The drought in western North America from 2000 to 2004 was the strongest in 800 years but may be a "new normal" for most of the coming century, scientists say.

Such climatic extremes as the extended drought that left dying forests and depleted river basins in its wake have increased as a result of global warming, a group of 10 researchers reported in Nature Geoscience.

The drought also decreased carbon sequestration by an average of 51 percent in a massive region of the western United States, Canada and Mexico, they reported.

"Climatic extremes such as this will cause more large-scale droughts and forest mortality, and the ability of vegetation to sequester carbon is going to decline," study co-author Beverly Law of Oregon State University said.

Tree-ring data showed the multiyear drought was unlike anything seen since the Middle Ages, from 977-981 and from 1146-1151, the researchers said.

But it may get even worse, they said.

"Areas that are already dry in the West are expected to get drier," Law said. "We expect more extremes. And it's these extreme periods that can really cause ecosystem damage, lead to climate-induced mortality of forests, and may cause some areas to convert from forest into shrublands or grassland."

These long-term trends are consistent with a 21st century "megadrought," the researchers said.