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As you saw in Key Concept 57.4, human-generated emissions of greenhouse gases are contributing to global climate warming, sea level rise, increased storminess, and ocean acidification, all of which are likely to become increasingly important causes of habitat loss and degradation, and ultimately species extinctions. Across North America, for example, average annual temperatures are predicted to increase by 2°C to 5°C by the end of the twenty-first century. If the climate warms to that extent, model projections show that the average temperature found at any given location in North America today could shift 500–800 kilometers to the north. Those species that cannot adapt to the warmer climate will have to shift their geographic ranges to stay within their physiological and ecological ranges. The shift in distributional range that organisms experience could result in habitat loss and fragmentation, especially if appropriate habitat does not shift as well or is lost altogether.

For example, as the globe warms, sea level is rising as a result of melting polar glaciers and warming seas. The Intergovernmental Panel on Climate Change (IPCC) estimates that the current rate of global sea level rise is 3 millimeters per year, resulting in a projected rise of 0.2–0.5 meters by 2100. As the seas slowly inundate the coast, coastal ecosystems will become increasingly flooded, putting them at great risk for habitat loss. Likewise, as polar regions warm, sea ice ecosystems are disappearing at an alarming rate, affecting species such as polar bears and seals dependent on that habitat (see the Future directions discussion in Chapter 57). Global warming has also been implicated in causing more frequent and extreme storm events, as seas warm and winds increase in intensity. Extreme storms such as Hurricane Sandy in 2012 are causing flooding and erosion hazards for both human and natural coastal communities. Finally, as discussed in Investigating Life: Food Webs in an Acidic and Warming Ocean in Chapter 57, as the ocean absorbs more atmospheric CO₂, seas become more acidic, leading to a host of individual species and ecosystem changes.

Scientists are beginning to explore how species and ecosystems will respond to climate change by predicting how it may affect organisms and looking for ways to mitigate those effects. Their research activities include analyses of past climate changes and studies of sites currently undergoing rapid climate change. It would be helpful to know, for example, how rapidly species responded to the end of the most recent ice age. Which species did and did not keep pace with the warming climate and rising seas? How much, and in what ways, do past ecological communities differ from those of today as a result of changes in climate?

Species that can disperse easily, such as birds, insects, and fish that can move considerable distances, may be able to shift their ranges as rapidly as the climate changes, provided they can find appropriate habitats. However, the ranges of other species, particularly plants, are likely to shift more slowly. For example, after the glaciers started to retreat in North America about 18,000 years ago, the ranges of plant communities slowly shifted northward (Figure 58.11). In addition, roughly 12,000 years ago, some novel, or what are termed "no analog" plant communities, formed under unique climate conditions that do not exist today. Thus it may be that as climate rapidly changes, unique combinations of species will come together to form similar novel communities.

Question

After the glaciers first started to retreat in North America about 18,000 years ago, the ranges of plant communities shifted northward and expanded considerably. Roughly 12,000 years ago, “no analog” plant communities, unlike any plant assemblage found today, formed under the unique climate conditions of the time. It may be that as climate changes, unique combinations of species will come together to form similar “no analog” communities.

Modern-day scientific observations have shown that for a wide variety of organisms, latitudinal and elevational distributions are shifting in ways that are consistent with climate change (Table 58.1). For example, a study that considered the range shifts in alpine plants in the European alps showed that plant species have been moving to higher elevations consistent with increasing temperature. Researchers compared current plant community species richness with historical data from the eighteenth and nineteenth centuries. They found that alpine plant species richness increased over time, suggesting that plants were moving from lower elevations to higher elevations. Likewise, observations of the distribution of nonmigratory butterfly species in Europe and North America showed that of the 39 species examined, 63 percent had moved their ranges northward, while only 3 percent had moved southward.

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Along with the shifts in distribution, evidence is mounting that important life history events are also occurring earlier in the spring (Table 58.2). The observations suggest that increases in temperature are triggering species to breed or migrate earlier than they have in the past few decades. There can also be physiological changes that reduce growth and reproduction. For example, since the mid-1980s the average minimum nightly temperature at La Selva Biological Station in the Caribbean lowlands of Costa Rica has increased from about 20°C to 22°C. On warmer nights, trees use more of their energy reserves to maintain themselves. As a result, even this small rise in temperature has reduced the average growth rate of six different tree species by about 20 percent.

Finally, climate change has the potential to cause species extinctions, although to date, none have been directly and definitively linked to this cause. However, as you saw in Figure 58.2, effects of climate change such as habitat loss or changes in life history or physiology could lead to lower effective population sizes, increased population extinctions, and eventually species extinctions.