Methodology For Impact Assessment Of Climate Change Scenarios

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Unlike assessments of current and historical events, impact assessment of projected climate change is based on scenarios, or plausible pictures of the future. This kind of research can include analysis of past observed events (e.g., the Dust Bowl in the United States during the 1930s), which could serve as societal analogs of a future warm climate (Glantz, 1988), or as climate change analogs superimposed on twenty-first-century society (Rosenberg, 1993), but there are also many studies at regional and global scales that use climate change scenarios as part of a forward-looking exercise to estimate future impacts.

Case studies of future climate scenarios have often been based on the outputs of climate models (IPCC, 1996a,b; Carter et al, 2000). These models (general circulation models or GCMs) produce estimates of climatic variables on a regular network of grid points for a base case (i.e., current climate), and as an "equilibrium" response to a doubling of carbon dioxide concentrations, or as a "transient" response over 50 years or more of incremental increases in carbon dioxide. Most assessments have taken the difference between the equilibrium or transient simulations and the base case simulation and combined this simulated "change" with baseline climate information from actual observations to produce a scenario (Carter et al, 1994, 2000).

These scenarios of changes in temperature, precipitation, and other elements are used as inputs to other analytical tools that would convert these climate changes to first-order changes in landscapes, ecosystems, renewable resources, and disease rates. Examples include (a) hydrologic models for streamflow and lake levels, (b) crop models for grain yields, (c) fire indicators for forest fire potential, and (d) pest indicators for seasonal ranges of insects. Outputs of first-order impact assessments have been applied to economic models (e.g., timber yields, food production, hydroelectric generation) to estimate impacts in monetary terms (IPCC, 1996a, 2001b).

There have also been attempts to combine these individual estimates into regional or national impact assessments. Scaling up from sectoral to national assessments adds complexity and uncertainty to an already difficult assignment. Economies and societies are composed of many stakeholders whose actions are not easily amenable to modeling. Governments, industries, and individuals may choose various response strategies depending on their knowledge and perceptions of the climate change issue and other forces of concern (e.g., population growth, changes in global trading patterns, and technology).

At the same time, concerns raised by atmospheric scientists about greenhouse gas emissions has led to international negotiations to establish a global strategy to reduce emissions. The main policy instrument, the United Nations Framework Convention on Climate Change (UNFCCC) has been ratified by more than 150 countries, and the emission targets for six greenhouse gases have been tentatively established for more than 30 industrialized countries by a recent agreement known as the Kyoto Protocol (negotiated in December, 1997, but not yet ratified). Critics of this agreement have argued that these targets will cause severe economic losses to most of these countries because, in their view, economic growth is directly linked to growth in energy consumption and hence increased greenhouse gas emissions. Two other concerns are raised as well: (a) global warming may not happen as predicted since there are uncertainties in climate models and their projections may be wrong, and (b) since societies in the past flourished during warm periods (e.g., medieval warm epoch), global warming (if it occurs) would actually be a good thing. Although the IPCC (1995, 1996a,b, 1998, 2001a,b) has published estimates of projected climate change, impacts of climate change scenarios, and impacts of greenhouse gas emission reductions, the rapid policy response has created a substantial new research challenge—determining the impacts of various policy options and compar ing these with the costs of doing nothing about emissions. Given the magnitude of the climate change problem and proposed responses to this, the demand for answers will be high.


Estimates of climate change impacts are summarized for several key sectors in Table 1. In fisheries and health, higher levels of impacts are estimated for developing countries, reflecting their vulnerabilities to climate change. Impacts in agriculture would result from damage due to heat stress, decreased soil moisture, increased incidence of pests and disease, and changes in plant growth cycles; but this could be offset in some circumstances by longer growing seasons and CO2 fertilization. Some developing countries, however, could experience significant increases in population at risk from hunger. Coastal zone costs are high in many regions, reflecting the growth in built structures in areas vulnerable to sea-level rise and extreme events, as well as land loss itself (e.g., coastal wetlands). Estimates are also available for changes in water supply, wetlands, electricity demand, and some other sectors (IPCC, 1996a,b).

In all cases, scenario estimates are dependent on a variety of assumptions about regional changes in climate (downscaling from GCMs), indirect effects of climate change (e.g., C02 fertilization effects), technological change, population growth, changes in infrastructure (e.g., health services in developing countries), and responses of stakeholders to other issues besides climate change (e.g., changing international markets). Table 1 should be considered as a first attempt at determining

TABLE 1 Range of Sectoral Impacts for Different World Regions (2.5°C warming scenario)0

Coastal Zone







(number of


(% loss in

(area lost,

costs 106







1000 s)

1000 ton)

European Union






United States




































GDP, gross domestic product; FSU, former Soviet Union; OECD, Organization of Economic Cooperation and Development (developed countries); non-OECD, developing countries. Source: Adapted from IPCC (1996b). Table 6.5

GDP, gross domestic product; FSU, former Soviet Union; OECD, Organization of Economic Cooperation and Development (developed countries); non-OECD, developing countries. Source: Adapted from IPCC (1996b). Table 6.5

TABLE 2 Overall Annual Economic Impacts in Different World Regions for a 2.5°C Warming Scenario (% of current GDP)a


IPCC, 1996b

IPCC, 2001

Developed countries

-2.8 to -1.3

-2.8 to 0.3


-0.7 to 0.3

-0.7 to 11.1

Developing countries

-8.7 to -4.1

-4.9 to 1.8


-1.9 to-1.4

-1.5 to 0.1

Source: Adapted from IPCC, 1996b (Table 6.6) and IPCC, 2001b (Table 19-4).

Source: Adapted from IPCC, 1996b (Table 6.6) and IPCC, 2001b (Table 19-4).

impacts, and changes in such estimates should be expected as new information becomes available.

Regional impact costs are summarized in Table 2. The range of uncertainty cannot be gauged from the existing literature, nor can the range of estimates provide a confidence interval. Some costs are hidden because of aggregation of communities and nations into large regions and because of lack of information on potential impacts of climate change scenarios on construction, insurance, nontropical extreme events (e.g., midlatitude river floods), transportation, and political institutions.

This work is still in its infancy, and economists have to make assumptions about markets, trading patterns, technological change, adaptation (direct, indirect), and the availability of information. Uncertainties associated with converting impacts into monetary units are due to many factors. One of the most controversial is the cost of health impacts. Is a premature death due to climate change (e.g., heat stress, tropical disease, etc.) worth the same monetary value if it were to occur in a developed or developing country? Since average incomes differ, initial estimates have used a higher "value of a statistical life" for a premature death in a developed country than in a developing country (IPCC, 1996b, Chapter 6). More recent assessments focus on global and regional development trends and their effects on vulnerability to climate change (IPCC, 2001b). These trends may increase adaptive capacity in some circumstances (e.g., community health), but may decrease it in others (e.g., protection of endangered species).

The dilemma of placing a value on life and death is an illustration of the political and social dimensions of the potential impacts of global climate change. Others include (a) the presence of different stakeholders with different visions and goals, (b) issues related to cultural preservation (especially in developing countries and the Arctic), (c) the influence of trade globalization on management of climate-sensitive resources (e.g., fish, water resources), (d) the market value of ecosystems (e.g., wetlands, rain forests, alpine tundra), and (e) intergenerational equity and the choice of discount rates (i.e., a percent change in monetary value due to depreciation, inflation, etc.) used in economic valuation of climate change damages and actions. All of these can influence development choices, including adaptation choices. Global climate change may have started as a theoretical problem of atmospheric science, but the transmission of information to the public has taken this issue outside of the laboratory and into the real world.


Climate impact research is a relatively young endeavor. The term climate impact assessment was coined only m the 1970s (Munn, 1979; Kates et al., 1985), and case studies of climate change scenarios have been undertaken for less than 20 years. Advances have been made in incorporating various natural and social science disciplines into the effort, and some important lessons have been learned in the process:

1. There is more than one scenario of future changes for any region, regardless of any scenarios of climate change.

2. There is more than one stakeholder, and one cannot assume that while temperatures change, stakeholders will continue historic patterns of activities. Historic and future decisions result from trade-offs and consensus reached by a broad array of decision makers with different visions.

3. Although there are some preferred options for assessing impacts on sectors, there is no single best method available to provide impact assessments for places. Parallel sectoral assessments have been the approach of choice in most countries, but some integration exercise needs to become part of this process. Integrated assessment models have become a highly visible option, but these tend to focus more on mitigation and are relatively weak on the impacts and adaptation dimensions, so alternative methods will continue to be important contributors.

Societal aspects of global climate change represent a significant interdisciplinary research challenge, in which atmospheric science and scientists will continue to play an important role. This collaboration will be beneficial for advancing the science as well as for providing better information for stakeholders as they grapple with the human dimensions of this issue.

As consumers of climate information, both from observations and models, researchers on impacts of and responses to climate change uncertainties represent a different type of client than those who work in the atmospheric sciences. This group needs value-added information that can be used as input to other analytical tools, which may or may not have been designed with climate as an explicit input element. Indeed, these tools (e.g., crop yield models, water management models) may be calibrated only to current climate conditions, and their response to climate change scenarios outside their calibration range represents one of many uncertainties in this process. Some of these tools require considerable amounts of data, often at spatial scales too fine to be visible in current GCMs. Impacts researchers are following the progress of downscaling activities with considerable interest (e.g., regional climate models, statistical techniques), and perhaps this can provide incentive to atmospheric scientists to continue research and development in this area. In the meantime, however, the urgent need for impacts information, as well as for testing and developing methodologies for impact and response assessments, means that currently available methods for scenario construction will continue to be used.

There will also be continued demand for assessments of historic events. Some of these events [e.g., effects of warm years, droughts (El Nino-Southern Oscillation (ENSO)] may serve as possible analogs of future climate change, but important questions arise. How can impacts and costs be attributed? Were these due to the climatic event (at what scale), or to changing vulnerabilities, or both? Did regional or global forces cause the climatic event, and was it consistent with modeled scenarios of future climate changes? The recent increase in insurance losses (Munich Re, 2000) is an example of an observed series of events that would benefit from such an analysis.

Finally, the Kyoto Protocol presents a challenge and an opportunity for new research into the potential impacts of measures to reduce emissions and improve adaptive capabilities. The decision to ratify or not ratify this agreement will be taken on the basis of technical information balanced against preset stakeholder interests. Atmospheric science, in collaboration with researchers from many other disciplines, will play an important role in determining the benefits and costs of various response scenarios.

Past and present impacts have occurred over landscapes and populations that are changing for many reasons. Such changes affect vulnerabilities and costs, as well as responses (e.g., changing land use, insurance programs). Future impacts will be influenced by global economic and institutional changes (e.g., globalization of trade) as well as policy initiatives at various scales. Stakeholders' responses will be determined by attitudes and beliefs about the importance of climate change in the context of other challenges. Atmospheric scientists were the first to call attention to global climate change. Now that this has become an international policy concern, there will be greater demands to make scientific views known not only in the traditional refereed literature, but in broader public forums as well.


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