Aerosols in the IPCC 2013 Summary for Policymakers

Whilst I don’t approve of “cherry picking” from important reports such as the IPCC WG1 Summary for Policymakers (SPM) that was published today, I do need to look particularly for the updates to the quotes from previous reports that have motivated much of what I do in my day to day job. Previously, the IPCC (2007) report said that aerosols were one of the most uncertain aspects of climate change. So what does the new report bring?

As I’ve said in my previous post, the IPCC have considered a phenomenal number of new publications since 2007. There has been a particularly large research effort since 2007 in trying to understand how aerosols affect climate, and to better represent them in models. The full WG1 report available on Monday 30th September 2013 will have an entire chapter concerning aerosols, and aerosol-cloud interactions, but the relevant parts that made it to the SPM are interesting.

1. Improved estimates of radiative forcing (perturbation to the energy balance of the planet) due to aerosols indicate a weaker net cooling relative to 1750 than was included in the last IPCC report (AR4)

2. The radiative forcing (RF) of the total aerosol effect in the atmosphere, which includes cloud adjustments due to aerosols is -0.9 [-1.9 to -0.1] Wm-2 with medium confidence and results from a negative forcing from most aerosols and a positive contribution from black carbon absorption of solar radiation. There is high confidence that aerosols and their interactions with clouds have offset a substantial portion of global mean forcing from well-mixed greenhouse gases. They continue to contribute the largest uncertainty to the total RF estimate

3. Climate models now include more cloud and aerosol processes, and their interactions, that at the time of the AR4, but there remains low confidence in the representation and quantification of these processes in models.

4. Observational and modelling evidence indicates that, all else being equal, locally higher surface temperatures in polluted regions will trigger regional feedbacks in chemistry and local emissions that will increase peak levels of ozone and PM2.5 (medium confidence). For PM2.5, climate change may alter natural aerosol sources as well as removal by precipitation, but no confidence level is attached to the overall impact of climate change on PM2.5 distributions.

5. A lower warming target, or higher likelihood of remaining below a specific warming target will require lower cumulative CO2 emissions. Accounting for warming effects of increases in non-CO2 greenhouse gases, reductions in aerosols, or the release of greenhouse gases from permafrost will also lower the cumulative CO2 emissions for a specific warming target

6. Solar Radiation Management (SRM) “geo-engineering”: Modelling indicates that these methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle and would not reduce ocean acidification. If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing. SRM methods carry side-effects and long-term consequences on a global-scale.

All in all, I’m probably not out of a job just yet…

You can find the 18 key IPCC headlines that were agreed by 110 governments in the form of tweets by @piersforster and storify form courtesy of Mark Brandon @icey_mark

The WG1 summary for policymakers is available here