BOX 5.1
IPCC (2000) Emission Scenarios

The IPCC Fourth Assessment Report projected global sea levels over the next 100 years based on 6 families of emission scenarios described in IPCC (2000). The A1 scenario family assumes high economic growth, low population growth that peaks mid century, and the rapid introduction of more efficient technologies. Within this family are scenarios designated as A1FI (fossil fuel intensive), A1B (balanced fuel), and A1T (predominantly nonfossil fuels). The A2 scenario family assumes slower economic growth and technological change, but high population growth. The B1 scenario family assumes the same low population growth as the A1 scenarios, but a shift toward a lower-emission service and information economy and cleaner technologies. Finally, the B2 scenario family assumes moderate population growth, intermediate economic growth, and slower and more diverse technological change than in the B1 and A1 scenarios. The A1FI scenario yields the highest carbon dioxide (CO2) emissions by 2100, and the B1 scenario yields the lowest CO2 emissions.

TABLE 5.1 IPCC (2007) Projected Contributions to Global Sea-Level Change, Relative to 2000

Projections for 2100 (cm)
Term B1 B2 MB A1T A2 A1FI
Thermal expansion 10–24 12–28 13–32 12–30 14–35 17–41
Glaciers and ice caps 7–14 7–15 8–15 8–15 8–16 8–17
Greenland Ice Sheet SMB 1–5 1–6 1–8 1–7 1–8 2–12
Antarctica Ice Sheet SMB -10–-2 -11–-2 -12–-2 -12–-2 -12–-3 -14–-3
Sea-level rise 18–38 20–43 21–48 20–45 23–51 26–59
Scaled-up ice sheet discharge 0–9 0–11 -1–13 -1–13 -1–13 -1–17

SOURCE: Adapted from Table 10.7 in Meehl et al. (2007).
NOTE: SMB = surface mass balance.

Steric Contributions

The IPCC Fourth Assessment Report used general circulation models (GCMs) of the ocean and atmosphere to estimate global steric response. Because the GCM simulations were only available for three emission scenarios, simplified climate models were used for the other three scenarios. Global ocean models compute both temperature and salinity, so their outputs can be used directly to calculate changes in sea level due to thermal expansion (thermosteric changes) and changes in salinity (halosteric changes). Thermosteric contributions from the ocean general circulation models used in the IPCC Fourth Assessment Report are shown in Figure 5.1 (Meehl et al., 2007). Note that the model results vary with time and emission scenario. The IPCC (2007) projected that thermal expansion would account for 55–69 percent of sea-level rise in 2100 (Table 5.1).

Cryospheric Contributions

The IPCC (2007) estimated the cryosphere response using models of ice sheet surface mass balance in Greenland and Antarctica and empirical models of the mass balance response of glaciers and ice caps to temperature and precipitation forcing. They projected that glaciers and ice caps would be the largest source of new water to the oceans throughout the 21st century (Table 5.1). The ice sheets were projected to contribute less new water than glaciers and ice caps, mainly because the Antarctic contribution was expected to be negative (i.e., mass gained because of increased snowfall would withdraw water from the ocean). However, recent observations of Antarctica show the opposite—a growing Antarctic contribution to sea-level rise due to the rapid transfer of ice from land to the ocean by glacier flow and iceberg calving, referred to here as “rapid dynamic response” (see “Glaciers, Ice Caps, and Ice Sheets” in Chapter 3).

At the time data were synthesized for the IPCC Fourth Assessment Report (until mid-2006), rapid transfers of ice at a global level were only beginning to be observed. In addition, the relatively simple treatment of land ice dynamics in the climate models precluded simulation of rapid dynamics. Although stand-alone



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