Aerosols, clouds and a digression into cosmic radiation

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.

a, CCN concentrations are more sensitive to emissions of sulphur precursor gases in the PI era because the condensation sink of the nucleating sulphuric acid vapour onto existing aerosol is lower. b, Cloud droplet concentrations are more sensitive to changes in CCN when droplet concentrations are low because higher droplet concentrations suppress in-cloud supersaturation and limit the activation of additional aerosol particles. c, Cloud albedo is more susceptible to changes in cloud droplet concentration when concentrations are low1. d, All three effects in a–c lead to a much higher sensitivity of albedo to precursor gas emissions in the PI era. e, Calculated effect of how the uncertainty in modelled aerosol affects the uncertainty in forcing. In this example, it is assumed that the CCN concentration scales in direct proportion with anthropogenic emissions (horizontal axis), as occurs approximately in the model. Uncertainties are then applied to CCN (±ΔCCN). The green lines shows the uncertainty in forcing when ΔCCN is proportional to the CCN concentration and the blue lines shows a case where ΔCCN is constant and independent of the anthropogenic emissions. The initial PI CCN concentration is 50 cm−3, rising to a maximum of 750 cm−3 in the PD. The cloud droplet number concentration (CDNC) is calculated as CDNC = 375 × (1 − exp(−0.0025 × CCN)) (ref. 28). The albedo A of the baseline cloud is assumed to be 0.5 and the albedo versus CDNC is dA/dln(CDNC) = A(1 − A)/3A (ref 1). The forcing is calculated according to  , where ΔA is the change in albedo from the PI value (0.5), Ta is the transmission of the atmosphere (assumed to be 0.75) and F0 is the radiative flux, assumed to be 340 W m−2. The black line shows the calculated forcing assuming the baseline aerosol number concentration. The green line shows the calculated forcing assuming ±30% uncertainty in CCN (35–65 cm−3 in the PI era to 525–975 cm−3 maximum in the PD). This calculation represents an uncertainty in aerosol concentrations due to a process that affects PI and polluted aerosol concentrations by the same factor, such as dry deposition. The blue line shows the calculated forcing assuming ±15 cm−3 uncertainty in CCN (35–65 cm−3 in the PI era, as in the scaled calculation, to a maximum polluted concentration of 735–765 cm−3). This calculation represents an uncertainty in aerosol concentrations due to a process or emission that affects PI and polluted aerosol by approximately the same absolute amount, such as caused by uncertainty in DMS or volcanic SO2 emissions. The small absolute change in aerosol has a much larger effect on forcing uncertainty than the scaled aerosol change.

a, CCN concentrations are more sensitive to emissions of sulphur precursor gases in the PI era because the condensation sink of the nucleating sulphuric acid vapour onto existing aerosol is lower. b, Cloud droplet concentrations are more sensitive to changes in CCN when droplet concentrations are low because higher droplet concentrations suppress in-cloud supersaturation and limit the activation of additional aerosol particles. c, Cloud albedo is more susceptible to changes in cloud droplet concentration when concentrations are low. d, All three effects in a–c lead to a much higher sensitivity of albedo to precursor gas emissions in the pre-industrial era. e, Calculated effect of how the uncertainty in modelled aerosol affects the uncertainty in forcing.

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.

 

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About richard telford

Ecologist with interests in quantitative methods and palaeoenvironments
This entry was posted in climate, Fake climate sceptics, Peer reviewed literature, solar variability, WUWT and tagged , , , . Bookmark the permalink.

One Response to Aerosols, clouds and a digression into cosmic radiation

  1. Paul S says:

    Don’t know if you’ve seen it but there was another relevant paper published a couple of weeks ago in Nature from a group involved with Kirkby’s CLOUD experiment at CERN.

    They’re not looking at the CCN stage yet, just at the basic aerosol nucleation level, but found that the presence of cosmic rays made had little impact on aerosol formation except where formation rates were low.

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