Geoscience Reference
In-Depth Information
water drawn into the ship's engine room. After the war, most of the temperature
data came from British ships, which dropped a bucket into the ocean and pulled it
aboard, during which process the water cooled enough to account for the temper-
ature discrepancy. Once the data from the two periods were adjusted to take this
difference into account, modeled temperatures for midcentury more closely replic-
ated observed temperatures.
10
As modeling continued to improve, the 2007 Fourth Assessment Report (AR4)
went the rest of the way, saying that “warming of the climate system is unequivoc-
al” and that its effects are visible in natural physical and biological systems. The
cause was “very likely [greater than 90%] due to the observed increase in anthro-
pogenic greenhouse gas concentrations.” Validating the controversial but accurate
“hockey stick” plot of sharply rising global temperatures, the report said that “av-
erage Northern Hemisphere temperatures during the second half of the 20th cen-
tury were very likely higher than during any other 50-year period in the last 500
years and likely the highest in at least the past 1300 years.”
11
AR4 had no sooner appeared than scientists unconnected to the IPCC began to
criticizethereportforunderplayingtherisksofglobalwarming.Surprisingly,AR4
had projected a lower sea level rise than had AR3. Had the IPCC in 2007 come to
see global warming as less threatening than it had in 2001? That would not seem to
make sense given the rest of AR4's conclusions. It turned out that articles showing
that Greenland and mountain glaciers were melting faster than previously thought
had come in too late to meet AR4's inclusion deadline. The extra meltwater, of
course, winds up in the oceans and raises sea level.
Reviewing the studies of future sea level rise that had appeared by 2010, Stefan
Rahmstorf of the Potsdam Institute of Climate Impacts Research concluded that
sea level might rise by well more than a meter by the end of the century. He
warned:
At the end of the last ice age, the Earth slowly warmed by 4-7°C globally and lost almost two-
thirdsofitslandiceintheprocess.Thatraisedsealevel by120meters, atratesoftenexceeding
a meter per century. Nothing in the present ice-sheet configuration would rule out similar rates
in future. How much of the remaining 65 meters' worth of land ice will humans melt if we
warm the planet by a further several degrees?
12
To project future temperature increases, AR4 reported four scenarios, each with
different assumptions for future energy sources and demand, the extent of global-
ization, population growthordecline, andsoon.Eachscenario hadvariants, bring-
ing the total number to forty. Scenario A1F1 forecast a temperature rise of from
2.4 to 6.4°C by 2100. Another variant, A1T, projected only 1.4°C. The range for
all scenarios was 1.1 to 6.4°C, wide enough to allow one to choose complacency
