|Home Library Strategic Plan 2003 Review Draft, November 2002 Chapter 2: Research Focused on Key Climate Change Uncertainties|
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The Climate Change Research Initiative (CCRI) will address key and emerging climate change science areas that offer the prospect of significant improvement in understanding of climate change phenomena, and where accelerated development of decision support information is possible. The purpose of CCRI accelerated science activities is to enhance the ongoing US Global Change Research Program (USGCRP) elements described in Part II where focused effort would rapidly lead to critical decision support information. At the request of the President, the National Research Council (NRC) identified "the areas in the science of climate change where there are the greatest certainties and uncertainties," (NRC, 2001a). This section outlines three key areas where the CCRI will address the specific uncertainties identified by the NRC, including: atmospheric concentrations of aerosols (see also Chapter 5); North American carbon sources and sinks (see also Chapter 9); and climate feedbacks and climate system sensitivities (see also Chapters 5, 6, and 7).
1. What aerosols are contributing factors to climate change and what is their relative contribution to climate change?
Aerosols play a unique role in the Earth's radiation (energy) budget, and scientists believe they play a large part in global and regional climate changes. However, because aerosols have a relatively short atmospheric residence time, have a spatially and temporally heterogeneous (non-uniform) distribution, and include a complex mixture of substances from numerous sources (e.g., black carbon, sulfate), there are substantial uncertainties in quantifying their role.
The Climate Change Science Program (CCSP) plan in Chapter 5 and the National Aerosol-Climate Interactions Program plan (NACIP, 2002) emphasize the importance of characterizing the distribution of all major aerosol species and their spatial and temporal variability, the separate contributions of aerosols from various anthropogenic activities and natural sources, and the processes by which the separate sources are linked to the global distribution of aerosols and their radiative characteristics.
Enhanced aerosol-climate research is needed to deliver focused information within 2-4 years that would be helpful in quantifying the role of aerosols in regional and global climate change in decision-relevant terms. The following research emphases will allow more meaningful assessments and projections, with a focus on decision-relevant products by 2006.
Intensive research to quantify and explain the processes controlling North America's carbon sources and sinks is a near-term priority. Accelerated research within the overall framework of the North American Carbon Program (NACP) will address fundamental questions relating to the buildup of CO2 and methane (CH4) in the atmosphere, and the fraction of fossil-fuel carbon being taken up by North America's ecosystems and coastal oceans.
Investments over the past decade have resulted in an unprecedented opportunity to study the carbon cycle over a scale not previously attempted -- that of continents and ocean basins. Observational capabilities such as the US forest and soil inventories, flux and tall tower networks, Atlantic and Pacific ocean time series and ships of opportunity, and vegetation and ocean color remote sensing have all contributed to a better understanding of components of the carbon cycle for North America and adjacent ocean basins. Current estimates of regional distributions of carbon sources and sinks derived from atmospheric and oceanic data and models differ from forest inventory and terrestrial ecosystem model estimates. Scientific understanding has now progressed to the point where targeted investments can yield major returns within five years. The CCRI will accelerate the observational, experimental, analytical, and data management activities needed to address uncertainties, reduce errors, and produce a consistent analysis of carbon sources and sinks for North America.
The integrated NACP requires enhanced observational networks and improved monitoring techniques; studies of key controlling processes and resource management regimes that regulate carbon storage and fluxes; modeling that integrates among atmospheric, land, ocean, and human systems; and periodic reporting. Priorities for an accelerated initial phase are:
An intensive regional-scale field program is needed and could begin as early as 2004. It will require in situ observations and process studies, intensive aircraft and remote sensing surveys, enhanced inventories, and modeling. It is also needed as a test bed for subsequent continent-wide implementation of the NACP. The NACP will leverage existing agency research activities and observational programs, but will require additional targeted investments to achieve the desired near-term results.
Accelerated research within the NACP will provide near-term information for decision support, scenario analysis, and carbon management. Results of this research will also establish the scientific underpinning needed to evaluate carbon management in US croplands, forests, rangelands, soils, and coastal systems and to support analyses of greenhouse gas trends and net emissions intensity. These results will contribute to decision analysis of the impacts of various resource management policies.
Water plays a key role in the radiative balance of the atmosphere: water vapor is the most important of the greenhouse gases, and clouds (whether liquid or ice) affect both vertical heating profiles and geographic heating patterns. In addition, results from climate models suggest there will be an overall increase in water vapor as the climate warms.
Predictions of climate change vary in large part because of differences in the way that the various feedback processes are represented in the models. The greatest differences are those associated with water vapor and cloud processes. For example, scientists do not know how the amount and distribution of clouds will change, both vertically and horizontally, as the water vapor in the atmosphere changes. More importantly, they do not know how the associated changes in radiative forcing and precipitation will affect climate. The feedback to the Earth's radiative balance and cloud structure from increased upper tropospheric water vapor is potentially quite large and could be positive or negative.
Basic understanding of the processes that control the atmospheric water vapor and clouds must be improved and incorporated into models. Better representation of the distribution of water vapor is critical given its contribution to temperature increases as an active radiative gas, as well as its role in cloud formation. Because the physical processes responsible for the horizontal and vertical transport of water vapor and cloud formation occur at scales that are not resolved by climate models, they must be parameterized (simplified for incorporation in the models). New, integrated, three-dimensional data sets of cloud properties and water vapor will be produced to reduce uncertainties due to the representation of clouds and water vapor in climate models. A combination of these data sets, new observations, and targeted process studies will be developed with a focus on model improvements.
While the studies described here will substantially improve understanding of feedbacks, other studies proposed as part of A Plan for a New Science Initiative on the Global Water Cycle (Hornberger et al., 2001) and the CCSP Strategic Plan (see Chapter 7) will be critical to predicting the impact of climate change on precipitation and water availability, for example, determining long-term trends in the global water cycle including the character of hydrologic events and their causes; developing the ability to bridge climate and weather modeling; and determining the relationship between the water cycle and biogeochemical/ecological processes.
The polar regions, particularly the Arctic, are especially sensitive to changes in climate, and models consistently predict future warming to be much more significant in these regions than elsewhere. This sensitivity arises primarily from the positive albedo (how much radiation is reflected by the surface) feedbacks associated with melting of snow and ice that blanket most of the region, which can as much as triple the amount of absorbed solar radiation. Compounding this sensitivity is the fact that sea ice cover modulates the exchange of heat and moisture between the ocean and atmosphere. The disappearance of insulating sea ice increases the transfer of energy and water vapor from the ocean to the atmosphere, enhancing atmospheric warming. Furthermore, Arctic soils serve as significant reservoirs of CO2 and CH4, and warming of the region could result in increased emission of these greenhouse gases, contributing to the carbon cycle in ways that are not yet clear.
In addition to high-latitude precipitation, and freshwater discharge from melting snow and ice, sea ice cover plays a major role in the Atlantic thermohaline circulation (controlled by temperature and salinity variations) in the Arctic, and the formation of Antarctic bottom water in the Southern Hemisphere. These are two dominant factors in ocean circulation that directly influence climate throughout the world. It is unclear how future polar climate changes, in particular changes in sea ice cover, will affect these oceanic drivers of the global climate system. In the case of the Arctic, for example, it is possible that increased surface freshening (reduced salinity) associated with enhanced melting and precipitation may suppress the overturning in the North Atlantic Ocean, which may lead to major abrupt changes in climate, such as has been observed in paleoclimatic data.
Ice on land is of critical importance for climate and sea level. The Greenland and Antarctic ice sheets contain enough ice to raise sea level by more than 70 meters (230 feet). The smaller glaciers and ice caps contain the equivalent of only about 0.5 meters (1.6 feet) of sea level rise, but they are far more susceptible to near-term changes and are disappearing rapidly. While global sea level is currently estimated to be rising at a rate of nearly 2 millimeters (0.08 inches) per year, there is evidence that in the past sea level has risen by as much as 50 millimeters (2 inches) per year in some locations. Such rapid rises, consistent with recently discovered abrupt climate changes, can only be attributed to changes in the Earth's larger ice masses. Given the potential economic consequences of sea level rise, there is a pressing need to understand changes in the amount of ice stored on land, and the mechanisms that drive these changes.
Representation of polar climate in climate models is not as advanced as that of the lower latitudes. This arises in part because of the limited data available for model development, refinement, and validation, and a limited understanding of the processes at work. An enhanced observation system and the use of existing and future satellite data sets should improve the representation of these areas in climate models, which is necessary to accurately predict future climate changes and assess the potential for these changes to be abrupt.
Warming temperatures may also affect Arctic land areas. If continuous permafrost areas become discontinuous and discontinuous areas experience complete summer thawing, the hydrology of northern land areas would be substantially altered. Many of the wetlands, marshes, and perched lakes in the Arctic are underlain by permanent ice. The reduction of this ice would lead to the infiltration of the water into the soil and widespread changes in vegetation patterns. The release of greenhouse gases such as CH4 associated with wetlands would expand in areas where melt water resulting from deeper and longer thaw periods does not have a natural drainage path to the ocean.
Warming could also lead to changes in the water cycle in polar regions. Reducing the uncertainties in current understanding of the relationships between climate change and Arctic hydrology is critical for evaluating the potential impacts of climate change on Arctic communities and their infrastructure. Further, a better understanding of these relationships may allow the development of monitoring procedures that use changes in the Arctic as a signal of the progress of global climate warming.
Hornberger et al., 2001. Hornberger et al., A Plan for a New Science Initiative on the Global Water Cycle (Washington, D.C., US Global Change Research Program).
IPCC, 2001. Intergovernmental Panel on Climate Change, Climate Change 2001. Third Assessment Report of the IPCC. (Cambridge, United Kingdom, and New York: Cambridge University Press). Includes:
NRC, 2001a. National Research Council, Committee on the Science of Climate Change, Climate Change Science: An Analysis of Some Key Questions (Washington, DC: National Academy Press).