There is broad consensus that the global climate is changing. Instrument records indicate significant globally averaged warming over the past century that has accelerated in the past few decades. A host of phenomenological observations confirm these conclusions. Although attributed by some to natural variability, objective scientific research points to human influences. What are the implications of global climate change for tropical storms and hurricanes? What changes in their frequency, intensity, and trajectories may be anticipated? What is the relative importance of natural variability and long-term trends, a question of vital importance for long-term reconstruction planning? In the context of global climate change, what can be said about regional changes and trends, particularly of the Gulf Coast? These are questions of vital importance for long-term reconstruction planning that require urgent, careful, and objective scientific analysis.
There are strong theoretical reasons, supported by modeling, to expect that the warming of the ocean would affect tropical storm characteristics. Because a hurricane acts as a heat engine that gains its energy from the ocean surface, increases in intensity are expected and probably inevitable. Recent observational studies have indicated that these increases already may be occurring and may be larger than originally predicted by theory and modeling studies.
Analysis of global data since 1970 shows little variation in the total annual number of tropical storms and hurricanes. Each year, 80 to 90 tropical storms and hurricanes occur in the tropical oceans with only about 11 percent occurring in the North Atlantic. Nevertheless, in the North Atlantic there has been a statistically significant increase in the annual number of hurricanes since 1995. The proportion of intense (categories 4 and 5) hurricanes has increased in all basins. The length of time that hurricanes remain high-category storms also has increased, by a half day. The duration of the North Atlantic hurricane season has lengthened, beginning earlier and ending later. In particular, July and October are now seeing more storms.
These observations are consistent with the hypothesis that increasing SST is leading to increased hurricane activity, though it appears likely that a substantial component of these changes is through indirect means, where the ocean changes modify the atmospheric circulation, which in turn drives the hurricane changes. Global numerical model experiments with doubled carbon dioxide and consequent warmer oceans produced a similar shift in hurricane intensity on a worldwide scale. The observed trends in SST and hurricane intensity cannot be explained by natural variability alone. It has been shown, for example, that the North Atlantic Oscillation explains only about 10 percent of the SST changes.
There is a strong basis in both theory and observation for projecting continued longer and more severe hurricane seasons for the foreseeable future. The hurricane climatology of the past decade, with increasing numbers of more intense hurricanes, should be considered the new normal. If present predictions of future greenhouse warming are correct, hurricane intensity will stay high and even increase as long as SSTs continue to increase.
European models are showing exceptional skill in predicting storm frequency 1 to 3 months in advance. It appears possible to give 3 to 4 months notice of an exceptionally intense hurricane season. Such predictions would be of significant value to coastal communities in planning and testing protection and response measures, training, and stocking of needed supplies. This information should be provided to decision-makers.
The climatology of the twentieth century should not be taken as the basis for planning and building for the 21st century. Existing hurricane response plans and plans for post-Katrina reconstruction should be reviewed in the light of apparently changing hurricane climatology.
Urgent attention needs to be given to improved projections of the likely impact of greenhouse warming on hurricane frequency and characteristics, particularly as they affect the North Atlantic. These projections must be coupled with and responsive to the needs of planning and design groups responsible for construction standards both onshore and offshore.
Many mysteries remain in our understanding of hurricanes. For example:
- Why are there about 85 hurricanes per year, and not 10 or 200?
- Can we explain the relationships among hurricane frequency, duration, and intensity?
- What is the role of hurricanes in the general circulation?
Without a better understanding of the fundamental physical basis for hurricane behavior, we cannot hope to make greatly improved predictions. Continued and adequately funded research by aircraft and satellites will enable scientists to help describe and in time understand the details of how hurricanes behave.