Estimated changes in reef area and CaCO3 production since the last glacial maximum (LGM) are presented for the first time, based on a model (ReefHab) which uses measured environmental data to predict global distribution of reef habitat. Suitable reef habitat is defined by temperature, salinity, nutrients, and the depth-attenuated level of photosynthetically available radiation (PAR). CaCO3 production is calculated as a function of PAR. When minimum PAR levels were chosen to restrict reef growth to 30 m depth and less, modern reef area totaled 584-746¿103 km2. Global carbonate production, which takes into account topographic relief as a control on carbonate accumulation, was 1.00 Gt yr-1. These values are close to the most widely accepted estimates of reef area and carbonate production and demonstrate that basic environmental data can be used to define reef habitat and calcification. To simulate reef habitat changes since the LGM, the model was run at 1-kyr intervals, using appropriate sea level and temperature values. These runs show that at the LGM, reef area was restricted to 20% of that today and carbonate production to 27%, due primarily to a reduction in available space at the lower sea level and secondarily to lower sea surface temperatures. Nonetheless, these values suggest that reef growth prior to shelf flooding was more extensive than previously thought. A crude estimate of reef-released CO2 to the atmosphere since the LGM is of the same order of magnitude as the atmospheric CO2 change recorded in the Vostok ice core, which emphasizes the role of neritic carbonates within the global carbon cycle. This model currently addresses only the major physical and chemical controls on reef carbonate production, but it provides a template for estimating shallow tropical carbonate production both in the present and in the past. As such, the model highlights several long-standing issues regarding reef carbonates, particularly in terms of better defining the roles of light, temperature, aragonite saturation state, and topography on reef calcification.¿ 1997 American Geophysical Union