Figure 6. Projections of surface emissions), which can variously lead to cooling and warming in the lower stratosphere. Volcanic aerosols have a direct radiative effect on the lower stratosphere, producing a warming. VERY PROBABLE (2) Global mean surface temperature warming will increase by the mid-21st century. The best available estimate is that global mean surface temperatures will increase by about 0.5 to 2°C (or about 1 to 3.5°F) over the period from 1990 to 2050 due to increases in the concentrations of greenhouse gases alone (note that point 15 indicates it is inappropriate to convert these estimates to a per-decade basis), assuming no significant actions to reduce the projected increase in the rate of emissions of these gases. The best available estimate for a climate change that is in equilibrium with two times the pre-industrial carbon dioxide concentration (or equivalent in terms of other greenhouse gases) is a warming of 1.5 to 4.5°C, with 2.5°C being the most probable estimate. Basis-This estimate of global warming is based on projections of emissions and on a combination of results from simple model studies, general circulation models, the observed record, and estimates of the response of the climate to various forcings in the geological past, based on reconstructions carried out in paleoclimate studies. The estimated warming would be reduced if sulfur emissions are not controlled, and the actual temperature change may be outside this range if natural climate variations (e.g., significant changes in the frequency of explosive volcanic eruptions) are large. (3) Global mean precipitation will increase. The distribution of this change is less certain (see below). 0 (4) Northern hemisphere sea ice will be reduced (the magnitude of the change will depend on the amount of the warming, and the reduced extent will initially be most evident in the transition seasons). Projected changes and their timing in the Southern Hemisphere sea-ice extent are less certain. Basis-Studies of past climates provide evidence for polar amplification of warming and reduced sea-ice extent. Modeling studies also suggest that this will occur, although some have suggested local sea-ice expansions. (5) Arctic land areas will experience wintertime warming. Basis-Paleoclimate and model studies provide evidence for polar amplification of warming and reduction in land-surface snow cover. The magnitude of the surface warming will be dependent on potential changes in the poleward heat flux, which are uncertain, and the magnitude of the global warming. Current models may significantly underestimate the magnitude of polar amplification while exaggerating tropical changes if poleward heat flux by the ocean-atmosphere system is not properly simulated by models. (6) Global sea level will rise at an increasing rate, although with some probability that the rate of rise may not be significantly greater than at present. The most reasonable estimates for the rate of sea-level rise are for a rise of 5-40 cm by 2050, as compared to a rise of 5-12 cm if rates of rise over the past century continue. Basis-The most tractable part of making response Figure 7. Model projec continuing (centuries-long response) rise in sea level as the deep ocean only slowly experiences the warming at the surface. These estimates also do not consider the sea-level rise that would result from a potential catastrophic collapse of the west Antarctic ice sheet, which has been proposed but remains a subject of considerable debate. (7) Solar variability over the next 50 years will not induce a prolonged forcing that is significant in comparison with the effects of the increasing concentrations of CO2 and other greenhouse gases. Basis-The magnitude of the forcing from known levels of solar variability (variations of roughly 0.2 W/m2 have been occurring as a result of the solar cycle) is small compared to projected changes in greenhouse gas forcing (forcing of roughly 2 to 3.5 W/m2 is projected to occur as a result of likely emission scenarios, according to the IPCC). Changes on centennial to millennial time scales may be as large as 0.5 W/m2, but are still small compared to projected greenhouse forcing. PROBABLE (8) Summer Northern Hemisphere mid-latitude continental dryness will increase. Basis-Evaporation increases strongly with temperature increases. Current models indicate some general agreement that summer mid-latitude dryness occurs because the evaporation increase is larger than the precipitation increase.2 However, uncertainties include the following: (a) increased atmospheric moisture may be transported into continental interiors, resulting in transients of increased precipitation; (b) the vegetation response to increased CO2 is not known outside of highly controlled conditions; and (c) land surface-atmosphere interactions, including the storage of wintertime moisture, are still poorly represented in models. (9) High-latitude precipitation will increase, with potential feedback effects related to the influence of additional freshwater on the thermohaline circulation and of increased snowfall or rain on the mass balance of polar ice caps. 2 In addition, models suggest that the precipitation increase will occur during the colder half of the year. I Unless stored in reservoirs and used for irrigation, this moisture would be unlikely to be retained in the upper soil layer, thus unable to offset the increased evaporation occurring due to the warmer summer temperatures. Basis-With global warming, there will be an increase in the atmospheric mixing ratio of water vapor, producing larger moisture fluxes and more precipitation than at present in high latitudes. (10) Antarctic and North Atlantic ocean regions will experi- Basis-Oceanic regions where surface waters mix downward and (11) Transient explosive volcanic eruptions will result in short term relative cooling. Basis-Historical volcanism records indicate cooling of a few tenths of a degree lasting up to a few years following major eruptions. The historical frequency of explosive volcanic events large enough to produce substantial increases in stratospheric aerosols suggests that a few to several such events could occur over the next several decades. 0.0 Figure 8. Model projec- UNCERTAIN (12) Changes in climate variability will occur. As yet there is no clear evidence that suggests how the character of interannual variability may change due to greenhouse warming, but there is the potential for multifaceted and complicated, even counter-intuitive, changes in variability. Basis-Many potential changes in variability can be identified, suggesting that some will occur. These possibilities include the following: (a) In all models, standing and transient eddy activity in the mid-latitudes decreases with the reduced meridional temperature gradient associated with global warming, which may lead to reduced wintertime variability for warmer climates; (b) El Niño Southern Oscillation (ENSO) frequency may be related to average temperature, as suggested in historical and some modeling studies (models have suggested both more and less persistence during warm climates); (c) variability associated with smaller scale convective activity may increase (e.g., thunderstorms) as a result of greater moisture content in the atmosphere; and (d) greenhouse warming and land-ocean temperature differences may increase the frequency of atmospheric "blocking" events, and may influence low frequency precipitation variability. (13) Regional scale (100-2000 km) climate changes will be different from the global average changes. However, at present there is only very limited capability to estimate how various regions will respond to global climate change. Basis-There is a significant mismatch of spatial scales between present climate system models and regional climate variations. This is especially important because of the dependence of regional-scale responses to the details of regional land-surface characteristics, especially orography, hydrological conditions, and land-surface features. The best estimates of regional change are currently based on the large-scale characteristics of model simulations, and differences between global and regional changes are uncertain but are expected to be present. (14) Tropical storm intensity may change. Basis-An increase in tropical storm intensity is plausible (e.g., model studies suggest increases in intensity associated with higher sea-surface temperatures), but are uncertain because of potential changes in poleward heat flux, uncertainties in tropical sea-surface temperature response, and the strength of the Hadley circulation in a greenhouse |