Arbeitsvorhaben Prof. Dr. Ulrich Platt

The Global Governance of Climate Engineering

My project for the active period as Marsilius fellow will be focussed on interdisciplinary research on "Climate Engineering"(CE). CE is the emerging concept of deliberately altering the global climate by technological means. While I strongly believe that CE is not a desirable
option compared to mitigation of climate change or adaption, society, nevertheless, has to be prepared for future scenarios where, for whatever reasons, CE measures may be applied. In particular, a responsible decision, whether CE measures should be considered can only be made on the basis of detailed knowledge on the side effects, technological feasibility, and societal cponsequences of CE measures. Recently, Climate Engineering has received increasing attention among scientists, policy-makers, and the public as an additional strategy for combating climate change [e.g. Boyd 2008]. A new Marsilius research project examines the chances and risks of Climate Engineering from several perspectives [Marsilius Project Proposal 2009]. Climate Engineering is inherently technical, but at the same time political and social in its consequences, therefore an interdisciplinary perspective is necessary to develop a more comprehensive understanding of the topic. In the fragmented world of the 21st century, regulating human-environment interactions involves a variety of institutions, actors and discourses on different scales.

The project as a whole addresses two interrelated questions: 1) how do the risk-benet perception of Climate Engineering - technologies differ and evolve across time, disciplines, and political actors. 2) how may these risk perceptions inform individual, societal and international capacities to foster a global governance of Climate Engineering?

My particular topics will be the Physical Basis of Climate Engineering. This will be aimed at understanding the basic physics of climate engineering and its intended and unintended consequences. The focus will be on the physics of climate engineering and Earth's radiation budget as well as on the hydrological impacts of climate engineering schemes. I will contribute to study the physical principles behind the proposed climate engineering schemes and some of their consequences. Here my long standing experience from studies of the physics of our environment will come to bear. Presently there is a growing body of literature describing various climate engineering schemes in greatly varying degree of detail. This literature provides a starting point for our work, but also needs to be critically reviewed, as in some cases even a supercial physical assessment of the proposed schemes reveals serious questions about their effectiveness and eciency. I see my main task in an analysis of the physics behind the proposed schemes in order to assess effectivity, side effects, cost, time scale, and other aspects of the proposed schemes.

This information will enable scientists from other disciplines (in particular from the societal sciences) to start and proceed from more accurate descriptions of the proposed CE schemes than presently given in the available literature and will thus help to focus the research on the relevant scenarios. The main methodology will be scrutinisation of the physical principles employed and assumptions made in the various proposals of climate engineering schemes. In detail I propose the following course of action:

Since changes of Earth's radiation budget are the main reason for the man-made climate change, many climate engineering proposals seek to inuence Earth's radiation budget. Therefore the radiation transport in the atmosphere and at the surface, and its modication by
aerosols and clouds will be studied. Changes due to elevated levels of greenhouse gases will be contrasted to reduced short-wave incoming radiation (e.g. due to shading measures). A particular topic are the effects of clouds and the feedback of temperature change on cloudcover and cloud properties [e.g. Wagner et al. 2007] and the absorption of radiation by clouds [e.g. Pfeilsticker et al. 1997, 1998].

Another topic will be the feasibility of various climate engineering proposals, as far as they are subject to physical limitations, for instance the size distribution of aerosols produced by different deployment schemes in the troposphere and stratosphere will be assessed. Together with Dr. W. Aeschbach-Hertig I will also study the relationship between the global radiation budget and the hydrological cycle. In current climate engineering proposals, temperature is the primary control variable. However, any climate engineering effort will not only affect temperature, but also many other climate variables, in particular the precipitation, with potentially
drastic consequences for human activities. Climate simulations show that climate engineering may lead to signicantly reduced precipitation [Matthews and Caldeira, 2007; Rasch et al., 2008]. However, the effects of rising CO2 and changing radiation regimes on the water cycle are complex and not fully understood. E.g., CO2 fertilization enables plants to use water more eciently, and thereby increases surface runoff [Gedney et al., 2006] but decreases evapotranspiration and precipitation over continents [Matthews and Caldeira, 2007]. Furthermore, reduced short-wave radiation or changes in cloud cover as a result of climate engineering measures will also affect the water cycle. A, particularly close connections are foreseen with colleagues from Economics, Geography, and International Law.

Literature

• Boyd P.W. (2008), Ranking geo-engineering schemes, Nature Geoscience 1 (Nov.), 722-724.
• Gedney, N., P. M. Cox, R. A. Betts, O. Boucher, C. Huntingford, and P. A. Stott (2006). Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.
• Marsilius Project Proposal, The Global Governance ofClimate Engineering, Final version: June
  19, 2009
• Matthews H.D. and Caldeira K. (2007), Transient climate-carbon simulations of planetary geoengineering, PNAS 104 (24), 9949-9954, www.pnas.org_cgi_doi_10.1073_pnas.0700419104.
• Pfeilsticker K., Erle F. and Platt U. (1997), Absorption of solar radiation by atmospheric O4, J. Atm. Sci., 54, 933-939.
• Pfeilsticker K. Erle F., Funk O., Veitel H. and Platt U. (1998), First Geometrical Path Lengths Probability Density Function Derivation of the Skylight from Spectroscopically Highly Resolved Oxygen A-band Observations. 1. Measurement technique, atmospheric observations and model calculations, J. Geophys. Res., 103, 11483 - 11504.
• Rasch, P. J., P. J. Crutzen, and D. B. Coleman (2008), Exploring the geoengineering of climate using stratospheric sulfate aerosols: The role of particle size, Geophys. Res. Lett., 35, L02809, doi:10.1029/2007GL032179.
• Wagner T., Beirle S., Deutschmann T., Grzegorski M., and Platt U. (2008); Dependence of cloud fraction and cloud top height on surface temperature derived from spectrally resolved UV/vis satellite observations, Atmos. Chem. Phys. 8, 2299-2312.