- Greenhouse gas emissions along with other deleterious anthropogenic impacts to Earth are driven by an ever-increasing demand for human products and services. This is further exacerbated by a growing population. Such trends challenge the long-term sustainability of modern civilization. The monumental mitigation technology challenge would be substantially moderated, if human needs were downscaled and/or population growth was reduced.
- Concentrations of CO2 have increased to 383 ppm from a pre-industrial value of 278 ppm. This increase is due to anthropogenic emissions of CO2 that can remain in the atmosphere more than 100 years. There is close to a scientific consensus that much if not all of the nearly 0.8° C global warming seen since the pre-industrial era is a result of increased concentrations of CO2 and other greenhouse gases.
- Global emissions of carbon dioxide have been accelerating at a rate of about 1.4% per year in the 1992-2002 time period. However, recent data suggests an acceleration of emission growth in recent years; 3.25% in the 2000-2008 period. China's major expansion of its coal-fired power generation capacity has been the key factor in this acceleration in growth rate. It will not be possible to have an effective global mitigation program without a serious commitment by the major emerging economies (e.g., China, India, and Brazil).
- Projections of warming have been made on a credible business-as-usual case based on IEA  projections extended to 2100. This base case assumes a global annual growth rate of 1.6% in the next 25 years. Under this assumption, CO2 concentration is projected to increase to 500 ppm in 2050 and 825 ppm by 2100. Such concentrations will yield best estimate average warming since the pre-industrial era, of 1.9°C in 2050 and 3.9°C in 2100.
- There is still a large range of uncertainty associated with these warming projections; the potential warming in 2100, since the pre-industrial era, could be as high as 4.9°C or as low as 2.5°C. Warming would continue into the next century, with equilibrium warming in the 2.7-10.5°C range, with the best estimate at 5.2°C above 1990 levels.
If current worldwide emission trends continue and grow at 3% per year until 2030 before moderating, then projected warming, and potential consequences, would be substantially higher. This scenario will yield a best-estimate average warming, relative to 1990, of 2.2°C in 2050 and 4.8°C in 2100. Warming would continue into the next century, with equilibrium warming in the 3.4-13.2°C range, with the best estimate at 6.6°C above pre-industrial levels. It is too late to prevent substantial additional warming; the most that can be achieved would be to moderate the projected warming. The best result that appears achievable with CO2 mitigation, assuming a major energy technology retooling, would be to constrain warming by about 2.4°C above pre-indus-trial levels (range 1.7°C and 3.1°C) by 2100. If other GHGs are aggressively mitigated, warming could be constrained to about 1.8°C. It is significant that an aggressive methane mitigation program could contribute in the order of an additional 0.3°C of warming avoidance. Aggressive mitigation of N2O and ozone precursors can yield an additional 0.3°C warming reduction. Although less certain, control of black carbon emissions from key combustion sources might also provide comparable potential mitigation.
Global impacts even for this constrained warming scenario are potentially serious. This suggests that the world community may have no remaining alternative other than to pursue both mitigation and adaptation approaches aggressively.
In order to limit warming to about 2.5 ± 0.7°C utilizing CO2 emission mitigation, it will be necessary for the world community to decrease annual emissions at a rate of between 2% and 3% per year for the rest of the century. The earlier the mitigation program starts, the less drastic the annual reductions would need to be. Since the base case assumes a roughly 1.6% positive growth rate, approximately one trillion tons of carbon (3.7 trillion tons of CO2) will have to be mitigated by 2100 relative to the base case. This would be an historic challenge. Never has the world community had to face the prospects of fundamental energy production and utilization transformations to such an extent and at such a pace. Recent publications were used to relate the implications of a 4 trillion-ton CO2 mitigation program needed, along with aggressive mitigation of the other GHGs, to constrain warming below 2°C, to the key energy sectors and the technologies within those sectors that can contribute to the major mitigation challenge. It is concluded that an aggressive, cost effective mitigation program relying on existing technologies is capable of mitigating only between about 25% and 45% of the required CO2, depending on projected business as usual CO2 growth rates. Therefore, in the absence of fundamental lifestyle changes, new technologies are required for the key energy-related sectors: power generation, transportation, industrial production, and buildings. The power-generation sector and transportation sectors are particularly important, since they are projected to grow at relatively high rates, driven especially by China and other actively developing countries.
The power-generation sector, which has been growing at 3.7% annually in the 2000-2008 period. Since the key source of emissions from this sector is coal combustion, it is critically important to develop affordable CO2 mitigation technologies for such sources and to develop economical alternatives to coal-based power generation. CCS offers the potential to allow coal use while at the same time mitigating CO2 emissions. The three major candidates for affordable CO2 capture are: PC boilers with advanced CO2 scrubbing, IGCC with carbon capture, and oxygen-fed (oxy-fuel) combustors. Of the three, only IGCC is being funded at levels approaching those needed. However, all three approaches rely on underground sequestration, an unproven technology at the scale required for coal-fired boilers, with many serious cost, efficacy, environmental, and safety issues. Nuclear power plants, natural gas/combined cycle plants, wind turbines and solar generators all have the potential to decrease dependence on coal use and make significant contributions to CO2 avoidance. An accelerated RD&D program is particularly important for advanced nuclear reactors, since serious safety, proliferation, and waste-disposal concerns remain, and for solar power systems given their long-term potential if costs can be substantially reduced.
The building sector, projected to grow globally at about 2% per year, is where much of the generated electricity is utilized and where there are many currently available technologies that can significantly reduce the use of electricity and other energy sources, with a corresponding decrease in CO2 emissions. The constraints here are less technological and more socioeconomic. However, to the extent RD&D can lower cost and raise efficiency of building components, it can help provide extra incentive for building owners to invest in the most efficient heating and cooling systems, lighting, and appliances.
Emissions from the transportation sector are projected to grow at 2% per year. The challenge in this sector is two-fold. The first challenge is that current propulsion systems all depend on fossil fuels with their associated CO2 emissions, suggesting that technologies based on renewable sources, such as biomass, would be important. The second challenge is that the automobile industry, driven by consumer preferences (especially in North America), have offered heavy, inefficient vehicles such as sport utility vehicles. A review of developing technologies suggests that hybrid and plug-in hybrid vehicles and biomass-to-diesel fuel via thermo chemical processing are the most promising in the near term. However, cellulosic biomass-to-ethanol and hydrogen/fuel cell vehicles offer longer-term potential, if key technical, economic, and environmental issues are resolved and, in the case of hydrogen, renewable sources are developed.
Industrial sector emissions are projected to grow at an annual rate of 1%. Although CO2 emission avoidance approaches can be specific to a particular industry, the following key commercial technologies can be applied to a large fraction of the industrial sector: efficient motors, steam generators, and enhanced use of cogeneration technology. For the larger, more energy-intensive industries such as blast furnaces, CCS offers the potential for mitigating large quantities of CO2. Developing and deploying new or modified industrial production processes can also yield important CO2 emission mitigation potential. Another attractive approach is to encourage utilization of products that have a lower life-cycle CO2 content (i.e., require less carbon intensive energy during product production, use, and disposal).
- If near-term mitigation of four trillion tons of CO2 is deemed a serious goal, a major increase in RD&D resources will be needed. Current CO2 mitigation research expenditures in the United States and globally have been relatively flat in recent years, and the U.S. federal research expenditures on energy technologies are 70% lower than research expenditures in response to oil shortages in the mid-1970's. U.S. private sector research has fallen even more precipitously in recent years. It is important that such RD&D be conducted at both the federal and private sector levels. Federal funding is particularly relevant for those technologies that require substantial funding due to high capital costs and/ or have a low probability of near term commercial impact and profitability. Examples include carbon capture and storage, and the next generation nuclear power technologies. Private sector funding for the lower cost, lower risk technologies could be encouraged by providing incentives, such as regulatory drivers and meaningful carbon prices. Technology research, development, and demonstration are of particular importance for coal generation technologies: IGCC, oxygen coal combustion, and CO2 capture technology for pulverized coal com-bustors. All of these technologies will have to be integrated with underground storage, a potentially breakthrough technology, but one which is at an early stage of development and faces environmental and cost issues. Also important are next generation nuclear power plants, solar technologies, biomass to diesel fuel processes, cellulosic biomass-to-ethanol production technology, and hydrogen production technology. Given their potential for wide scale utilization, all of these emerging technologies must evolve with full consideration of the tradeoffs associated with their unique environmental characteristics and their carbon mitigation potential, economics, safety, etc. Toward this end, concurrent research to assess potential environmental impacts and to identify risk management approaches is needed.
- Given the monumental challenge and uncertainties associated with a major mitigation program, it may be prudent to consider all available and emerging technologies. This suggests that fundamental research on energy technologies in addition to those currently in advanced stage of development, be part of the global research portfolio, since breakthroughs on today's leading edge technologies could yield tomorrow's alternatives. In addition, it is the author's opinion that it is prudent to consider geoengineering options, which although radical in concept, could potentially buy the time we may need to make the necessary adjustments in our energy and industrial infrastructure.
- Finally, availability of key technologies will be necessary but not sufficient to limit CO2 emissions. Since many of these technologies have higher costs and/or greater operational uncertainties than currently available carbon intensive technologies, robust regulatory/incentive programs will be necessary to encourage their utilization.
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