Global Ecosystem Response to Changing Light Environments and the Implications for Climate Feedbacks

San Francisco State University, San Francisco CA

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Productivity of terrestrial ecosystems through photosynthesis represents the largest single planetary annual carbon flux, accounting for about 120 gigatons of carbon per year. Recent research into the role of light in ecosystem-scale photosynthesis has revealed a significant enhancement of carbon uptake under conditions of diffuse light, where light has been scattered by clouds and aerosols. This enhanced diffuse-use efficiency (DUE) can make a significant difference in annual gross primary production (GPP), and in the case of volcanic aerosols has an observable global impact on atmospheric carbon dioxide concentrations. It is not known how universal the DUE is, however, as well as how it varies across ecosystem types, hydroclimates, and light regimes or how large the global terrestrial GPP enhancement is due to diffuse light. As a result, researchers are unsure what impact future changes in light conditions might have on the global carbon budget and ultimately atmospheric concentrations of carbon dioxide. The objectives of this project are to produce a ground-up estimate of the net global DUE; to investigate the geographic variability in DUE as a function of ecosystem characteristics, climate, and light regime; and to estimate potential carbon feedbacks under scenarios of future changes to regional light environments. This will be accomplished through analysis of a dataset of more than 200 carbon dioxide flux tower sites around the world, each providing multiple years of 30-minute carbon fluxes, radiation and meteorological data. This new global dataset is the product of large contributions by hundreds of researchers around the world and covers all major ecoregions, most climate classes, a latitudinal range of 110 degrees, and all the continents except Antarctica. Analysis will first involve a site-by-site parameterization of the DUE primarily using carbon fluxes and solar radiation data. These will be synthesized with a spatial ecoregion model to estimate the magnitude of the global DUE and will be analyzed with ecosystem, climate, and light attributes to help understand the geographic distribution and mechanisms controlling DUE. A global spatial model of DUE parameterizations also will be combined with scenarios of cloud and aerosol change to approximate the global carbon budget impacts of future changes in light regimes. This project will provide foundational parameterizations of an important carbon budget control for use in ecosystem-climate models. This will improve basic understanding of the mechanisms controlling DUE and the carbon budget implications, including atmospheric concentrations of this important greenhouse gas. This will lead to future accounting of this important feedback in global climate models, which is critical for understanding ecosystem responses to environmental change and for improved prediction of future climates. The project will provide education and teaching opportunities for students from traditionally underserved communities, and it will provide valuable information and insights for environmental researchers and policy makers.