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Why are projections of local changes more uncertain than projections of global changes?
In general terms, there is greater agreement
(and so less uncertainty) between climate models at the global scale
and less agreement (and so greater uncertainty) when results are downscaled to more local scales.
Each time the projections are downscaled, additional assumptions are made in the modelling. This can be thought of as the cascade of uncertainty as the projections are downscaled to finer and finer resolutions.
In detail
This can be illustrated using the IPCC
AR4 results for 21 global climate models for Northern Europe. Each map
below shows projected change in annual precipitation for the same
future time period (2080–2099) for the same emissions scenario (A1B – labelled as Medium in UKCP09). The only difference between the maps is that each is produced by a different global climate model.
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Despite differences in how each of the models is constructed and
about how they represent climate processes, the patterns of change look
similar, although the positions and intensities are not. In general,
all models indicate an increase in precipitation in the northern Europe
(green) and a decrease in precipitation in the southern Europe (brown).
This similarity between model projections suggests that there is a
relatively low amount of uncertainty about changes in the gross pattern
of changes in precipitation at a large-scale (i.e. Europe-wide).
However, the models disagree on the magnitude and geographical details of this change – in particular the position of the dividing line between increased and decreased precipitation. As shown below, in some models the orange dividing line
appears north and east of the UK while in other models in appears south
and west of the UK.Models which have similar patterns of change can
have very different magnitudes.
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This large disagreement between global models has large implications
for the UK – it is the difference between a projected wetter or drier
future climate. It illustrates the sensitivity of UK climate change to
large-scale atmospheric and ocean processes.
Find out more
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Last Updated Tuesday, 11 January 2011 |
