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This paper confronts the wide political
support for the 2C objective of global increase in
temperature, reaffirmed in Copenhagen, with the consistent
set of hypotheses on which it relies. It explains why
neither an almost zero pure time preference nor concerns
about catastrophic damages in case of uncontrolled global
warming are prerequisites for policy decisions preserving
the possibility of meeting a 2C target. It rests on an
optimal stochastic control model balancing the costs and
benefits of climate policies resolved sequentially in order
to account for the arrival of new information (the RESPONSE
model). This model describes the optimal abatement pathways
for 2,304 worldviews, combining hypotheses about growth
rates, baseline emissions, abatement costs, pure time
preference, damages, and climate sensitivity. It shows that
26 percent of the worldviews selecting the 2C target are not
characterized by one of the extreme assumptions about pure
time preference or climate change damages.
