Unformatted text preview: dent of the United States (16), at a time when the Earth
may be nearing a point of dangerous human-made interference
with climate (17).
The congressional testimony in 1988 (13) included a graph (Fig.
2) of simulated global temperature for three scenarios (A, B, and
C) and maps of simulated temperature change for scenario B. The
three scenarios were used to bracket likely possibilities. Scenario A
was described as ‘‘on the high side of reality,’’ because it assumed
rapid exponential growth of GHGs and it included no large volcanic
eruptions during the next half century. Scenario C was described as
‘‘a more drastic curtailment of emissions than has generally been
imagined,’’ specifically GHGs were assumed to stop increasing after
2000. Intermediate scenario B was described as ‘‘the most plausible.’’ Scenario B has continued moderate increase in the rate of
GHG emissions and includes three large volcanic eruptions sprin- Annual Mean Global Temperature Change: ΔTs (°C) Fig. 2. Global surface temperature computed for scenarios A, B, and C (12),
compared with two analyses of observational data. The 0.5°C and 1°C temperature levels, relative to 1951–1980, were estimated (12) to be maximum global
temperatures in the Holocene and the prior interglacial period, respectively. Hansen et al. kled through the 50-year period after 1988, one of them in
Real-world GHG climate forcing (17) so far has followed a
course closest to scenario B. The real world even had one large
volcanic eruption in the 1990s, Mount Pinatubo in 1991, whereas
scenario B placed a volcano in 1995.
Fig. 2 compares simulations and observations. The red curve, as
in ref. 12, is the updated Goddard Institute for Space Studies
observational analysis based on meteorological stations. The black
curve is the land–ocean global temperature index from Fig. 1, which
uses SST changes for ocean areas (5, 6). The land–ocean temperature has more complete coverage of ocean areas and yields slightly
smaller long-term temperature change, because warming on average is less over ocean than over land (Fig. 1B).
Temperature change from climate models, including that reported in 1988 (12), usually refers to temperature of surface air over
both land and ocean. Surface air temperature change in a warming
climate is slightly larger than the SST change (4), especially in
regions of sea ice. Therefore, the best temperature observation for
comparison with climate models probably falls between the meteorological station surface air analysis and the land–ocean temperature index.
Observed warming (Fig. 2) is comparable to that simulated for
scenarios B and C, and smaller than that for scenario A. Following
refs. 18 and 14, let us assess ‘‘predictions’’ by comparing simulated
and observed temperature change from 1988 to the most recent
year. Modeled 1988–2005 temperature changes are 0.59, 0.33, and
0.40°C, respectively, for scenarios A, B, and C. Observed temperature cha...
View Full Document