The spread in future climate projections done with general climate models is large, and is a key measure of uncertainty regarding climate change. In order to reduce this spread: Falsifying particular models through direct comparison to future climate observations is not possible. An alternative means of eliminating unrealistic models and reducing model spread through "emergent constraints" is discussed. The goal of emergent constraints is to discover observable parameters of the current climate that are strongly linked to key aspects of future climate behavior in contemporary climate models. Links must be proven both through statistical analyses of the model ensemble and physical argumentation demonstrating the link is not merely a statistical artifact. If a model falls outside the uncertainty range associated with the observed value of the current climate parameter, then that models' associated future climate behavior is deemed unrealistic. This method can be used to reduce model spread and associated uncertainty in key processes affecting the future climate response to increasing greenhouse gases. These processes include climate feedbacks determining warming, factors shaping the intensification of the global hydrologic cycle, and biogeochemical feedbacks.

Alex Hall's research is focused on reducing uncertainty in processes determining the climate system's response to increases in greenhouse gases. Dr. Hall was a Contributing Author to the 2007 IPCC 4th scientific assessment of climate change Working Group I report, where his work on climate sensitivity is featured. He was also a Lead Author for Chapter 14 of the Working Group I component of the IPCC 5th Assessment Report, entitled "Climate Phenomena and their Relevance for Future Regional Climate Change," and a Contributing Author of Chapter 9, "Evaluation of Climate Models." He is a member of the executive committee of the UCLA-JPL Joint Institute for Regional Earth System Science and Engineering, and the faculty director of the UCLA Center for Climate Change Solutions and UCLA Earth Systems Institute.