Wall Street Journal from 9/23/2014
Sept. 19, 2014 12:19 p.m. ET
The idea that "Climate science is
settled" runs through today's popular and policy discussions.
Unfortunately, that claim is misguided. It has not only distorted our
public and policy debates on issues related to energy, greenhouse-gas
emissions and the environment. But it also has inhibited the scientific
and policy discussions that we need to have about our climate future.
My
training as a computational physicist—together with a 40-year career of
scientific research, advising and management in academia, government
and the private sector—has afforded me an extended, up-close perspective
on climate science. Detailed technical discussions during the past year
with leading climate scientists have given me an even better sense of
what we know, and don't know, about climate. I have come to appreciate
the daunting scientific challenge of answering the questions that policy
makers and the public are asking.
The
crucial scientific question for policy isn't whether the climate is
changing. That is a settled matter: The climate has always changed and
always will. Geological and historical records show the occurrence of
major climate shifts, sometimes over only a few decades. We know, for
instance, that during the 20th century the Earth's global average
surface temperature rose 1.4 degrees Fahrenheit.
Nor is the crucial question whether
humans are influencing the climate. That is no hoax: There is little
doubt in the scientific community that continually growing amounts of
greenhouse gases in the atmosphere, due largely to carbon-dioxide
emissions from the conventional use of fossil fuels, are influencing the
climate. There is also little doubt that the carbon dioxide will
persist in the atmosphere for several centuries. The impact today of
human activity appears to be comparable to the intrinsic, natural
variability of the climate system itself.
Rather,
the crucial, unsettled scientific question for policy is, "How will the
climate change over the next century under both natural and human
influences?" Answers to that question at the global and regional levels,
as well as to equally complex questions of how ecosystems and human
activities will be affected, should inform our choices about energy and
infrastructure.
But—here's the
catch—those questions are the hardest ones to answer. They challenge, in
a fundamental way, what science can tell us about future climates.
Even
though human influences could have serious consequences for the
climate, they are physically small in relation to the climate system as a
whole. For example, human additions to carbon dioxide in the atmosphere
by the middle of the 21st century are expected to directly shift the
atmosphere's natural greenhouse effect by only 1% to 2%. Since the
climate system is highly variable on its own, that smallness sets a very
high bar for confidently projecting the consequences of human
influences.
A second challenge to
"knowing" future climate is today's poor understanding of the oceans.
The oceans, which change over decades and centuries, hold most of the
climate's heat and strongly influence the atmosphere. Unfortunately,
precise, comprehensive observations of the oceans are available only for
the past few decades; the reliable record is still far too short to
adequately understand how the oceans will change and how that will
affect climate.
A third fundamental
challenge arises from feedbacks that can dramatically amplify or mute
the climate's response to human and natural influences. One important
feedback, which is thought to approximately double the direct heating
effect of carbon dioxide, involves water vapor, clouds and temperature.
But feedbacks are uncertain. They
depend on the details of processes such as evaporation and the flow of
radiation through clouds. They cannot be determined confidently from the
basic laws of physics and chemistry, so they must be verified by
precise, detailed observations that are, in many cases, not yet
available.
Beyond these observational
challenges are those posed by the complex computer models used to
project future climate. These massive programs attempt to describe the
dynamics and interactions of the various components of the Earth
system—the atmosphere, the oceans, the land, the ice and the biosphere
of living things. While some parts of the models rely on well-tested
physical laws, other parts involve technically informed estimation.
Computer modeling of complex systems is as much an art as a science.
For
instance, global climate models describe the Earth on a grid that is
currently limited by computer capabilities to a resolution of no finer
than 60 miles. (The distance from New York City to Washington, D.C., is
thus covered by only four grid cells.) But processes such as cloud
formation, turbulence and rain all happen on much smaller scales. These
critical processes then appear in the model only through adjustable
assumptions that specify, for example, how the average cloud cover
depends on a grid box's average temperature and humidity. In a given
model, dozens of such assumptions must be adjusted ("tuned," in the
jargon of modelers) to reproduce both current observations and
imperfectly known historical records.
We
often hear that there is a "scientific consensus" about climate change.
But as far as the computer models go, there isn't a useful consensus at
the level of detail relevant to assessing human influences. Since 1990,
the United Nations Intergovernmental Panel on Climate Change, or IPCC,
has periodically surveyed the state of climate science. Each successive
report from that endeavor, with contributions from thousands of
scientists around the world, has come to be seen as the definitive
assessment of climate science at the time of its issue.
For the latest IPCC report (September
2013), its Working Group I, which focuses on physical science, uses an
ensemble of some 55 different models. Although most of these models are
tuned to reproduce the gross features of the Earth's climate, the marked
differences in their details and projections reflect all of the
limitations that I have described. For example:
•
The models differ in their descriptions of the past century's global
average surface temperature by more than three times the entire warming
recorded during that time. Such mismatches are also present in many
other basic climate factors, including rainfall, which is fundamental to
the atmosphere's energy balance. As a result, the models give widely
varying descriptions of the climate's inner workings. Since they
disagree so markedly, no more than one of them can be right.
•
Although the Earth's average surface temperature rose sharply by 0.9
degree Fahrenheit during the last quarter of the 20th century, it has
increased much more slowly for the past 16 years, even as the human
contribution to atmospheric carbon dioxide has risen by some 25%. This
surprising fact demonstrates directly that natural influences and
variability are powerful enough to counteract the present warming
influence exerted by human activity.
Yet
the models famously fail to capture this slowing in the temperature
rise. Several dozen different explanations for this failure have been
offered, with ocean variability most likely playing a major role. But
the whole episode continues to highlight the limits of our modeling.
•
The models roughly describe the shrinking extent of Arctic sea ice
observed over the past two decades, but they fail to describe the
comparable growth of Antarctic sea ice, which is now at a record high.
•
The models predict that the lower atmosphere in the tropics will absorb
much of the heat of the warming atmosphere. But that "hot spot" has not
been confidently observed, casting doubt on our understanding of the
crucial feedback of water vapor on temperature.
•
Even though the human influence on climate was much smaller in the
past, the models do not account for the fact that the rate of global
sea-level rise 70 years ago was as large as what we observe today—about
one foot per century.
• A crucial
measure of our knowledge of feedbacks is climate sensitivity—that is,
the warming induced by a hypothetical doubling of carbon-dioxide
concentration. Today's best estimate of the sensitivity (between 2.7
degrees Fahrenheit and 8.1 degrees Fahrenheit) is no different, and no
more certain, than it was 30 years ago. And this is despite an heroic
research effort costing billions of dollars.
These
and many other open questions are in fact described in the IPCC
research reports, although a detailed and knowledgeable reading is
sometimes required to discern them. They are not "minor" issues to be
"cleaned up" by further research. Rather, they are deficiencies that
erode confidence in the computer projections. Work to resolve these
shortcomings in climate models should be among the top priorities for
climate research.
Yet a public official
reading only the IPCC's "Summary for Policy Makers" would gain little
sense of the extent or implications of these deficiencies. These are
fundamental challenges to our understanding of human impacts on the
climate, and they should not be dismissed with the mantra that "climate
science is settled."
While the past two
decades have seen progress in climate science, the field is not yet
mature enough to usefully answer the difficult and important questions
being asked of it. This decidedly unsettled state highlights what should
be obvious: Understanding climate, at the level of detail relevant to
human influences, is a very, very difficult problem.
We
can and should take steps to make climate projections more useful over
time. An international commitment to a sustained global climate
observation system would generate an ever-lengthening record of more
precise observations. And increasingly powerful computers can allow a
better understanding of the uncertainties in our models, finer model
grids and more sophisticated descriptions of the processes that occur
within them. The science is urgent, since we could be caught flat-footed
if our understanding does not improve more rapidly than the climate
itself changes.
A transparent rigor
would also be a welcome development, especially given the momentous
political and policy decisions at stake. That could be supported by
regular, independent, "red team" reviews to stress-test and challenge
the projections by focusing on their deficiencies and uncertainties;
that would certainly be the best practice of the scientific method. But
because the natural climate changes over decades, it will take many
years to get the data needed to confidently isolate and quantify the
effects of human influences.
Policy
makers and the public may wish for the comfort of certainty in their
climate science. But I fear that rigidly promulgating the idea that
climate science is "settled" (or is a "hoax") demeans and chills the
scientific enterprise, retarding its progress in these important
matters. Uncertainty is a prime mover and motivator of science and must
be faced head-on. It should not be confined to hushed sidebar
conversations at academic conferences.
Society's
choices in the years ahead will necessarily be based on uncertain
knowledge of future climates. That uncertainty need not be an excuse for
inaction. There is well-justified prudence in accelerating the
development of low-emissions technologies and in cost-effective
energy-efficiency measures.
But climate
strategies beyond such "no regrets" efforts carry costs, risks and
questions of effectiveness, so nonscientific factors inevitably enter
the decision. These include our tolerance for risk and the priorities
that we assign to economic development, poverty reduction, environmental
quality, and intergenerational and geographical equity.
Individuals
and countries can legitimately disagree about these matters, so the
discussion should not be about "believing" or "denying" the science.
Despite the statements of numerous scientific societies, the scientific
community cannot claim any special expertise in addressing issues
related to humanity's deepest goals and values. The political and
diplomatic spheres are best suited to debating and resolving such
questions, and misrepresenting the current state of climate science does
nothing to advance that effort.
Any
serious discussion of the changing climate must begin by acknowledging
not only the scientific certainties but also the uncertainties,
especially in projecting the future. Recognizing those limits, rather
than ignoring them, will lead to a more sober and ultimately more
productive discussion of climate change and climate policies. To do
otherwise is a great disservice to climate science itself.
Dr.
Koonin was undersecretary for science in the Energy Department during
President Barack Obama's first term and is currently director of the
Center for Urban Science and Progress at New York University. His
previous positions include professor of theoretical physics and provost
at Caltech, as well as chief scientist of
BP, where his work focused on renewable and low-carbon energy technologies.
