Global Warming Effects on weather

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.

Extreme weather

Storm strength leading to extreme weather is increasing, such as the power dissipation index of hurricane intensity. Kerry Emanuel writes that hurricane power dissipation is highly correlated with temperature, reflecting global warming.. However, a further study by Emanuel using current model output has concluded that the increase in power dissipation in recent decades cannot be completely attributed to global warming. Hurricane modeling has produced similar results, finding that hurricanes, simulated under warmer, high-CO2 conditions, are more intense; models also show that hurricane frequency will be reduced.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 has increased by 7% over the twentieth century. The extent to which this is due to global warming as opposed to the Atlantic Multidecadal Oscillation is unclear. Some studies have found that the increase in sea surface temperature may be offset by an increase in wind shear, leading to little or no change in hurricane activity.

Increases in catastrophes resulting from extreme weather are mainly caused by increasing population densities, and anticipated future increases are similarly dominated by societal change rather than climate change.The World Meteorological Organization explains that “though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.” They also clarified that “no individual tropical cyclone can be directly attributed to climate change.” However, Hoyos et al. (2006) have linked the increasing trend in number of category 4 and 5 hurricanes for the period 1970-2004 directly to the trend in sea surface temperatures.

Highest ACE hurricane seasons
(since 1850, source)
Rank Season ACE
1 2005 248
2 1950 243
3 1893 231
4 1995 227
5 2004 224
6 1926 222
7 1933 213
8 1961 205
9 1955 199
10 1887 182

Thomas Knutson and Robert E. Tuleya of NOAA stated in 2004 that warming induced by greenhouse gas may lead to increasing occurrence of highly destructive category-5 storms. Vecchi and Soden find that wind shear, the increase of which acts to inhibit tropical cyclones, also changes in model-projections of global warming. There are projected increases of wind shear in the tropical Atlantic and East Pacific associated with the deceleration of the Walker circulation, as well as decreases of wind shear in the western and central Pacific.The study does not make claims about the net effect on Atlantic and East Pacific hurricanes of the warming and moistening atmospheres, and the model-projected increases in Atlantic wind shear. 

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. Increases in temperature are expected to produce more intense convection over land and a higher frequency of the most severe storms.

Stephen Mwakifwamba, national co-ordinator of the Centre for Energy, Environment, Science and Technology — which prepared the Tanzanian government's climate change report to the UN — says that change is happening in Tanzania right now. "In the past, we had a drought about every 10 years", he says. "Now we just don't know when they will come. They are more frequent, but then so are floods. The climate is far less predictable. We might have floods in May or droughts every three years. Upland areas, which were never affected by mosquitoes, now are. Water levels are decreasing every day. The rains come at the wrong time for farmers and it is leading to many problems".

Greg Holland, director of the Mesoscale and Microscale Meteorology Division at the National Center for Atmospheric Research in Boulder, Colorado, said on April 24, 2006, "The hurricanes we are seeing are indeed a direct result of climate change," and that the wind and warmer water conditions that fuel storms when they form in the Caribbean are, "increasingly due to greenhouse gases. There seems to be no other conclusion you can logically draw." Holland said, "The large bulk of the scientific community say what we are seeing now is linked directly to greenhouse gases."  (See also "Global warming?" in tropical cyclone)

Increased evaporation

Over the course of the 20th century, evaporation rates have reduced worldwide this is thought by many to be explained by global dimming. As the climate grows warmer and the causes of global dimming are reduced, evaporation will increase due to warmer oceans. Because the world is a closed system this will cause heavier rainfall, with more erosion. This erosion, in turn, can in vulnerable tropical areas (especially in Africa) lead to desertification. On the other hand, in other areas, increased rainfall lead to growth of forests in dry desert areas.

Scientists have found evidence that increased evaporation could result in more extreme weather as global warming progresses. The IPCC Third Annual Report says: " average water vapor concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected.