Rick D. Russotto

Relevant climate response tests for stratospheric aerosol injection: A combined ethical and scientific analysis

In this paper, we focus on stratospheric sulfate injection as a geoengineering scheme, and provide a combined scientific and ethical analysis of climate response tests, which are a subset of outdoor tests that would seek to impose detectable and attributable changes to climate variables on global or regional scales. We assess the current state of scientific understanding on the plausibility and scalability of climate response tests. Then we delineate a minimal baseline against which to consider whether certain climate response tests would be relevant for a deployment scenario. Our analysis shows that some climate response tests, such as those attempting to detect changes in regional climate impacts, may not be deployable in time periods relevant to realistic geoengineering scenarios. This might pose significant challenges for justifying SSI deployment overall. We then outline some of the major ethical challenges proposed climate response tests would face to be considered properly socially licensed forms of research. We consider what levels of confidence would be required to ethically justify approving a proposed test; whether the consequences of tests are subject to similar questions of justice, compensation and informed consent as full scale deployment; and whether questions of intent and hubris are morally relevant for climate response tests. We suggest further research into laboratory-based work and modeling may help to narrow the scientific uncertainties related to climate response tests, and help inform future ethical debate. However, even if such work is pursued, the ethical issues raised by proposed climate response tests are significant and manifold.

Sensitivity of Thin Cirrus Clouds in the Tropical Tropopause Layer to Ice Crystal Shape and Radiative Absorption

Subvisible cirrus clouds in the tropical tropopause layer (TTL) play potentially important roles in Earth’s radiation budget and in the transport of water into the stratosphere. Previous work on these clouds with 2D cloud-resolving models has assumed that all ice crystals were spherical, producing too few crystals greater than 60 microns in length compared with observations. In this study, the System for Atmospheric Modeling (SAM) cloudresolving model is modified in order to calculate the fall speeds, growth rates, and radiative absorption of non-spherical ice crystals. This extended model is used in simulations that aim to provide an upper bound on the effects of ice crystal shape on the time evolution of thin cirrus clouds and to identify the physical processes responsible for any such effects. Model runs assuming spheroidal crystals result in a higher center of cloud ice mass than in the control, spherical case, while the total mass of ice is little affected by the shape. Increasing the radiative heating results in less total cloud ice mass relative to the control case, an effect which is robust with more extreme perturbations to the absorption coefficients. This is due to higher temperatures reducing the relative humidity in the cloud and its environment, and greater entrainment of dry air due to dynamical changes. Comparisons of modeled ice crystal size distributions with recent airborne observations of TTL cirrus show that incorporating non-spherical shape has the potential to bring the model closer to observations. c ©2016 American Geophysical Union. All Rights Reserved. Keypoints: We extended a model of optically thin TTL cirrus to include non-spherical ice crystal shapes. Shape has little impact on cloud dynamics due to competing effects and small ice crystal size. Increasing the radiative heating lofts the cloud higher but reduces its total mass. c ©2016 American Geophysical Union. All Rights Reserved.