Effects of global warming
The predicted effects of global warming are many and various, both for the environment and for human life. There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. A northwards branch of the gulf stream, the North Atlantic Drift, is part of the thermohaline circulation (THC), transporting warmth further north to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe.
Increasing temperature is likely to lead to increasing precipitation but the effects on storms are less clear. Extratropical storms partly depend on the temperature gradient, which is predicted to weaken in the northern hemisphere as the polar region warms more than the rest of the hemisphere.
The Intergovernmental Panel on Climate Change (IPCC) third annual assessment report "Climate Change 2001" stated "there is no compelling evidence to indicate that the characteristics of tropical and extratropical storms have changed." There is, however, limited evidence from a relatively short time period that storm strength is increasing, such as the Emanuel (2005) "power dissipation index" of hurricane intensity. Worldwide, the proportion of hurricanes reaching categories 4 or 5 -- with wind speeds above 56 metres per second -- has risen from 20% in the 1970s to 35% in the 1990s.Precipitation hitting the US from hurricanes increased by 7% over the twentieth century.
A substantially higher risk of extreme weather does not necessarily mean a noticeably greater risk of slightly-above-average weather. However, the evidence is clear that severe weather and moderate rainfall are also increasing.