The U.S. Climate Change Science Program released its second major U.S. climate change report to Congress, which provides 12 different greenhouse gas stabilization scenarios intended to serve as reference data for climate policy decision making and private sector technology development. The report also evaluates the process used for developing the scenarios and offers suggestions to improve future studies.

On July 10, 2007, the U.S. Climate Change Science Program (CCSP) released its second major U.S. climate change report to Congress. The report, titled Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations, and Review of Integrated Scenario Development and Application (the Report), provides 12 different greenhouse gas (GHG) stabilization scenarios intended to serve as global reference data, and an evaluation of the process through which the scenarios were developed and used.

CCSP was established in 2002 by President Bush to integrate federal research on global environmental change, and to provide science-based information to evaluate and manage the risks and opportunities of climate change and related environmental systems.

The Report is bifurcated. Part A (Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations) presents computer-based scenarios that evaluate four alternative GHG stabilization levels (low to high) and the impact on the energy sector and the economy for achieving each level. Part B (Review of Integrated Scenario Development and Application) evaluates the manner in which the scenarios were developed, the level of effectiveness and suggests ways to make future scenarios even more useful.

These scenarios are meant to contribute to climate change decision making by providing insight with regard to key unknowns, such as future emissions and climate, as well as economic and environmental concerns. The different types of decisions that can be informed by these scenarios include those surrounding climate impact and adaptation, those related to emissions mitigation policy and those focused on private sector technology development.

Report Details

The CCSP selected three separate modeling groups to create a "reference scenario" in which all climate policies were assumed to expire in 2012—thereby leaving no climate policy after 2012. Each modeling group then developed four stabilization scenarios demonstrating stabilization of GHG at four alternative levels (450, 550, 650 and 750 parts per million by volume (ppmv)). The stabilization scenarios in the Report go beyond past studies by providing modeling for the six primary anthropogenic GHG – carbon dioxide (CO2), nitrous oxide, methane, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride, instead of simply modeling for CO2.

The Report notes that the various scenario outcomes, estimated over a 100-year period, reflect simplified assumptions about economic, societal and political behavior, including a presumption of perfect economic efficiency and policy agreement among nations. The results therefore cannot be considered predictions or forecasts of the most likely outcomes within the national and global political system.

Highlights from the Report
Absent the adoption of new climate policies (the reference scenario), changes occur to the Earth’s radiation level that are 3 to 4 times that already experienced since the beginning of the industrial age based on economic growth and continued fossil fuel use. Non-fossil energy use increases from over 4 to nearly 9 times over the century, but this growth is insufficient to supplant fossil fuels as the major source of energy.

In the stabilization scenarios, CO2 emissions peak and decline in the 21st century or soon thereafter. Under the most stringent stabilization levels, CO2 emissions start to decline immediately or within a matter of decades. Under the least stringent stabilization levels, CO2 emissions do not peak until late in the century and are 1.5 to 2.5 times today’s levels in 2100.

Each of the stabilization scenarios require a transformation of the global energy system, including modification to the mix of energy technologies and fuels, such as increased reliance on nuclear, solar, wind, biomass and other renewable sources, and significant reductions in demand.

Significant differences in GHG emissions prices and economic costs arise between the modeling groups for each stabilization level. Among the more significant factors creating the variations in economic costs include differences in assumptions regarding economic growth over the century, the behavior of the planet in terms of "taking up" CO2 and technology advances to shift demand towards low-carbon energy sources. All other things being equal, scenarios utilizing more low-cost technology options and lower mandated emissions reductions result in lower economic costs.

Across the stabilization scenarios, CO2 emissions from electric power generation are reduced at relatively lower costs than CO2 emissions in other sectors, such as transportation and industry. Emissions are reduced from electric power generation by increased reliance on technologies such as CO2 capture and storage, nuclear energy and renewable energy.

Other sectors respond by reducing their demand for fossil fuels—replacing it with low- or non-emitting energy sources such as hydrogen and bioenergy and utilizing CO2 capture and storage technology when possible.

The scenarios in the Report represent "but one step in a long process of research" in the climate area. They will need to be updated as knowledge advances and conditions change.

Recommendations for Future Analysis
Greater transparency should be added regarding the underlying assumptions and probability judgments utilized when constructing future stabilization scenarios. Increased transparency is necessary to support the credibility of the scenarios.

The capacity to distribute and evaluate future scenarios and other similar decision-support tools should also be significantly expanded. Climate change scenarios should be communicated to multiple audiences with diverse interests and information needs.

Future scenarios should analyze the sensitivity of results to assumptions about cost, performance and environmental issues concerning key technologies such as biofuels, nuclear power, and carbon capture and storage.

Scenarios based on different assumptions than those used in this Report should be conducted concerning how the burdens of emissions mitigation may be shared among nations and over time.

Future scenarios would benefit from an uncertainty analysis and consideration of decision-making under these conditions.

A copy of the Report can be found on CCSP’s website: http://www.climatescience.gov/