U.N. Climate Panel Endorses Ceiling on Global Emissions

STOCKHOLM — The world’s top climate
scientists on Friday formally embraced
an upper limit on greenhouse gases for
the first time, establishing a target level
at which humanity must stop spewing
them into the atmosphere or face
irreversible and potentially
catastrophic climatic changes. They
warned that the target is likely to be
exceeded in a matter of decades unless
steps are taken soon to reduce
Unveiling the latest United Nations
assessment of climate science, the
experts cited a litany of changes that
are already under way, warned that
they are likely to accelerate and
expressed virtual certainty that human
activity is the main cause.
“Climate change is the greatest
challenge of our time,” said Thomas F.
Stocker, co-chairman of the
Intergovernmental Panel on Climate
Change, the United Nations-sponsored
group of scientists that produced the
report. “In short, it threatens our
planet, our only home.”
The panel, in issuing its most definitive
assessment yet of the risks of human-
caused warming, hoped to give impetus
to international negotiations toward a
new climate treaty, which have
languished in recent years in a swamp
of technical and political disputes. The
group made clear that time is not on
the planet’s side if emissions continue
“Human influence has been detected in
warming of the atmosphere and the
ocean, in changes in the global water
cycle, in reductions in snow and ice, in
global mean sea level rise, and in
changes in some climate extremes,” the
report said. “It is extremely likely that
human influence has been the
dominant cause of the observed
warming since the mid-20th century.”
Despite the urgency of the panel’s
findings, the course of future
international negotiations remains
unclear and heated opposition both to
established climate science and to
strong actions to reduce emissions is
certain to continue.
Going well beyond its four previous
analyses of the emissions problem, the
panel endorsed a “carbon budget” for
humanity — a limit on the amount of
the primary greenhouse gas, carbon
dioxide, that can be produced by
industrial activities and the clearing of
To stand the best chance of keeping the
planetary warming below an
internationally agreed target of 3.6
degrees Fahrenheit (2 degrees Celsius)
above the level of preindustrial times,
the panel found, no more than one
trillion metric tons of carbon can be
burned and the resulting gas released
into the atmosphere.
Just over half that amount has already
been emitted since the beginning of the
Industrial Revolution, and at the rate
energy consumption is growing, the
trillionth ton will be released
somewhere around 2040, according to
calculations by Myles R. Allen, a
scientist at the University of Oxford
and one of the authors of the new
report. More than three trillion tons of
carbon are still left in the ground as
fossil fuels.
To keep using fossil fuels beyond the
trillionth ton of emissions, companies
would have to develop potentially
expensive technology to capture and
store carbon dioxide from emissions
sources like power plants. Such efforts
have been lagging badly; only last
week, Norway scaled back one of the
most ambitious such projects because of
soaring costs.
But a considerable body of research
suggests that in principle it could be
done, and in the United States, the
Obama administration is moving
toward rules that would essentially
require utilities to develop the
technology if they want to keep
burning coal to produce electricity. In
response, the president’s Republican
opponents have accused him of waging
a “war on coal.”
The report is a 30-page synopsis of a
larger, 900-page report that is to be
released next week on the physical
science of climate change. That will be
followed by additional reports in 2014
on the likely impacts and on possible
steps to limit the damage.
The group has now issued five major
reports since 1990, each of them
finding greater certainty that the world
is warming and greater likelihood that
human activity is the chief cause. The
new report finds a 95 to 100 percent
chance that most of the warming of
recent decades is human-caused, up
from the 90 to 100 percent chance
cited in the last report, in 2007.
But the new document also
acknowledges that climate science still
contains huge uncertainties, including
the likely magnitude of the warming
for a given level of emissions, the rate
at which the ocean will rise, and the
likelihood that plants and animals will
be driven to extinction. The scientists
emphasized, however, that those
uncertainties cut in both directions and
the only way to limit the risk is to limit
The group won the Nobel Peace Prize in
2007, along with Al Gore, the former
vice president, for highlighting the
climate problem. But it came under
attack in recent years by climate-
skeptic organizations, who assailed the
new report as alarmist even before it
was published.
The Heartland Institute, a Chicago
organization that once compared
climate scientists to the Unabomber,
issued a document last week saying that
any additional global warming would
likely be limited to a few tenths of a
degree and this “would not represent a
climate crisis.”
The new report did not take direct aim
at such climate doubters, but it did
curtly dismiss some of their favorite
theories, like the notion that cosmic
rays are warming the planet.
On an issue much cited by the climate
doubters, a slowdown in global
warming that has occurred over the
past 15 years, the report acknowledged
that it was not fully understood, but
said such pauses had occurred in the
past and the natural variability of
climate was a likely explanation.
“People think that global warming
means every year is going to be
warmer than the year before,” said
Gerald A. Meehl, an American scientist
who helped write the report. “It doesn’t
work that way. It’s more like a stair-
step kind of thing.”
Climate scientists not involved in
writing the new report said that the
authors had made a series of cautious
choices in their assessment of the
scientific evidence.
Regarding sea level rise, for instance,
they gave the first firm estimates ever
contained in an intergovernmental
panel report, declaring that if
emissions continue at a rapid pace, the
rise by the end of the 21st century
could be as much as three feet. They
threw out a string of published papers
suggesting a worst-case rise closer to
five feet.
Similarly, the authors went out of their
way to include a recent batch of papers
suggesting the earth might be somewhat
less sensitive to carbon dioxide
emissions than previously thought, even
though serious questions have been
raised about the validity of those
The new report lowered the bottom end
of the range of potential warming that
could be expected to occur over the
long term if the carbon dioxide level in
the atmosphere were to double,
reversing a decision the panel made in
the last report and restoring a
scientific consensus that had prevailed
from 1979 to 2007. Six years ago, that
range was reported as 3.6 to 8.1
degrees Fahrenheit; the new range is
2.7 to 8.1 degrees.
In Washington, the White House praised
the new report. President Obama’s
science adviser, John P. Holdren, cited
increased scientific confidence “that
the kinds of harm already being
experienced from climate change will
continue to worsen unless and until
comprehensive and vigorous action to
reduce emissions is undertaken
Ban Ki-moon, the United Nations
secretary general, spoke to delegates at
the meeting Friday by video link,
declaring his intention to call a
meeting of heads of state in 2014, in a
risky attempt to push such a treaty
forward. The last such high-level
meeting, in Copenhagen in 2009, ended
in disarray.