Climate sensitivity in the media: A case of mistranslation?
- 09 Apr 2013, 18:30
- Roz Pidcock
Climate sensitivity is a measure of how sensitive earth is to
greenhouse gases - and is key to forecasting future temperature
rise. But as the media discusses climate sensitivity, some of the
scientific detail is being lost. Carbon Brief looks at what
scientists really mean when they talk about climate
Estimating climate sensitivity
Understanding how sensitive the climate is to greenhouse gases
isn't just about atmospheric emissions. A warming atmosphere has
knock on effects that can amplify or dampen warming, such as the
loss of sea ice which reflects sunlights and helps keep global
temperatures down. These are known as
feedback processes - and they make climate sensitivity
inherently difficult to quantify.
In 2007, the Intergovernmental Panel on Climate Change
estimated a doubling of carbon dioxide above
pre-industrial levels - 280 parts per million - was likely to raise
global temperature by between two and 4.5 degrees Celsius. But it
didn't rule out higher values of up to six or seven degrees.
The amount of temperature rise following a doubling of carbon
dioxide levels is called "climate sensitivity". If climate
sensitivity is higher, climate change will happen faster. And if
it's lower, the planet will warm more slowly as greenhouse gases
are put into the atmosphere.
three main ways to estimate climate sensitivity. One
method is to look at how earth responded to natural greenhouse gas
changes in its geological past. Another matches global atmospheric
temperatures with greenhouse gas concentrations over the last
century or so. The third uses climate models to predict the
theoretical effect of a doubling of carbon dioxide.
As the graphic below shows, the different methods produce
generally comparable results.
Source: Skeptical Science adapted from Knutti and Hegerl (2008)
But there is still some important uncertainty in climate models
- mainly to do with how they simulate clouds, which can have a
warming or cooling effect. A few models project higher values,
which means the right hand side of the yellow curve in the graph
above extends beyond 4.5 degrees. Scientists have called the small
chance that these high values could be right the "
long tail" of climate sensitivity.
The "long tail"
The long tail idea has been around for a while. But now,
scientists are discussing
two possibilities. The first is whether new
research means this long tail of climate sensitivity can now be
As climate scientist James Annan explains on his
"It's increasingly difficult to
reconcile a high climate sensitivity (say over four degrees
Celsius) with the observational evidence for the planetary energy
balance over the industrial era."
Essentially, Annan believes that looking at how the climate
changed over the past few centuries gives enough information to
rule out the long tail of climate sensitivity.
Some skeptic bloggers have suggested Annan's perspective is
controversial among climate scientists. But climate science
Science explains, "there are few if any mainstream climate
scientists who would disagree with [Annan's suggested upper
limit]". Andy Revkin's
Dot Earth blog also has a useful discussion of what scientists
think the likely range is.
The second strand of scientific discussion is whether a subset
of climate models projecting higher temperature rise than has
occurred recently could be overestimating climate sensitivity.
The last decade or so has seen a fairly muted rise in global
atmospheric temperature, meaning temperatures are now approaching
lower boundary of where IPCC models predict they should be.
Source: Ed Hawkins,
Climate Lab Book
speculating about reasons for the mismatch. Oxford University
climate scientist Myles Allen
told the Guardian recently:
"[R]ecent observations [are] indeed
suggesting that very high values of the so-called "climate
sensitivity" (the long-term warming we should expect on doubling
carbon dioxide), values greater than five Celsius or so, [are]
looking less likely."
So this might be another reason to rule out the very high 'long
tail' of climate sensitivity - values above five degrees, in this
case. But it's important to flag that it's only one possible
explanation. We wrote more about this
here after a
Mail on Sunday piece suggested the recent mismatch meant
climate models "were wrong all along" - which is clearly not the
To summarise, both scientific discussions point to the
possibility of ruling out the higher values of climate sensitivity
- above 4.5 degrees - not about changing the likely range of
climate sensitivity according to the IPCC. That's one mistake
recent articles in both the
Economist and the
Telegraph appear to have made.
Some high-profile papers have emerged recently supporting values
at the lower end of the IPCC range, which both articles picked up
on to support their conclusions. One is an un-peer reviewed and
unpublished study by Norwegian researchers suggesting a value of
1.9 degrees. We reported on the study
here. The Economist also mentions - among others -
a paper by Julia Hargreaves, suggesting a mean estimate of 2.5
But as this Skeptical
Science blog explains, the lower estimates are based on
atmospheric temperature change and don't include the role of the
oceans. This means they could
underestimate recent warming. A recent
paper suggests the rate of ocean warming has accelerated during
the current slowdown in atmospheric temperature rise.
The bottom line is that the IPCC likely range can't be narrowed
down any further just yet. A number of studies still point towards
both ends of the IPCC range. As a recent blog post from the
Committee on Climate Change says:
"While it is true that recent studies
based on temperature observations question the very highest model
projections of warming, even if confirmed, these would not justify
a wholesale downward revision of the [IPCC] range..."
In any case, many
argue the policy response should be the same
regardless of the climate sensitivity. Atmospheric carbon
dioxide concentration is already approaching 400 ppm and
the IPCC's high emissions scenario (A1F1) suggests we
could see a doubling of carbon dioxide above pre-industrial levels
Even with a climate sensitivity at the lower end of the IPCC
range, by mid century the world would be committed to two degrees
of warming above pre-industrial levels. That's the target
set by the United Nations Framework Convention on Climate Change
(UNFCCC) to avoid serious climate change. As
Piers Forster, climatologist at the university of Leeds,
"Even with a suggested [climate
sensitivity] of around 2.5 Celsius or so we can end up with a very
significant climate change by 2100 if we don't do something."
And if climate sensitivity is at the upper end of the IPCC's
likely range - even excluding the higher 'long tail' - reports
suggesting a four
degree temperature rise above pre-industrial levels by the end
of the century look optimistic given current emission
A scientific question
Scientists have been grappling with what the likely value of
climate sensitivity is for a long time now - and it's proving a
tricky one to pin down. But the question surrounding the true value
of climate sensitivity is ultimately a scientific one not a
political one - if emissions continue unabated we're looking as
serious climate change this century whatever happens.