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36 Lessons from the Distant Past Much of what we know about the current ecological impacts of climate change comes from long- term observations and experiments. Our ability to predict future ecological impacts of climate change, however, largely stems from what we know about the effects of past climate changes. Although climate changes over recent geological history were generally more modest and slower than the changes we currently face, studying the geologic record of climate and ecosystem change provides a valuable way to understand how large-scale climate change affects natural vegetation and ecosystems. Climatic records spanning the last 50,000 or so years can be generated from sediments, tree-rings, cave formations, corals, ice cores, and many other natural recorders of climate. These climate records can be dated and compared with vegetation and ecosystem records like those based on fossil pollen, plants, animals, and other organisms. These physical records allow scientists to understand what ecosystems were like in the past and together make up what is called the paleoecological record. Past ecological responses to climate change Thousands of years ago The last 20,000 years on Earth have seen the demise of the last ice age, along with a global warming of 4-7°C (7.2-12.6° F) into the current interglacial period, all driven by subtle changes in the orbit of Earth (Jansen et al. 2007). This warming caused significant and widespread changes in dominant plant types as the ecosystems of the continent reorganized in response to climate warming and associated changes in the water cycle (Overpeck et al. 2003). This millennia-long climate shift caused some plant and animal species to become rare on the landscape and restricted to just a few isolated locations, a situation that can put species at greater risk of extinction. Some vegetation communities shifted to locations with more favorable conditions, others became extinct, and new communities emerged. The paleoecological record of the last 20,000 years makes clear that each plant species adapts to large-scale climate change in its own way, and that the climate conditions needed for any individual species’ survival and reproduction can move hundreds of kilometers or more across the landscape in response to climate change (Jackson and Overpeck, 2000). In extreme cases whole biomes (assemblages of plant species) can go extinct. For example, around 12,000 years ago, much of the U.S. Midwest was covered by a mixed forest of spruce and hardwoods unlike anything that can be found there today. This major vegetation biome went extinct as summers warmed and ice sheets retreated about 10,000 years ago. Of course, the converse is also true: when novel new climates emerge, ecosystems with entirely new mixtures of species may arise. Interestingly, the long-term climate changes that occurred in the 20,000 years before the Industrial Revolution are known to have caused only one North American plant species—a type of spruce tree—to go extinct in the last 20,000 years (Jackson and Weng 1999). In contrast, many species of North American mammal—most notably Pleistocene megafauna such as wooly mammoths and mastodons—are known to have gone extinct over the last 20,000 years. Although debate continues on the exact cause of these extinctions, it appears most likely that climate change caused major reductions in the favored habitats of these animals, and that human hunting acted as a compounding stress (Koch and Barnosky 2006). This highlights the potentially deadly
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Lessons From the Distant Past 37 challenge that rapid climate change (even faster than over the last 20,000 years), combined with human activities (now more varied and pervasive than ever in the past), could pose to North American biodiversity in the future. Millions of years ago Another important lesson emerges from the Paleocene-Eocene Thermal Maximum, approximately 55 million years ago. At this time rapid and large increases in atmospheric carbon dioxide caused an abrupt, sustained global warming of over 5°C (9°F), and also acidification of the world’s oceans—similar to what we currently face. Ocean chemistry took over 100,000 years to return to a less acidic state, and in the meantime many marine animals became extinct. Ecological impacts on land were also substantial, apparently including the appearance of the first primates on Earth, although the details of cause and effect remain uncertain (Jansen et al. 2007). The global warming that occurred after the last ice age was large, particularly with respect to the magnitude of global warming over the last 100 years, but it was also likely at least 10 times slower than what could happen in the future. Such rapid changes in conditions place greater stress on ecosystems, since not all of the individual species that make up the ecosystem will be able to adapt or migrate at the same speed. In addition to unprecedented rates of warming, species now face human-caused fragmentation of the landscape and other barriers to migration, invasive species, groundwater and stream flow reductions, pollution, and other pervasive human influences that will inhibit the natural ability of ecosystems to adjust to climate change.