Climate models predict that global warming will upset the delicately balanced atmospheric circulation that controls global climate and weather patterns. Now scientists are finding evidence that man-made greenhouse gas emissions may already be tipping the equilibrium, which could have severe repercussions, including altered weather patterns and a decline in important fishing grounds. A key feature of Earth’s atmospheric circulation is a steady flow of tropical air known as the Walker circulation. Warm air rises over the equatorial western Pacific and cools and sinks in the east. This sets up a flow of air over the ocean from high pressure in the east to low pressure in the west. The winds push water from east to west, which causes an upwelling of cold, nutrient-rich water in the eastern Pacific. Simple climate theories predict that global warming will weaken this circulation. But is it actually happening? A team led by climate scientist Gabriel Vecchi of the U.S. National Oceanic and Atmospheric Administration examined historical weather records from the equatorial Pacific Ocean dating back to 1861. They found that the difference in pressure between the east and west Pacific has declined since that time, suggesting a weakening Walker circulation. Vecchi’s […]