What are scenarios for assessment of climate change impacts and potential responses?

Scenarios are quantitative and narrative descriptions of plausible future conditions that provide assumptions for analyses of potential impacts and responses to climate change. Scenarios are ways to help understand what future conditions might be, with each scenario an example of what might happen under different assumptions. Scenarios are not predictions or forecasts, and no probabilities are associated with them. Instead, they provide a range of future conditions to bound uncertainty. The scenarios here include climate, sea level change, land use, and socioeconomic conditions. They are based on peer-reviewed, published sources including materials prepared by the Intergovernmental Panel on Climate Change (IPCC).

These scenarios are developed using a wide range of assumptions about population growth, economic development, the evolution of technology, and decisions about environmental protection, among other factors. These assumptions give rise to different patterns of energy production and use, agriculture, and other activities that change land use, which in turn lead to different levels of the greenhouse gas emissions that cause the human component of climate change.

For more information on the scenarios selected for the Fourth NCA (due in 2018), please refer to this technical memo. Further information on the new scenarios will be provided on this site as soon as it is available.

Most of the information currently available on these pages was developed for the Third National Climate Assessment (2014). For that report, two emission levels (developed by the IPCC Special Report on Emission Scenarios [SRES]) were used to illustrate differing possible impacts of changing emissions in the future. These are not the highest and lowest possible emission levels over the next 100 years, but simply represent a range of estimates. In the higher end of the range from the SRES, the "A2 scenario" has higher emissions and results in high levels of climate change. The scenario represents a divided world with high population growth, low economic growth, slower technology improvements and diffusion, and other factors that contribute to high emissions and lower adaptive capacity (e.g., low per capita wealth). At the low end of the SRES range, the "B1 scenario" represents a world with lower population growth, higher economic development, a shift to efficient energy technologies that are diffused rapidly around the world through free trade, and other conditions that yield slower growth of greenhouse gas concentrations and reduce the rate and level of changes in climate averages and extremes and increase capacity for adaptation.

Perspectives on “plausible” emissions evolve over time and newer emissions scenarios have been prepared and recently released by the research community (newer scenarios will be used for the Fourth NCA), few studies of impacts and response options using these new scenarios have been prepared. Because there is substantial overlap in the range of conditions in the SRES and the new emissions scenarios, the climate scenarios presented here, and that are provided as input to the 2014 NCA, mainly use SRES-based scenarios. In some cases, comparison is offered between the SRES-based scenarios and those from the new emission scenarios. It is certainly important to note that the A2 and B1 scenarios do not encompass the full range of possible futures: emissions can change less than those scenarios imply, or they can change even more.

Scenarios for the National Climate Assessment include information on:

Climate – plausible representations of future climate conditions (temperature, precipitation, and other variables) produced using a variety of techniques including mathematical models and downscaling methods.

Sea Level – estimates of future increase in global average sea level resulting from climate-related processes such as thermal expansion of the oceans and melting of grounded ice sheets as well as descriptions of selected regional and local anomalies from the global trend resulting from land subsidence or uplift and other factors.

Land Cover, Land Use, and Socioeconomic Factors – projections of the extent and distribution of different categories of land uses such as agriculture, forestry, and settlement, as well as different vegetative covers resulting from human management, climate change, and other factors. Socioeconomic factors include assumptions about potential future demographic, economic, institutional, and other characteristics resulting from different patterns of economic growth and social change.

Materials on Participatory Scenario Planning – a short description of the use of scenarios to consider the implications of uncertain future climate and socioeconomic conditions in planning and decision making. Additional materials are under development and will be released when available.

Links on the right panel provide access to further information about many of these scenario components. Other useful links can be found on the menu at the left side of the page. Frequently Asked Questions (FAQs) are available here for the climate scenarios, and the sea level scenarios, including more information about their development and guidance for use. The IPCC Technical Guidelines are an additional reference on the use of scenarios.

Background for the Scenarios

The U.S. Global Change Research Program (USGCRP) is mandated by Congress to produce a periodic assessment that summarizes the state of knowledge about climate and global change, potential impacts on the U.S., and response options. To facilitate preparation of the Third National Climate Assessment report (2014) and to build resources for an ongoing assessment process, a number of Federal agencies convened a preparatory workshop that resulted in a report on options for scenarios to use in preparing the 2014 NCA report. The National Climate Assessment Development and Advisory Committee (NCADAC) , a Federal advisory committee responsible for producing the report, adopted a scenario strategy for the 2014 NCA. In 2015, USGCRP made decisions about scenarios of focus for the Fourth NCA, due in 2018.