"Teller and Wood are technical optimists, and pessimists about human behaviour," says Caldeira, who saw Wood present their work in 1998, and was disturbed by the re-emergence of these Sun-blocking proposals. "My hope was to show that it wouldn't work," he says, "so people would give up on it."
Caldeira thought he saw a central flaw in all of these Sun-dimming ideas. While reducing sunlight might have an overall cooling effect on the planet as a whole, he argued, it wouldn't counteract the local heating effect from CO2. The two forces act in a completely different way on different parts of the globe. The capacity of CO2 to trap heat increases with temperature, so it's more effective near the equator. The Sun, on the other hand, shines more in the northern hemisphere in June, more in the southern hemisphere in December. Surely, thought Caldeira, these very different phenomena couldn't exactly cancel each other out? Not on a country-sized scale, or over the different seasons. Instead, he expected the winters to get warmer and the summers cooler, the poles warmer and the tropics cooler. In short, many of the problems the Earth faces from global warming-the seasons becoming more and more alike, and a variety of weird weather (New Scientist, 16 September, p 26)-would be made worse.
To try to prove his hypothesis, Caldeira went to work with his colleague Bala Govindasamy and a hefty climate-modelling program. They invented three future worlds to test. In the first, the control model, levels of atmospheric CO2 were set at just 280 parts per million-the pre-industrial value-and the Sun shone at a constant 1367 watts per square metre at the Earth's surface. In the second, there was twice as much CO2, adding an extra 4.17 watts per square metre to the Earth-bound radiation. In the final, geoengineered scenario, the doubled CO2 was countered by cutting 1.8 per cent of the Sun's light, again worth 4.17 watts per square metre.
Contact: Claire Bowles