Scientists: How Matt Ridley misinterpreted new climate sensitivity paper
- 22 May 2013, 16:30
- Roz Pidcock
A new paper suggesting temperatures might not rise as much as
some models predict in the near future has been interpreted in some
corners - notably by
Matt Ridley in the Times - as a sign that climate
change no longer poses a problem. But the authors have spoken out
against Ridley's arguments, highlighting why pinpointing earth's
sensitivity to greenhouse gases relies on more than one
The story stems from a
letter in Nature Geoscience on Sunday with a new
estimate of what scientists call equilibrium climate
sensitivity. That's the total warming we can expect from a
doubling of carbon dioxide in the atmosphere, compared with
Climate sensitivity is important because the higher it is, the
more warming there will be. In its 2007 report, the
Intergovernmental Panel on Climate Change (IPCC) estimated a likely
range of between
two and 4.5 degrees Celsius, with a best estimate of three
More than one way to estimate climate
There are different ways to go about calculating climate
sensitivity - which is part of the reason the IPCC's range is so
large. The new study used what's called an energy budget
model, which used land, atmosphere, ice and ocean temperatures
between 1970 to 2009 to see how warming in all parts of the climate
system has changed in that time. We wrote more about the new paper
Scientists can also use natural recorders of temperature -
climate proxies - to look at how earth's temperature
has changed in response to greenhouse gases throughout its distant
past. The third way is using complex climate models to simulate how
processes affecting temperature rise are likely to evolve in the
Uncertainties about some of these processes -
particularly how clouds affect the rate of warming - means climate
model estimates can vary quite widely.
Most of the media coverage surrounding the new
paper focusses on earth's Transient Climate Response (TCR). This is
a simplified measure of climate sensitivity that estimates the
amount of surface warming we can expect at the point of a doubling
of carbon dioxide.
Importantly, TCR doesn't take into account heat that's
going into the deep oceans rather than staying in the atmosphere.
Scientists think natural
cycles are driving the deep oceans to take up more heat at the
moment, causing surface temperatures to rise slower recently than
in previous decades.
The new paper suggests a best estimate of TCR
based on data from the last decade of 1.3 degrees, but because of
uncertainty around the measurements it could be as low as 0.9
degrees or as high as two degrees.
The authors suggest that unlike their estimate of equilibrium
climate sensitivity - which is largely consistent with model
projections - their range of TCR is
lower than some models predict. This is the point some parts of
the media have picked up on.
In a piece for the Times, Conservative peer and
member of skeptic thinktank the Global Warming Policy
Matt Ridley, compared the new paper's
estimate of TCR with that of the Met Office's
HadGem2 model, which suggests a value of 2.5
In a blog for yesterday's
Guardian, co author on the new paper Myles
Allen disagrees with the point Ridley's trying to make with his
comparison. Allen says:
is worried that government policies are misguided because they
place their faith in climate models, like one of the Met Office
models that puts the warming instead at 2.5 degrees Celsius, almost
twice our estimate."
But, as Allen explains, scientists don't place
all their confidence in just one model. He says:
scientists are all well aware the Met's model (HadGEM2) is at the
top end of the current range. The Met Office's advice to government
is based on the range of results from current climate models, not
just their own."
Allen is making the point that scientists use a
number of climate models to make robust temperature projections -
and if a comparison is to be made, it should be with the average of
all the models, not just HadGem2. He says:
relevant comparison is not with the 2.5 degrees Celsius response of
one model, but with the average of climate models used by the UN's
climate science panel [the IPCC] in its upcoming major report,
which is 1.8 degrees Celsius. Now, 1.3 degrees is 30 per cent less
than 1.8 degrees Celsius, but this is hardly a game
Allen's explains that at 1.8 degrees, the
average value of climate sensitivity from all the models is within
the 0.9 to two degree range of uncertainty that accompanies the new
paper's estimate. In other words, the two estimates aren't
inconsistent with each other.
As part of a longer discussion on the paper's
conclusions, lead author on the paper Alexander Otto makes a
similar point in an article
for the Met Office.
The fact that the uncertainty ranges of
different methods of estimating climate sensitivity are still quite
large is another reason the IPCC's range is unlikely to be refined
Ridley goes on to suggest that given the new
lower estimate of sensitivity, the negative effects of climate
change we're likely to experience before the end of the century are
negligible. He says:
little doubt that the damage being done by climate-change policies
currently exceeds the damage being done by climate change, and will
for several decades yet … At this rate, it will be the last decades
of this century before global warming does net harm."
Part of Ridley's argument is that rolling out renewables
"dwarf[s] any possible [economic] effects of worse weather, for
which there is still no actual evidence anyway: recent droughts,
floods and storms are within historic variability."
Although it's certainly difficult to attribute
particular events to climate change, scientists suggest rising
temperatures are stacking the decks towards more extreme weather.
As Allen puts it:
explaining to a casino bouncer that it doesn't matter you are using
loaded dice because a triple-six is within historic
Warming in the pipeline
While TCR gives a useful indication of the surface warming we
can expect in the next few decades, scientists turn to equilibrium
climate sensitivity for the full picture of the total warming - at
earth's surface and in the oceans - we can expect in the long term.
We wrote a bit more about the difference between the two measures
The new paper's estimated range of equilibrium climate
sensitivity is consistent with that of current climate models.
Contrary to Ridley's suggestion that scientists are "backing away
from rapid warming", this means projections of how much warming we
can expect in the future remain unchanged - at least for now.
While the precise value of climate sensitivity poses an
interesting scientific question, many
argue tackling the policy response is by far the bigger
problem. As Allen puts it:
means we can move on from a sterile debate about the global
response to much more interesting questions about regional impacts,
the rights of different generations, and, most interesting of all,
what to do about it, that's great. Ridley, welcome to the real