Climate change back in the spotlight

Few of you would have missed the news that the Intergovernmental Panel on Climate Change has released its latest Assessment Report. Only the Summary for Policy Makers has been released thus far, with the unedited version of the remainder of the report due for release on Monday.

The findings are largely unsurprising and are consistent with the last assessment report published in 2007. This latest version strengthens its statement of confidence about the role of humans in causing global warming, and (as was expected) has increased its estimates of projected sea level rise. In some way this consistency is reassuring, as the fundamental science is standing the test of time. In fact, many of the key results, such as the equilibrium climate sensitivity (the equilibrium global temperature change for a doubling of CO2) of 1.5-4.5°C, have changed very little since the first IPCC assessment report in 1990.

The findings are also deeply distributing. The estimates of temperature change are all given relative to 1986-2005 levels, and thus it is necessary to add 0.61°C to get an estimate relative to temperatures before the 20th century. After taking this into account, the lowest emission scenario (RCP2.6) gives a projected range of between 0.9 and 2.3°C warming by 2081-2100. However this emission scenario is extremely optimistic. It assumes that CO2 concentrations will stabilise to a concentration of 421ppm by 2100 (CO2 concentrations have increased by 40 % since pre-industrial times,  to nearly 400 ppm now, with a current rate of increase of about 2 ppm per year). This would require that emissions peak before 2020, and fall rapidly after that – something that is becoming more difficult to achieve by the day.

Therefore, I think it is time to accept that global temperatures will probably increase by more than 2°C by the end of this century. And as hydrologists, we will need to take an active role in helping society adapt to these changes. The report from the second Working Group entitled “Impacts, Adaptation and Vulnerability”, to be released in March 2014, will provide much more detail on the local impacts, but here is what the current report says will happen in the coming decades:

(1)    The contrast in precipitation between wet and dry regions, and between wet and dry seasons, will increase (see figure below). This has been dubbed “the rich-get-richer” phenomenon in which those who have abundant rainfall will get more, and those who already have to manage water scarcity will get even less.

(2)    Extreme precipitation over most of the mid-latitudes and over the wet tropics will likely become more intense and more frequent. Much of this can already be detected in the observations.

(3)    Precipitation variability related to the El Niño-Southern Oscillation phenomenon (such as the 2010/11 Queensland flooding rains) will likely intensify because of increased moisture availability.

(4)    The best estimates of sea level rise are roughly half a metre, with a big range (from 0.26 to 0.82 m relative to 1986-2005 levels) because of uncertainty in key physical processes associated with ice sheet dynamics. This is likely to have a significant impact on flood risk in Australia’s coastal zone.

Changes in average precipitation (1986-2005 to 2081-2100) for RCP2.6 (left panel) and RCP8.5 (right panel). Hatching indicates regions where the multi-model mean is small compared to internal variability, and stippling indicates region where the multi-model mean is large compared to internal variability and where 90% of models agree on the sign of the change.

Changes in average precipitation (1986-2005 to 2081-2100) for RCP2.6 (left panel) and RCP8.5 (right panel). Hatching indicates regions where the multi-model mean is small compared to internal variability, and stippling indicates region where the multi-model mean is large compared to internal variability and where 90% of models agree on the sign of the change.

We can only hope that the world will take stronger action in reducing greenhouse gas emissions in the coming decades. Because the ‘best case’ scenario will prove to be challenging enough.

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