What Would it Take to Limit Global Warming to 2°C ?

       To limit global temperatures from rising more than 2C above their pre-industrial levels, the global economy would have to immediately cut the amount of carbon dioxide (CO2) that it emits to the atmosphere, and sustain more concerted cuts through the 21st Century, according to an international team led by Glen P. Peters of the Norwegian Center for International Climate and Environmental Research. In a commentary they wrote1 for Nature Climate Change, Peters compares the track record of emissions of CO2 since 1990 with four widely used sets of projections for CO2 emissions in the future.

        Emissions of CO2 have not gone down, nor has the growth rate declined over the last several decades. In fact, the growth rate has accelerated since 2000, by +3.1% per year, after being +1% in the 1990s and +1.9% in the 1980s. The amount of CO2 released has consistently been at the high end of the range of various projected growth curves dating back to 1990. Estimates were prepared for the Intergovernmental Panel on Climate Change (IPCC) at four different times (1990, 1992, 2000, and 2012-13). Each estimate involved a projection to the year 2050 or beyond.

      The highest estimates in each of these four sets of projections would lead to a global warming anywhere from +3.5° to +6.2°C by 2100. Clearly, a global “warming cap” of +2°C or less will require reductions in CO2 emissions from today’s rates, and some think that the 2° goal will require “net negative emissions,” which means that humans would directly remove CO2 from the atmosphere through some intervention.

       According to Peters, the goal of +2C or less of global warming is possible if global emissions of CO2 are cut by 3% per year, starting by 2020. If the peak rate of CO2 emissions is not achieved by 2020 before the emissions start to decline, then the goal of limiting global warming to 2C “may become unfeasible,” he adds.

       The authors cite examples of countries that cut their CO2 emissions over 10-year periods. France, Belgium, and Sweden managed to cut their emissions by 4 to 5% per year in the 1970s and 1980s, after the oil crises of 1973 caused utilities to shift from oil to nuclear power. Electric utilities in the United States reduced emissions by 1.4% per year after 2005, as cheaper natural gas was substituted for coal to generate electric power at less cost.


       Peter’s team concludes that “unless large and concerted mitigation efforts are initiated soon, the goal of remaining below 2°C will very soon become unachievable.”

       A more substantial case for policies and pathways over the next 7 or 8 years that might succeed in limiting global warming to 2C over the long term was articulated in an article in Nature Climate Change2 this year. Joeri Rogelj and three co-authors explored how different amounts of greenhouse gases emitted from now until 2020 will affect the feasibility of meeting the 2C target in the long term. Within a range of CO2 emissions that would be permitted in 2020, from 20% below to 6% above the current (2012) emissions3, the option of limiting the warming of the planet to 2C or less is still open. The size of this "feasibility window" depends on whether certain energy technologies (nuclear power, biogas, etc.) can be reliably used or not, and whether energy efficiency practices succeed in limiting the growth in energy demand.

       The Swiss and Austrian team concluded that “improving the efficiency of energy” is key, because limiting the demand for energy had the largest impact on the size of the feasibility window. Next in importance was the availability of CO2 capture and storage techniques, and the “immediate participation of all regions” in agreements to cut greenhouse gas emissions. By this they mean that China, India, and the rest of the developing world would have to reduce their carbon emissions along with the industrialized First World. Without both changes, the team added, “it is infeasible to achieve 2C in our framework.” Finally, nuclear power does not seem to be absolutely necessary to meet the 2 target, unless CO2 emissions in 2020 exceed what they are today. (Editor's note: CO2 emissions certainly are on a course to do that.)

 CITATIONS                                                                            Top

1. “The challenge to keep global warming below 2°C” (2013), commentary by G. P. Peters and eight others, Nature Climate Change, vol. 3, 4-6 (2013). Doi:10.1038/nclimate1783.

2. “2020 Emissions Levels Required to Limit Warming to below 2°C” (2013), by Joeri Rogelj, D.L. McCollum, B.C O’Neill and K. Riahi, Nature Climate Change, vol. 3, 405-412, April 2013.

3. The current amount of greenhouse gases emitted by human activity was estimated to be equivalent to 51.8 billion tons of CO2 in 2012.

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