AN important question in the Earth sciences is the role of the biota in the chemical weathering of silicate rocks, which affects atmospheric CO2 and therefore climate1-10. No comprehensive study of biotic influences, however, has quantitatively examined the climatic consequences were weathering to take place under completely abiotic conditions. Here we calculate that if today's weathering is 10, 100 or 1,000 times the abiotic weathering rate, then an abiotic Earth would be, respectively, ~15, 30 or 45 °C warmer than today. The upper two temperatures are preferred estimates because of the probable almost complete absence of soil under abiotic conditions, suggesting that without a biota that significantly enhances weathering rates, the Earth today would be uninhabitable for nearly all but the most primitive microbes. Life may have been crucial in cooling early Earth and maintaining relatively cool conditions.