A recent comparison of global temperature observations and model simulations on this blog prompted a rush of media and wider interest, notably in the Daily Mail, The Economist & in evidence to the US House of Representatives. Given the widespread misinterpretation of this comparison, often without the correct attribution or links to the original source, a more complete description & update is needed.
The two figures below compare observed global temperature changes with the latest projections from 42 different climate models (CMIP5) for the 1950-2050 period, using a common reference period of 1961-1990. One figure shows the individual simulations, and the other shows an estimate of the 50% and 90% confidence intervals.
What can be learnt from this comparison? Simply, global temperatures have not warmed as much as the mean of the model projections in the past decade or so and are currently at the lower edge of the ensemble of simulations. However, there are simulations which are consistent with the observations.
Interpretation of this comparison is not straightforward for many reasons (see below).
But, what does this mean for the future? I welcome suggestions for a ‘likely’ range for the global temperature mean of the 2016-2035 period – the IPCC AR5 will give their verdict on this in early October…
UPDATE (11/08/13): The second figure below was recently used in a U.S. Senate hearing (at about 3h27m).
UPDATE (23/09/13): A version of this comparison was shown on BBC News today as part of their preparatory coverage for the IPCC AR5.
Role of aerosols and other radiative forcings
These particular simulations use historical radiative forcings (greenhouse gases, solar activity etc) until 2005, and an assumed ‘scenario’ of radiative forcings (named RCP 4.5) from 2005-2050. Firstly, this is just one scenario of post-2005, and there are others – this uncertainty is not represented here.
One of the key uncertainties is in the role of aerosols (small particles emitted through fossil fuel burning which tend to reflect sunlight, interact with clouds and cool the climate). All the RCP scenarios assume that atmospheric concentrations of aerosols decline at slightly different rates post-2005, but all are perhaps optimistic. At the very least this does not fully sample the uncertainty in post-2005 aerosol emissions.
Why has the globe warmed less than projected in the last decade?
So, why might the simulations appear too warm? One possibility is that aerosol emissions have not declined as rapidly as assumed since 2005, causing the simulations to appear too warm. However, observations of aerosol emissions are somewhat uncertain. Also, no volcanic eruptions are present in the post-2005 simulations, although it is possible that recent moderate eruptions have had a cooling influence. In addition, solar activity post-2005 has been weaker than the simulations assume. All of these effects would make the simulations appear too warm.
There is also some recent evidence that the models with the very highest climate sensitivities may be inconsistent with the observations. Uncertainty in the observations of global temperatures is also not negligible, as shown by the red lines.
The final possible explanation is that internal climate variability has reduced the rate of warming this decade, and that some of the additional energy may be in the deep ocean instead of the atmosphere.
Some more technical considerations:
Although the broad qualitative picture is fairly robust, note that there is also sensitivity of the quantitative results to:
(1) choice of reference period (1961-1990 is used here)
(2) choice of observational datatset (HadCRUT4 is used here)
(3) choice of ensemble members – I have picked 1 ensemble member per model, but note that significant differences are possible in different ensemble members
with (1) being perhaps the most important. Also note that these 42 different simulations are not independent, due to similarities between the models used. For example, 8 simulations are from the NASA-GISS family of models. Also note that I have ‘masked’ the simulations to only use data at the same locations where gridded observations in the HadCRUT4 dataset exist.