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Author
Date
2016Type
- Doctoral Thesis
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Abstract
This thesis evaluates the option of modifying cirrus clouds in order to counteract part
of the anthropogenic global warming, also known as cirrus seeding. The feasibility
of cirrus seeding is assessed with the general circulation model ECHAM6-HAM with
coupled aerosol-cloud interactions. The warming effect of cirrus clouds on climate led
part of the research community to the idea of artificially decreasing their frequency
and optical thickness by seeding them with ice nucleating particles. Cirrus seeding re-
lies on the competition between two distinct cirrus formation pathways: homogeneous
nucleation of soluble aerosols and heterogeneous ice crystal nucleation with the help
of solid ice nucleating particles. Seeding attempts to transform optically thicker, longer
lived homogeneously nucleated cirrus clouds to thinner and shorter lived heteroge-
neously nucleated cirrus clouds.
The cirrus seeding effectiveness depends on the correct representation of cirrus
clouds in climate models. We evaluated the ECHAM6-HAM model against CALIPSO
satellite data and found that the model reproduces the cirrus cloud occurrence fraction
and ice water content well, but overestimates their extinction. Most importantly, we
found that a large fraction of cirrus clouds formed in environments with liquid water,
which cannot be modified by cirrus seeding. Moreover, modelled in situ cirrus formed
by heterogenous ice nucleation on dust aerosols in most of the world, limiting the cir-
rus geoengineering potential. Meanwhile, homogeneous cirrus were dominant only in
the tropical tropopause layer and over mountain regions.
Seeding cirrus clouds with ice nucleating particles of radii below 10 μm did not lead to
a significant cooling effect on climate, due to radiative warming effects of increasing cir-
rus cloud cover and decreasing ice crystal radius, which neutralized the cooling gained
by the ice crystal number decrease. However, when seeding with larger ice nucleating
particles, cirrus cloud cover, ice crystal number concentration, and ice water content
decreased, due to increased ice crystal sedimentation velocity, leading to a cooling
of up to 0.7◦ C globally. In addition, an idealized cirrus geoengineering setup with in-
creased ice crystal sedimentation velocities at temperatures colder than -35◦ C could
fully counteract the temperature increase by 1.5 x CO2 concentrations. Furthermore,
seeding with ice nucleating particles larger than 10 μm could counteract up to 55% of
the temperature and precipitation damage caused by a 1.5 x CO2 increase, without
causing negative side effects in any of the analysed regions. Thus, cirrus geoengi-
neering was found to be, not considering its problematic engineering details and large
modelling uncertainties, an attractive method to counteract part of the anthropogenic
warming. Show more
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https://doi.org/10.3929/ethz-b-000000052Publication status
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Publisher
ETH ZurichSubject
climate modelling; cirrus clouds; geoengineering; aerosol-cloud interactions; cloud radiative effects; climate engineeringOrganisational unit
03690 - Lohmann, Ulrike / Lohmann, Ulrike
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