Geoscience Reference
In-Depth Information
Modelling uncertainty
In the most recent IPCC Fifth Assessment report, the RCPs were inputted into about 40
GCMs. Each of these models has their own independent design and parameterizations of
key processes. The independence of each model is important, as confidence may be derived
from multiple runs on different models providing similar future climate predictions. In ad-
dition the differences between the models can help us to learn about their individual limita-
tions and advantages. Within the IPCC, due to political expediency, each model and its out-
put is assumed to be equally valid. This is despite the fact that some are known to perform
better than others when tested against reality provided by the historic and palaeoclimate re-
cords. Moreover though we understand uncertainty within a single model the notion of
quantifying uncertainty from many models currently lacks any real theoretical background
or basis. The IPCC combines all the models used for each run and then presents the mean
and the uncertainty between the models. This way it is clear that there are difference in the
model output but that in general they agree and show very different futures based on which
RCP we take. The uncertainties in the IPCC 2013 report are slightly higher than those in
the 2007 report and this is because of our greater understanding of the processes and our
ability to quantify that knowledge. So though our confidence in the climate models has in-
creased, so has the range of possible answers for any specific GHG forcing. Dr Dan Row-
lands (Oxford University) and colleagues in 2012 recently explored the amount of uncer-
tainty inherent in complex models by running one specific climate model through nearly
10,000 simulations (as opposed to the usual handful of runs that can be usually be man-
aged). While his average results matched well with the IPCC projections, they found that
more extreme results, including warming of up to 4°C by 2050, were just as likely as the
less extreme results.
