Amplification of Heat Extremes by CO2 Physiological Forcing

Continued increases in anthropogenic CO₂ emissions are expected to enhance the magnitude and frequency of extreme temperature events. Given the potential acute impacts on human and natural systems, a process-based understanding of where and how extreme temperature events may change in response to elevated CO2 is critical.  In this work, we utilize a set of idealized AOGCM simulations from the carbon-climate feedback experiment within CMIP5 and a suite of AGCM simulations with the Community Climate System Model 4 to identify and elucidate the processes linked to projected future high-impact temperature extremes.  Model simulations reveal that the response of plant physiology to CO₂, CO₂ physiological forcing, has an important role in shaping the geographic distribution and intensity of extreme heat events under projected elevated CO₂. Surface flux and atmospheric circulation patterns associated with physiological-driven temperature extremes are analyzed in detail. Specifically, we find that reduced stomatal conductance from projected physiological forcing limits evaporative cooling at the Earth’s surface and increases the flux of sensible heat to the atmosphere, making vegetated regions of the tropics and Northern Hemisphere mid-latitudes particularly susceptible to increases in future heat extremes. Robust increases in excess of 15 extreme heat days per year (greater than 99^th percentile temperature events) are common throughout tropical forests, northern North America and Eurasia in response to CO₂ physiological forcing alone. Our results point to a critical role for vegetation in shaping future climate change impacts, and highlight the need to further refine our understanding of the relationship between leaf-level vegetation responses to CO₂ and land-atmosphere energy and water fluxes.