Aerosols and Climate

Global cloud condensation nuclei

Cloud condensation nuclei (CCN) are the particles upon which cloud droplets form, so fundamentally control the magnitude of the indirect forcing of climate. Models like GLOMAP enable us to understand how and where in the atmosphere CCN are formed. The key challenge is to calculate the sources of particle number and not just mass, as most models have done.

• Cooling effect of soot aerosol: Our analysis of global aerosol data shows that soot emitted from fossil fuel and biofuel combustion produces a large fraction of the world's cloud drops, thereby cooling the climate and possibly cancelling out the warming effect of the black carbon in the particles.
• First quantification of global CCN sources: CCN are critical for clouds and climate. Here, for the first time, we quantified the global sources of these particles.
• Regional differences in "CCN formation potential": The amount of new CCN produced by sulphur dioxide emissions varies significantly between different regions.
• CCN from boundary layer nucleation: Nucleation of new particles in the lower atmosphere is very widely observed. Here we quantified the impact on CCN.
• Large contribution of nucleation to global aerosol: The first global estimate of how nucleation affects global particle number concentrations.

Volcanic impacts on global aerosol

• Degassing volcanoes have a large effect on climate: The sulphur emissions have a surprisingly large effect on pre-industrial to present-day cloud forcing.
• Health impacts of large volcanic eruptions: A modern-day long-lasting Icelandic volcanic eruption could cause the premature death of up to 140,000 people in Europe due to degradation of air quality.

Marine aerosol, plankton and climate feedbacks

• Aerosol over marine regions is particularly important for climate because most of the low-level clouds that cool the climate exist there. Marine aerosol is also potentially important in climate regulation because the natural aerosol sources (from phytoplankton and sea spray) could change in a future climate.
• Is the CLAW hypothesis dead? CLAW is a 20-year-old hypothesis that plankton can regulate the climate. We show that the effect is very weak.
• How the Antarctic ozone hole can cause changes in marine clouds: Trends in southern hemisphere wind speed over the last two decades have caused an acceleration of the westerly jet, more sea spray and brighter clouds.
• Quantifying the contribution of DMS to the CCN annual cycle: The long-term monitoring site at Cape Grim shows a strong correlation between CCN and DMS. Here we used GLOMAP to understand how much of that correlation is really driven by DMS.
• A significant global marine organic aerosol source: Previous studies showed that the oceans produce organic aerosol in addition to sea salt. We used models to estimate the global source.

Geoengineering

Various radical proposals have been made for "solar radiation management" to cool the climate. In these papers we have used GLOMAP to test whether the mechanisms could work.

• Iron fertilization of the ocean as a climate geoengineering option: This idea has been proposed to cool the climate. We showed that it is unlikely to work.
• Sea spray geoengineering predicted to be very weak: The first assessment of geoengineering including the aerosol processes, rather than just adjusting cloud drop concentrations.
• Do high latitude forests warm or cool the climate? Formation of aerosol from terpenes emitted by forests can affect the net climatic effect of the forests.

Arctic aerosol

The climate of the Arctic is changing faster than any other place on Earth. Large changes in aerosol almost certainly play a role, but unfortunately the aerosol in the Arctic is very poorly handled in most models.

• What controls the Arctic aerosol particle size distribution? Observations show a sharp transition in aerosol properties in spring. We used GLOMAP to test what might be causing that change.

Dust

Wind-blown dust aerosol is a very important component of the Earth system, providing a source of nutrients to oceans, causing a significant direct radiative effect and acting as an ice nucleus or giant CCN, thereby affecting precipitation.

• Changes in iron solubility not caused by physical size sorting: Our laboratory and modelling study shows that chemistry must be responsible for observed changes in iron solubility away from the source.
• Dust storms have a minor effect on other aerosol: By analysing dust observations from the ACE-Asia campaign we showed that dust storms have a surprisingly small effect on other CCN particles.

Cosmic rays, clouds and climate

Ken Carslaw has been involved with the CERN CLOUD project  since 2000. We got our first exciting results in 2011. The next steps for us are to implement these data in GLOMAP to study global effects.

• Results of the first CERN CLOUD experiment: Rates of nucleation in the H2SO4-NH3 system far below expected. A large impact of ioninsation (in this inorganic system at least).
• Reviews of the cosmic ray-cloud hypothesis: When you don't have your own data, at least you can review other people's.

Uncertainty quantification

The aerosol forcing of climate has been dominated by uncertainty. To address this problem, we have been using some new statistical techniques to quantify the sources of uncertainty so that we can focus research effort on the most important processes.

• Using an emulator to quantify uncertainty: We have pioneered the use of statistical emulation to extract as much information about uncertainty in our model as you would get from Monte Carlo approaches.