acts of climate change onecosystem structure and functionNancy B Grimm1*, F Stuart Chapin III2, Britta Bierwagen3, Patrick Gonzalez4, Peter M Groffman5, Yiqi Luo6,Forrest Melton7, Knute Nadelhoffer8, Amber Pairis9, Peter A Raymond10, Josh Schimel11, andCraig E Williamson12Recent climate-change research largely confirms the impacts on US ecosystems identified in the 2009 NationalClimate Assessment and provides greater mechanistic understanding and geographic specificity for thoseimpacts. Pervasive climate-change impacts on ecosystems are those that affect productivity of ecosystems ortheir ability to process chemical elements. Loss of sea ice, rapid warming, and higher organic inputs affectmarine and lake productivity, while combined impacts of wildfire and insect outbreaks decrease forest pro-ductivity, mostly in the arid and semi-arid West. Forests in wetter regions are more productive owing to warm-ing. Shifts in species ranges are so EXTENSIVE that by 2100 they may alter biome composition across 5–20% ofUS land area. Accelerated losses of nutrients from terrestrial ecosystems to receiving waters are caused by bothwinter warming and intensification of the hydrologic cycle. Ecosystem feedbacks, especially those associatedwith release of carbon dioxide and methane release from wetlands and thawing permafrost soils, magnify therate of climate change.Climate fundamentally controls the distribution ofecosystems, species ranges, and process rates onEarth. As a component of the US National ClimateAssessment, to be released in 2014, a group of over 60ecological experts from academic, governmental, andnongovernmental organizations assessed the state ofknowledge about how climate change has affected andwill affect species, biodiversity, and ecosystem structure,function, and services in the US. Here, we summarize keyfindings on the impacts of climate change on ecosystems,focusing on the fluxes of matter and energy and the bioticand abiotic parts of ecosystems that contribute most tothose fluxes.Ecosystem patterns and processes, such as rates of pri-mary productivity or input–output balance of chemicalelements, respond in complex ways to climate changebecause of multiple controlling factors. For example,whether a forest is a carbon (C) source or sink dependson the balance of primary production and ecosystemrespiration, processes that respond to different drivers.Physical changes in ecosystems – for instance, changesin thermal STRATIFICATION patterns in lakes and oceans,flood and drying regimes in streams and rivers, or inten-sification of the hydrologic cycle across LARGE basins –lead to changes in ecosystem structure and functionthat have economic and human consequences. Oftenthe extremes or changes in timing have greater impactthan changes in average conditions and incur greatersocietal impacts and costs. Recognizing these issues, cli-mate-change ACTION plans and management strategieshave begun to account for forecasted changes inextremes or seasonality.n Seven key impactsAlthough climate change is affecting US ecosystems innumerous ways, seven findings emerged from our assess-ment as representing the most critical climate-changeimpacts on ecosystem structure and function in the US,supported by compelling evidence from the past 4 years(Figure 1). Only a few of the important references can becited in this article due to space limitations, and we referreaders to the WebReferences for additional supportingreferences.Climate effects on sea ice, lakes, and coastalecosystemsThe late summer extent of Arctic sea ice continues todecline, with a record low set in 2012 (www.climate-watch.noaa.gov/article/2012/arctic-sea-ice-breaks-2007-record-low). This low extent exceeds the previous recordset in 2007 (Figure 2a). The Arctic Ocean is projected tobe ice-free in late summer before the middle of the 21stcentury, radically changing patterns of marine productiv-ity associated with ice edges (Arrigo et al. 2012). In theSouthern Hemisphere, the population size of krill – a keycomponent of whale and other marine vertebrate diets – ispositively correlated with the extent of sea ice (Atkinsonet al. 2004). As the oceans warm and land-based ice melts,sea level is rising steadily (Figure 2b) and threateninghabitat-forming species such as corals and mangroves incoastal ecosystems, as well as infrastructure and liveli-hoods of people living on coasts (Doney et al. 2012).Ecosystem state change Many of the aforementionedbiome shifts are stabilized byfeedbacks that maintain theseecosystems in their new state,making it difficult to reverse thechanges. For instance, the Sahelchanged from a TROPICAL forest tograssland and then to desertwithin a few thousand years(Kröpelin et al. 2008). Rapid orabrupt transitions, such as deser-tification or collapse of coralreefs, may occur when a thresh-old is crossed (Scheffer et al.2001).