Date: 7 November 2005
Subject: Hurricanes

Of the six most powerful Atlantic hurricanes of the last 70 years, three (Katrina, Rita and Wilma) came in 2005, with a fourth category 5 (Emily) also developing.

Katrina formed over the Bahamas on August 23, 2005, and crossed southern Florida at Category 1 intensity before strengthening rapidly in the Gulf of Mexico, becoming, at that time, the strongest hurricane ever recorded in the Gulf. By the time it hit Louisiana it had reduced in power, but was enough to breach the levees and let Lake Pontchartrain pour into New Orleans on August 29th. Katrina killed nearly one and a half thousand people, and caused over �£40bn of damage.

ember 21st. It clipped Lousiana again on its way to Texas. Its effects were less overall than feared, and the net cost was probably about �£6bn

began to form on October 18th, with an incredibly fast fall in pressure (78mbar in a day). By some measurements it was, at its height, the biggest Atlantic cyclone ever. Wilma hit Cuba and Florida, costing some ВЈ10bn of damage, though comparatively much less costly in lives than Katrina.

For Christians there are some disasters that we do not seem to have affected. The tragic Tsunami that claimed so many lives on Boxing Day 2004 was caused by the movements of the earth's "plates". These have not been affected by human activity. Other disasters, like some famines or mudslides, may be caused eg by deforestation which is putting economic gain for some above welfare for others. What kind of category does the effects of such hurricanes come into?

Opinion is divided on whether global warming, now widely regarded by scientists as a fact, has or will cause an increase in number and/or severity of Atlantic hurricanes. In 1998 a paper in the Bulletin of the American Meterological Society, William Gray and Kerry Emmanuel argued that even as much as a doubling of the carbon dioxide levels in the atmosphere would leave unaffected the frequency of tropical cyclones, and would not increase their intensity more than ten percent. In August this year, however, Emmanuel published in Nature(vol 436 p.686) a study showing that storms are 60% longer and wind speeds up to 15% higher. Since the damage done by hurricanes is proportional to the cube of the wind speed, this is significant. Emmanuel (and Gray), though more hesitant, seem to still stick to the idea that this is cyclical. In a recent interview, for example, (http://www.democracynow.org/article.pl?sid=05/08/29/145206 ) Emmanuel accepted that intensities were clearly rising but though "it's very hard to say"still thought that probably the current upsurge was part of a cycle rather than due to global warming.

The question can be approached in one of two ways. One is purely statistical (and Emmanuel says "the problem is a purely statistical one"). This simply looks at whether in the past we have seen similar periods of high hurricane force вЂ" though this is hampered by comparative lack of earlier measurements. The other would be to look directly at the physical science of the hurricane. We know that humid air rises from the surface of a tropical ocean, and as it rises it condenses, the latent heat released driving hurricane formation. Emmanuel speaks of a pillar of humid air forming from the sea surface up to the stratosphere, with low pressure at its based sucking in more air. If this process is more than a few degrees either side of the equator then Coriolis forces (which result from the earth's rotation) and other factors may cause the pillar to begin to spin, and a hurricane develops. To continue it needs warm moist air, and so will diminish if it goes across land. The initial pillar generally begins only if the sea surface temperature exceeds 26 degrees C, and develops strongly only if the warmth goes down some way, so on the face of things it seems likely that global warming will affect this. However, global warming may also increase wind speeds in the upper atmosphere, which may tend to disrupt hurricanes and diminish them. Global warming could also affect El NiГ±o, the periodic reversal of ocean currents and wind in the Pacific. This means that the whole process is multifaceted, and there may be too many unknowns to be sure of what will happen.

Some have, however, concluded that global warming is having an effect. Kevin Trenberth (of the National Center for Atmospheric Research ) wrote an article in Science in June this year (http://www.sciencemag.org/cgi/content/full/308/5729/1753). He concluded: "Trends in human-influenced environmental changes are now evident in hurricane regions. These changes are expected to affect hurricane intensity and rainfall, but the effect on hurricane numbers remains unclear." Following this Webster et al, also in Science (http://www.sciencemag.org/cgi/content/full/309/5742/1844) wrote: "A large increase was seen in the number and proportion of hurricanes reaching categories 4 and 5. The largest increase occurred in the North Pacific, Indian, and Southwest Pacific Oceans, and the smallest percentage increase occurred in the North Atlantic Ocean." They produced a graph showing mean sea surface temperatures as follows:

Fig. 1. Running 5-year mean of SST during the respective hurricane seasons for the principal ocean basins in which hurricanes occur: the North Atlantic Ocean (NATL: 90В° to 20В°E, 5В° to 25В°N, June-October), the Western Pacific Ocean (WPAC: 120В° to 180В°E, 5В° to 20В°N, May-December), the East Pacific Ocean (EPAC: 90В° to 120В°W, 5В° to 20В°N, June-October), the Southwest Pacific Ocean (SPAC: 155В° to 180В°E, 5В° to 20В°S, December-April), the North Indian Ocean (NIO: 55В° to 90В°E, 5В° to 20В°N, April-May and September-November), and the South Indian Ocean (SIO: 50В° to 115В°E, 5В° to 20В°S, November-April.

They conclude: "We conclude that global data indicate a 30-year trend toward more frequent and intense hurricanes, corroborated by the results of the recent regional assessment (29). This trend is not inconsistent with recent climate model simulations that a doubling of CO2 may increase the frequency of the most intense cyclones (18, 30), although attribution of the 30-year trends to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state."

Where does all this leave us? . Firstly, the mean sea temperature is rising, this global warming being attributable to human activity. Secondly, hurricanes are increasing in intensity. Thirdly, whilst everyone admits that no certainly is possible, an increasing number of informed commentators believe that global warming is affecting hurricanes вЂ" though that the longer term effect will be is again not certain.

Human activity has certainly affected the immediate effects of the hurricanes on New Orleans. This is not, as some politicians have claimed, an "unforseable disaster". Various experts have been predicting exactly the kind of disaster than ensued. The problem is that no one can quite say when it will happen. It is like predicting eg when a volcano like St Helens will erupt. Sciences cannot yet say the exact time of what day it will blow, and it needs a bold political decision to evacuate when actually it may not happen yet. Likewise with hurricanes, no one can predict exactly what path they will take. What was predictable is that at some stage a strong hurricane would hit New Orleans, and a city which is protected by levees from a large body of water is very vulnerable. It was also known that various policies regarding the Mississippi had lessened the silt carried down, and led to the erosion of offshore banks and islands which once protected the city by lessening the hurricanes. It was a disaster waiting to happen. Whilst one cannot but have every sympathy for the victims, and their wish to re-establish their stricken but historic city, without some radical change in the levees or other systems, moving back and simply shoring up the levees would be folly.

On a human level, many Christian organisations and churches have welcomed victims of the Atlantic hurricanes. On the broader and political level we should be encouraging Governments to take seriously climatic threats, especially those relating to human economic activity and carbon emissions, and to seek to deal with them even though the climatic hammer may or may not fall within their own present administration. A weakness of our kind of democracy is that the temptation is to concentrate on marginal constituencies, or on 5-year gains. In many cases the science is there or can be developed to prevent disaster, it needs the political will to commit sufficient resources to it.