Warm air can hold more water than cold air. That means when it rains, it rains harder, resulting in more flooding from more severe storms. And where there is no rain, the warmer temperatures means droughts are even more severe. Effects are being felt far from human population centers. Ice caps are thinning so much that polar bears that were hunted and trapped weigh an average of 200 pounds less than they did 20 years ago, McKibben said.
Scientists think the weight loss is a result of the bears having less ice from which to hunt seals.
All that from a single degree difference. Four degrees would be considerably worse. To keep the change at the low end of the scale will mean overhauling how we live, McKibben said. That means promoting alternate energy sources. That means riding bikes and buses, not ever-larger sport utility vehicles.