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By Amy Lynd Luers
A growing body of scientific evidence indicates that sustained global warming of more than two degrees Celsius (°C), or 3.6 degrees Fahrenheit, above pre-industrial levels (i.e., those that existed prior to 1860) could have such damaging effects as the extinction of many species and extensive melting of the Greenland and West Antarctic ice sheets—causing global sea level to rise between 12 and 40 feet. In light of this evidence, policy makers in the European Union have already committed their countries to a long-term goal of limiting warming to 2ºC.
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Implications of Delay |
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Under a "business as usual" scenario, the United States would use nearly all of its emission budget by 2030, requiring unrealistically drastic cuts thereafter to achieve the 450ppm CO2eq stabilization target by 2050. In contrast, the emissions cuts required by S. 309 (the Global Warming Pollution Reduction Act) would allow reductions to proceed in a more gradual fashion, providing greater flexibility in the method and timing of reductions.
(click here for larger image) | The United States has agreed in principle to work with more than 180 other nations under the United Nations Framework Convention on Climate Change, but has done little to live up to that agreement thus far. And although there is growing momentum within the United States to pursue deep reductions in emissions of carbon dioxide (CO2) and other heat-trapping gases that cause global warming, U.S. policy makers have not had access to any rigorous scientific assessments that would provide a sound rationale for specific emissions reduction targets. This is one reason for the wide variation in state-level targets that have been put into law and federal targets that have been proposed.
Step One: Set a Global Target
A new UCS analysis fills the information gap by proposing U.S. targets based on atmospheric levels of heat-trapping gases that existing scientific studies suggest would give us a reasonable chance of preventing a temperature increase of more than 2?C above pre-industrial levels. To be precise, stabilizing the concentration of heat-trapping gases in the atmosphere at or below the CO2 equivalent of 450 parts per million (450 ppm CO2eq—a measurement that expresses the concentrations of all heat-trapping gases in terms of CO2) would provide a roughly 50 percent chance of keeping the global average temperature from rising more than 2°C.
Given current levels of heat-trapping gases in the atmosphere, meeting this stabilization target will likely require atmospheric concentrations to peak above 450 ppm CO2eq briefly before returning to the target. Recent studies indicate that, to follow such a path while still maintaining a reasonable chance of keeping temperatures from rising more than 2°C, cumulative global emissions must not exceed approximately 1,700 gigatons (Gt) CO2eq for the period 2000–2050. Constraining cumulative global emissions (i.e., those of industrialized and developing nations) in this way will require reductions on the order of 40 to 50 percent below 2000 levels by 2050.
Step Two: Divide Up the Work
If we assume that the world’s developing nations pursue the most aggressive reductions that can reasonably be expected of them, the world’s industrialized nations will have to reduce their emissions an average of 70 to 80 percent below 2000 levels by 2050. Industrialized nations’ share of the cumulative emissions budget must be no more than 700 GtCO2eq (approximately 40 percent of the budget).
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Cumulative U.S. Emissions in 2050 under Federal Proposals |
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Only two current climate policy proposals (H. R. 1590 and S. 309) would stay within the emissions budget of 160 to 265 GtCO2eq defined by our analysis, and even these proposals would result in emission well below the low end of the range. For S. 1776, the potential range of cumulative emissions is provided (the lower portion of the bar represents the best case scenario, assuming all contingencies in the bill occur; the color gradient in the upper portion of the bar represents additional emissions that could occur under other scenarios).
(click here for larger image) | This 70 to 80 percent range for reductions by 2050 also assumes that industrialized nations’ emissions will peak in 2010 before starting to decline, and that those from developing nations will peak between 2020 and 2025. A delay in the peak of either group would require increasingly steep and unrealistic global reduction rates in order to stay within the cumulative emissions budget.
Step Three: Define Our National Goal
There are several ways to determine the United States’ share of the industrialized nations’ emissions budget. Our analysis explored three methods, including allocations based on the current U.S. share (among industrialized countries) of population, gross domestic product (GDP), and heat-trapping emissions. Using these criteria, the U.S. cumulative emissions budget ranges from 160 to 265 GtCO2eq for the period 2000–2050, of which approximately 45 GtCO2eq has already been emitted.
Given our aggressive assumptions about reductions by other nations and the fact that a stabilization target of 450 ppm CO2eq represents the upper limit needed to avoid a potentially dangerous temperature increase, we propose that the United States should reduce its emissions at least 80 percent below 2000 levels by 2050.
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What Happens If Temperatures Rise More Than 2ºC?
Scientific studies indicate that crossing this temperature threshold will increase the risk of potentially severe impacts in the following areas.
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Sea level. Sustained warming between 1.6ºC and 5.2ºC above pre-industrial levels could initiate widespread destabilization of the Greenland and West Antarctica ice sheets, leading to sea level rise between 12 and 40 feet. While the full increase may take centuries to occur, even an increase of one meter (about three feet) would threaten major cities including Mumbai, New York, and Tokyo, and inundate some small islands. Rising seas also magnify the destructive potential of coastal storms; projections show a mere 7- to 14-inch rise could produce coastal flooding in Boston and Atlantic City, NJ, equivalent to today’s 100-year flood almost every year on average.
Wildlife. Even 2ºC to 3ºC of warming could threaten 20 to 30 percent of Earth’s species with extinction, according to the Intergovernmental Panel on Climate Change (IPCC). Many coral reefs would become bleached at these temperature thresholds, and marine ecosystems could suffer from increasing ocean acidification.
Weather-related events. Warming in this range has been linked to increases in the severity of floods, droughts, fires, and heat waves. A 2.4°C increase, for example, would cause many cities across the northeastern United States to experience a projected tripling in the number of days with high temperatures above 32°C (90°F), increasing the risk of heat-related illness and death among vulnerable populations.
Water resources. In the continental United States, drought-prone ecosystems are projected to expand approximately 11 percent in area for each degree Celsius of additional warming. Worldwide, 1 to 2 billion people would be at risk of increased water scarcity. | The costs of delay are high. To meet this minimum target, starting in 2010 the United States must reduce its emissions, on average, approximately 4 percent per year (equivalent to an average absolute reduction of approximately 0.16 GtCO2eq per year, or about 2 percent of current levels). If, however, U.S. emissions continue to increase until 2020—even on a “low-growth” path projected by the Energy Information Administration (EIA)—the United States would have to make much sharper cuts later: approximately 8 percent per year on average from 2020 to 2050, or double the annual reductions that would be required if we started promptly.
The Way Forward
Of the six climate policy proposals currently being considered by the U.S. Congress, only two—the Safe Climate Act (H.R. 1590) and the Global Warming Pollution Reduction Act (S. 309)—would require reductions consistent with staying below the upper limit of our proposed U.S. cumulative emissions budget (265 GtCO2eq). Not one comes close to the lower end of the proposed budget (160 GtCO2eq).
On the other hand, several of the proposals (H.R. 1590, S. 309, and S. 485, the Global Warming Reduction Act) do provide for periodic review by the National Academy of Sciences, which could maintain or strengthen U.S. targets as needed to meet the goal of preventing a 2°C temperature increase—an essential element of any robust climate policy. Other proposals provide for review but fail to specify the 2°C goal or allow the targets to be strengthened.
It is clear that the United States must quickly overcome its current impasse on climate policy and actively pursue the many solutions for reducing heat-trapping emissions that are already available to us (e.g., greater energy efficiency, increased use of renewable energy, reductions in deforestation). These changes can be encouraged by a wide range of market-based and complementary policies including cap-and-trade programs, renewable electricity standards, efficiency standards for electricity and vehicles, and incentives for cleaner technologies and international cooperation on emissions reductions.
For the United States to be fully engaged in the fight against global warming, however, Congress must support legislation that requires the deep emissions reductions needed to stay within the budget described here and protect future generations from the risks of dangerous climate change.
Amy Lynd Luers, manager of the UCS California climate program, wrote this article based on the work of several report authors.
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