Evaluation of the effect of climate change on agro-climatic indices is a key factor studying crop response to future climatic conditions. Monthly variation of mean for temperature and rainfall under doubling CO2 concentration were predicted for Iran using Goddard Institute for Space Studies (GISS) and Geophysical Fluid Dynamics Laboratory (GFDL) models. The coefficients were applied to the long term climatic data and the effects of those changes on growth, maturity duration and water requirements in different crops such as sugar beet, potato, sunflower and chickpea (Cicer arientinum) were simulated by a crop simulation model (OSBOL). Based on the prediction made by GISS and GFDL, mean annual temperature increased by 4.6 and 4.3˚C, respectively. These values were 32% and 1% increase for rainfall. Doubling CO2 concentration reduced number of days from planting to maturity and increased biological yield, root yield and water requirement of sugar beet, but number of days to maturity and water requirement were reduced for potato. The biomass for rainfed sunflower increased, while days to maturity and harvest index were reduced. Doubling CO2 caused a reduction in days to maturity, biomass, and harvest index and water requirements for irrigated chick-pea. Length of growing period (LGP) based on temperature would be increased in all rainfed areas of country; however, including the water availability in the calculations was led to a lowered LGP. Reduction of LGP for the studied stations was in the range of 8-36 and 19-55 days for years 2025 and 2050, respectively. Rainfall deficit index for2025 and 2050 was varied, respectively at 8.3-17.7 and 21.1-32.3 mm. It was estimated that under climatic condition of years 2025 and 2050 the cultivated areas in the main rainfed production regions of the country would be reduced by 16-25 and 23-33%, respectively.

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