PNNL

Abstract

Understanding the role of climate extremes and their impacts on the carbon (C) cycle is increasingly a focus of Earth system science. Climate extremes such as droughts, heat waves or heavy precipitation events but also fires can cause substantial changes in terrestrial C fluxes. On the other side, extreme changes in C fluxes are often, but not always, driven by extreme climate conditions. Here we present an analysis of how extremes in temperature and precipitation, and extreme changes in terrestrial C fluxes are related to each other in 10 state-of-the-art terrestrial carbon models, all driven by the same climate forcing for the 20th century. We use models from the North American Carbon Program (NACP) Multi-scale synthesis and Terrestrial Model Intercomparison Project (MsTMIP). Our results show that at global scale droughts largely decrease both gross primary production (GPP) and total respiration (TR) while heat waves decrease GPP but increase TR, both resulting in an anomalous source of CO2 to the atmosphere. Cold and wet periods have a smaller opposite effect. Extreme changes in GPP and TR are often caused by strong shifts in water availability but for extremes in TR also shifts in temperature are important. Only NEE extremes are equally strongly driven by deviations in temperature and precipitation. Models mostly agree in sign of the C flux response to climate extremes, but model spread in absolute magnitude is large. A regionalized analysis shows that extremes in tropical forests are stronger associated to water availability whereas in boreal forests temperature plays a more important role. Models are particularly uncertain in the C flux response to extreme heat in boreal forests.