The report did note, however, that there are some serious questions over adverse effects with this method, particularly depletion of stratospheric ozone.

Volcanic eruptions provide evidence that sulfate particle injection leads to reductions in globally-averaged surface temperatures. However, there are concerns that there will be substantial regional impacts, on temperatures, rainfall and other aspects of climate. There are also uncertainties concerning timescales e.g. how rapidly injection might act, how quickly it could be turned off and whether the climate responds differently to continued injection of aerosols compared with the episodic nature of volcanic eruptions.

In terms of geo-engineering, the natural volcanic analogue of sulfate particle injection may not be optimum in terms of radiation management, and there may be better candidate particles for injection. Related to this, there are significant issues of cost and feasibility of injecting candidate particles into the stratosphere and the sustainability of particular injection technologies that require much further investigation.

The SPICE project will investigate the effectiveness of stratospheric particle injection. It will address the three main challenges in solar radiation management:

1. How much, of what, needs to be injected where into the atmosphere to effectively and safely manage the climate system?
2. How do we deliver it there?
3. What are the likely impacts?

SPICE addresses these questions through three coordinated and inter-linked work packages:

1. WP1. Evaluating candidate particles. What is the “perfect” particle that maximizes solar radiation scattering, minimizes the greenhouse effect and the impact on the stratospheric ozone layer and has minimal impact on climate, weather, ecosystems and human health?

   Critical in this project is the understanding of the interaction of radiation (of various wavelengths) with aerosol particles, and likely chemical effects on the stratosphere of injection of significant additional surface area as a potential catalyst for multiphase chemistry. Researchers will develop metrics of the suitability of various particle compositions, sizes and surface properties for stratospheric aerosol geoengineering (including scattering efficiency, greenhouse effect, chemical reactivity, lifetime, cost of fabrication, health impact, capability to serve as ice nuclei, etc) and perform an assessment of candidate aerosol particles (sulphuric acid, sea-salt, other salts, minerals, and metal oxides) from the available literature and simple modelling of the key reactions, surface properties, agglomeration and sedimentation rates, and light scattering theory.

2. WP2. Delivery Systems. What are the various options for delivery of particles? ( A range of delivery systems have at various times been proposed including batch delivery by aircraft, balloons or ballistics and steady-state delivery by thermal plumes, or from a fixed tower or pipe supported by a balloon.) What is the feasibility of using a tethered-balloon pipe to inject particles and/or gases into the stratosphere in a more cost-effective and sustainable way than alternative methods?

   Particles are to be injected into the stratosphere at heights upwards of 10km (mid-latitude) and 18km (equatorial). The rate required for global climate modification is upward of 1Mte p.a. and at this rate the delivery costs for batch methods are estimated to be well above £1 billion (US$1.5 billion) p.a. but an order of magnitude less than 1/10 of this for the pipe delivery method.

   If current thinking is validated, the WP will proceed with a more detailed design of a tethered balloon approach which will involve the design of a pipe and associated pumping and deployment systems. The WP will begin with a preliminary technical evaluation of the pipe-balloon system and the associated fluid/structural mechanics. The pipe will be up to 25 km long, subject to high tensile and bursting pressures. It will be a sealed unit, abrasion resistant, insulated to prevent freezing and made of a braided or filament wound fibre composite.

3. WP3. Climate and environmental modelling. What are the most effective locations for injection? How can we best use past volcanic analogues? What are the climate and environmental impacts of stratospheric particles?

   In this work package essential groundwork will be carried out to assess and optimize the accuracy of the model and an extensive list of metrics and diagnostics will be developed to assess the impact of particle injection in the atmosphere.

Principal investigator on SPICE is Dr. Matthew Watson from the University of Bristol, who also is heading work on WP1. Dr. Hugh Hunt from the University of Cambridge is in charge of WP2 and Prof.Lesleyy Gray from the University of Reading is in charge of WP3.