So New Orleans survived Hurricane Gustav. But was it by dumb luck or foresight? Despite initial fears, Gustav didn't pose much of a test for the defences erected since Hurricane Katrina in 2005. Katrina hit the coast as a formidable category 3 hurricane, whereas Gustav faded to a category 2 by landfall. And while Katrina passed east of New Orleans, pushing the waters of Lake Pontchartrain into the city, Gustav passed south and west. The levees have been strengthened in the last three years, and although the waves from Gustav topped parts of the city's floodwall, they caused only minor flooding.
Yet there is no cause for complacency. It won't be until 2011 at the earliest that the Hurricane Protection Office of the US Army Corps of Engineers will finish installing the level of protection that should have been in place for Katrina. And no walls can protect the whole Louisiana coast, which is sinking even as sea levels rise.
Besides, Gustav was just a taste of things to come. The effect of global warming on hurricane frequency and intensity has been highly contentious, but a new study shows that the strongest tropical storms have been growing stronger since 1981, particularly in the north Atlantic and northern Indian ocean (where Cyclone Nargis killed more than 100,000 people in Burma in May).
Previous studies have looked at changes in mean wind speed for all cyclones. The new focus on the strongest has revealed the upward trend, which is blamed mostly on rising ocean temperatures. Since 1970, the tropical oceans have warmed on average by around 0.5 degrees Celsius. Climate models predict a further rise of 2 degrees by 2100.
The US's Climate Prediction Centre expects this year's hurricane season to be busier than normal, predicting 14-18 named storms as opposed to the usual average of 11, and three to six major hurricanes. All this suggests that the citizens of New Orleans might have to get used to evacuation. And although they may acquiesce while disaster is still fresh in their minds, memories fade fast.
Wind-powered ships to the rescue
The dangers of global warming posed to places like New Orleans provide another reason to take geoengineering solutions seriously. Global-scale climate technofixes are widely regarded as both dangerous and an evasion of responsibility, not to mention prohibitively expensive. But veteran scientist James Lovelock has voiced cautious approval of the principle, calling it a kind of crude "planetary medicine" comparable to the efforts of physicians before the 1940s.
Now John Latham of the National Centre for Atmospheric Research in Colorado and engineers from Edinburgh have proposed a form of geoengineering that sounds relatively benign, economically feasible and easily controllable. Their plan is to offset the warming caused by greenhouse gas emissions by increasing the amount of sunlight reflected back into space from clouds. They propose spraying fine droplets of sea water into the sky from a fleet of wind-powered, remote-controlled ships. The water will quickly evaporate to leave airborne sea-salt particles, which seed cloud droplets. This wouldn't create new cloud, but gives existing clouds smaller, more numerous droplets, which makes them whiter and thus more reflective.
That's the idea. But if, as seems possible, pre-industrial levels of atmospheric carbon dioxide double before the end of the century, preventing any change in temperatures would mean reflecting an amount of solar power equivalent to the output of 2m nuclear power stations.
Yet the calculations of Latham and his colleagues suggest that their scheme could do that. Ordinary sailing vessels cannot be remote-controlled because of the need to rearrange sails and tackle as winds shift. Latham's ships would have vertical spinning cylinders called Flettner rotors instead of sails. The rotors generate a propulsive force perpendicular to the wind direction through the same aerodynamic effect that makes spinning cricket balls curve. The first such boat was made by German inventor Anton Flettner in the 1920s. The wind-turbine manufacturer Enercon is developing a Flettner rotor ship, E-Ship1, due to be launched by the end of the year.
The energy needed to spin the rotors would come from electricity generated by propellers turning in the water as the ships move—a kind of bootstrapping process. This energy also sends jets of sea water through billions of micro-nozzles formed from perforated silicon. Filtration clears the sea water of particles that would otherwise block the nozzles.
To create the maximum impact, say the researchers, the boats should be located off California and Peru in the Pacific ocean, and off Namibia in the Atlantic. Each vessel might cost £1-2m, and a fleet of about 1,500 would be enough for the job if increased by 50 per year to keep pace with rising CO2 levels.
The advantage of this method of geoengineering is that it can be fine-tuned, and turned on and off at short notice. And it could be targeted at areas especially vulnerable to ocean surface warming, such as polar ice or coral reefs. As technofixes go, it may be the best on offer.
