24 25Droughts and heat waves have important impacts on multiple sectors including water 26 resources, agriculture, electricity generation, and public health, so it is important to 27 understand how they will be affected by climate change. However, there is large 28 uncertainty in the projected changes of these extreme events from climate models. We 29 compare historical biases in models against their future projections to understand and 30 attempt to constrain these uncertainties. Historical biases in precipitation, near-surface air 31 temperature, evapotranspiration, and a land-atmospheric coupling metric are calculated 32 Similarly, it is challenging to predict greenhouse gas emissions because they depend on 85 human development and mitigation efforts. Uncertainty from climate models occurs 86 because they include different sets of physical processes, use different parameterizations, 87 or have different spatial and vertical resolutions, even though they share significant 88 components and seek to solve the same general physical equations (Knutti et al. 2013). 89
90The work presented here focuses on the uncertainty in future projections of droughts and 91 heat waves derived from the diversity of climate models in CMIP5, and seeks to 92 5 constrain it by using information on the models' historical biases. These biases are 93 calculated using observationally constrained model output for key variables of the land 94 surface, near-surface atmosphere, and their interactions, which are important in 95 representing and controlling the occurrence of droughts, heat waves, and their feedbacks. 96Land-atmospheric (L-A) coupling is one of the main physical processes examined. This 97 represents how much influence the land surface has on the lower part of the atmosphere 98 and vice versa. The type of coupling determines whether a region is water-limited 99