Hurricanes, which are circular storms spinning around a region of low atmospheric pressure, are powered by energy released by spiralling surface winds that draw heat from the ocean. Warmer seas provide more energy and make hurricanes stronger. This is what happened during Hurricane Katrina in August 2005, which submerged New Orleans and the vicinity. In fact, according to climate scientists, both the intensity and destructiveness of hurricanes have increased markedly since the 1970s. In other words, the energy released by an average hurricane appears to have increased by about 70 per cent within the past 30 years. This increase correlates very closely with rises in sea surface temperatures. Furthermore, tropical oceans have warmed about one degree Fahrenheit in the past 50 years, a rise that is believed to be chiefly the result of global warming.