This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.
This project examines changes in hydroclimate in model simulations prepared for the AR5, using observational datasets to evaluate model performance in variables including precipitation, evapotranspiration, soil moisture, and runoff. A key issue to be addressed is the effect of land use and land cover change (LULC), which is expected to be significant for the hydrological cycle on regional spatial scales. Between 1700 and 2000, the global extent of natural vegetation decreased by 45%, and agricultural land area increased by 500%. In addition, a number of LULCC impacts have been found in model sensitivity studies, including radiative cooling of global and regional climate as a result of increase in surface albedo, regional warming due to decrease in evapotranspiration, and global warming due to CO2 emission from land cover change. The simulation ensemble prepared for the AR5 is the first of its kind to include simulations of the response to LULC alone, so that the climatic impact of this potentially important forcing can be assessed through comparisons of model simulations forced by LULC, greenhouse gas increases, and other forcing factors. Thus, work under this grant will explore both the ability of climate models to simulate important hydroclimate variables and the extent to which these variables are affected by LULC.
The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. Decision makers will benefit from an assessment of the quality of the climate models used to project climate changes due to global warming, as well as an evaluation of the impact of LULC on climate (particularly the hydrological cycle) at regional and global scales.
The primary goal of this project is to explore fidelity of latest climate models (CMIP5 climate models) in simulating hydroclimatic variability and change at global and regional scales. Specifically, we have compared 20th century temperature and precipitation trends from CMIP5 climate simulations with the observations. We have also explored effects of land use land cover change on regional hydroclimate in CMIP5 climate simulations. Followings are keys outcomes from the project. The multimodel ensemble-mean results from CMIP5 climate models capture the observed 20th century global land average warming well (Figure 1). The global average annual mean precipitation trends are insignificant (~0) in both model simulations as well as the observations. There are substantial intermodal spread or uncertainties among different models’ climate simulations (Kumar et al. 2013a). There are substantial uncertainties in model simulated temperature and precipitation trends at local/regional scales (Figure 2). The pattern correlation between simulated trends and observed trends, as well as between two different CMIP5 models’ simulated trends are generally not significant. This result in conservative estimate of multimodel ensemble-mean temperature and precipitation trends compared to the observation at local/regional scales. Further, exploration focusing on eastern United States showed that natural climate variability introduces a major source of uncertainties in regional climate change. There is no reason why natural climate variability component should sycronize among different climate simulations. Hence, uncertanties in regional climate projection is expected. We have developed a new methodology to assess Land use change impacts in CMIP5 simulations. The new methodology is based on comparing climate change impacts between two neighboring regions in which one region has experienced Land use change and other has not. In the highest emission scenario (RCP8.5) model simulations show higher summer warming in North America and Eurasia historical land use change regions than in the surrounding regions. We found substantial uncertainties in the Africa and South America future land use change regions, where most climate models show a net decrease in summer temperature due to land use change (Figure 3). In the highest emission scenario 21st century projected climate the rate of near-surface atmospheric warming is 8 times faster (0.58±0.03°C/decade) compared to the 20th century warming rate (0.07±0.01°C/decade). We found that the water cycle of the earth system could potentially see a major change in the projected climate, where land surface evapotranspiration contribution would become increasingly water limited, significantly changing the behavior of the climate system (Figure 4). Besides the specific outcomes described above, this project provided partial support for one post-doctoral researcher to develop skills in analyzing climate models with respect to regional and global hydro climatology and land use impacts. In addition, results from this study present a clarified understanding of regional climate uncertainties. The newly developed understanding can be helpful while downscaling global climate models’ projections at regional scale for water resource applications
