Observed Impacts of Climate Change in the United States

For over a century, scientists have documented the important role that that the climate plays in the geographic distribution of the world’s ecosystems and the wildlife they support.  Yet, it is now quite evident that the climate these species depend upon is changing.  Global temperatures increased by over 1oF during the past century and are projected to increase 2.5-10.4oF by 2100 as a result of human emissions of greenhouse gases.  Given the reliance of plants and animals on their natural environment, they are often early barometers of the effects of climate change.

“Observed Impacts of Global Climate Change in the U.S.” is the twelfth in a series of Pew Center reports examining the impacts of climate change on the U.S. environment.  While past Pew Center reports have reviewed the potential impacts of future climate change, this report provides compelling evidence that ecosystems are already responding to climate change and provides insights into what we can expect from future changes in the Earth’s climate.  Looking specifically at the United States, report authors, Drs. Camille Parmesan and Hector Galbraith find:

A number of ecological changes have already occurred in the United States over the past century in concert with increases in average U.S. temperature and changes in precipitation.  Warmer temperatures have resulted in longer growing seasons at the national level, altered carbon cycling and storage in the Alaskan tundra, and increased the frequency of fires and other disturbances in U.S. forests.  Individual species such as Edith’s checkerspot butterfly and the red fox have shifted north or to higher altitudes. Other species including Mexican jays and tree swallows have experienced changes in the timing of reproduction, as have plants such as forest phlox and butterfly weed.  While these changes illustrate efforts by species to adapt to a warming climate, these responses may alter competition and predator-prey relationships and have other unforeseen consequences.

These observed changes have been linked to human-induced warming of the global climate.  There is increasingly strong evidence that the observed global climate change, particularly that of the past 50 years, is primarily the result of human emissions of greenhouse gases.  Changes in U.S. climate have also been linked with human activities.

Changes in natural systems will continue and become even more apparent in the future, resulting in the degradation and loss of U.S. biodiversity. With continued and more severe changes in the climate, the ability of U.S. wildlife to adapt through migration and physiological change will be increasingly limited.  Furthermore, because of adaptive migration, species such as the red fox are now competing for habitat previously dominated by the arctic fox, threatening the arctic fox’s long-term survival.  The challenge is even greater when considered along with the broad range of other environmental threats currently affecting wildlife, such as habitat loss, environmental contamination, and invasive species.

Efforts to reduce greenhouse gas emissions, protect U.S. ecosystems and wildlife, and provide refuge for sensitive species are all necessary to limit the future ecological consequences of climate change.  Curbing greenhouse gas emissions can reduce the rate and magnitude of future climate change, consequently reducing the severity of, but not preventing, climatic stresses to wildlife.  Meanwhile, the expansion of nature reserves and habitat conservation efforts can alleviate some non-climate stresses and enable species to better adapt to the effects of climate change.

The authors and Pew Center gratefully acknowledge the input of Drs. Lou Pitelka and Walter Oechel on this report.  The Pew Center would also like to thank Joel Smith of Stratus Consulting for his assistance in the management of this Environmental Impacts Series.