Surface warming slowdown doesn’t affect climate sensitivity, study says
- 21 May 2013, 12:00
- Roz Pidcock
A group of scientists has just published a new
estimate of how sensitive earth is to rising greenhouse gases.
Their value for 'climate sensitivity' is at the low end of what
scientists have previously suggested - but there's still a lot of
Importantly, the study shows slower surface
temperature rise over the past decade doesn't change scientists'
projections of how much warming we can expect in the long term.
What scientists call 'equilibrium climate sensitivity' is the
warming we can expect from a doubling of carbon dioxide in the
atmosphere, compared with pre-industrial levels. It's a number
scientists are still trying to pin down - and it's important,
because the higher climate sensitivity is, the more warming there
In its last report, published in 2007, the Intergovernmental
Panel on Climate Change (IPCC) estimated a likely range of climate
sensitivity of between
two and 4.5 degrees Celsius, with a best estimate of three
degrees. But it didn't rule out lower or higher values and since
then, scientists have continued to try to narrow the range of
uncertainty surrounding the number.
new study, published as a letter in Nature Geoscience, is based
on measurements of how much heat the land, ocean, ice and
atmosphere have absorbed between 1970 and 2009.
The new calculations produced a best estimate for climate
sensitivity of 1.9 degrees Celsius, which is similar to a couple of
other studies taking a similar approach that have attracted
media attention recently.
A value of 1.9 degrees Celsius sits just below the IPCC's likely
range. But the new estimate also comes with a large uncertainty
range - of 0.9 to five degrees. One of the co-authors on the new
study, Professor Piers Forster, tells us:
"The message is that the uncertainties
are really too large to offer tight constraints on climate
sensitivity - a wide range is still possible."
Hence, the paper isn't a conclusive argument for a lower value
of climate sensitivity. But lead author Alexander Otto tells us it
might support lowering the bottom boundary of the IPCC's likely
range a bit:
"[T]he energy budget
allows lower values than two degrees, but it is also consistent
with values up to four or five degrees."
Other scientists argue that these new results make the upper end
of the IPCC range for climate sensitivity look unlikely. Professor
James Annan says on his blog:
[the new estimate] reflects a change in thinking from the IPCC
authors involved … [The result] implies a marked lowering of the
IPCC 'likely' range."
Sensitivity and the surface warming
Despite surface temperatures rising relatively
slowly in the last decade and a half, the new study finds
temperature data from the last decade suggests a best estimate of
climate sensitivity that's consistent with the previous three
decades - around two degrees Celsius. The uncertainty range is a
bit narrower though at 1.2 to 3.9 degrees.
The similarity between decades doesn't support some media
suggestions that the recent surface warming slowdown might be
explained by a lower value of climate sensitivity. As Otto
"[T]he important message
on equilibrium climate sensitivity is that there isn't [a message]
… [T]here isn't any obvious inconsistency between the energy budget
of the past decade and the conventional range."
In other words, looking at the whole climate
system, the earth warmed by a similar amount during the last decade
as it did in previous ones. That's because estimating climate
sensitivity the way the authors did takes into account all elements
of the climate system - not just the atmosphere. While surface
temperatures may have risen relatively slowly recently, the study
suggests the slower pace of warming is being
compensated by faster temperature rise
The authors' top line conclusion is that
although their best estimate of climate sensitivity for the last
decade is relatively low, the range accompanying the data is
consistent with the range projected by current climate models. The
overlap for the whole period - 1970 to 2009 - with climate models
is even stronger, say the authors, which means there's no reason to
revise down scientists' estimates of the warming we can expect in
the long term.
Scientists say the most likely reason for
the shift from more to less atmospheric warming in the last decade
is that natural climate variability is causing more heat to enter
Climate sensitivity calculated for surface
temperatures alone is called the Transient Climate Response (TCR).
TCR is easier to estimate than equilibrium climate sensitivity
because it doesn't consider long term climate feedbacks, like the
delay in atmospheric warming caused by heat entering the oceans.
But for that same reason, it only gives part of the
The study calculates TCR based on the last decade, giving an
estimate of 1.3 degrees with an uncertainty range of 0.9 to 2.0
degrees. The new TCR range is slightly lower than some IPCC models
project. Otto tells us:
"[H]ence we conclude
that the more extreme versions of these models might be
inconsistent with the data and less likely … [but] we need to be
careful about interpreting just one decade, and the 1970 to 2009
period as a whole suggests 0.7 to 2.5 degrees."
It's this result that much of the media coverage of the paper
has picked up on. It's arguable that TCR gives a useful indication
of the amount of warming we'll directly experience in the short
term. But for a complete impression of the warming we can expect in
the long term, equilibrium climate sensitivity remains the relevant
Sensitivity is not the same as eventual
The overall scientific view of the likely range
of equilibrium climate sensitivity won't be altered on the back of
one study. It seems fair to say scientists' projections of long
term temperature rise remain largely unchanged - for now at
Finally, it's worth stressing that equilibrium
climate sensitivity is not equivalent to total warming, it's the
eventual warming expected per doubling of carbon dioxide after all
feedbacks have been accounted for.
The IPCC's high emissions scenario (A1F1
) suggests we could see a doubling of carbon dioxide above
pre-industrial levels by 2050. That means that even with a climate
sensitivity sitting at or just below the lower end of the IPCC
range, by mid century the world would be committed to two degrees
of warming above pre-industrial levels.
Two degrees is the target
set by the United Nations Framework Convention on Climate
Change (UNFCCC), which is often suggested as a threshold to avoid
dangerous climate change. And after 2050, unless emissions had
fallen to zero, further warming would occur.
As Forster puts it:
"[The new study] has an
effect but not a massive effect on projections. We still need to
mitigate carbon dioxide emissions very significantly to keep below
two degrees [of warming above pre-industrial levels]."
Meanwhile, if climate sensitivity is at the
upper end of the IPCC's likely range, reports suggesting a
degree temperature rise above pre-industrial
levels by the end of the century look optimistic given current