Spring is usually prime food time for some 1,200 polar bears along Canada’’s Hudson Bay. Each year they plunder the bay’’s ice floes, smash open the snow caves of seals, and stuff themselves on seal pups. But in recent years the bears’’ feast has turned into slimmer pickings. Why Temperatures at Hudson Bay have risen by one half degree Fahrenheit every decade since 1950. Winter ice on the bay melts three weeks earlier than it did just 25 years ago, which means three fewer weeks of polar bear mealtime. Result: Polar bears are 10 percent thinner and produce 10 percent fewer cubs than they did 20 years ago. And though climatologists hotly debate the causes behind Earth’’s Arctic meltdown, "these changes are startling and unexpected, " says James McCarthy, co-leader of the United Nations Intergovernmental Panel on Climate Change. The plight of polar bears is just the tip to the iceberg when it comes to mounting evidence of global warming. "There’’s definitely a stark contrast with the way things were at the start of the 20th century," says atmospheric scientist Leonard Druyan, of Columbia University. Recent data show the volume of Arctic sea ice has shrunk 20 percent since the 1950s; glaciers around the world are melting at rapidly increasing rates. Rivers and lakes in North America, Asia, and Europe now freeze about nine days later and thaw 10 days earlier than they did a century ago. Most scientists believe the only effective strategy to halt global warming is to drastically reduce emissions of powerful air pollutants like carbon dioxide, which accounts for two-thirds of all greenhouse gases. In the last 150 years, the surging use of fossil fuels coal, oil, and natural gas -- has released 270 billion tons of carbon into the air in the form of carbon dioxide. Fortunately, oceans, plants, and soils absorb more than half of all atmospheric carbon dioxide -- without them world temperatures might have already soared at an alarming rate. What has kept temperatures from rising at an even higher rate