The Muted Precipitation Increase in Global Warming Simulations: A Surface Evaporation Perspective

Both observations and climate simulations suggest an increase of atmospheric moisture content by 7% per degree surface warming. The simulated precipitation, on the other hand, increases at a much slower rate. This muted response of the hydrological cycle to increased greenhouse gas forcing is investigated from a surface evaporation perspective, using simulations participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) under the A1B forcing scenario. A 90-year analysis of surface evaporation based on a standard bulk formula reveals that the following atmospheric changes act to slow down the increase in surface evaporation over ice free oceans: surface relative humidity increases by 1.0%, surface stability, as measured by air-sea temperature difference, increases by 0.2 K, and surface wind speed decreases by 0.02 m/s. As a result of these changes, surface evaporation increases by only 2% per Kelvin of surface warming rather than 7%/K. Particularly the increased surface stability and relative humidity are very robust among models. The potential impact of relative humidity trends on the surface energy budget and temperature is addressed in a set of sensitivity tests with the NCAR Community Atmosphere Model (CAM3) coupled to a slab ocean model. In these CO2 doubling experiments surface relative humidity is prescribed either from present day or CO2 doubling climatology. The results indicate that the increase in surface relative humidity, through inhibiting latent heat flux, contributes approximately 20% of the total warming under CO2 doubling. Our findings underscore the importance of the air-sea interface in the global warming problem and call for observational efforts to detect and monitor changes in surface relative humidity and stability over the World Ocean.