Geoscience Reference
In-Depth Information
What the sceptics say
One of the best ways to summarize the perceived problems of modelling climate change is
to review what the sceptics say.
(1) Clouds can have negative feedbacks on global climate which will reduce the effects of climate change
As has been the case since the very first IPCC report in 1990, the greatest uncertainty in fu-
ture predictions is the role of the clouds and their interaction with radiation. Clouds can
both absorb and reflect radiation, thereby cooling the surface, and absorb and emit long-
wave radiation, thus warming the surface. The competition between these effects depends
on a number of factors: height, thickness, and radiative properties of clouds. The radiative
properties and formation and development of clouds depend on the distribution of atmo-
spheric water vapour, water drops, ice particles, atmospheric aerosols, and cloud thickness.
The physical basis of how clouds are represented or parametrized in the AOGCMs has
greatly improved through the inclusion of representations of cloud microphysical properties
in the cloud water budget equations. Clouds still represent a significant source of uncer-
tainty in climate simulations. However, as
Figure 14
shows, even if the most extreme cool-
ing value is applied for clouds, the warming factors are still three times larger.
(2) Different models give different results, so how can we trust any of them?
This is a frequent response from many people not familiar with modelling, as there is a
feeling that somehow science must be able to predict an exact future. However, in no other
walk of life do we expect this precision. For example, you would never expect to get a per-
fect prediction of which horse will win a race or which football team will emerge tri-
umphant. The truth is that none of the climate models is exactly right. But what they
provide is the best estimate that we have of the future. Now this view of the future is
strengthened by the use of more than one model, because each model has been developed
by different groups of scientists around the world, using different assumptions and different
computers, and thus they provide their own particular future prediction. What causes sci-
entists to have confidence in the model results is that they all roughly predict the same
trend in global temperature and sea level for the next 100 years. One of the great strengths
of the 2013 IPCC report is that it used over 40 models, while the 2007 IPCC report used 23
international models, compared to seven in 2001. Another strength of this large-scale mul-
tiple model approach is that scientists can also give an estimation of how confident they are
in the model results and also a range of possible predictions, discussed earlier. One key test
of climate models is the equilibrium climate sensitivity (ECS) whereby the model predicts
