usaid/nambia environmental threats and opportunities assessment

ANNEX C
Interpretation of CC projections for Namibia (After Turpie et al 2010)
Because of the high level of uncertainty in projections for future climate in southern Africa, particularly
of rainfall change, it seems useful from a policy perspective to estimate the potential ranges of impacts,
including high, median and low impacts, by 2050, as it is unlikely that the uncertainty range will be
reduced in the near future, and because of the impacts of current levels of climate variability in the
region. Variability is likely to dominate the climate signal for at least a few decades until clear climate
change signals become evident. Using high resolution spatially downscaled climate information seems of
little use in this regard, as it is more important for policy development to estimate the impacts
particularly at the median and “tails” of the distribution of possible future climate scenarios. Estimates at
the tails of the distribution can provide an assessment of impacts that have a low probability but a high
societal relevance if they do occur.
Climate scenarios that are currently generated using General Circulation Models (GCMs) have two main
sources of uncertainty that result in a relatively wide range of projections, especially for rainfall futures,
for southern Africa in particular. These are 1) the GCM design itself, which varies between the several
models used in the IPCC AR4, and 2) the emissions scenarios used to drive the GCMs. The largest
source of uncertainty by the middle of this century is due to GCM design, and rather little is due to
emissions scenario. Emissions scenario is however an important source of uncertainty and variation for
simulations towards the end of the century. As mentioned above, due to the potentially large range of
uncertainty in scenarios, it seems of little value to focus on fine spatial scales for climate scenarios and
impacts studies, as by far the largest source of uncertainty is at large spatial scale. It is also of limited
value to consider a range of emissions scenarios, but rather to focus on understanding the range of GCM
variation, and to attempt to represent impacts that might relate to the median and the extremes of that
range for policy relevant information.
Unfortunately it is currently difficult to obtain spatially downscaled climate projection data for measures
other than rainfall or temperature for southern Africa outside of South Africa for the IPCC AR4 climate
projections, especially for the middle of this century. We have thus compared the best available
information for the IPCC AR4 generated by GCM’s for the year 2100 (median projections of 21 GCMs,
driven by the A1B emissions scenario) with the interpolated HADCM3 GCM data used for the previous
most comprehensive impacts assessment on Namibia for 2050 ( Midgley et al 2005 . Because this
comparison (Figure A) shows that the HADCM3 GCM used by Midgley et al. (2005) represents roughly
a median climate future for the 21 AR4 GCMs, climate surfaces representing rainfall and temperature
change at the monthly temporal scale for 2050 and 2080 have been created for this project using the
HADCM3 GCM (as driven by the A2 scenario). These have been overlaid on a current climate surface
that is taken from the recognized and quality-controlled WorldClim data set and used for impact
assessments of species-level change.
Comparison of IPCC AR4 scenarios with those used by Midgley et al (2005) reveal that the median
rainfall change projected for 2100 by the IPCC AR4 (between 5 and 20% reduction) is comparable to the
least extreme median rainfall change used by Midgley et al. (2005), represented by the HAD CM3 model
for 2050, under an A2 emissions scenario. By 2080, this scenario suggests a more extreme rainfall change
of between a 10 and 30% reduction. The 2050 scenario used by Midgley et al. (2005) shows a relatively
spatially uniform rainfall change, with the largest reductions of ~ 20% across the centre of Namibia, with
more severe drying suggested in the northwest and on the central coast. This contrasts with the IPCC
USAID/NAMIBIA ENVIRONMENTAL THREATS AND OPPORTUNITIES ASSESSMENT 95