Wednesday, September 3, 2008

Relation Global Warming and Hurricane

"Temperature increases are taking place all over the world, and that's causing stronger storms," Gore declares. It's a clear and powerful message: Global warming is not just a looming disaster we can palm off on future generations. It's here now, and people are dying because of it.

Many scientists already believed: that the atmosphere, warmed up by mankind's relentless pumping of carbon dioxide into the air, is also warming up the world's prime hurricane breeding grounds. (Tropical sea surface temperatures have risen by about 1 degree Fahrenheit since 1970, but a direct link to the warming atmosphere had not been so clearly established.) A media firm working for one of the study's sponsoring institutions turned the hype up a notch, billing the revelation as "the final piece of the puzzle" connecting an upsurge in powerful hurricanes to global warming.

The hurricane-warming link isn't settled at all. Rather, it's a very contentious debate between two groups of scientists—computer-modeling atmospheric scientists versus meteorologists—who have very different methods, ideas, and priorities. The debate has been raging for months, with attacks and counterattacks—albeit very polite ones—appearing regularly in top scientific journals. Because the issue has massive policy implications and the particulars are difficult to understand and explain, the competing groups have also resorted to dueling press releases and other forms of media outreach.

Given the high degree of interest in the possible relationship between climate change and tropical cyclones (including hurricanes and typhoons).

The strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere. Although we cannot say at present whether more or fewer hurricanes will occur in the future with global warming, the hurricanes that do occur near the end of the 21st century are expected to be stronger and have significantly more intense rainfall than under present day climate conditions.

Tropical cyclone wind-speed monitoring has changed dramatically over the last few decades leading to difficulties in determining accurate trends.

Weather events will always result from a combination of deterministic factors (including greenhouse gas forcing or slow natural climate cycles) and stochastic factors (pure chance).

In particular, the available scientific evidence indicates that it is likely that global warming will make - and possibly already is making - those hurricanes that form more destructive than they otherwise would have been.

The basic connection between the two is actually fairly simple: warm water, and the instability in the lower atmosphere that is created by it, is the energy source of hurricanes. This is why they only arise in the tropics and during the season when SSTs are highest (June to November in the tropical North Atlantic).

SST is not the only influence on hurricane formation. Strong shear in atmospheric winds (that is, changes in wind strength and direction with height in the atmosphere above the surface), for example, inhibits development of the highly organized structure that is required for a hurricane to form. In the case of Atlantic hurricanes, the El Nino/Southern Oscillation tends to influence the vertical wind shear, and thus, in turn, the number of hurricanes that tend to form in a given year. Many other features of the process of hurricane development and strengthening, however, are closely linked to SST.

The frequency of the strongest (category 5) hurricanes roughly triples in the anthropogenic climate change scenario relative to the control. This suggests that hurricanes may indeed become more destructive (1) as tropical SSTs warm due to anthropogenic impacts. The frequency of all tropical storms and hurricanes (lumping the weak ones in with the strong ones) rather than a measure of changes in the intensity of the storms.

Trends in the strongest category storms, maximum hurricane winds, and changes in minimum central pressures, suggest a systematic increase in the intensities of those storms that form.

As tropical SSTs have increased in past decades, so has the intrinsic destructive potential of hurricanes.

The key question then becomes this: Why has SST increased in the tropics? Is this increase due to global warming (which is almost certainly in large part due to human impacts on climate)? Or is this increase part of a natural cycle?

So the question to ask here is: why is the Gulf of Mexico so hot at present - how much of this could be attributed to global warming, and how much to natural variability? More detailed analysis of the SST changes in the relevant regions, and comparisons with model predictions, will probably shed more light on this question in the future. At present, however, the available scientific evidence suggests that it would be premature to assert that the recent anomalous behavior can be attributed entirely to a natural cycle. But ultimately the answer to what caused Katrina is of little practical value. Katrina is in the past. Far more important is learning something for the future, as this could help reduce the risk of further tragedies. Better protection against hurricanes will be an obvious discussion point over the coming months, to which as climatologists we are not particularly qualified to contribute. But climate science can help us understand how human actions influence climate. The current evidence strongly suggests that:
(a) hurricanes tend to become more destructive as ocean temperatures rise, and
(b) an unchecked rise in greenhouse gas concentrations will very likely increase ocean temperatures further, ultimately overwhelming any natural oscillations.

Scenarios for future global warming show tropical SST rising by a few degrees, not just tenths of a degree. That is the important message from science. What we need to discuss is not what caused Katrina, but the likelyhood that global warming will make hurricanes even worse in future.


1 comments:

JEANETTE ALKASSEM said...

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