Based on a "large and compelling body of evidence from within the field of climate science," Pravda reports this week, "many sources of data which provide our knowledge base of long-term climate change" indicate that the current 12,000-year-long warming trend is coming to an end.

Pravda points to three astronomical "Milankovich cycles" for the coming cool-down:

• The tilt of the Earth, which varies over a 41,000-year period.

• The shape of the Earth's orbit, which changes over a period of 100,000 years, "separated by intervening warm interglacials, each lasting about 12,000 years."

• The Precession of the Equinoxes, also known as the earth's "wobble," which gradually rotates the direction of the earth's axis over a period of 26,000 years.

Erik Brown, a professor of geological sciences at the University of Minnesota, Duluth, today characterized the report as telling a "rather sensational story."

"Certainly much of what is discussed regarding Milankovich cycles is essentially correct. However, a critically important point is that the conditions of the Earth's orbit about the Sun do not return to the exact same settings every 100,000 years. This leads to natural variability in the length of the warm interglacial periods."

Brown also disputed the report's implication that the observed increase in carbon dioxide over the past century "may be explained by natural processes, and is comparable to past variations."

"This is simply wrong," Brown said. "If you were to plot changes in CO2 [carbon dioxide] over the past century ... CO2 concentrations would increase ... well beyond levels in the past million years at a rate completely unlike anything seen in nature."