According to the Intergovernmental Panel on Climate Change (IPCC), warming of the global climate system is indisputable and many natural systems, on all continents and in some oceans, are being affected by regional climate changes. At continental, regional and ocean basin scales, numerous long-term changes have been observed. Most of the global average warming over the past 50 years is very likely due to anthropogenic greenhouse gas increases. Models and other evidence show that it is virtually certain that continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century. Moreover, these changes would very likely be larger than those observed during the 20th century.
The IPCC states that projected warming in the 21st century shows patterns similar to those observed over the past several decades. Models currently project a warming of about 0.2°C per decade for the next two decades. Based on a range of models, the impacts of climate change are very likely to increase due to increased frequencies and intensities of some extreme weather events, such as heat waves and heavy precipitation events. However, questions still remain as to how this predicted warming and related environmental changes will affect future hurricane activity.
One simple way of making a projection is to use the observed relationship between hurricane activity and sea surface temperature (SST) data and extrapolate it forward in time. Two different statistical projections of 21st century Atlantic tropical cyclone activity are shown in Figure 5. One projection (top graph) is based on a direct relationship between tropical Atlantic sea surface temperature and tropical cyclone activity. If this projection holds through the 21st century, a dramatic increase of over 300% in power dissipation may be possible. Scientists also have studied an alternative statistical relationship that exists between Atlantic hurricane activity and a sea surface temperature that uses both tropical Atlantic sea surface temperature and the average sea surface temperature elsewhere in the tropics. This alternative relationship describes hurricane activity fluctuations roughly as well as the local sea surface temperature relationship during the historical period since 1950. However, much smaller changes in hurricane activity are projected for the late 21st century by the alternative relationship. Studies to date using more complex dynamical model of the atmosphere suggest relatively modest future changes in Atlantic hurricane activity, such as in the projections in the lower panel of Fig. 5. Determining which of these projections in Fig. 5 is more realistic is a key question in the hurricane science field.
Current models project decreases in the global frequency of tropical cyclones, ranging from 6 to 34%, by the late 21st century, but in individual ocean basins, these models project that the frequency may either increase or decrease by a substantial percentage. However, scientists have a low confidence in projected changes to tropical storm activity in individual ocean basins. There is some agreement among hurricane climate scientists that it is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged in response to 21st century climate warming.
For hurricane intensity, a recent assessment concluded that “an increase in the mean maximum wind speed of tropical cyclones is likely (+2 to +11% globally)” (Knutson et al., 2010) with projected climatic warming over the next century. However, this may not occur in all ocean basins. “The frequency of the most intense storms will more likely than not increase by a substantially larger percentage in some basins.” (Knutson et al., 2010).
This increase in tropical cyclone intensity is primarily related to a projected increase in sea surface temperature (SST). However, if the sub-surface ocean temperature increase differs from the increase at the surface, then hurricane intensities may be affected due to changes in the local sea surface cooling caused by hurricane-induced mixing (see Interaction between a Hurricane and the Ocean).
Rising sea surface temperatures have also been linked to an increase in the overall water vapor content of the tropical atmosphere, which, according to hurricane modeling studies, is likely to increase hurricane rainfall rates. It has been estimated that water vapor over the oceans has increased by 4% since 1970. Tropical cyclone rainfall rates in model studies typically increase by roughly 20% within 100 km of the tropical cyclone center by the end of the 21st century.
Another concern is the complication of sea level rise due to climate warming. Rising ocean temperatures cause the ocean to expand, causing sea level rise. Increased melt water from melting ice sheets, is also projected to cause the sea level to continue rising. Higher sea levels mean that storm surges and waves ride on a higher base level, causing storm surge impacts such as coastal erosion--even from minor storms--to increase, possibly dramatically. Low-lying coastal ecosystems are greatly threatened by continued sea level rise and increased risk from extreme weather events.
Increased hurricane rainfall rates and storm surge levels would increase the risk of inland flood damage and coastal flood damage, respectively, in areas affected by landfalling hurricanes.
There are some physical reasons why a warming of sea surface temperatures in the tropical oceans by 35.6 ºF (2°C), as projected for the 21st century by climate models, may not lead to dramatic future increases in hurricane intensity and frequency. According to existing theory and models, while a hurricane’s intensity is generally higher if the sea surface temperature is higher, hurricane intensity also depends on the environmental temperature profile throughout the troposphere. Climate models project that with greenhouse warming, the upper tropical troposphere will warm substantially more than near the surface. Accordingly, this typically causes the potential intensity to increase, but only by a few percent, even with a very substantial (35.6°F/2°C) increase in the sea surface temperature (see Hurricane Development: From Birth to Maturity). Related to this issue, models suggest that although the current minimum sea surface temperature threshold for tropical storm genesis or intensification is ~26°C (~79°F), this threshold is likely to increase along with the global warming so that by the late 21st century, the threshold may be ~ 82°F (~28°C).
Warming tropical sea surface temperatures may also change the environmental wind field, which could affect hurricane genesis, intensity, and track (see Hurricane Movement). For example, If the vertical wind shear in the environmental wind field increases in the hurricane development region, as projected by recent climate models, then hurricane intensity and frequency could be suppressed (see also Hurricane Decay: Demise of a Hurricane). However, one recent study (Bender et al. 2009) suggests that the storms that do develop and survive long enough to move away from the area of increased shear could reach much more favorable regions for intensification. According to the study, this could lead to large (~100%) increases in the occurrence rate of relatively rare but very intense category 4 and 5 Atlantic hurricanes (Fig. 7). Another factor is the number of tropical disturbances or ‘seed disturbances’ for hurricanes. If this number increases or decreases, then the number of tropical storm or hurricane formations might change is a similar manner (see Hurricane Genesis: Birth of a Hurricane). However, such an effect has not been reported in existing climate change studies.
In addition to climate change affecting hurricanes, hurricanes also may influence global and regional climate. Hurricanes extract heat from the ocean (see Hurricane Development: From Birth to Maturity), and redistribute the heat and water vapor upward and poleward in the troposphere (see Hurricane Movement); they also can redistribute heat downward in the ocean (see Interaction between a Hurricane and the Ocean). Therefore, if the frequency, intensity, size, or tracks of hurricanes change over time, the transports of heat in the atmosphere and ocean may change as well.
The climate system remains dauntingly complicated. Climate models, together with sustained observations, reconstructions of past climates and hurricane activity, and theoretical approaches all will be needed to better understand and predict these complex interactions and Earth’s future climate.