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Ev 46 Science and Technology Committee: Evidence
A 50% chance of avoiding dangerous outcomes is a very low level of protection for an intergenerational endowment. Yet international negotiations to date have implicitly targeted concentrations of 450 ppme and above. Further, the modelling behind these estimates excludes what are termed “slow” feedback eVects— changes to ice sheets and vegetation. If climate models are required to deliver an appropriately high chance of keeping within the 2)C target, and also to account for slow feedback eVects, the world’s “carbon” budget is already overspent. Citing a changed understanding of the timescale for slow feedback eVects, James Hansen of NASA last year revised down his estimate of a “safe” concentration for CO2 alone to 350 ppm—meaning less than 400 ppme for all gases. This is below today’s level and would require that a significant volume of greenhouse gases be extracted from the atmosphere. In contrast, the plan to confront global warming that is the current focus of international negotiations is based simply on cutting emissions. It assumes that the atmosphere can absorb suYcient additional carbon to allow a prompt but unhurried transition to a low carbon economy. However, a truly low risk and precautionary plan would require not just cutting emissions but also reducing the existing concentration by extracting CO2 from the atmosphere. Can Sequestration Bridge the Gap? Avoiding dangerous climate change will require a new plan that makes use of a mixture of options. There is a clear hierarchy of preferred measures: 1. Abatement: reducing greenhouse gas emissions; 2. Sequestration: removing greenhouse gases from the atmosphere; and 3. Reflection: limiting warming by reflecting or blocking sunlight. Immediate options for large-scale sequestration are biologically based and dominated by aVorestation, while the capture of CO2 from the air by chemical reaction is seen as the key emerging technology. If all the sequestration capacity estimated to be available were used together with responsible rates of abatement, this would be almost enough to absorb both new and historic emissions at a rate suYcient to deliver a 380 ppme concentration by 2050. While new techniques may well raise future estimates of the available capacity, it remains uncertain what proportion of the technical potential could be accessed in practice and the extent to which financial constraints would limit uptake. Further, if the concentration is already above a safe level and it will take many years to reduce this significantly under any plan, there is the risk that significant feedback eVects could be triggered in the meantime—such as the release of additional greenhouse gases from thawing permafrost. A precautionary plan would therefore also need to investigate reflection options. Such a plan would place emphasis on managing risk, especially feedback eVects. These are triggered by what might be termed the Apparent Concentration—the warming eVect felt today as compared to that which can ultimately be expected. In particular, half the warming eVect of the greenhouse gases added to the atmosphere since the industrial revolution is suppressed by aerosol pollutants and other agents that reflect sunlight. It will be important to separately manage the net eVect of these two, the Apparent Concentration, while maintaining a long-term focus on reducing the greenhouse gas concentration. Facing the Mirror Reflection options receiving serious attention include injecting aerosols into the stratosphere and enhancing the reflectivity of clouds. If these techniques can be proven, each is estimated to have the capacity to neutralise the warming expected from the current concentration of greenhouse gases. They are both estimated to be low cost compared to sequestration and would lower average temperatures relatively rapidly. The fundamental diVerence between sequestration and reflection, however, is that sequestration addresses the root issue (the concentration) while reflection merely treats the symptom (warming). Problems arising from this include: — Reflection does not address the acidification of oceans that results from excess CO2 in the atmosphere being absorbed by the sea; — Schemes that inject particles into the atmosphere are likely to alter the distribution of rainfall and also cause some reduction in the global quantity of rainfall (as reflecting sunlight is not that same as reducing the CO2 concentration); and — Many reflection techniques will need to be replenished constantly over their lifetime and, if this is not kept up, extremely rapid warming could ensue. Even if the physical eVects were considered an acceptable near-term trade-oV, the haunting concern lying behind reflection is that its use could severely undermine incentives for both sequestration and abatement such that fair and aVordable action on these would not be undertaken in the near term. Reflection schemes may be low cost but if the concentration needs to be lowered from its present level, someone will ultimately have to pay for this. Deferring abatement and sequestration passes debt and risk to future generations.