Climate, ecosystems, and planetary futures: The challenge to predict life in Earth system models

BACKGROUND Earth system models (ESMs) simulate physical, chemical, and biological processes that underlie climate and are the most complex in a hierarchy of models of Earth’s interacting atmosphere–land–ocean–sea ice system. As terrestrial and marine ecosystems have been added to ESMs, the distinction between the physical basis for climate change, mitigation, and vulnerability, impacts, and adaptation (VIA) no longer necessarily holds. The same global change stresses that affect terrestrial and marine ecosystems are critical processes that determine the magnitude and trajectory of climate change, and many of the interventions that might lessen anthropogenic climate change pertain to the biosphere. Here we describe environmental changes that are stressing terrestrial and marine ecosystems. We discuss how these stressors are being included in ESMs, initially with an emphasis on climate processes, but also show their emerging utility for VIA analyses and examine them in the context of Earth system prediction. ADVANCES Terrestrial ecosystems face stresses from changing climate and atmospheric composition that alter phenology, growing season length, and community composition; these stresses enhance productivity and water-use efficiency in some regions, but also lead to mortality and increased disturbances from wildfires, insects, and extreme events in other regions. The addition of reactive nitrogen, elevated levels of tropospheric O 3 , and anthropogenic land-use and land-cover change stress ecosystems as well. The terrestrial biosphere models included in ESMs simulate the ecological impacts of these stresses and their effects on Earth system functioning. Ocean ecosystems and living marine resources face threats from ocean warming, changing large-scale circulation, increased vertical stratification, declining oxygen, and acidification, which alter nutrient supply, the light environment, and phytoplankton productivity; result in coral bleaching; and produce novel marine communities. Three-dimensional ocean models simulate the carbon cycle and associated biogeochemistry. Plankton ecosystem models both drive biogeochemistry models and characterize marine ecological dynamics. OUTLOOK The untapped potential of ESMs is to bring dispersed terrestrial and marine ecosystem research related to climate processes, VIA, and mitigation into a common framework. ESMs offer an opportunity to move beyond physical descriptors of atmospheric and oceanic states to societally relevant quantities such as habitat loss, water availability, wildfire risk, air quality, and crop, fishery, and timber yields. To do so, the science of climate prediction has to be extended to a more multifaceted Earth system prediction, including the biosphere and its resources. ESMs provide the means not just to assess the potential for future global change stresses, but also to determine the outcome of those stresses on the biosphere. Such Earth system prediction is necessary to inform sound policy that maintains a healthy biosphere and provides the food, energy, and fresh water needed for a growing global population without further exacerbating climate change. Substantial impediments that must be overcome include advancing our knowledge of biosphere-related climate processes; reducing model uncertainty; and effectively communicating among, rather than across, the disparate science communities of climate prediction, global biosphere modeling, VIA analyses, and climate change mitigation.