WATER-VAPOR FEEDBACK AND THE ICE-AGE SNOWLINE RECORD

The CLIMAP reconstruction of the Earth's climate during the last major glaciation 18 000 years ago (ybp) found that while average tropical s ea surface temperatures were no more than about 1-degrees-C cooler tha n at present, snow lines on high peaks had descended 1 km. This sugges ts a temperature reduction of about 5-degrees-C in the mid-troposphere . Since the tropical atmosphere (above the tradewind boundary layer) c annot sustain large horizontal temperature gradients, this feature is likely to be characteristic - implying the lapse rate in the lower hal f of the tropical troposphere during the last glaciation was about 20% greater than at present. We note that such a reduction in static stab ility requires a reduction of radiative cooling rate at these levels. Using a radiative-convective model with a physical parameterization fo r tropical convection, we find that only a significant increase of rel ative humidity in the middle and upper troposphere (20-40% greater tha n at present) can lead to the needed increase of lapse rate in the low troposphere. It is noted that the profiles observed in CLIMAP have mu ch more convective available potential energy (CAPE) than present prof iles, which can lead to enhanced production of hydrometeors in the mid dle and the upper troposphere whose reevaporation can moisturize the f ree troposphere. It is shown that the moisture change leads to an extr emely strong negative feedback which greatly reduces the response to a doubling of CO2. Finally, we discuss the apparent contradictions betw een the present results and existing studies suggesting a positive wat er vapor feedback.