A new study requested by the U.S. Congress is helping clear up some of the controversies regarding global warming.

The 150-page study was put together by scientists with the National Academy of Sciences. It finds the warming of the northern hemisphere in the last decades of the 20th century was unprecedented in the past thousand years. It also says the decade of the 1990s was the warmest on record. Authors believe human activities are at least partly responsible for recent warming.

Extreme flooding

The study comes as extreme weather has struck many parts of the United States. There was flooding in parts of the Midwest last week, and more heavy rains this week in the southern and eastern United States. Scientists say that the warmer the air, the more evaporated water it holds. Winds pick up more moisture from the hotter ocean surfaces, resulting in heavier and more frequent downpours.

Kevin Trenberth

Kevin Trenberth of the National Center for Atmospheric Research said, "Very heavy rains…the top one percent has gone up 20 percent in the last century, in the United States. When it rains it pours much more so that it did 30 years ago."

Recent data compiled on wildfires indicate an increasing frequency in North America and elsewhere around the world.

Wildfire

One California firefighter said, "The snow is melting earlier in the year on very regular intervals now, and we're getting much longer fire seasons. It dries out much more than before."

Scientists say generally over the last five decades snows are melting sooner and faster, producing hot, dry drought conditions in many parts of the world. Weather patterns have natural cycles, but this latest study suggests more frequent extremes of too dry and too wet conditions may exist for the rest of the decade and beyond.