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Scientific American Volume 310, Issue 4
>> Features <http://www.scientificamerican.com/department/feature-articles>
See Inside[image: Scientific American Volume 310, Issue
Will Cross the Climate Danger Threshold by 2036
The rate of global temperature rise mayhave hit a plateau, but a climate
crisis still looms in the near future
Mar 18, 2014 |By Michael E.
*Click here to view a larger version of this graphic
*Credit: Pitch Interactive; SOURCE: MICHAEL E. MANN*
- The rate at which the earth's temperature has been rising eased
slightly in the past decade, but temperature is still increasing; calling
the slowdown a "pause" is false.
- New calculations by the author indicate that if the world continues to
burn fossil fuels at the current rate, global warming will rise to two
degrees Celsius by 2036, crossing a threshold that will harm human
- To avoid the threshold, nations will have to keep carbon dioxide
levels below 405 parts per million.
More In This Article
- [image: Enlarge Graphic] Enlarge
- [image: Enlarge Graphic] Enlarge
- [image: Why Global Warming Will Cross a Dangerous Threshold in 2036] Why
Global Warming Will Cross a Dangerous Threshold in
"Temperatures have been flat for 15 years--nobody can properly explain it,"
the *Wall Street Journal* says. "Global warming 'pause' may last for 20
more years, and Arctic sea ice has already started to recover," the *Daily
Mail* says. Such reassuring claims about climate abound in the popular
media, but they are misleading at best. Global warming continues unabated,
and it remains an urgent problem.
The misunderstanding stems from data showing that during the past decade
there was a slowing in the rate at which the earth's average surface
temperature had been increasing. The event is commonly referred to as "the
pause," but that is a misnomer: temperatures still rose, just not as fast
as during the prior decade. The important question is, What does the
short-term slowdown portend for how the world may warm in the future?
The Intergovernmental Panel on Climate Change (IPCC) is charged with
answering such questions. In response to the data, the IPCC in its
September 2013 report lowered one aspect of its prediction for future
warming. Its forecasts, released every five to seven years, drive climate
policy worldwide, so even the small change raised debate over how fast the
planet is warming and how much time we have to stop it. The IPCC has not
yet weighed in on the impacts of the warming or how to mitigate it, which
it will do in reports that were due this March and April. Yet I have done
some calculations that I think can answer those questions now: If the world
keeps burning fossil fuels at the current rate, it will cross a threshold
into environmental ruin by 2036. The "faux pause" could buy the planet a
few extra years beyond that date to reduce greenhouse gas emissions and
avoid the crossover--but only a few.
*A Sensitive Debate*
The dramatic nature of global warming captured world attention in 2001,
when the IPCC published a graph that my co-authors and I devised, which
became known as the "hockey stick." The shaft of the stick, horizontal and
sloping gently downward from left to right, indicated only modest changes
in Northern Hemisphere temperature for almost 1,000 years--as far back as
our data went. The upturned blade of the stick, at the right, indicated an
abrupt and unprecedented rise since the mid-1800s. The graph became a
lightning rod in the climate change debate, and I, as a result, reluctantly
became a public figure. In its September 2013 report, the IPCC extended the
stick back in time, concluding that the recent warming was likely
unprecedented for at least 1,400 years.
Although the earth has experienced exceptional warming over the past
century, to estimate how much more will occur we need to know how
temperature will respond to the ongoing human-caused rise in atmospheric
greenhouse gases, primarily carbon dioxide. Scientists call this
responsiveness "equilibrium climate sensitivity" (ECS). ECS is a common
measure of the heating effect of greenhouse gases. It represents the
warming at the earth's surface that is expected after the concentration of
CO2 in the atmosphere doubles and the climate subsequently stabilizes
The preindustrial level of CO2 was about 280 parts per million (ppm), so
double is roughly 560 ppm. Scientists expect this doubling to occur later
this century if nations continue to burn fossil fuels as they do now--the
"business as usual" scenario--instead of curtailing fossil-fuel use. The
more sensitive the atmosphere is to a rise in CO2, the higher the ECS, and
the faster the temperature will rise. ECS is shorthand for the amount of
warming expected, given a particular fossil-fuel emissions scenario.
It is difficult to determine an exact value of ECS because warming is
affected by feedback mechanisms, including clouds, ice and other factors.
Different modeling groups come to different conclusions on what the precise
effects of these feedbacks may be. Clouds could be the most significant.
They can have both a cooling effect, by blocking out incoming sunlight, and
a warming effect, by absorbing some of the heat energy that the earth sends
out toward space. Which of these effects dominates depends on the type,
distribution and altitude of the clouds--difficult for climate models to
predict. Other feedback factors relate to how much water vapor there will
be in a warmer atmosphere and how fast sea ice and continental ice sheets
Because the nature of these feedback factors is uncertain, the IPCC
provides a range for ECS, rather than a single number. In the September
report--the IPCC's fifth major assessment--the panel settled on a range of
1.5 to 4.5 degrees Celsius (roughly three to eight degrees Fahrenheit). The
IPCC had lowered the bottom end of the range, down from the two degrees C
it had set in its Fourth Assessment Report, issued in 2007. The IPCC based
the lowered bound on one narrow line of evidence: the slowing of surface
warming during the past decade--yes, the faux pause.
Many climate scientists--myself included--think that a single decade is too
brief to accurately measure global warming and that the IPCC was unduly
influenced by this one, short-term number. Furthermore, other explanations
for the speed bump do not contradict the preponderance of evidence that
suggests that temperatures will continue to rise. For example, the
accumulated effect of volcanic eruptions during the past decade, including
the Icelandic volcano with the impossible name, Eyjafjallajökull, may have
had a greater cooling effect on the earth's surface than has been accounted
for in most climate model simulations. There was also a slight but
measurable decrease in the sun's output that was not taken into account in
the IPCC's simulations.
Natural variability in the amount of heat the oceans absorb may have played
a role. In the latter half of the decade, La Niña conditions persisted in
the eastern and central tropical Pacific, keeping global surface
temperatures about 0.1 degree C colder than average--a small effect compared
with long-term global warming but a substantial one over a decade. Finally,
one recent study suggests that incomplete sampling of Arctic temperatures
led to underestimation of how much the globe actually warmed.
None of these plausible explanations would imply that climate is less
sensitive to greenhouse gases. Other measurements also do not support the
IPCC's revised lower bound of 1.5 degrees C. When all the forms of evidence
are combined, they point to a most likely value for ECS that is close to
three degrees C. And as it turns out, the climate models the IPCC actually
used in its Fifth Assessment Report imply an even higher value of 3.2
degrees C. The IPCC's lower bound for ECS, in other words, probably does
not have much significance for future world climate--and neither does the
For argument's sake, however, let us take the pause at face value. What
would it mean if the actual ECS were half a degree lower than previously
thought? Would it change the risks presented by business-as-usual
fossil-fuel burning? How quickly would the earth cross the critical
*[image: Predicting the Future]
Date with Destiny: 2036*
Most scientists concur that two degrees C of warming above the temperature
during preindustrial time would harm all sectors of civilization--food,
water, health, land, national security, energy and economic prosperity. ECS
is a guide to when that will happen if we continue emitting CO2 at our
I recently calculated hypothetical future temperatures by plugging
different ECS values into a so-called energy balance model, which
scientists use to investigate possible climate scenarios. The computer
model determines how the average surface temperature responds to changing
natural factors, such as volcanoes and the sun, and human
factors--greenhouse gases, aerosol pollutants, and so on. (Although climate
models have critics, they reflect our best ability to describe how the
climate system works, based on physics, chemistry and biology. And they
have a proved track record: for example, the actual warming in recent years
was accurately predicted by the models decades ago.)
I then instructed the model to project forward under the assumption of
business-as-usual greenhouse gas emissions. I ran the model again and
again, for ECS values ranging from the IPCC's lower bound (1.5 degrees C)
to its upper bound (4.5 degrees C). The curves for an ECS of 2.5 degrees
and three degrees C fit the instrument readings most closely. The curves
for a substantially lower (1.5 degrees C) and higher (4.5 degrees C) ECS
did not fit the recent instrumental record at all, reinforcing the notion
that they are not realistic.
To my wonder, I found that for an ECS of three degrees C, our planet would
cross the dangerous warming threshold of two degrees C in 2036, only 22
years from now. When I considered the lower ECS value of 2.5 degrees C, the
world would cross the threshold in 2046, just 10 years later [*see graph on
pages 78 and 79*].
So even if we accept a lower ECS value, it hardly signals the end of global
warming or even a pause. Instead it simply buys us a little bit of
time--potentially valuable time--to prevent our planet from crossing the
These findings have implications for what we all must do to prevent
disaster. An ECS of three degrees C means that if we are to limit global
warming to below two degrees C forever, we need to keep CO2 concentrations
far below twice preindustrial levels, closer to 450 ppm. Ironically, if the
world burns significantly less coal, that would lessen CO2 emissions but
also reduce aerosols in the atmosphere that block the sun (such as sulfate
particulates), so we would have to limit CO2 to below roughly 405 ppm.
We are well on our way to surpassing these limits. In 2013 atmospheric
reached 400 ppm for the first time in recorded history--and perhaps for the
first time in millions of years, according to geologic evidence. To avoid
breaching the 405-ppm threshold, fossil-fuel burning would essentially have
to cease immediately. To avoid the 450-ppm threshold, global carbon
emissions could rise only for a few more years and then would have to ramp
down by several percent a year. That is a tall task. If the ECS is indeed
2.5 degrees C, it will make that goal a bit easier.
Even so, there is considerable reason for concern. The conclusion that
limiting CO2below 450 ppm will prevent warming beyond two degrees C is
based on a conservative definition of climate sensitivity that considers
only the so-called fast feedbacks in the climate system, such as changes in
clouds, water vapor and melting sea ice. Some climate scientists, including
James E. Hansen, former head of the nasa Goddard Institute for Space
Studies, say we must also consider slower feedbacks such as changes in the
continental ice sheets. When these are taken into account, Hansen and
others maintain, we need to get back down to the lower level of CO2 that
existed during the mid-20th century--about 350 ppm. That would require
widespread deployment of expensive "air capture" technology that actively
removes CO2 from the atmosphere.
Furthermore, the notion that two degrees C of warming is a "safe" limit is
subjective. It is based on when *most* of the globe will be exposed to
potentially irreversible climate changes. Yet destructive change has
already arrived in some regions. In the Arctic, loss of sea ice and thawing
permafrost are wreaking havoc on indigenous peoples and ecosystems. In
low-lying island nations, land and freshwater are disappearing because of
rising sea levels and erosion. For these regions, current warming, and the
further warming (at least 0.5 degree C) guaranteed by CO2 already emitted,
constitutes damaging climate change today.
Let us hope that a lower climate sensitivity of 2.5 degrees C turns out to
be correct. If so, it offers cautious optimism. It provides encouragement
that we can avert irreparable harm to our planet. That is, if--and only
if--we accept the urgency of making a transition away from our reliance on
fossil fuels for energy.
This article was originally published with the title "False Hope."
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