Geoscience Reference
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1.2°C. But very strong amplifi ers are at work in the climate-change pro-
cess, and they can boost that temperature increase to the range of esti-
mates shown in Figure 7.
TEMPERATURE PROJECTIONS FOR THE NEXT CENTURY
We now have the two basic building blocks for projecting future cli-
mate change. First, we saw how energy specialists project future CO 2
emissions, and how these are translated into future concentrations of CO 2
and other GHGs. Second, we described how climate modelers take these
projected concentrations and compute the path of climatic variables such
as temperature, precipitation, and sea-level rise in the decades ahead.
The next step is to integrate the two parts to develop projections of
climate change. For these estimates, we calculate the path with no cli-
mate policy, or the baseline scenario. In other words, we examine what
would happen if countries take no steps to slow the growth in CO 2 and
other GHG emissions. While no one would recommend this as an ap-
propriate policy, it provides an important reference to estimate the tra-
jectory of climate variables, such as temperature, when countries sit back
and roll the climatic dice.
It is useful to start with the instrumental temperature record. The
basic trends in global temperature since the late nineteenth century, as
recorded by thermometers and synthesized by three different research
groups, are displayed in Figure 8. 10 The rising trend over this period is
clear. However, the year-to-year movements are erratic and sometimes
diffi cult to explain (like the stock market).
Now move to projections of future climate. One set of baseline pro-
jections uses the standardized IPCC-SRES scenarios. These scenarios
are from a Special Report on Emissions Scenarios (SRES), which have
been extensively used by climate modelers as a way of standardizing
inputs into their analyses. These have been updated with a new set of
IPCC reference scenarios that are developed from paths of GHG concen-
trations, but the projections have changed little over the last decade.
Standardized scenarios may not be the most accurate projections, but
they generate a range of emissions trajectories to test the models—like
using a wind tunnel to test aircraft.
 
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