The HadCM3L climate model was used to investigate climate response to an abrupt quadrupling of atmospheric CO_2 and 4% increase in solar irradiance. Modeling results show that a quadrupling of CO_2 and 4% increase in solar irradiance cause approximately the same global mean surface temperature change by the end of 1000-year simulations, but the precipitation responses are substantially different. The difference is mainly due to the different fast response of the climate system, which occurs over a short time period (about one month) before sea surface warms significantly. During this period, over land, the physiological effect of CO_2 reduces the plant transpiration. Over ocean, the radiation effect of CO_2 leads to the increase of the temperature in the lower atmosphere, which occurs much faster than the warming of sea surface temperature. This increases vertical stability of the lower atmosphere, suppressing evaporation over ocean. Consequently, during the period of fast response (about one month), precipitation decreases in land and ocean areas. Compare to the effects of physiological effects of CO_2, the radiation effect of CO_2 has more important effect on the climate system at the time scale longer than a few years. But on a short time scale of one month, the physiological effect of CO_2 has a greater impact over land.