Twenty-first Century Challenges in Water Resources - A Land-Atmosphere Interaction Perspective

Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models show skill in simulating the 20th century observed warming rate (0.08 K/decade, global average). In the 21st century, the projected warming rate is significantly higher. For example, the 21st century global average temperature trend is four times higher in the 4.5 watts/m2 Representative Concentration Pathway (RCP 4.5), and nine times higher in 8.5 watts/m2 Representative Concentration Pathway (RCP 8.5), compared to the 20th century simulated temperature trend. In this study, we have investigated changes in land-atmosphere interactions, which have important consequences for fresh water availability in the 21st century. A multi-model analysis of results from 16 CMIP5 climate models that have detailed land surface schemes has been examined. A correlation between changes in annual evapotranspiration and temperature (a surrogate for available energy) or changes in precipitation (a surrogate for water availability) among different climate simulations; a total of 103 climate realizations is used as metric to study land-atmosphere interactions. We found that in the highest emission scenario (RCP 8.5), the earth system could potentially see a major change during the 21st century, where much of the land surface will transition to a predominantly water-limited system, i.e., despite increases in precipitation the land surface transitions to a drier state (water-limited). Details of the transition from water and energy limited systems in the 20th century to only water limited systems in the 21st century is presented. Results from the intermediate emissions scenario (RCP4.5) will be also discussed. These results have important consequences for fresh water availability, such as decreased water availability for ground water recharge and runoff.