March 1 2006
Japan Agency for Marine-Earth Science and Technology
Improved estimation of
the degree of temperature rise due to global warming
- An improved estimate of climate sensitivity -
James Annan and Julia Hargreaves, and colleagues in the Global Warming
Research Program of Frontier Research Center for Global Change (FRCGC),
Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have
made an improved estimate of climate sensitivity (Note
1), showing for the first time that this value is less than
4.5C at the 95% confidence level (Note 2)
| Two recent papers have contributed to this work. The
first, using the MIROC3.2 GCM and the Earth Simulator, demonstrated
the use of past climate data in estimating climate sensitivity, and
limited climate sensitivity to less than 7C. Other recent research
by other researchers had proposed that climate sensitivity could be
as high as 10C.
In the second paper, the researchers used climate data from a number
of historical periods and showed that, by correctly combining information
from multiple sources using Bayesian methods (Note
3), climate sensitivity can be even more tightly constrained
to be very likely (95% confidence) less than 4.5C, with a lower limit
of 1.8C. A likely range of 2.2-3.9C was also established at the 70%
confidence level. In the future, increasingly confident and useful
climate forecasts are expected.
The first paper was published in Scientific Online Letters on the
Atmosphere in December 2005 and the second paper is shortly to be
published in Geophysical Research Letters.
In 1979 it was first stated that climate sensitivity was likely to
lie in the range 1.5-4.5C. This range was a very subjective estimate
based on the limited climate modelling performed at that time. Subsequent
IPCC Assessment Reports have supported this estimate without being
able to narrow its range, and the Third Assessment Report (TAR) quantified
"likely" to mean a confidence in those bounds only at the
66-90% level. Furthermore, recent research based on observational
data had indicated that much higher values for climate sensitivities
could not be reasonably ruled out, suggesting that this subjective
range might be too optimistic. Values for climate sensitivity of up
to 10C have been proposed, which would have disastrous consequences
for the future of the planet.
This failure to decrease the uncertainty on the estimate of climate
sensitivity, as well as the possibility of the true value being very
high, has caused considerable alarm in the global warming debate at
both the scientific and political level.
In this new research, it has been shown that by combining observational
evidence from a wide range of sources, a greatly improved observationally-based
estimate can be formed. For the first time, an upper bound of 4.5C
(at the 95% confidence level) has been established from an analysis
of observational data.
The greatest achievement of the experiment is to rule out the extremely
high estimates of climate sensitivity that have previously been proposed.
The work also describes the methods that can be used to combine information
so that the full benefit can be gained from available data and models.
The Earth Simulator enabled ensembles of models to be run, greatly
opening up the possibility for exploration of the problem using state
of the art climate models. The value of climate sensitivity has now
been demonstrated to lie within a reasonably narrow range, and this
range is highly consistent with the results achieved from the state
of the art climate models run at climate research centres throughout
the world. This results should increase confidence in the model results
and enable researchers to move forward in forecasting the impacts
of climate change in greater detail.
The result increases the confidence that we can have in climate models
and it is now hoped that researchers can use this information to start
making accurate prediction of climate change on regional scales and
over the time scales of interest to policy makers. It is also likely
that the methods described will lead to a more precise estimate of
climate sensitivity in the near future.
J. D. Annan, J. C. Hargreaves, R. Ohgaito, A. Abe-Ouchi, S. Emori.
Efficiently constraining climate sensitivity with paleoclimate simulations.
SOLA Vol 1 pages 181-184.
J. D. Annan and J. C. Hargreaves. Using multiple observationally-based
constraints to estimate climate sensitivity. In press, GRL
|Notes 1: Climate sensitivity
is defined as the globally-averaged surface temperature change that
would result in the long-term from a doubling of the atmospheric CO2
concentration. It describes the most fundamental response of the climate
system to anthropogenic forcing, and so determining its value is of
great importance in predicting future climate change.
Note 2: Confidence level is the probability
value associated with a confidence interval.
Note 3: Bayes' Theorem is a fundamental
result in probability theory which explains how multiple sources
of information can be combined to generate an improved estimate.
Observations of recent global warming, short-term cooling after
major volcanic eruptions, cooling at the Last Glacial Maximum and
other periods in the historical record, and the seasonal variation
in climate, all provide some information which helps to determine
the value of climate sensitivity.
Japan Agency for Marine-Earth Science and Technology
Mr. Katsuhiko Masuda, Director
Frontier Research Promotion Department,
Frontier Research Center for Global Change
Mr. Kenichi Takahashi, Manager
Public Relations Division, Administration Department
Tel:046-867-9050 Fax: 046-867-9055