Although many worry how global warming will affect the planet, University of California researchers theorizing why it took so long for agriculture to develop say that global cooling is a bigger problem for human survival.

Agriculture was impossible 13,000 years ago during the Ice Age, although humans were otherwise quite sophisticated, say UC Davis Professors Peter Richerson and Robert Bettinger and UCLA Professor Robert Boyd. They are authors of an article on the subject published in the July issue of American Antiquity, an archeology journal.

Paleoclimatologists studying ice cap and ocean cores have recently discovered that during the Ice Age, the climate changes were erratic, with temperatures swinging sometimes from warm to glacial within a decade.

"We argue that climate had been making things worse off and on over the whole of human existence," Bettinger says. "Agriculture developed when the climate became dramatically nicer."

The researchers hypothesize that "the reduction in climate variability, increase in carbon dioxide in the atmosphere and increases in rainfall rather abruptly changed the Earth from a regime where agriculture was impossible everywhere to one where it was possible in many places."

But developing agriculture took more than those climatic changes. Humans needed to create functional systems integrating social institutions and agricultural technology. "People can't change that quickly. Full-scale agriculture took thousands of years to develop, and social and agricultural innovation is ongoing today," Richerson says.

However, once agriculture took hold, groups that efficiently used plant resources were able to increase their populations dramatically.