Conclusions

1) Sufficient studies now exist to conclude that the consequences of climate change are already detectable within U.S. ecosystems. This report reviews more than 40 studies that associate climate change with observed ecological impacts in the United States, and, using objective evaluation criteria, finds that more than half provide strong evidence of a direct link. These studies span a broad range of plant and animal species from various regions of the United States. Yet, despite the diversity among studies, the observed ecological responses are consistent with one another, as well as with the changes that one would expect based on the nature of U.S. climate change observed to date. 

2) The timing of important ecological events, including the flowering of plants and the breeding times of animals, has shifted, and these changes have occurred in conjunction with changes in U.S. climate. If these timing shifts are synchronous across species that normally interact with each other (for example, if adult butterflies and the flowers they depend on for nectar both emerge two weeks earlier), then these species’ interactions are preserved, and the system may remain healthy. On the other hand, if responses to temperature increases vary across species (for example, if butterflies emerge before the flowers they depend on for survival), then species’ interactions may become out of synchrony and could lead to population declines. Both types of situations have been documented.

3) Geographic ranges of some plants and animals have shifted northward and upward in elevation, and in some cases, contracted. One of the most detailed and best-studied examples is the Edith’s checkerspot butterfly in the western United States. As temperatures have increased over the last century, many southern and lower-elevation populations of this species have disappeared entirely. The effect of this shift has been a contraction of the species’ range to the north (i.e., it is disappearing from Mexico but thriving in Canada).  The red fox, another example, has shifted northward and is now encroaching on the arctic fox’s range, threatening its survival. Similar range shifts within the United States have also been observed in organisms as diverse as birds, mammals, intertidal invertebrates, and plants. Such major shifts alter species’ interactions and potentially threaten U.S. biodiversity.

4) Species composition within communities has changed in concert with local temperature rise. As species within a community change abundances or, ultimately, are added or lost, the relationships among species also change. In particular, such shifts in composition are likely to alter important competitive and predatory/prey relationships, which can reduce local or regional biodiversity. A particularly compelling example of this is the change observed over more than 60 years in the intertidal communities of Monterey, California, where a community previously dominated by northern colder-water species has been “infiltrated” by southern warmer-water species in response to oceanic warming. Similar changes have also been observed in nearby offshore marine fish communities. Thus, many protected lands, such as the marine reserve in Monterey Bay, are experiencing a shift in the communities that they protect.

5) Ecosystem processes such as carbon cycling and storage have been altered by climate change. The lengthening of the growing season has altered the annual cycle of carbon-dioxide (CO^₂) levels in the atmosphere, because plants are a major intermediary for carbon flow through ecosystems. The Alaskan tundra has switched from being a net sink of CO^₂ (absorbing and storing more carbon from the atmosphere than is released) to being a net source of CO^₂ (releasing more carbon than is stored), because warmer winters have allowed previously stored dead plant matter in the soil to decompose and release CO^₂. Like the tundra, boreal forests have become carbon sources because of reduced growth due to climate-mediated increases in water stress, pest outbreaks, and wildfires. Conversely, many of the forests of the lower 48 states have switched in the opposite direction—becoming carbon sinks in recent decades. This transition is attributed to regrowth of forests following logging and abandonment of agricultural fields.  However, it is expected to stop as soon as the forests mature.

6) The findings that climate change is affecting U.S. biological systems are consistent across different geographic scales and a variety of species, and these U.S. impacts reflect global trends. Even against a background of apparently dominating forces such as direct human-driven habitat destruction and alteration, a climate “fingerprint” is discernible in natural systems. The most rigorous studies within the United States provide strong evidence that climate change has affected the timing of biological events in at least three taxa (i.e., groups of related species). They also provide strong evidence that at least three taxa have shifted their ranges in response to climate change and that climate change has altered ecological communities and processes. Further, very few instances of biotic change run completely counter to climate-change predictions, and the findings of many of the U.S. studies are mirrored by studies elsewhere around the world. Climate change has the potential to degrade ecosystem functions vital to global health. If the observed biological changes are merely one phase in a cyclical pattern of warming and cooling periods, then they may not represent a threat to long-term species and ecosystem health. If, however, they are linked to anthropogenic climate change, they will continue along the same path. Thus, it is essential to address the extent to which the U.S. climate change responsible for observed ecological responses can be attributed to global emissions of anthropogenic greenhouse gases.

7) There is an emerging link between observed changes in wild plants and animals across the United States and human-driven global increases in greenhouse gases.   In 2001, the Intergovernmental Panel on Climate Change concluded that the global rise in average yearly temperature over the past 50 years was primarily due to increased concentrations of anthropogenic greenhouse gases. U.S. climate trends are consistent with global climate trends. Global biological trends are predicted by (and match) observed climate trends, indicating that anthropogenic global climate change has affected natural systems. Recent research focusing on North America has also shown a significant greenhouse gas signal in North American climate trends over the past 50 years. The combination of strong consistency across climate and biological studies and across scales (from regional to global), coupled with new climate analyses specific to the United States, links U.S. biological changes to anthropogenic climate change. The implications of this link are that current biological trends will continue over future decades as greenhouse gas emissions continue to rise.

8) The addition of climate change to the mix of stressors already affecting valued habitats and endangered species will present a major challenge to future conservation of U.S. ecological resources. Many if not most of the ecosystems and organisms in the United States are already suffering from other anthropogenic stressors such as habitat destruction or fragmentation, introduction of invasive species, and contamination. As yet, scientists do not have a clear idea how climate change might affect this already fragile situation. It is likely, however, that in many cases climate change may exacerbate current conditions, further stressing wild species and their associated ecosystems. There is a growing consensus within the scientific community that climate change will compound existing threats and lead to an acceleration of the rate at which biodiversity is lost.

9) In the future, range contractions are more likely than simple northward or upslope shifts.  During historic glacial cycles, range shifts of hundreds to thousands of miles were common, and species extinction was rare. However, achieving such massive relocation is much more problematic across the human-dominated, artificially fragmented landscapes of today. The large reduction in the areas of natural habitats and the growth of barriers to species’ dispersal (urban and agricultural zones) makes simple range shifts unlikely. Species that are not adapted to urban and agricultural environments are likely to be confined to smaller total geographic areas as climate causes them to contract from their southern and lower boundaries.  Already rare or endangered species, or those living only on high mountaintops, are likely to have the highest risk of extinction.

10) Reducing the adverse effects of climate change on U.S. ecosystems can be facilitated through a broad range of strategies, including adaptive management, promotion of transitional habitat in nonpreserved areas, and the alleviation of nonclimate stressors. The protection of transitional habitat that links natural areas might assist in enabling species migration in response to climate change. Meanwhile, promoting dynamic design and management plans for nature reserves may enable managers to facilitate the adjustment of wild species to changing climate conditions (e.g., through active relocation programs). Also, because climate change may be particularly dangerous to natural systems when superimposed on already existing stressors, alleviation of the stress due to these other anthropogenic factors may help reduce their combined effects with climate change.