इन्क्रेअसिंग storms

Are rising temperatures favouring more and stronger hurricanes? A study published in Nature this week attempts to quantify the relationship between Atlantic hurricane activity and ocean temperature to help answer this question. Nature News examines where we are in the debate and what it means for us.
Have hurricanes become more frequent and fierce?

Yes. The number of major — category 4 or 5 — hurricanes (or cyclones, as hurricanes are called in the Pacific region) has increased worldwide by around 75% since 1970. The largest increases were in the North Pacific, Indian and Southwest Pacific Oceans. There is also a global trend since the mid-1970s towards longer storm duration.

In the North Atlantic, Caribbean Sea and the Gulf of Mexico, hurricane activity was below average in the 1970s and 1980s, but increased substantially after 1995. The number of North Atlantic hurricanes has been above normal in 9 of the last 11 years, reaching a peak in 2005 — a record number of 15 hurricanes formed between June and December 2005, making it the most active hurricane season on record.
What is behind the increase?

It is difficult to attribute the observed changes to specific causes because of the large natural variability in hurricane activity, which is typical of many atmospheric and oceanic processes.

It is now widely accepted, however, that rising sea surface temperatures play the main role. Hurricanes can only form from pre-existing atmospheric disturbances in regions where sea surface temperatures exceed 26 °Celsius. The warmer the water, the more moisture and energy is available for intense storms to develop.

Between June and August of 2005, sea surface temperatures in the tropical North Atlantic hurricane-development region were 0.9 °Celsius above the 1901 to 1970 average — one of many record-breaking figures of this season. Atmospheric conditions were also favourable for hurricanes that season.

Kerry Emanuel, a leading hurricane researcher at the Massachusetts Institute of Technology in Cambridge, showed that the strength of hurricanes is correlated with sea surface temperature and that hurricanes have grown more intense over the past 30 years1.
What does this new paper tell us?

Mark Saunders and Adam Lea of University College London, UK, have quantified, for the first time, the contribution of sea warming to the increase in hurricane activity. They found that local sea surface warming was responsible for around 40% of the rise in number of storms between 1996 and 2005 (compared to the 1950-2000 average) in the North Atlantic2.

Saunders and Lea used a statistical model to disentangle the two main hurricane predictors: sea surface temperature and near-surface trade wind speed. Together, these two variables explain about 80% of the variance observed in tropical Atlantic hurricane activity between 1965 and 2005.

Their result indicates that a 0.5 °Celsius increase in August-September sea surface temperature should result in an average 40% increase in hurricane activity — a measure including both number and severity of storms.
How much have the seas warmed?

Globally, the world's oceans have warmed by around 0.5 °Celsius since 1970, and scientists believe that they will continue to warm throughout the century. In the tropical North Atlantic, sea surface temperatures in the 1996-2005 period were 0.27 °Celsius above average, the highest ten-year anomaly since records began in 1950. Computer models suggest that sea surface temperatures in the Atlantic hurricane-forming region could warm by 2 °Celsius by 2100.
So can we expect an ever growing number of severe hurricanes in a warmer future?

Not necessarily. Natural variability continues to play a role in hurricane activity. And some scientists think that as the sea continues to warm, atmospheric conditions in a warmer climate might become more unfavourable for hurricanes.

As the speed of near-surface trade winds increases, so-called shear wind force (caused by the difference between wind speeds at different altitudes) over hurricane track regions might increase as well. Once wind shear gets strong enough, it will suppress the cyclonic rotation of winds. If, and at what point, this might outweigh the impact of rising sea temperatures is unknown and hotly contested. At last week’s annual meeting of the American Meteorological Society, a new study proposing that global warming may eventually decrease the number of hurricanes in this way created quite a stir.
Will climate change result in more hurricanes reaching land, causing more damage?

The proportion of storms that hit the land versus those that stay at sea is unlikely to change. But the area of ocean surface warm enough to form hurricanes is likely to expand, making more coastline susceptible to these storms.
If these waters warm by 2 °Celsius by 2100, maximum wind speeds of hurricanes could increase by 6%3. That might not seem like much, but damage from hurricanes rises in proportion to the cube of the wind speed.
Has debate over the link between climate change and hurricanes died down in the United States?

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No. Political debate may have taken a break during a relative lull after Hurricane Katrina in 2005, but the link is still hotly contested in the scientific community.
Are hurricanes bad for the planet?

Hurricanes are bad news for people who find themselves in harm’s way. But hurricanes contribute to the functioning of the planet much as do wildfires, volcanic eruptions or tectonic movements. They play an important role in mixing the oceans, and during past hotter climates they may have helped to cool down the otherwise excessively hot tropics.

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References
1. Emanuel, K. Nature 436, 686-688 (2005). | Article | PubMed | ISI | ChemPort |
2. Saunders, M. A. & Lea, A. S. Nature 451, 557-560 (2008). | Article |
3. Knutson, T. R. & Tuleya, R. E. J. Climate 17, 3477-3495 (2004). | Article |