In the three and a half years since the third US National Climate Assessment (NCA) was initiated, record fires in the West, a serious drought in Texas, Superstorm Sandy, and massive flooding in the Midwest, Northeast, and Colorado have punctuated news reports. Nature is reminding us that our activities – particularly those accelerating the rise of atmospheric CO2 concentrations – are leading to changes in the climate system that have repercussions for both ecosystems and people. Economic impacts are clearly evident, reflected in the mounting costs of fighting fires, managing water supplies, and protecting coastal communities from rising sea levels. The general public's dawning realization that intact or restored ecosystems can help moderate these impacts has brought the concept of ecosystem services to the fore.
This third NCA process was specifically designed to help communicate scientific knowledge – about the impacts, risks, and opportunities associated with changes in the global environment – to decision makers. The NCA author teams were asked to identify the changes of greatest concern and to analyze the implications of observed and projected changes from various perspectives. Clearly, the consequences of climate change for ecosystems are not well understood by the human communities that depend on them, and the risks may be greater for ecosystems than for people. But people and ecosystems are interdependent in a variety of ways, and it is these interdependencies that are discussed in the papers that make up this Special Issue.
Written by a diverse group of scientists and other stakeholders, these papers represent the findings of a US Geological Survey-sponsored technical report to the NCA. The authors explore compelling evidence about the impacts of climate change on ecosystems: their species, communities, and biodiversity; their structure and function, in terms of the flow of energy and fluxes of materials; and the services they provide to people.
Assessments differ in important ways from primary research and scientific literature reviews; ecologists who have not participated in assessments may therefore appreciate an overview of the process and its goals (Grimm et al. 456–464). Major climate-related impacts on species have been documented, including shifts in range and timing of life-cycle events for more than 4000 species worldwide, even in the short amount of time that has passed since the last assessment in 2009. Many shifts are species specific, producing changes in community structure and species interactions. Staudinger et al. (465–473) consider biodiversity at multiple scales, from genes to ecosystems. Ecosystem processes that result from interactions among organisms and abiotic elements may integrate the kinds of shifts that are seen for individual species, yet there are clear signals of change at the ecosystem level as well. For example, productivity of lakes, marine systems, and forests is responding to changes in drivers such as thermal stratification, organic inputs, and disturbance regimes. Hydrologically mediated connections among ecosystems are altered, with consequent impacts on structure and processes (Grimm et al. 474–482). All of these changes in ecosystem structure and function affect people through the loss or gain of ecosystem services. Nelson et al. (483–493) describe several of the well-known benefits of ecosystems in modulating the severity of weather events and in providing food and fiber, and offer a forward-looking menu of ways that this knowledge can help society adapt to climate change.
Because climate change is just one of several drivers of change in ecosystems (albeit an increasingly important one), Staudt et al. (494–501) view impacts through the lens of multiple stressors. The ways in which ordinary people, ecosystem managers, and other decision makers can prepare for the likely impacts of climate change on ecosystems is also considered (Stein et al. 502–510).
The third NCA introduces several major innovations in process and content, but perhaps the most important conceptual contribution is an explicit focus on systems impacts and the interlocking and cascading effects across sectors. Seven chapters in the NCA focused on complex cross-sectoral issues; in the case of ecosystems and ecosystem services, major shifts in public policy are already underway as new concepts – such as “green infrastructure” – that clearly recognize the economic and social value of the buffering capacity of ecosystems are rapidly being adopted. And at the highest levels of the US Government, climate resilience and preparedness activities for human systems are now incorporating ecosystem-based approaches. These innovations provide evidence for what ongoing, rigorous, and interdisciplinary assessment activities can mean for ecosystems and the human communities that depend on them.