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  An Introduction to Climate Change Legislation

Table of Contents | Foreword | Preface | Executive Summary | Overview | Contributors | Participants and Staff

U.S. Climate Mitigation in the Context of Global Stabilization

Richard G. Newell and Daniel S. Hall


This issue brief examines recent studies of long-term scenarios for stabilizing atmospheric concentrations of greenhouse gases (GHGs) to understand whether and how near-term U.S. climate policy can translate into environmentally significant climate outcomes. Specifically, the focus is on modeling analyses that have attempted to quantify the emissions reductions necessary to achieve a defined set of stabilization targets. The scenarios analyzed include information on the path of emissions reductions, changes in technology, and prices for emissions needed to reach different stabilization levels. As such, they provide insight on the near-term actions - particularly with regard to carbon prices and technology developments - that would be consistent with achieving long-term environmental objectives.

The broad picture given by the model scenarios can help inform near-term policy. Although the models differ in their details, several messages emerge.

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U.S. Climate Mitigation in the Context of Global Stabilization

  • Given current estimates of the relationship between GHG concentrations and global temperature change, stabilizing atmospheric concentrations of carbon dioxide (CO2) at 450-650 parts per million (ppm) by volume significantly reduces the expected change in global average surface temperature and associated impacts relative to baseline projections for increased GHG concentrations.

  • Most modeling scenarios for cost-effectively achieving a 550 ppm CO2 (670 ppm CO2-e1) stabilization target show U.S. and global emissions leveling off over the next several decades, with a slight initial rise in emissions that peaks by 2020-2040, and a declining trajectory thereafter. Stabilizing at lower concentration levels would require that emissions start declining sooner; while a less protective (higher concentration) target would allow for a longer period of continued emissions growth and/or slower decline.

  • To cost-effectively stabilize atmospheric CO2 at about 550 ppm, most models require that global carbon prices rise to $5-$30 per metric ton of CO2 in the next 20 years, increasing to $20-$90 per metric ton by 2050, and continuing to rise thereafter. These modeling scenarios assume an idealized, flexible, comprehensive, least-cost approach to reducing emissions. Costs could therefore be significantly higher in the context of real-world policy where countries set different levels and trends of policy stringency, do not cover all sectors, do not include all GHGs, or employ relatively costly policy instruments. For example, limiting mitigation to CO2 (rather than all GHGs) could roughly double the CO2 prices needed to achieve a given stabilization goal.

  • The more stringent the stabilization target, the higher the CO2 price required to achieve it and vice versa. Models suggest that the global carbon price levels needed for stabilization at 450 ppm CO2 (530 ppm CO2e) could be 3-14 times higher by 2050 than the price levels needed to stabilize at 550 ppm, assuming emissions reductions are implemented cost-effectively. Likewise, a less stringent 650 ppm CO2 (830 ppm CO2e) target could be achieved with CO2 prices that are 50-75 percent lower than the prices modeled for a 550 ppm target, since considerably less action would be required relative to baseline expectations.

  • Although the models show differing degrees of utilization for different technology strategies, all of them indicate that achieving the requisite emissions abatement will necessitate reductions in both overall energy use (through efficiency and conservation) and in the carbon intensity of remaining energy use (through greater reliance on low- or non-carbon resources such as nuclear power, fossil-fuel systems with carbon capture and storage, and renewable electricity and biofuels). Scenarios that assume higher rates of baseline economic growth require pushing harder on each of these technological fronts to achieve a given stabilization goal, with commensurately higher emissions prices.

  • Concerted global action including all large emitters will be required in the medium and long term to cost-effectively stabilize atmospheric GHG concentrations. Nonetheless, delaying reductions by developing countries in the near term would not significantly impede the prospects for CO2 stabilization at levels of about 550 ppm or higher. However, if the stabilization target is close to current levels (450 ppm) flexibility is considerably reduced, and early participation by developing countries becomes essential if much higher costs are to be avoided.

    1. CO2 equivalence is a means of measuring the total concentration of all GHGs, not solely CO2.

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