Climate Change Over The Next Few Decades
Human society is highly dependent on the Earth's climate. Climate patterns and human adaptations determine the availability of food, fresh water, and other resources for sustaining life. The social and economic characteristics of society have also been shaped largely by adapting to the seasonal and year- to-year (interannual) patterns of temperature and rainfall. While anomalous variations in these shorter time scale patterns can have serious effects on society, the vulnerability of society to longer-term climate change, occurring over periods of decades to centuries, will depend on its ability to understand and respond to this change. Thus, it is imperative that society develop the strongest possible scientific understanding of the causes and dynamics of climate change and greenhouse warming, the potential ecological and socioeconomic impacts of change, and the implications of alternative courses of action to mitigate and adapt to change.
Scientists have determined that climate can be influenced by both natural forces and human activities. For example, habitable temperatures are maintained on Earth by a natural phenomenon known as the "greenhouse effect." Solar radiation is absorbed by the Earth's atmosphere and land and water bodies. The resultant heat is re-emitted as long-wave radiation, some of which escapes to space and some of which is absorbed and trapped by atmospheric gases such as water vapor, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and ozone. While some of these gases are present naturally, increased concentrations of greenhouse gases as a result of human activity can enhance this natural greenhouse effect, creating additional warming of the surface and the atmosphere(see figure). Human activities such as fossil fuel combustion and land-use change have resulted in a 30% increase in atmospheric carbon dioxide and have contributed to more than a doubling of the methane concentration since preindustrial times.
Human-induced and natural changes in global land cover (such as deforestation and desertification) and emissions of aerosols (from fossil fuel burning and volcanic eruptions) also influence climate. Although much has been learned, there are still significant improvements to be made in estimating how human activities will combine with natural influences to affect the future global climate.
IPCC assessments conducted with the participation of thousands of scientists from more than 150 countries and with significant USGCRP participation, suggest that emissions of greenhouse gases and sulfate aerosols could, by the end of the next century, lead to an increase in global mean temperatures of about 1-4°C, a rise in sea level, and a global change in precipitation patterns. Among other uncertainties, the magnitude and location of projected shifts in global precipitation patterns remain particularly difficult to predict.
Observational data show an increase in global average temperature of about 0.5°C over the last 100 years. The likelihood that this warming is due primarily to natural variability is low. This observed warming trend is continuing despite the influence of the Mt. Pinatubo volcanic eruption, which caused volcanic emissions to reduce incoming solar radiation for nearly two years. The most recent climate model simulations have been able to explain the magnitude and temporal pattern of this observed trend reasonably well. To build greater confidence in predictions of future climate, further improvements are needed in modeling the influence of atmospheric aerosol concentrations, the cycling of atmospheric water in all its phases, cloud-radiation interactions, ocean-atmosphere coupling, and in predicting the expected range of natural climate variability.
Projected climate change over the next few decades, including changes in temperature, precipitation, and sea level, can add to other stresses on natural systems caused by other factors such as population growth, land-use changes, and pollution, posing risks to managed and unmanaged resource systems (see Potential Impacts on Various Sectors Associated with Climate Change Predictions over the Next Century). Although temperature changes of the magnitude expected from the enhanced greenhouse effect have occurred in the distant past, the evidence suggests that the changes generally took place over centuries or millennia instead of decades. Because rates of natural migration and adaptation of species and communities appear to be much slower than may be forced by the predicted rate of climate change, populations of many species and inhabited ranges could decrease as the climate to which they are adapted effectively shifts northward or to higher elevations.
Overall, various strategies for coping with climate change can be identified for "intensively managed" systems (such as agriculture, water resources, and developed coastlines). For these systems, technological and management options exist to some extent today, although they may be costly to implement. By comparison, fewer options have been identified for natural systems such as wetlands and wilderness areas.
Changes in climate may also have significant impacts on human health. These impacts may include increases in mortality and morbidity as a result of a higher frequency of heat waves and synergistic effects from higher temperatures and air pollutant mixtures (higher temperatures may cause changes in urban air chemistry). There is also evidence that a changing climate will cause a migration into higher latitudes and altitudes of some diseases, such as malaria and dengue, the incidence of which is highly correlated with rainfall and elevated nighttime temperatures.
Proposed Future Research on Climate Change over the Next Few Decades Highlights of USGCRP research in FY 1996 include programs to:
- Continue to reconstruct past climates of the Earth to improve understanding of the dynamics of climate change by further analyzing ice core data and other historical records relevant to understanding of the Earth's past climates and climate changes, and comparing simulated climates with the geologic records to evaluate model performance.
- Analyze observational evidence of changes in the climate system over the past century to better distinguish natural variability from the influence of human activities. In addition to space-based and in-situ observations, existing climate data will be reanalyzed for trends and patterns of change and compared with the results of model simulations.
- Evaluate natural and human-induced factors that lead to climate change, including the sources, sinks, and lifetimes of greenhouse gases and atmospheric aerosols, and their collective effects on the Earth's radiation balance.
- Refine predictions of global climate change over the next few decades by extending model simulations to comprehensively link oceanic, atmospheric, terrestrial, glacial, and sea-ice influences and to include improved representation of important processes, such as cloud- radiation and chemistry interactions.
- Assess the vulnerability of society to the projected patterns and rates of climate change at the regional, national, and international levels. Conduct research to assess the vulnerability of Earth systems, including economic, human health, and ecological systems, to the predicted rates and magnitudes of climate change.
- Develop new integrated assessment tools to bring together complex information and research results from natural, social, and policy sciences into an evaluation framework.