- What assumptions have to be made to estimate future atmospheric concentrations of CO2?
Atmospheric CO2 Levels for Different Emission Scenarios
- The Intergovernmental Panel on Climate Change, in its 2007 report, described global climate model projections of 21st century climate under a number of CO2 emission "scenarios". What assumptions does the "low" emissions scenario make about CO2 emissions through the end of the current century? What is the projected concentration of CO2 by 2100 under this scenario? [The global average CO2 concentration in 2018 is about 389 parts per million (ppm), and the pre-industrial concentration was about 280 ppm.] Will it have stabilized by then?
- What assumptions does the IPCC's "medium" emissions scenario make about CO2 emissions through the end of the 21st century? What is the projected CO2 concentration by 2100? By what year would CO2 concentrations reach twice their pre-industrial levels?
- What assumptions does the IPCC's "high" emissions scenario make about CO2 emissions through the end of the 21st century? (How much of the known "recoverable" reserves of fossil fuels would have been consumed by then?) What is the projected CO2 concentration by 2100? By what year would CO2 concentrations reach twice their pre-industrial levels?
- The IPCC considered a scenario with even higher emission rates than the "high" scenario. Over the last 5-6 years of the first decade of the 21st century, how have actual CO2 emissions (so far) compared to this most "pessimistic" scenario?
- Based on simple, one-dimensional, radiative-convective climate model simulations (which provide a rough first estimate but are not as sophisticated as global climate models), what is the range of projected global average temperature increases by the end of the 21st century (relative to the 1850 baseline value) in response to the range of CO2 emission scenarios considered by the IPCC? How much of this projected increase has already occurred? Based on these projections, how would the rate of temperature increase compare to what has already occurred?
- Do the projections described above, made in response to various CO2 emission scenarios, account for contributions from other greenhouse gases?
Increases in Other Trace Gases
- How have concentrations of methane, nitrous oxides, and chlorofluorocarbons (all greenhouse gases with at least some sources related to human activities) varied over the last several decades?
The Concept of Radiative Forcing
- What is meant by radiative forcing? What is the radiative forcing associated with a doubling of CO2 concentrations? How large a surface temperature increase does this produce in one-dimensional radiative-convective climate models? How have the radiative forcings for methane, nitrous oxide, and halocarbons each increased between 1750 and 2005? What has contributed to negative radiative forcing (which produces cooling) since 1750? Are these as well understood?
- What has been the net effect of the factors above on radiative forcing? Is there significant uncertainty about the sign of the net radiative forcing?
- What proportion of the radiative forcing from all greenhouse gases combined has been offset by the effects of aerosols (which increase atmospheric albedo in two different ways)? What has (probably) been the most visible effect of aerosols on the history of global temperature changes since 1850? Why is the offsetting effects of aerosols likely to have less impact in the future?
AOGCM Predictions of Global Warming
- What is meant by thermal drag exerted by the ocean? What effect does thermal drag have on the response of atmospheric temperatures to increases in greenhouse gas concentrations?
- How do the global average temperature projections made by coupled atmospheric-ocean global climate models (AOGCMs) compare to those made by the simple, one-dimensional radiative equilibrium models?
- What is the temperature change projected by AOGCMs if concentrations of aerosols and greenhouse gases are held constant at their levels in the year 2000? Why should the temperature continue to rise at all?
- What is the range of uncertainty in global average temperature projections for the year 2100, due to differences among AOGCMs and among CO2 emission scenarios?
Long-Term Climate Warming
- CO2 concentrations are projected to continue to rise beyond the year 2100. By how much might they eventually rise, compared to the preindustrial value?
- What increase in temperature do AOGCMs project as a result? How does this change in temperature compare to surface temperature change during the last 100 million years?
Possible Changes in the Thermohaline Circulation
- In some scenarios, why might northern Europe experience significant cooling while the rest of the world undergoes significant warming?