Geoscience Reference
In-Depth Information
pre-industrial levels ought not to exceed 2 degrees C.” These statements
are representative of the aspirational targets set by many governments.
5
THE SCIENTIFIC BASIS OF THE 2°C TARGET
The declarations quoted above refer to “the scientifi c view” on the
appropriate target. Where does this scientifi c view come from? Was the
2°C target based on a strong body of evidence that suggests there is a
threshold at 2°C? Would there be “dangerous” or at least serious conse-
quences if the earth's climate system passes this threshold?
The surprising answer is that the scientifi c rationale for the 2°C
target, the most recent report of the U.S. National Academy of Science
did little more than connect the circularity of the argument: “Subse-
quent scientifi c research has sought to better understand and quantify
the links among GHG emissions, atmospheric GHG concentrations,
changes in global climate, and the impacts of those changes on human
and environmental systems. Based on this research, many policy mak-
ers in the international community recognize limiting the increase in
global mean surface temperature to 2°C above preindustrial levels as an
important benchmark; this goal was embodied in the Copenhagen Ac-
the politicians refer to the science, and the scientists refer to the politics.
If we sift through the arguments, we fi nd three justifi cations for the
temperature target. The fi rst is that the maximum experienced global
temperature for the last half-million years is about 2°C more than today,
and to exceed that would be potentially dangerous. A second rationale is
that ecological adjustments may be diffi cult beyond this temperature
increase. A fi nal reason is that many dangerous thresholds will be
crossed once the temperature surpasses 2°C.
We begin with the fi rst reason, which is based on historical climate
data. Figure 28 shows a reconstruction of global temperature change
over the last half-million years. These estimates are derived from Ant-
measure trends over Antarctica; additionally, they are based on a re-
gional temperature proxy rather than on actual temperature measure-
