Aerosols, both natural and anthropogenic, provide the condensation nuclei around which cloud droplets form. Changes in emissions of aerosols affect the abundance and properties of cloud condensation nuclei (CCN), and consequently the radiative properties of the clouds that form. There are several different aspects to this effect. On aggregate the indirect aerosol effect, as it is known, is probably negative, but its magnitude is uncertain. If it is large, then the increase in aerosol emissions since 1750 will mask some of the effects of greenhouse gases that have been emitted, and climate sensitivity to a doubling of CO2 may be larger than estimated from the temperature increase over the last century.
Carslaw et al (2013), published in Nature this week, explore this uncertainty with a sensitivity analysis. They use multiple runs of an aerosol-cloud model coupled to climate reanalysis data; each run differs in the value of 28 parameters related to aerosols and cloud formation. This analysis is done under 1750 (pre-industrial) and 2000 (present day) emissions – forcing is conventionally defined as the difference from the pre-industrial. They find that almost half of the uncertainty in indirect aerosol forcing is because of uncertainty relating to natural emissions, which makes the pre-industrial state rather uncertain.
The implication is that we need to better understand the role of natural emissions to constrain the uncertainty on indirect aerosol forcing. This will not be easy, as it is doubtful that any sufficiently pristine atmosphere remains where the natural processes still dominate.
What caught my attention in this paper was the schematic figure 3 (focus of 3d) showing that climate is more sensitive to variability in aerosol emissions when emissions are low. The system saturates with high emissions.
This aerosol saturation has implications for the potential for cosmic radiation to affect current climate. Cosmic radiation, whose flux is modulated by the sun’s activity, ionises the atmosphere and can initiate CCN formation and hence affect the radiative properties of clouds. At least that is what several climate contrarians hypothesis, pushing this potential cosmic radiation-cloud relationship as an alternative explanation of the increase in 20th century warming despite the lack of a trend in the flux of cosmic radiation in the instrumental record. This mechanism has further serious problems if the climate system is approaching saturation with CCN, as the extra CCN attributable to cosmic radiation will have little effect. So even if variability in cosmic radiation had a significant impact on pre-industrial climate variability, it is much less likely to do so in the present day.
This week, WUWT pushed a 2011 video by Jasper Kirkby (of the CERN CLOUD experiment) as if it were news. The video details various Holocene proxy climate reconstructions that have been linked to solar activity (no comment of the quality of the purported relationships), and claims the link to be through cosmic radiation. For example, when discussing Bond’s ice-rafted debris (3:45′-4:59′), Kirkby claims
“cosmic ray intensity is associated with the temperature of the North Atlantic”
appearing to ignore the other (arguably more plausible) mechanisms by which solar variability. But even if cosmic radiation affected climate in the Holocene, and can generate CCN in the ultra-clean CLOUD experiment chambers, that is no guarantee that it has a material effect on the present-day polluted atmosphere.
UPDATE: via John Cook @amp;skepticscience, I found Krissansen-Totton & Davies (2013) who present an investigation of the cosmic-radiation cloud hypothesis using some new satellite data. No statistically significant relationships are found. Not a surprise really.