3.5. Scenarios of the 21st Century

In 2000, the IPCC completed a Special Report on Emissions Scenarios (SRES) to replace the earlier set of six IS92 scenarios developed for the IPCC in 1992. These newer scenarios consider the period 1990 to 2100 and include a range of socioeconomic assumptions (e.g., global population and gross domestic product). Their implications for other aspects of global change also have been calculated; some of these implications are summarized for 2050 and 2100 in Table TS-1. For example, mean ground-level ozone concentrations in July over the industrialized continents of the northern hemisphere are projected to rise from about 40 ppb in 2000 to more than 70 ppb in 2100 under the highest illustrative SRES emissions scenarios; by comparison, the clean-air standard is below 80 ppb. Peak levels of ozone in local smog events could be many times higher. Estimates of CO₂ concentration range from 478 ppm to1099 ppm by 2100, given the range of SRES emissions and uncertainties about the carbon cycle (Table TS-1). This range of implied radiative forcing gives rise to an estimated global warming from 1990 to 2100 of 1.4-5.8°C, assuming a range of climate sensitivities. This range is higher than the 0.7-3.5°C of the SAR because of higher levels of radiative forcing in the SRES scenarios than in the IS92a-f scenarios -- primarily as a result of lower sulfate aerosol emissions, especially after 2050. The equivalent range of estimates of global sea-level rise (for this range of global temperature change in combination with a range of ice melt sensitivities) to 2100 is 9-88 cm (compared to 15-95 cm in the SAR). [3.2.4.1, 3.4.4, 3.8.1, 3.8.2]

Table TS-1: The SRES scenarios and their implications for atmospheric composition, climate, and sea level. Values of population, GDP, and per capita income ratio (a measure of regional equity) are those applied in integrated assessment models used to estimate emissions (based on Tables 3-2 and 3-9).
Date	Global Population billions)a	Global GDP (1012 US$ yr-1)b	Per Capita Income Ratioc	Ground Level O3 Concentration (ppm)d	CO2 Concentration (ppm)e	Global Temperature Change (°C)f	Global Sea-Level Rise (cm)g
1990	5.3	21	16.1	--	354	0	0
2000	6.1-6.2	25-28	12.3-14.2	40	367	0.2	2
2050	8.4-11.3	59-187	2.4-8.2	~60	463-623	0.8-2.6	5-32
2100	7.0-15.1	197-550	1.4-6.3	>70	478-1099	1.4-5.8	9-88
a Values for 2000 show range across the six illustrative SRES emissions scenarios; values for 2050 and 2100 show range across all 40 SRES scenarios. b See footnote a; gross domestic product (trillion 1990 US$ yr-1). c See footnote a; ratio of developed countries and economies-in-transition (Annex I) to developing countries (non-Annex I). d Model estimates for industrialized continents of northern hemisphere assuming emissions for 2000, 2060, and 2100 from the A1F and A2 illustrative SRES emissions scenarios at high end of the SRES range ( Chapter 4, TAR WG I). e Observed 1999 value ( Chapter 3, WG I TAR); values for 1990, 2050, and 2100 are from simple model runs across the range of 35 fully quantified SRES emissions scenarios and accounting for uncertainties in carbon cycle feedbacks related to climate sensitivity (data from S.C.B. Raper, Chapter 9, WG I TAR). Note that the ranges for 2050 and 2100 differ from those presented by TAR WGI ( Appendix II), which were ranges across the six illustrative SRES emissions scenarios from simulations using two different carbon cycle models. f Change in global mean annual temperature relative to 1990 averaged across simple climate model runs emulating results of seven AOGCMs with an average climate sensitivity of 2.8°C for the range of 35 fully quantified SRES emissions scenarios ( Chapter 9, WG I TAR). g Based on global mean temperature changes but also accounting for uncertainties in model parameters for land ice, permafrost, and sediment deposition ( Chapter 11, WG I TAR).

In terms of mean changes in regional climate, results from GCMs that have been run assuming the new SRES emissions scenarios display many similarities with previous runs. The WGI contribution to the TAR concludes that rates of warming are expected to be greater than the global average over most land areas and will be most pronounced at high latitudes in winter. As warming proceeds, northern hemisphere snow cover and sea-ice extent will be reduced. Models indicate warming below the global average in the north Atlantic and circumpolar southern ocean regions, as well as in southern and southeast Asia and southern South America in June-August. Globally, there will be increases in average water vapor and precipitation. Regionally, December-February precipitation is expected to increase over the northern extratropics, Antarctica, and tropical Africa. Models also agree on a decrease in precipitation over Central America and little change in southeast Asia. Precipitation in June-August is estimated to increase in high northern latitudes, Antarctica, and south Asia; it is expected to change little in southeast Asia and to decrease in central America, Australia, southern Africa, and the Mediterranean region.

Changes in the frequency and intensity of extreme climate events also can be expected. Based on the conclusions of the WGI report and the likelihood scale employed therein, under GHG forcing to 2100, it is very likely that daytime maximum and minimum temperatures will increase, accompanied by an increased frequency of hot days (see Table TS-2). It also is very likely that heat waves will become more frequent, and the number of cold waves and frost days (in applicable regions) will decline. Increases in high-intensity precipitation events are likely at many locations; Asian summer monsoon precipitation variability also is likely to increase. The frequency of summer drought will increase in many interior continental locations, and droughts -- as well as floods -- associated with El Niño events are likely to intensify. Peak wind intensity and mean and peak precipitation intensities of tropical cyclones are likely to increase. The direction of changes in the average intensity of mid-latitude storms cannot be determined with current climate models. [Table 3-10]

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