Volcanoes, Dust, Clouds and Climate

What are aerosols?

Aerosols are fine, airborne particles consisting at least in part of solid material. Density of the basic materials of aerosols range from 1.0 g/cm3 (for soot) to 2.6 (for minerals). The ocean is a major source of natural aerosols. Air-sea exchange of particulate matter contributes to the global cycles of carbon, nitrogen, and sulfur aerosols, such as dimethylsulfide (DMS) produced by phytoplankton. Ocean water and sea salt are transferred to the atmosphere through air bubbles at the sea surface. As this water evaporates, the salt is left suspended in the atmosphere. Four other significant sources of aerosols are terrestrial biomass burning, volcanic eruptions, windblown dust from arid and semi-arid regions, and pollution from industrial emissions (Fig 1).

Clean continental air often contains less than 3,000 particles per cubic centimetre (of which half are water-soluble), polluted continental air typically 50,000/cm3 (of which two-thirds are soot, and the rest mostly water-soluble). Urban air typically contains 160,000/cm3, mostly soot, and only 20% is water-soluble. Desert air has about 2,300/cm3 on average, almost all water-soluble. Clean marine air generally has about 1,500/cm3, about all water-soluble. The lowest sea-level values occur over the oceans near the subtropical highs (600/cm3 on average, but occasionally below 300/cm3). Arctic air has about 6600/cm3 (including 5,300 soot) and on the Antarctic plateau only 43/cm3 occur (about all sulphate) (1).

Fig 1. Main sources and types of aerosols that affect climate.

Aerosols: Volcanoes, Dust, Clouds and Climate

Haze from small particles surely affected climate, but how? Old speculations about the effects of smoke from volcanoes were brought to mind in the 1960s, when urban smog became a major research topic. Some tentative evidence suggested that aerosols emitted by human industry and agriculture could change the weather. A few scientists exclaimed that smoke and dust from human activities would cause a dangerous global cooling. Or would pollution warm the atmosphere? Theory and data were too feeble to answer the question, and few people even tried to address it. Among these few, the uncertainties fueled vigorous debates, in particular over how adding aerosols might change the planet’s cloud cover. Starting in the late 1970s, powerful computers got to work on the ferociously complex calculations, helped by data from volcanic eruptions. By the 1990s it was clear that overall, human production of aerosols was cooling the atmosphere. Pollution was significantly delaying, and masking, the coming of greenhouse effect warming.

Aerosols and Incoming Sunlight (Direct Effects)

The Sun provides the energy that drives Earth’s climate, but not all of the energy that reaches the top of the atmosphere finds its way to the surface. That’s because aerosols—and clouds seeded by them—reflect about a quarter of the Sun’s energy back to space.

Climate change and aerosols

Dr Jim Haywood, Aerosol Research Manager

Dr Jim Haywood Atmospheric aerosols are microscopic particles suspended in the Earth’s atmosphere, which generally act to cool the climate by reflecting sunlight back to Space and also by affecting clouds. The net impact of human activities, including greenhouse gases and aerosols, has been to warm the world’s climate.

Deep Carbon Emissions from Volcanoes
Michael R. Burton
Istituto Nazionale di Geofisica e Vulcanologia
Via della Faggiola,
Pisa, Italy
Georgina M. Sawyer
Laboratoire Magmas et Volcans, Université Blaise Pascal
5 rue Kessler, 63038 Clermont Ferrand, France
Istituto Nazionale di Geofisica e Vulcanologia
Via della Faggiola,
Pisa, Italy
Domenico Granieri
Istituto Nazionale di Geofisica e Vulcanologia
Via della Faggiola,
Pisa, Italy
In recent years, measurements of CO2 flux from volcanoes and volcanic areas have greatly increased, particularly on persistently degassing volcanoes, of which ~22% have had their CO2 flux quantified. Notwithstanding this progress, it is clear that the CO2 emissions from the majority of volcanic sources are still unknown. Using the available data from plume measurements from 33 degassing volcanoes we determine a total CO2 flux of 59.7 Mt/yr.

Extrapolating this to ~150 active volcanoes produces a total of 271 Mt/yr CO2. Extrapolation of the measured 6.4 Mt/yr of CO2 emitted from the flanks of 30 historically active volcanoes to all 550 historically active volcanoes produces a global emission rate of 117 Mt/yr. Perez et al. (2011) calculated the global emission from volcanic lakes to be 94 Mt/yr CO2. The sum of these fluxes produces an updated estimate of the global subaerial volcanic CO2 flux of 474 Mt/yr. Emissions from tectonic, hydrothermal and inactive volcanic areas contribute a further 66 Mt/yr to this total, producing a total subaerial volcanic emission of 540 Mt/yr. An extrapolation to a global estimate is not straightforward for tectonic-related degassing, as the number of areas which produce such emissions is not known. Given the fact that ~10 Mt/yr is produced by Italy alone it is possible that the global total is significant, and this merits further investigation. We highlight also that the magnitude of CO2 emissions from both cold and hot
non-MOR submarine volcanic sources are currently effectively unknown.

Our subaerial volcanic CO2 flux matches well with estimates of CO2 removal rates of 515 Mt/yr due to silicate weathering, which, over timescales of 0.5 Ma, should balance lithospheric CO2 emissions. However, inclusion of the metamorphic CO2 flux of 300 Mt/yr calculated by Morner and Etiope (2002) produces a total subaerial lithospheric flux of 840 Mt/yr, suggesting that, assuming steady-state, weathering rates might be slightly higher in order to absorb all the CO2 emitted from the lithosphere.

The global subaerial CO2 flux we report is higher than previous estimates, but remains insignificant relative to anthropogenic emissions, which are two orders of magnitude greater at 35,000 Mt/yr (Friedlingstein et al. 2010). Nevertheless, it is clear that uncertainties in volcanic CO2 emission rates remain high and significant upward revisions of the lithospheric CO2 flux cannot be ruled out. This uncertainty also limits our understanding of global volcanic carbon budgets and the evolution of the distribution of CO2 between the crust and the mantle.

Furthermore, with the notable exception of continuous CO2 flux monitoring at a handful of volcanoes we have very little data with which to assess CO2 flux variations across different timescales. It is clear that there is much further work to be done surveying CO2 emissions from both active and inactive volcanoes


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