Climate Change Affects Terrestrial Biodiversity

In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) left no doubt that global warming is occurring and that climate change is human-induced, concluding that “warming of the climate system is unequivocal” and stating with 90 percent confidence that the net effect of human activities on Earth since 1750 has been warming.1 And it is increasingly clear that this warming climate is having significant impacts on the world’s biodiversity.

In 2005 the Millennium Ecosystem Assessment (MA), which involved 1,360 scientists from 95 countries, concluded that climate change has affected biodiversity in all ecosystems over the last century, though the magnitude of the changes varied across ecosystem types.2 (See Table 1.)

The most studied and best understood impact is phenological changes, which are alterations in the timing of periodic biological events, such as the onset of animal migration or plant blooming, in response to climatic conditions. These events are typically linked to climate and are predicted to occur increasingly early in response to Earth’s steady warming; unfortunately, an increasing number of scientific studies present evidence consistent with this prediction. In plants, for instance, the flowering of cherry trees at the Royal Court in Kyoto, Japan, for which records have been maintained for at least 600 years, has advanced steadily since 1952.3 And in the western United States, the flowering of lilacs and honeysuckles has advanced by 2 and 3.8 days per decade, respectively. 4 Increased warming has also extended the growing season of some plants, as in the eastern deciduous forest of the United States, where the growing season has lengthened most notably since 1966, and in the colder, most northerly zones at 42°–45° latitude.5

Among invertebrate animals, a study of 35 butterfly species in the United Kingdom found that the date of first appearance for 26 species has grown earlier by between 1.0 and 15.8 days per decade between 1976 and 1998, while for the remaining 9 species it either has not changed (2 species) or has gotten later by between 0.1 and 3.6 days per decade.6 Likewise, between 1988 and 2002, the dates of first appearance for 17 Spanish butterfly species have advanced, as have the dates of first flight for 16 of 23 butterfly species in central California over 31 years.7 In vertebrate animals, studies have documented that the males of four American frog species are initiating calling 10–13 days earlier than before and that migrant birds in the North Sea have been passing 0.5–2.8 days earlier per decade since 1960.8 Other studies of birds have shown significant advances in the onset of breeding: by over eight days from 1971 to 1995 in 20 of 63 European bird species and by nine days from 1959 to 1991 for North American tree swallows.9 In a third study, of 23 European pied flycatcher populations, there was a significant correlation between changes in the local spring temperature and the onset of egg-laying—the warmer the local temperature, the earlier the onset of egg-laying within the local population.10

A second category of climate change effects are shifts in the range of a species, with movement in the long term expected to be toward each pole and to higher altitudes.11 Once again, a growing body of scientific evidence supports this. Near Antarctica, for instance, data indicate that several species of penguins, both sea ice–dependent and ocean-going species, have moved southward toward the pole.12 Among more temperate birds, data indicate that 12 species in the United Kingdom have moved northward by, on average, 18.9 kilometers over 20 years.13 Among insects, 23 species of dragonflies and damselflies in the United Kingdom expanded their ranges northward by on average 88 kilometers between 1960 and 1995.14

Cases of elevational shifts are also well documented. Among 16 Spanish butterflies, the lower elevational range has risen by, on average, 212 meters over 30 years.15 In mammals, 7 of the 25 populations of the pika in the western United States have gone extinct since being recorded in the 1930s, but the populations that disappeared were at significantly lower elevations than those that survived.16

These numerous studies indicate quite clearly that climate change has had a significant effect on terrestrial biodiversity, causing signifi- cant changes in a number of organisms across a wide array of ecosystems. Will these changes continue? And what will be their long-term effects? The IPCC and MA reports address the first of these questions. The IPCC concluded that it is “virtually certain” that recent warming trends will continue, and the MA projected that the impacts of climate change on biodiversity across all ecosystems will increase very rapidly.17

Answers to the second question are less clear. In some cases, the economic effects of climate change are likely to be positive; longer growing seasons and range shifts for agricultural crops benefit farmers in certain geographic regions, for example. Yet many of the impacts will be negative, including the direct and indirect effects of species relationships being disrupted and direct species extinctions.18 One study that synthesized the changes of 11 multispecies interactions found that more than 60 percent of the interactions had been disrupted and become less synchronous over time due to the different responses of individual species to climate change, producing, in some cases, significant negative consequences.19 Habitat loss and other climate change effects can also result in significant population declines. Polar bears, for instance, are dealing with decreases in the extent and thickness of sea ice, resulting in both shrinking population sizes and reductions in mean body weight.20 Thus climate change can significantly increase the probability of species extinction.

These individual accounts clearly demonstrate specific effects that climate change is having on terrestrial biodiversity. The larger scope of the problem is better illustrated by the IPCC, however. In an assessment of 29,000 observed changes in terrestrial biological systems, more than 89 percent of the significant changes were consistent with the direction of change expected as a response to global warming.21 The IPCC concluded “with high confidence that anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems.”22 Some of these changes are likely to have, at least in the short term, positive economic and biological impacts. But many of the long-term impacts will undoubtedly be negative. Among the most significant of those are a 5 out of 10 chance of an increased extinction risk for 20–30 percent of plants and animals and an 8 out of 10 chance of major changes in ecosystem structure and function.23

With increased understanding of the longterm consequences of climate change and the greater probability of negative biological outcomes should climate change continue at its current rate, the need to grapple with the challenges of global climate change also increases. The IPCC noted that a wide array of technological, behavioral, managerial, and policy options are currently available; however, they also noted that more extensive action is required in order to reduce human vulnerability to future climate change.24 As Tom Lovejoy of the Heinz Center for Science, Economics, and the Environment puts it, “Life on Earth is sending an urgent warning signal that climate change needs to be engaged with—and with an urgency and scale hitherto not contemplated.”25