What led to Katrina? Jury still out on global warming


William Gray of the Tropical Meteorology Project at Colorado State University has shown that hurricane activity waxes and wanes over 25 to 30 years. The 1910s and ’20s were bad for hurricanes. Then came a period of calm, and another bad period in the 1940s and ’50s. From the 1960s to 1995 was a period of calm.