‘No, climate sensitivity is not smaller, it is higher than we thought’ – because organic aerosol feedbacks mask warming

Says an international research group led by Gothenburg University.

organic aerosols climate sensitivityIt serves to show individual climate sensitivity studies are never conclusive but add up bits of fresh understanding to an already enormous pile of data and knowledge.

Right in the middle of that pile is the IPCC. Should the entire pile shift or change shape – rest assured, then the official IPCC climate sensitivity range [now around 3 degrees Celsius for a doubling of atmospheric CO2] will do the same in their 2013 climate report.

The new Swedish climate sensitivity research has just been published (PDF) in Atmospheric Chemistry and Physics, the open-access journal of the European Geosciences Union.

The authors argue that climate sensitivity could be ‘greater than previously believed’ because in the initial phases of the current CO2-induced warming plant life has emitted larger amounts of precursor gases that lead to the formation of reflective or blocking* secondary organic aerosols (SOA) in the atmosphere, thereby acting as a negative climate feedback, and masking part of the ‘warming’ that’s occurring underneath.

[*) These SOAs are also more important to cloud formation than previously thought, other recent research shows – but such ‘organic clouds’ may in fact only complicate the climate story further, as clouds may be positive feedbacks to our planet’s warming. Lead author Kent Salo of Gothenburg University summarises this complexity as follows: “Particles in the atmosphere basically have a cooling effect on the Earth, and they affect cloud formation. The greater the number of particles in the air, the greater will be the number of cloud droplets. This affects the lifetime of the clouds and the amounts of precipitation, and consequently, the climate. Today, we do not have a fundamental understanding of how SOA particles are formed and the properties they have, despite them being an important component of, for example, climate models.” So basically they cool and elsewhere they may at least interfere with precipitation patterns, but no one is really sure to what effect.]

Because of the increased production of organic aerosols the sensitivity of our Holocene climate system to a doubling of the atmospheric CO2 concentration can not simply be extrapolated* from the observed temperature rise, but would in fact be higher.

[*) Noteworthy at this point is that there are many other reasons why climate sensitivity should not be derived from that temperature graph – most notably the fact that our planet’s warming is a much delayed response to the rise of atmospheric greenhouse gases – because of the large thermal mass of our oceans. It is estimated that delay could be about 4 decades, which implies the current warming is the effect of combined emissions up to around 1970, which have about doubled since – inevitable future warming.]

Of course we would also like to know if an increase in organic aerosols leads to climate cooling, can we count on this aerosol increase to continue as we keep emitting ever more climate-warming greenhouse gases?

The new study shows the chemical processes involved in the formations of SOAs favour higher temperatures. But as the aerosol precursor gases are a by-product of plant growth, let’s not ignore plants too live in ecosystems – and yesterday we learned ecosystems will change dramatically under continued climate change.

So perhaps the answer is no, and this cooling is just as temporary as the thermal inertia of the oceans, and warming will increase just about at the point where our planet’s many positive climate feedbacks from the ecological carbon cycle will start to pick up. Doesn’t this only confirm the theoretical nature of ‘climate senstivity’?

At some point it will in fact be the icecaps, and indeed your backyard thermometer that will do a much better job at explaining what is going on…

© Rolf Schuttenhelm | www.bitsofscience.org

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