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Stratospheric controlled perturbation experiment: a small-scale experiment to improve understanding of the risks of solar geoengineering - PubMed

  • ️Wed Jan 01 2014

Stratospheric controlled perturbation experiment: a small-scale experiment to improve understanding of the risks of solar geoengineering

John A Dykema et al. Philos Trans A Math Phys Eng Sci. 2014.

Abstract

Although solar radiation management (SRM) through stratospheric aerosol methods has the potential to mitigate impacts of climate change, our current knowledge of stratospheric processes suggests that these methods may entail significant risks. In addition to the risks associated with current knowledge, the possibility of 'unknown unknowns' exists that could significantly alter the risk assessment relative to our current understanding. While laboratory experimentation can improve the current state of knowledge and atmospheric models can assess large-scale climate response, they cannot capture possible unknown chemistry or represent the full range of interactive atmospheric chemical physics. Small-scale, in situ experimentation under well-regulated circumstances can begin to remove some of these uncertainties. This experiment-provisionally titled the stratospheric controlled perturbation experiment-is under development and will only proceed with transparent and predominantly governmental funding and independent risk assessment. We describe the scientific and technical foundation for performing, under external oversight, small-scale experiments to quantify the risks posed by SRM to activation of halogen species and subsequent erosion of stratospheric ozone. The paper's scope includes selection of the measurement platform, relevant aspects of stratospheric meteorology, operational considerations and instrument design and engineering.

Keywords: balloon; geoengineering; ozone depletion; solar radiation management; stratosphere.

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Figures

Figure 1.
Figure 1.

Schematic of interactions between green house gas (GHG)-driven climate change, SRM and stratospheric ozone. A red arrow denotes an interaction where an increase in the quantity on the left generally causes an increase in the quantity on the right; a blue arrow denotes the converse; and a grey arrow is used for indeterminate cases. Sulfate aerosol causes direct radiative heating of the lower stratosphere and perhaps of the tropical tropopause layer (TTL). SRM would introduce a net negative radiative forcing that would offset some impacts of the positive forcing from increased GHGs. The combined effects of increased surface aerosol density, stratospheric temperature decreases and water vapour increases could substantially increase photochemical ozone losses. Conversely, SRM aerosol might decrease stratospheric water vapour, an offsetting effect. The purpose of SCoPEx is to reduce the uncertainty in our knowledge of relevant aerosol processes and this photochemistry through in situ perturbation experiments.

Figure 2.
Figure 2.

Results of CFD calculations for balloon physical configuration and propulsion assuming 1 m s−1 at 20 km altitude. (a) The plume radius—defined by a passive tracer concentration of 5×10−3 of the initial peak concentration found on the centreline of the well-developed plume—as a function of the distance downstream (km). Plume dispersal will be dominated by wakes generated by balloon motion. The plume initially expands rapidly, slowing after a few hours towards an asymptotic radius. (b) The tracer concentration at distances of 1000 and 3000 m downstream as a function of plume radius. (c) The concentration of a passive tracer (arbitrary units) released from the balloon gondola as it travels right to left.

Figure 3.
Figure 3.

The StratoCruiser propulsion module (a) contains the docking enclosure for the suspended payload, the articulated solar panels for power, Li–Po batteries for energy storage, dual high-efficiency propellers for concerted directional control, the winching system for suspended payload reeldown as well as all electronics support and command/control requirements. A cutaway of the suspended payload (b) shows representative in situ instruments and their associated inlet systems, meteorological measurements, electronics support, communication command and control, and safety parachute. The configuration of sensors for SCoPEx will be finalized in future engineering studies.

Figure 4.
Figure 4.

The concept of operations for the proposed experiment is initiated by seeding a 1 km length of stratospheric air with a combination of water vapour and sulfate aerosol using the propulsive capability of the StratoCruiser (a). Using a combination of its altitude and propulsive capabilities, the StratoCruiser manoeuvres past and above the seeded volume, which continues to expand owing to the turbulent wake generated by the propellers. The suspended instrument payload is reeled through the seeded volume to measure aerosols, water vapour and chemical species including HCl and ClO (b). The propulsion capability together with the LIDAR surveillance is used to track the seeded volume as it drifts with ambient wind and to make repeated measurements with the suspended payload, resolving the chemical evolution within the seeded volume as a function of time (c).

Figure 5.
Figure 5.

Calculated concentrations (ppbv) of HCl, NO, NO2 and ClO under background conditions (thin solid lines), and ‘slow’ (solid thick lines) and ‘fast’ (dashed thick lines) perturbed conditions for 48 h following injection that occurs just before dawn. See details in §4a. Note that ClO concentrations have been scaled up by a factor of 10 for clarity. The background conditions are 5 ppmv H2O and 2 μm2 cm−3 SAD sulfate aerosol. ‘Slow’ case has T=208 K and 15 SAD μm2 cm−3; ‘fast’ case has T=204 K and 50 μm2 cm−3 SAD. Both cases have 10 ppmv H2O inside plume.

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