Comparison of climate change signals in CMIP3 and CMIP5 multi-model ensembles and implications for Central Asian glaciers

Central Asian water resources largely depend on
melt water generated in the Pamir and Tien Shan mountain
ranges. To estimate future water availability in this region,
it is necessary to use climate projections to estimate the future
glacier extent and volume. In this study, we evaluate
the impact of uncertainty in climate change projections on
the future glacier extent in the Amu and Syr Darya river
basins. To this end we use the latest climate change projections
generated for the upcoming IPCC report (CMIP5) and,
for comparison, projections used in the fourth IPCC assessment
(CMIP3). With these projections we force a regionalized
glacier mass balance model, and estimate changes in the
basins’ glacier extent as a function of the glacier size distribution
in the basins and projected temperature and precipitation.
This glacier mass balance model is specifically developed
for implementation in large scale hydrological models,
where the spatial resolution does not allow for simulating individual
glaciers and data scarcity is an issue. Although the
CMIP5 ensemble results in greater regional warming than
the CMIP3 ensemble and the range in projections for temperature
as well as precipitation is wider for the CMIP5 than
for the CMIP3, the spread in projections of future glacier
extent in Central Asia is similar for both ensembles. This
is because differences in temperature rise are small during
periods of maximum melt (July–September) while differences
in precipitation change are small during the period
of maximum accumulation (October–February). However,
the model uncertainty due to parameter uncertainty is high,
and has roughly the same importance as uncertainty in the
climate projections. Uncertainty about the size of the decline
in glacier extent remains large, making estimates of future
Central Asian glacier evolution and downstream water
availability uncertain.