous reports. The evidence is only moving in one direction.
Not only is it increasingly clear that human actions have increased temperatures, we haven’t even seen the full effect:
It is likely that increases in greenhouse gas concentrations alone would have caused more warming than observed because volcanic and anthropogenic aerosols have offset some warming that would otherwise have taken place.
And contrary to what climate change deniers have claimed:
The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past fifty years can be explained without external forcing, and very likely that it is not due to known natural causes alone.
“Extremely unlikely” means less than 5% chance, very unlikely is less than 10%.
If carbon dioxide concentrations double, average temperatures are expected to rise between 2 and 4.5 degrees Celsius, or 3.6 to 8.1 degrees Fahrenheit.
The report also indicates that even under the most optimistic scenario for the future, we are likely to see substantial warming over the next century. The yellow line would involve maintaining carbon dioxode concentrations at current levels, a task presently impossible. The difference between scenarios in which society moves to cleaner technologies quickly is substantially cooler than the scenario in which business proceeds as usual.
It is also worth noting that all models project a substantial warming of the central parts of North America, and little change in precipitation for this area.