[FONT face=Arial size=5]Trade winds weaken with global warming[/FONT]
[FONT face=Geneva,Arial,sans-serif size=2]Ian Sample, science correspondent
Thursday May 4, 2006
[/FONT] [DIV id=GuardianArticleBody]Trade winds that sweep around half the globe are weakening as global warming disrupts normal atmospheric circulation, scientists report today. The winds, which bring rains to the west and churn up the oceans, turning surface waters into rich feeding grounds, cover 20,000km to drive weather conditions around the world. Scientists fear that as the winds lose their puff, weather patterns will become less predictable and marine organisms will suffer, as fewer vital nutrients are forced up to the surface from the ocean depths. [DIV class=hide_class id=spacedesc_mpu_div style="DISPLAY: none"] [DIV class=hide_class id=spacedesc_mpu_i-frame style="DISPLAY: none"] [/DIV][/DIV]Gabriel Vecchi, a climate scientist at the National Oceanic and Atmospheric Administration in New Jersey, used air pressure records, including measurements from the Royal Navy dating back to the 1860s, to work out how a major air current, known as the Walker circulation, had changed with time, particularly since the Industrial Revolution. The air current drives the Pacific Ocean's trade winds from the western coast of South America to Indonesia. Along the way, the winds pick up moisture from the warm waters, which falls as rain as the air rises over Indonesia. But the winds have another feature that directly affects the marine ecosystems of the Pacific. As the wind passes over the ocean surface, it pushes the water and sets up ocean currents. In the tropical region of the Walker current, the ocean flow causes cool, nutrient-rich water from the depths of the Pacific to circle up to the surface, where it provides vital nourishment for marine life. Dr Vecchi found that since the mid-1800s, the Walker current had weakened by 3.5% and was expected to fall by 10% by the year 2100. Any drop in the strength of winds has a larger effect on ocean currents, and calculations show that the flow of the ocean has dropped by 7.5% because of the slacking trade wind. The study appears in the journal Nature today. The weakening of the Walker current is one of the most consistent predictions of climate change models. "One of the most robust predictions of climate change is that this Walker circulation should slow down, so we wanted to test that prediction to see how good the models are. Right now they are the only thing we have to guide us into the future," said Dr Vecchi. In follow-on experiments, the researchers teased apart the contribution to the current's slowing of natural climate variability and that caused by human activity, such as greenhouse gas emissions from the burning of fossil fuels. They used different simulations to predict what would happen to the Walker circulation with and without the hike in greenhouse gas emissions. The simulations showed that the Walker current only weakened when recent increases in greenhouse gas emissions were taken into account. "Nearly all of this is down to increases in greenhouse gas emissions," said Dr Vecchi. "We know this air current has an influence on weather in many regions, so we can expect there to be changes locally. Ecosystems are complex, but our concern would be that this could also affect biological productivity over a large part of the Pacific," he added.
