Human activities are significantly perturbing all of these biogeochemical cycles as well as other earth system processes, both directly through industrial processes and indirectly through changing distributions and abundance of life. The atmosphere is of particular importance to the perturbations due to its crucial role in mediating all energy that enters and leaves earth. Overall, the atmosphere is the component that controls the dominant energy flow in the earth's climate system, and solar radiation from the sun provides the energy to make the weather machine work. Embedded in this process are the biogeochemical cycles we have described that operate on a variety of time and space scales and help to regulate flows of energy and materials throughout the earth system (Figure 1). Yet, while we understand much about the functioning of separate parts of this system, there is still a great deal to be discovered about the feedbacks and linkages that allow these interconnected parts to function as a whole and, in turn, how they will respond to human modification.

Life influences the amount of CO2 in the atmosphere through photosynthesis, respiration, and oceanic absorption. As ecosystems are altered, the balance of these processes will be altered. Human activities are upsetting this balance and increasing CO2 in the atmosphere through the burning of fossil fuels and clearing of forests.A significant increase in CO2 could have dramatic consequences. Mathematical models of the climate suggest that when CO2 (or its heat-trapping equivalent in other greenhouse gases) doubles (sometime in the middle of the next century should population, economic and technology trends continue as typically projected), the world will warm up somewhere between 1 and 5 degrees Celsius by 2100 A.D. unless other factors counteract or amplify the CO2 - induced change (IPCC, WGI 1995).

Even the lower end of that range is a projected warming at the rate of one degree per hundred years, a factor of ten faster than the one degree per thousand that has been the typical average rate of natural sustained global temperature change from the end of the ice ages to warmer interglacial times. Should the higher end of the 1 to 5 degree Celsius range occur, then we could see rates of climate change some fifty times faster than sustained, natural average conditions. Climate largely determines the types of ecosystems that occupy an area. Global climate change at such a rapid rate would force many species to shift their ranges in an attempt to keep up with changing climatic conditions, as occurred during the ice age - interglacial transition ten to fifteen thousand years ago. Migrations of species such as slow growing trees with large seeds would have to occur much faster than they did in the past to keep up with rapidly shifting climates. Other species could move more easily, raising the likelihood that communities of species could be disassembled (e.g. Root and Schneider, 1993). Estimating the rates of global warming in the next century, however, are very controversial because of the uncertainties involved with multiple interacting feedback mechanisms (IPCC, WGI 1995).

Humanity can control climate in ways other than changing greenhouse gas concentrations. Consider the amount of moisture released to the atmosphere through transpiration in the tropical rainforests. The dense vegetation in areas such as the Amazon basin typically recycles the precipitation that falls on it many times over, helping to form heavy cloud cover in the region. The clouds, in turn, reflect sunlight and produce more rain, directly influencing regional climate as well as indirectly perturbing global climate through altering large-scale circulation patterns over the tropics. As humanity deforests regions like the Amazon, not only is CO2 released into the atmosphere, but changes in the hydrologic cycle will almost certainly affect regional climate and possibly even global climatic patterns. In deforested areas of northeastern Brazil, the cutting of the tropical forests has led to desertification, changing both surface reflectivity and the rate of transpiration. This change in ecosystem character can lead to a destabilizing positive feedback, which may cause an even further reduction in precipitation.

In a recent study on the possible climatic impacts of tropical deforestation, researchers suggest that conversion of forest into crop land or pastures would cause significant changes in the local microclimate (Salati and Nobre, 1991). Expected changes include reduction in soil moisture, larger diurnal fluctuation of surface temperature and humidity deficit, and increased surface runoff during the rainy season and decreased runoff during the dry season. Results from general circulation model simulations of large-scale deforestation and conversion to grassy vegetation in the Amazon basin indicate an increase in surface temperature, decrease in evapotranspiration, and significant reduction in precipitation (Lean and Warrilow, 1989; Shukla, Nobre, and Sellers, 1990). Depending on the scale of the disturbed areas, local climate changes can lead to regional climate changes which, in turn, may cause alterations in the global climate through atmospheric connections between tropical circulation and large-scale circulation patterns outside of the tropics. The effect on the ecological systems through changes in the hydrological cycle, an increase in the dry season, and the disruption of plant-animal interactions may make it difficult for the rainforests to re-establish themselves if they are destroyed. Climate change aside, the implications of this scenario for the conservation of biodiversity are serious.

The provision of fresh water and regulation of its flows through precipitation, evaporation, transpiration, and run off is mediated by all ecosystems. Forests and other vegetation types are critical components of this ecosystem service providing free flood and drought relief among other things. The loss of these services, through landuse change, can exacerbate disasters like spring floods in the Midwest and Southeast resulting from large expanses of land cleared for agriculture as well as the drainage of wetlands and swamps which otherwise might have acted as reservoirs for holding excess water or filtering toxic wastes.

The combination of potentially very rapid rates of human induced climate change at the same time natural habitat has been fragmented for agriculture and development activities, and assaulted with a host of chemical agents is unprecedented. It is for these reasons that it is essential to understand not only how much climate change is likely, but just as importantly, how to characterize and analyze the value of the ecosystem services that might be disrupted. How the biosphere will respond to human-induced climate change is fraught with uncertainty. One thing that is clear is that life, biogeochemical cycles, and climate are linked components of a highly interactive system. An illustration of this linked behavior can be seen in the simultaneous variation of CO2, CH4, temperature, and SO4 2- found over time in Antarctic ice cores (see Charlson et al., 1992). Temperature, CO2, and CH4 are positively correlated with one another, while each are negatively correlated with SO4 2-. More recent data of N2O, CH4, and CO2 over the past 300 years show an increase in these trace gases that matches the magnitude of the changes in composition that occurred between the ice-age and interglacial periods. This change in composition causes more heat to be trapped near the earth's surface. Since the Industrial Revolution the build-up of these and other greenhouse gases has increased the flow of energy to earth's surface by an average of roughly two watts per square meter. Climatologists also generally agree that the global air temperature at the surface has warmed up on average approximately 0.5 +/- 0.2 degrees Celsius in the past century. It is this rate of change that appears very large compared to the sustained temperature changes from the ice ages to the interglacials in recent earth history.

Uncertainties become more significant when considering projections of climatic impacts. The combination of increasing population and increasing energy consumption per capita is expected to contribute to increasing CO2 and sulfate emissions over the next century, but projections of the extent of the increase are very uncertain. Central estimates of emissions imply a doubling of current CO2 concentrations by the middle of the 21st century, leading to typical projected warming ranging, as mentioned earlier, from 1 degree to more than 5 degrees Celsius by the second half of the 21st century. Warming at the low end of this uncertainty range could still have significant implications for species adaptations, whereas warming of 5 degrees Celsius or more could have catastrophic effects on natural and managed ecosystems, produce serious coastal flooding, and involve other impacts on natural and human systems. The overall cost of these impacts in "market sectors" of the economy could easily run into many tens of billions of dollars annually (Smith and Tirpak, 1988; IPCC, WG II 1995). Although fossil fuel use contributes substantially to the cause of the impacts, associated costs are not included in the price of conventional fuels; they are externalized. Internalizing these environmental externalities into economic benefit-cost analyses (see Goulder and Kennedy, chapter X, this volume) is a principle goal of international climate policy advocates. We now turn to analyzing a few of the specific ecosystem services that link climate and life, and use the subjective probabilities of potential climate change impacts to provide a crude metric for assigning dollar values to certain aspects of these services.

On to Economic Analyses!