AGU highlights: Effects of particle nucleation and cosmic rays on clouds

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This past week the annual AGU (American Geophysical Union) fall meeting was held in San Francisco. There were a number of interesting climate related talks that I attended. Here’s a short briefing of some of these, related to the climate effects of aerosol, and possibly of cosmic rays. This post is more technical than other ones on this blog. Meeting abstracts can be searched here.

 

Background

Aerosol nucleation refers to the formation of a stable aerosol particle (miniscule liquid droplet of a few nanometers in diameter) in the atmosphere. These particles can grow larger in size to then affect cloud formation, and thus climate. A controversial hypothesis sais that the decreasing flux of cosmic rays from outer space has decreased the amount of particles produced by nucleation, and thus decreased the cloud cover and thereby warmed the climate. The flux of cosmic rays has remained constant over the last 50 years (apart from the 11 year cycle mirroring solar min/max conditions), so they can’t have influenced the warming during this time period. The micro-physics of the processes involved are poorly understood, but important/interesting for a number of other reasons as well, eg the climate effects of aerosols in general.

 

AGU highlights

The Finnish group (headed by Kulmala) gave an overview of long term measurements of particle nucleation at their Boreal forest site, reporting that there was no relation whatsoever with cosmic rays. Sulfuric acid (a prime agent in the nucleation process) had a slightly decreasing trend over the past decade, whereas both particle nucleation and growth rates slightly increased, suggesting that the role of organics in both these processes may have increased. According to their analyses, the role of ion induced nucleation (relevant to the hypothetical cosmic rays – cloud link) can explain 10 to 20% on average of the rate of production of 2 nm sized particles.

 

Bondo, from the Danish group headed by Svensmark, reported on laboratory (chamber) studies of ion induced nucleation under exposure of ionizing radiation from a radioactive source. The presentation focused mainly on comparing theoretical calculations with the measurements. To the speaker’s credit, no far reaching climate conclusions were drawn.

 

Yu and Turco analyzed data from the Finnish group, and whereas the Fins calculate a less than 10% contribution of ion induced nucleation (to the total amount of particles produced), Yu and Turco arrived at the opposite conclusion. If anything, this indicates that the nucleation processes are very poorly understood, partly because of the strong non-linearity (and thus strong dependencies on uncertain parameters) involved. Yu cautioned on the use of the nucleation theorem (which sais that the log-log slope of the nucleation rate to the sulfuric acid concentration is equal to the number of sulfuric acid molecules in the critical cluster), because other factors (if they’re not constant) may influence the slope found.

 

There were two talks on global modeling of aerosol nucleation. Spracklen showed that only including primary emissions of aerosol underestimates the number of aerosol particles typically measured, so nucleation contributes significantly to the aerosol number budget. Moreover, only including binary homogeneous nucleation (according to the classical nucleation theory) still leads to an underestimation of particle numbers; some sort of empirical nucleation scheme is needed to reach reasonable agreement. That is something I recognize from most other (including my own) research of both field and laboratory measurements.

 

Pierce, using a different model (based on the GISS II GCM), investigated the potential effect of cosmic rays on cloud condensation nuclei and cloudiness. Two different parameterizations for ion induced nucleation (Modgil et al and an ‘ion-limit’ assumption that all ions go on to form a new particle), somewhat surprisingly, both lead to a change in nucleation of about 20%, in response to a prescribed change in cosmic ray flux. This difference in cosmic ray flux was of the order of the difference between solar minimum and solar maximum, which happens to be comparable to the change in cosmic ray flux over the first half of the 20th century. If this 20% change in nucleation would translate directly in a 20% change in the number of cloud droplets, one might expect a change in cloud cover of about 2% (which is similar to the magnitude suggested by Svensmark et al as having been caused by cosmic rays). However, as one might expect, the number of cloud condensation nuclei (CCN) in the model changed by much less than 20% in response to the 20% change in nucleation. Let alone the number of cloud droplets and the cloud cover.

 

Penner gave a very interesting talk in which she showed that satellite derived estimates of the aerosol indirect effect (i.e. their effect on clouds and thus on climate) are substantially smaller than model calculated estimates (the former are in the range of -0.2 W/m2, whereas the latter range between roughly -0.5 and -1.5 W/m2. The negative sign means it causes cooling.) However, satellite estimates are based on the aerosol optical depth, and this may cause an underestimation of the aerosol indirect effect (AIE). The model calculated AIE is very sensitive to nucleation and to primary aerosol emissions.

 

Concerning the second indirect effect, based on satellite measurements a 5% difference in the probability of precipitation was found between clean and polluted clouds (where the latter has a lower probability than the former, because of the enhanced droplet number and thus reduced droplet size). Rosenfeld claims that this so called cloud lifetime effect is stronger than the cloud albedo effect, whereas others claim the opposite.

 

At the last EGU meeting in Vienna Philipona presented empirical evidence from several mid-European sites that the direct effect of aerosol (scattering of sunlight) may be stronger than their indirect effect via clouds, whereas modeling results usually point to the opposite. They suggested that a decrease in aerosol loading may have contributed significantly to the warming of the past 30 years, and that when the aerosol loading stabilizes again, the rate of warming will decrease as a consequence. This fits in with the global dimming – global brightening picture (the amount of solar radiation received at the surface depends on the aerosol loading because of their light scattering properties).

 

The large uncertainties in the aerosol effects on climate may affect estimates of the climate sensitivity (equilibrium temperature change for a given change in climate forcing) (Penner) and/or of the ocean response time (lag in temperature response caused by thermal inertia of the oceans) (Hansen). The three are connected, and their magnitudes can be traded off in trying to match the 20th century temperature record by models. For example, a larger negative aerosol forcing (and thus a weaker net positive forcing) would need to be combined with a higher climate sensitivity and/or a shorter ocean response time in order to still provide a good match. Of course, there are other constrains on these processes as well that have to be taken into account.

 

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