CGD Climate Highlights

Hurricanes

Why do we believe the climate is changing?

Hurricanes and Climate Change

Tropical cyclones form over tropical oceans where sea surface temperatures (SSTs) are greater than about 80°F. When the maximum sustained near-surface wind speed exceeds 74 miles per hour (33 meters per second) these storms are called hurricanes in the Atlantic Ocean, typhoons in the Pacific Ocean, and cyclones in the Indian Ocean. The fuel that drives tropical cyclones is the heat released as rain forms when water vapor, evaporated from the warm ocean, condenses. Two of the factors that promote intensification of tropical storms - tropical ocean heat content and water vapor - have both increased over the past several decades.

Human activities, such as burning fossil fuels and clearing forests, have caused carbon dioxide (CO²) levels in the atmosphere to become significantly higher than at any time during the past 650,000 years, and probably much longer. CO2; and other greenhouse gases trap heat in the lower atmosphere, warming land and ocean, and therefore raise temperatures and/or dry out the surface. Observations show that the world's oceans are warming down to at least 1500 feet depth. In particular, the SSTs in the critical region for hurricanes are increasing largely because of global warming. As this warming occurs the oceans expand and raise sea level. Melting land ice also raises sea level, currently at a rate of 1.5 inches over the past 12 years. Rising seas means that storm surges ride on a higher base level, turning even relatively minor storms into more flood events thereby increasing storm damage along coasts.

Many factors influence tropical cyclone behavior, and three factors can cause them to intensify:

  1. warm ocean temperatures,
  2. relatively unstable atmosphere with little wind shear, and
  3. high water vapor content.

An important process driving a tropical cyclone is the evaporation of moisture from the ocean's warm surface into the atmosphere.

Warm air can hold more water vapor than cold air, by 4% per 1°F increase in temperature. Observations of water vapor content within the atmosphere over the oceans exhibit an increase estimated to be 4% since 1970. On the other hand, unfavorable winds and especially wind shear in the atmosphere can prevent a disturbance from developing into a vortex. ENSO and variations in monsoons as well as other factors also affect where storms form and track. Other factors that diminish a tropical cyclone include moving over or churning up colder ocean water, dry air migrating to the hurricane's core, and moving over land, which creates high frictional drag.

While attention has often been focussed simply on the frequency or number of storms, the intensity, size and duration likely matter more. The power dissipation of a storm is proportional to the wind speed cubed, as the main dissipation is from surface friction and wind stress effects, and is measured by a Power Dissipation Index (PDI). Consequently, the effects of these storms are highly nonlinear and one big storm may have much greater impacts on the environment and climate system than several smaller storms. For land-falling tropical cyclones the damage from winds, flooding and storm surges are especially of concern, but often depend more on human factors, including whether people place themselves in harms way, their vulnerability, and their resilience through such things as building codes.

Changes in tropical cyclones are difficult to track prior to the satellite era (about 1970) except over the Atlantic, where aircraft surveillance produces reliable reports back to about 1944. Recent studies of the post-1970 period show clearly that the destructive power of tropical cyclones has increased by 70% in the Atlantic and Pacific, owing to increases in intensity and duration. The changes in the power destructive index are very highly correlated with SSTs in the critical region where tropical cyclones form, adding confidence that the result is real. Another study revealed that the global percentage of category 4 and 5 hurricanes have increased over the past 30 years, again correlating with the rise in sea surface temperatures in the tropical cyclone generation regions. Researchers have also suggested that some category 4 and 5 tropical cyclones may have been underestimated previously, but results are large enough and so strongly related to the SST changes that they are thought to be robust to the data uncertainties. Another recent study determined factors for the bonanza 2005 North Atlantic record breaking season and found that North Atlantic SSTs were 0.92°C above normal: less than 0.1°C arose from the regional Atlantic Multidecadal Oscillation, while 0.2°C came from the earlier 2004-2005 El Niño, and 0.45°C from global warming. The remainder arose from normal weather variations.

The record breaking numbers of named storms in 2005, the record number of hurricanes, and the unprecedented 4 category 5 storms along with record damage, most notably from Katrina, is part of the upward trend toward much more active hurricane seasons in the North Atlantic since 1995. Given the loss of life and the huge costs of rebuilding from the cleanup hurricanes, it is essential to do whatever we can to avoid dangerous warming and preserve healthy and prosperous coastal communities for ourselves and our children. It is crucial that we combine aggressive emission reduction efforts with improved measures to protect coastal communities. These measures-including building codes, storm drainage plans, and preservation and restoration of wetlands, dunes, and barrier islands-must be designed to cope with both increasing sea level rise and greater storm intensity due to global warming.

References

Barnett, T.P., D.W. Pierce, and R. Schnur. 2001. Detection of Anthropogenic climate change in the world's oceans. Science 292: 270-274.

Emanuel, K. 2005a. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436:686-688.

Emanuel, K. 2005b. Emanuel Replies. Nature 438:E13.

Landsea, D.W., B. A. Harper, K. Hoarau and J.A. Knaff. 2006. Can we detect trends in extreme tropical cyclones? Science 313:452-454.

Scharroo, R. 2005. Satellite altimetry and the intensification of hurricane Katrina. EOS Transactions of the American Geophysical Union, 86:366-367.

Trenberth, K. 2005. Uncertainty in Hurricanes and Global Warming. Science 308: 1753-1754.

Trenberth, K.E. and D.J. Shea. 2006. Atlantic hurricanes and natural variability in 2005. Geophysical Research Letters 33: L12704 10.1029/2006GL026256.

Webster, P.J., Holland, G.J., Curry, J.A. and H.-R. Chang. 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science 309:1844-1846.

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CGD | Climate & Global Dynamics