Interrelations of global climate change and Earth's ecosystems.
Jerry M. Melillo*
Science
Jan 8 1999: 183-184.The climate of Earth is changing. Climatologists are confident that over the past century, the global average surface temperature has increased by about half a degree Celsius (1). This warming is thought to be at least partly the result of human activities, such as the burning of fossil fuels and the clearing of forests for agriculture. As the global population grows and national economies expand, the global average temperature is expected to continue increasing by an additional 1.0 to 3.5oC by the year 2100 (1).
Climate change is one of the most important environmental issues facing humankind. Understanding the potential impacts of climate change for natural ecosystems is essential if we are going to manage our environment to minimize the negative consequences of climate change and maximize the opportunities that it may offer. Because natural ecosystems are complex, nonlinear systems, it follows that their responses to climate change are likely to be complex. Climate change may affect natural ecosystems in a variety of ways (see figure). In the short term, climate change can alter the mix of plant species in land ecosystems such as grasslands. In the long term, climate change has the potential to dramatically alter the geographic distribution of major vegetation types--savannas, forests, and tundra. Climate change can also potentially alter global ecosystem processes, including the cycling of carbon, nitrogen, phosphorus, and sulfur. Moreover, changes in these ecosystem processes can affect and be affected by changes in the plant species of the ecosystem and vegetation type. All of the climate change-induced alterations of natural ecosystems affect the services that these ecosystems provide to humans.
Interrelations of global climate change and Earth's ecosystems.
The global average surface temperature increase of half a degree Celsius observed over the past century has been in part due to differential changes in daily maximum and minimum temperatures, resulting in a narrowing of the diurnal temperature range. Decreases in the diurnal temperature range were first identified in the United States, where large-area trends showed that maximum temperatures have remained constant or increased only slightly, whereas minimum temperatures (TMIN) have increased at a faster rate (2). On page 229 of this issue, Alward et al. (3) report on the different sensitivities of rangeland plants to TMIN increases.
On the basis of a decade of measurements at the National Science Foundation's (NSF) Long-Term Ecological Research site in the short-grass steppe in northeastern Colorado, Alward et al. concluded that increased spring TMIN was correlated with a reduction in the abundance of buffalo grass, Bouteloua gracilis, and an increase in native and exotic forbs. This alteration in species composition of the rangeland affects its ability to provide an ecosystem service that ranchers have come to rely on--the availability of a productive, palatable, drought-resistant grass, buffalo grass, which is important to livestock production in the region.
From their work at the Toolik Lake site in the Alaskan arctic (another NSF Long-Term Ecological Research site), Chapin et al. have also reported that climate change can alter plant species composition (4). Over a 9-year period, they increased the mean daily air temperature above the vegetation by 3.5oC at a tussock tundra site by placing clear plastic tents over the vegetation. One of the major effects of the warming was to increase the availability of nitrogen to plants by speeding up its release from decaying organic matter. The enhanced nitrogen availability increased the dominance of the four plant species that were initially most abundant and decreased abundance of (or eliminated) plants that were initially least abundant, including forbs and lichens. Forbs in the tundra are nutritionally important and selectively grazed by caribou during lactation, whereas lichens are critical to the over-winter nutrition of caribou. The loss of forbs and lichens from the tundra as a result of climate change could lead to reductions in the caribou herds that are important to the lives of Alaska's native peoples.
Over decades to centuries, climate change may cause large-scale alterations in the distribution of major vegetation types such as grasslands and forests. Global-scale simulations (the new dynamic global vegetation models, for example) predict how changes in climate parameters such as maximum and minimum temperatures and the spatial and temporal patterns of precipitation affect the distribution of major vegetation types across the globe (5). Using these models with scenarios of future climate change, researchers have identified many potential consequences of large-scale vegetation shifts. The composition of one-third of Earth's forest could change markedly as a result of climate changes associated with a doubling of atmospheric CO2. Over the next 100 years, the ideal range for some North American forest species could shift as much as 300 miles to the north. Economically and aesthetically important species, such as the sugar maple, could be lost from New England by the end of the next century (6).
One of the most robust predictions of the new dynamic global vegetation models is that by 2100, boreal forest will occupy a substantial portion of the land that is now covered by tundra vegetation. Because boreal forests absorb much more solar radiation than tundra does, poleward shifts in the location of the forest-tundra boundary during a period of warming can amplify climate changes by as much as 50% (7).
Researchers are using three main approaches for investigating the consequences of climate change on natural ecosystems. Through long-term observations, including those taken from the paleorecord, scientists have begun to detect some of the effects of warming on the structure and function of natural ecosystems. Through experimental manipulations of the environment and the use of ecological simulation models, scientists are gaining insights into possible future consequences of warming and other aspects of climate change for our life support system, the biosphere. These three approaches--observation, experimental manipulation, and simulation modeling--are complimentary and are all needed to improve our understanding of the consequences of climate change for Earth's life support system, the biosphere.
References
The author is in The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA
02543, USA. E-mail: jmelillo@mbl.edu