Ken Caldeira, M. Granger Morgan, Dennis Baldocchi, Peter G. Brewer, Chen-Tung Arthur Chen, Gert-Jan Nabuurs, Nebojsa Nakicenovic, and G. Philip Robertson
Continuation of current trends in fossil-fuel and land use is likely to lead to significant climate change, with important adverse consequences for both natural and human systems. This has led to the investigation of various options to reduce greenhouse gas emissions or otherwise diminish the impact of human activities on the climate system. Here, we review options that can contribute to managing this problem and discuss factors that could accelerate their development, deployment, and improvement.
There is no single option available now or apparent on the horizon that will allow stabilization of radiative forcing from greenhouse gases and other atmospheric constituents. A portfolio approach will be essential. The portfolio contains two broad options:
• Reducing sources of carbon (or carbon equivalents) to the atmosphere (e.g., reduce dependence on fossil fuels, reduce energy demand, reduce releases of other radiatively active gases, limit deforestation)
• Increasing sinks of carbon (or carbon equivalents) from the atmosphere (e.g., augment carbon uptake by the land biosphere or the oceans over what would have occurred in the absence of active management)
A variety of options could make a significant contribution in the short term. These include: changing agricultural management practice to increase carbon storage and reduce non-CO2 gas emission; improving appliances, lighting, motors, buildings, industrial processes, and vehicles; mitigating non-CO2 greenhouse gas emissions from industry; reforestation; and geoengineering Earth's climate with stratospheric sulfate aerosols.
Longer-term options that could make a significant contribution include separating carbon from fossil fuels and storing it in geologic reservoirs or the ocean; developing large-scale solar and wind resources with long-distance electricity transmission and/or long-distance H2 distribution and storage; ceasing net deforestation; developing energy-efficient urban and transportation systems; developing highly efficient coal technologies (e.g., integrated gasifier combined cycle, or IGCC, discussed later in this chapter); generating electricity from biomass, possibly with carbon capture and sequestration; producing transportation fuels from biomass; reducing population growth; and developing next-generation nuclear fission.
As long as we continue to use fossil fuels, there are relatively few places to put the associated carbon.
• If CO2 is put directly into the atmosphere, about one-third stays in the atmosphere, causing climate change. Another one-third currently goes to the biosphere, but this sink will eventually saturate, leaving CO2 to accumulate in the atmosphere, where it can cause climate to change. The remaining one-third quickly enters the ocean (and most of the increased atmospheric burden will end up in the ocean on longer timescales). This movement causes significant acidification of the biologically active surface waters before mixing and diluting in the deep ocean on timescales of centuries.
• If CO2 is put directly into the deep ocean (through deep injection), most of it will stay there without first producing a substantial acidification of biologically more active surface waters, but risks to deep ocean biota are not well understood.
• If CO2 is put into deep (>1 km) geological formations (through geologic sequestration) it may be effectively sequestered, but there is uncertainty about the available geological storage capacity, about how much of the injected carbon dioxide will stay in place and for how long, and what ecological and other risks may be associated if and when reservoirs leak.
• If CO2 could be mineralized to a solid form of carbonate (or dissolved forms in the ocean), it could be effectively sequestered on geological timescales, but currently we do not know how to mineralize carbon dioxide or accelerate natural mineral weathering reactions in a cost-effective way.
In the short run (<20 years), management of emissions of non-CO2 greenhouse gases and black carbon may hold as much or more potential to limit radiative forcing than management of carbon dioxide. Continued management of these non-CO2 greenhouse gases and particulates will remain essential in the long run.
Management strategies must be regionally adaptive since sources, sinks, energy alternatives, and other factors vary widely around the world. In industrializing and industrialized countries, the largest sources of CO2 are from fossil fuel. In less-industrialized countries the largest sources involve land use.
Technologies and approaches for achieving stabilization will not arise automatically though market forces. Markets can effectively convert knowledge into working solutions, but scientists do not currently have the knowledge to efficiently and effectively stabilize radiative forcing at acceptable levels. Dramatically larger investments in basic technology research, in understanding consequences of new energy systems, and in understanding ecosystem processes will be required to produce the needed knowledge. Such investments would enable creation of essential skills and experience with innovative pilot programs for technologies and options that could be developed, deployed, and improved to facilitate climate stabilization while maintaining robust economic growth.
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