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we are producing for centuries. The fossil-fuel issue is different from other types of
environmental pollution – we cannot just “stop” and be assured the natural systems will absorb
the gases quickly.
Most of the warming already in place has occurred in the upper layers of the oceans, which will
retain the excess heat for a long time, so warming is not easily reversible.
The absorption of anthropogenic carbon dioxide by the oceans has already begun to acidify the
ocean. This chemical process, with still-uncertain ecological impacts, is inevitable and
inexorable. The ocean's buffering mechanisms have long, probably millennial, time scales.
This long-term perspective should also warn us that action to forestall harmful effects of human-
induced global warming must begin now and will have to be combined with adaptation
strategies because we have already “locked in,” for good or ill, some change due to global
warming.
A human impact is already discernible in the data. It’s not just that the earth has been warming;
it’s the pattern of warming, “fingerprint,” that identifies the human impact. The stratosphere, the
atmosphere above about 10 kilometres, has been cooling while the troposphere, the part we
live in, has been cooling. This pattern is consistent with the heat-trapping action of greenhouse
gases, and inconsistent with other warming mechanisms, like changes in the dust content of the
atmosphere or the sun’s output – these would warm the whole atmosphere, not just the lower
altitudes. Similarly we can reproduce the temperature changes of the past century with
simulations of climate only by including the buildup of greenhouse gases as well as other
natural and man-made influences.
Global warming might have remained an esoteric scientific debate, if not for its policy
implications. But it has become a politically polarized argument because of them. There’s
confusion over who or what to believe about global warming in the face of so many conflicting
narratives of the science. We scientists could do a better job of communicating how science
works. So it’s worth clarifying what science is and is not.
Science is not about “consensus;” we do not take a vote over scientific hypotheses. It is not
about “authority” or “credentials.” The elegant theory of the highest-ranking professor can, and
often is, brought down by the work of a humble grad student. That is the beauty and strength of
science: it respects no authority, only the rigour of intellectual argument and the strength of
data. Scientific theories stand only on their ability to withstand the challenges and constraints of
evidence. The sociology of science is adversarial. We are all trying to knock down any
prevailing theory or paradigm, not defend them. If I had the evidence right now that the idea of
human-induced climate change was wrong, I’d publish it and make myself famous. That is why I
think this theory is robust.
Proponents and opponents of fossil-fuel emissions limitations need to beware of a philosophical
pitfall: the assumption that settling the scientific issues will make any particular policy step
inevitable. Some in the pro-Kyoto camp assume that if only policymakers were convinced that
global warming is real and due to human action, the Kyoto Protocol would be universally ratified
and implemented. Not necessarily – climate science is not the only factor driving energy and
carbon-emissions policy, and Kyoto is not the only possible carbon limitation scheme.
Similarly, some opponents of Kyoto try to discredit climate science to forestall action. But there
are other, more immediate reasons than climate change to cut back our dependence on fossil
fuels. Examples include local and regional pollution which pose public health risks, and the
geopolitical and economic risk posed by dependence on imported fossil fuels.
The issue of whether global warming is occurring, and if so, is attributable to human action, is a
scientific question. What actions, if any, to take in response to the problem, if any, come down
to political, economic, ethical, even spiritual considerations. It is possible the Intergovernmental
Panel on Climate Change (IPCC) estimates of future climate are wrong, and we must accept
that possibility. Although the climate puzzle is incomplete, as citizens and policy-makers we
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