Importance of tropospheric volcanic aerosol for indirect radiative forcing of climate

Abstract

Abstract.Observations and models have shown that continuously degassing volcanoes have a potentially large effect on the natural background aerosol loading and the radiative state of the atmosphere. We use a global aerosol microphysics model to quantify the impact of these volcanic emissions on the cloud albedo radiative forcing under pre-industrial (PI) and present-day (PD) conditions. We find that volcanic degassing increases global annual mean cloud droplet number concentrations by 40 % under PI conditions, but by only 10 % under PD conditions. Consequently, volcanic degassing causes a global annual mean cloud albedo effect of −1.06 W m −2 in the PI era but only −0.56 W m −2 in the PD era. This non-equal effect is explained partly by the lower background aerosol concentrations in the PI era, but also because more aerosol particles are produced per unit of volcanic sulphur emission in the PI atmosphere. The higher sensitivity of the PI atmosphere to volcanic emissions has an important consequence for the anthropogenic cloud radiative forcing because the large uncertainty in volcanic emissions translates into an uncertainty in the PI baseline cloud radiative state. Assuming a −50/+100 % uncertainty range in the volcanic sulphur flux, we estimate the annual mean anthropogenic cloud albedo forcing to lie between −1.16 W m −2 and −0.86 W m −2 . Therefore, the volcanically induced uncertainty in the PI baseline cloud radiative state substantially adds to the already large uncertainty in the magnitude of the indirect radiative forcing of climate.

Occurrence of pristine aerosol environments on a polluted planet

“…Although our model suggests that even the clean region of the Pacific studied by Koren et al (6) has a different aerosol state and response to parameter perturbations today compared with the PI, it may still behave in a way that is sufficiently similar to the PI to be informative. However, clouds appear to be highly sensitive to aerosols at very low concentrations (3,4,6) and strong regime shifts can occur in some aerosol-cloud systems (8,41), so even small perturbations of the aerosol state away from PI conditions may be important.…”

“…Improved understanding of how natural emissions determine aerosol concentrations in different environments is important for reducing the uncertainty in model estimates of cloud radiative forcing over the industrial period (3,4). Even under the assumption that natural emissions do not change with time, the magnitude of preindustrial (PI) to present-day (PD) aerosol−cloud forcing is very sensitive to natural emissions and processes in the PI because of the nonlinear relationship between aerosol concentrations, cloud drop concentrations, and cloud albedo (3)(4)(5)(6)(7). Other more complex cloud adjustments are also likely to respond sensitively to small changes in aerosol under clean conditions (6,8).…”

“…Based on the results of a global aerosol microphysics model (3,4,25), we use two measures of similarity of aerosol between the PI and PD. First, we compare the aerosol states: On how many days are PD CCN concentrations similar to those in the PI?…”

Inverting for volcanic SO<sub>2</sub> flux at high temporal resolution using spaceborne plume imagery and chemistry-transport modelling: the 2010 Eyjafjallajökull eruption case study

“…This type of sporadic volcanic activity is one of the factors able to force climate variability with effects persisting from a few weeks to a few years according to the intensity, the duration and the location of the event. However, recent studies suggest that more frequent eruptions of lower magnitude (restricted to the low stratosphere) as well as persistent volcanic degassing in the troposphere may also significantly impact the radiative state of the atmosphere (Vernier et al, 2011;Schmidt et al, 2012). Consequently, studying volcanic gas emissions during eruptions, but also during periods of quiescent degassing, is of importance to assess the impact of volcanism as a whole on the atmosphere.…”

The aerosol-cyclone indirect effect in observations and high-resolution simulations

“…Further observational evidence is supplied by the natural laboratory provided by a degassing volcano in the North Atlantic. Some recent studies have used volcanoes to show that 5 transient changes in aerosol load lead to changes in cloud properties (Mace and Abernathy, 2016;Gassó, 2008;Ebmeier et al, 2014;Yuan et al, 2011;Schmidt et al, 2012;Malavelle et al, 2017), but due to the difficulty in separating meteorological and aerosol effects this is the first time that a liquid water response has been demonstrated for extratropical cyclones.…”