Projections of Climate Change
Because we can't know the future for certain, scientists use computer-based climate models to project plausible scenarios for the coming century and beyond.
Recently, the IPCC developed a series of 24 future climate change projections, a much broader set than previously considered. This elaboration was intended to convey a broader range of possibilities while accounting for the important uncertainties still outstanding. The scenarios are based on a range of estimations of heat-trapping-gas and aerosol emissions and assumptions about future population, energy use, economic growth, land use changes, and so on.
The IPCC grouped these 24 scenarios into 6 "families" and showed each represented in a line graph together with an earlier reference scenario (IS92a). In the charts below, you see the projections for each of these scenarios regarding changes in CO2 emissions, the resulting atmospheric carbon dioxide concentrations, SO2 emissions, temperature change and sea-level rise between 1990 and 2100. Take the red dashed line -- the A1T scenario -- for example( - - -).
Chart (a) This red line basically says that CO2 emissions will be radically reduced after about 2040.
Chart (b) tells us that only those kinds of radical emission reductions will lead to a stabilized CO2 concentration in the atmosphere.
Chart (c) says that this reduction would be accompanied by a strong reduction in SO2 - probably due to a reduction in burning coal and other CO2- and SO2-rich fossil fuels.
Chart (d) then indicates that with such strong reductions we would still see a temperature increase, but a more modest one than under some of the other scenarios, and that the warming trend would level off late in the 21st century.
Chart (e), finally, shows how this degree of warming would produce a mid-range scenario of sea-level rise. Because of the slow response of the oceans to atmospheric warming, that upward trend is still continuing and will continue for many more years after heat-trapping gases are stabilized and the upward temperature trend levels off.
The growing empirical evidence of climate change that is consistent with model projections, and other recent advances in the understanding of climate science have led to increased confidence in the use of global circulation models to project future climate change, but predicting the future remains inherently risky.
The current estimate of the IPCC is that an unmitigated rise in atmospheric greenhouse gas concentrations will cause temperatures to rise between 2.4 and 10.5 degrees Fahrenheit over the 21st century. This projection is significantly larger than what the panel predicted in 1996 at the time of its second assessment report, primarily based on improved models and the assumed reduction of aerosol emissions (most of which have a cooling effect).
As a result of this warming, scientists predict:
Sea level could rise by 3.5 to 34.6 inches between 1990 and 2100, making coastal groundwater saltier, endangering wetlands, and inundating valuable land and coastal communities;
Changes in precipitation patterns could have a significant impact, especially in already water-scarce regions that are likely to suffer from further decreasing rainfall;
The ranges and abundance of plants and animals could change dramatically under changing climate conditions, and some of them are likely to be unable to adapt or migrate to new locations;
Future adverse impacts from projected global warming also include possible increases in forest productivity for very modest temperature increases but likely severe stress on forests for rapid and greater warming;
There could also be serious human health impacts, such as from increasing heat stress, worsening air pollution, declining water quality, and the spread of infectious diseases into regions previously free from them.
The impacts of global warming will vary by country and region. In some higher latitudes, agricultural productivity could actually increase while farmers in many subtropical and tropical regions could experience significant declines in their yields. Small island states and countries close to sea level -- such as Bangladesh, with extensive low-lying coastal areas -- are especially vulnerable to sea-level rise. Many high-mountain regions could experience significant changes in ecosystems and water resources.
IPCC Working Group II -- which addresses the impacts of climate change, as well as vulnerability and adaptation issues -- found in its 2001 report that "adaptation has the potential to reduce adverse impacts of climate change and enhance beneficial impacts, but will incur costs and will not prevent all damages." In particular, "those with the least resources have the least capacity to adapt and are the most vulnerable." In fact, Working Group II co-chair, Dr. James McCarthy (Harvard University), concluded, "most of the earth's people will be on the losing side."
Contents:
- Consensus Versus Certainty in a Complex World
- What We Do Know about Climate Change
- Why the Climate Changes: Emissions of Heat-Trapping Gases and Aerosols
- Each Country's Share of Global CO2 Emissions
- Has the Climate Changed Already?
- Future Projections of Climate Change
- The Role of the Intergovernmental Panel on Climate Change
- Consensus in the IPCC—What Does It Mean?
- The Need for More Research and Clarification
- Sound Science for Public Policy and Decision-Making
