How might global warming have an effect on hurricane activity in the North Atlantic?

Global warming will change our physical environment in a variety of ways. The most recent Intergovernmental Panel on Climate Change (IPCC) reports provide the best and most thorough summary of expected changes. The reports suggest that we know fairly well what to expect for some changes, for example, an increase in global average surface temperature. We are less certain about other changes, such as an increase in the frequency of intense rainfall events. And we are very uncertain about other changes, for example, how temperature and precipitation will be altered on very local scales. Finally, we should remember that there are other possibilities that can be described using the terms “known unknowns and unknown unknowns” derived from a now “infamous” quote from Donald Rumsfeld.

With regard to hurricanes in the North Atlantic, global warming will most likely change a variety of factors that influence hurricane activity. The easiest to understand is probably the increase in sea surface temperature (SST). All other things being equal, an increase in SST will provide more energy to a hurricane and cause it to have stronger winds. However, many other factors play a role in the intensity of a hurricane. One example is the amount of wind shear. The expected response of wind shear to global warming is not as well constrained as the expected increase in SST. However, an increase in wind shear around a hurricane in the North Atlantic could easily counteract an increase in SST.

Which factor or factors will dominate is not known with 100 percent certainty, but most modelling results suggest that hurricane intensity and rainfall will increase as the globe warms in response to increase in atmospheric greenhouse gases such as carbon dioxide.

Which factor has been more detrimental to global warming, the human contribution or natural contributions (if any)?

Climate has always changed and there is a large amount of natural climate variability at all time scales. For example, El Ni|0xf1|o and La Ni|0xf1|a come and go in the equatorial Pacific on the time scale of a few years whereas ice ages come and go over a period of tens of thousands of years or longer. So far, the human contribution to global warming is less than that produced by natural variability in the past.

>Will global warming cause similar changes in hurricane activity in the Pacific?

Model studies of the response of hurricanes to global warming suggest that on average hurricane wind and rainfall will increase. It is not possible to describe the response of individual storms and changes in each basin will differ.

In 2006, the hurricane season was relatively mild — can you explain wh$>

People have gone back and examined the factors that likely contributed to the mild season in 2006. For example, SSTs in 2006 were not as warm as in 2005 and there was an El Ni|0xf1|o event. However, it is not possible to state why those factors were as observed in 2006.

What ramifications does that hold for the 2007 hurricane seas<$>

What happened in 2006 has no real impact on what might happen in 2007. Most seasonal forecasts suggest that Atlantic hurricane activity in 2007 will be higher than last year and higher than the long-term average. This seems reasonable as it appears that a La Ni|0xf1|a event, which is associated with greater Atlantic hurricane activity, is developing in the Pacific and because Atlantic SSTs are expected to be warmer than the long-term average. However, as we saw in 2006, seasonal forecasts can be wrong. Regardless of what a seasonal hurricane forecast states, one should always be prepared. Even if there was only one hurricane in 2006, it might be the one that hits you.

Director of the North Caroline Aquarium at Pine Knoll Shores, author and hurricane historian Jay Barnes said that more heat could generate more storms and more intense hurricanes. If not caused by global warming, what do you feel might be the reason for increased hurricane activity in the North Atlantic over the last 30 ye?<$>

An analysis of Atlantic SST over the past 100 years shows that the temperatures vary over a period of several decades. If you look at the relationship between hurricane activity and SST you see that when SSTs are warm you tend to have more hurricanes, and when SSTs are cool you tend to have fewer hurricanes. The exact cause for the variation in SST over time is open for debate. Some people believe that essentially all the variation is due to natural variability. Other people believe that a significant part of the variability in SST is due human activity. I believe that both factors are playing a role in forcing SST and that the relationship between SST and hurricane activity is complicated and related to oceanic and atmospheric changes.

If global warming continues to go unchecked, what can we look forward to in terms of hurricane actiy?<$>

This is a good question. We’ll have to wait to learn the correct answer. I suspect that the models will be correct in that there will be an increase in the average of hurricane intensity. However, we don’t really know what changes will occur in other important hurricane characteristics such as frequency, size, and track.

Kerry Emanuel of MIT in Cambridge said that the duration and strength of hurricanes have increased by about 50 percent over the last 30 years, but this defies the existing models of measuring hurricane and typhoon intensity that say there should be an increase of five percent for every one degree Centigrade rise in sea surface temperature. Can you explain this?

No. But, people are trying. For example, at the end of this month the Risk Prediction Initiative at BIOS is co-sponsoring a week-long meeting on hurricanes and climate change. Scientists from around the world will gather for discussion and debate as one step towards answers for this and other questio

If ocean temperatures increase more rapidly than atmospheric temperatures, might this be an explanation for the increase in hurricane activity?

All other things being equal and if you’re referring to temperatures in the upper atmosphere, then yes. But all other things are not equal.