2025 Greenhouse Capping Proposal Rejects 2 Degree C ‘Dangerous Warming’ Threshold

President Bush, well into what is widely seen as his lame-duck period, last month proposed his administration’s first concrete plans to cap U.S. greenhouse gas emissions. Policy makers and many in the news media appear to have largely written-off the proposal as too little, too late, in effect saying it would amount to further delay rather than serious action.

News Analysis

The Bush proposal to cap emissions by 2025 in effect amounts to a rejection of the 2 degrees C (3.6 degrees F) threshold of “dangerous warming” (relative to preindustrial average temperatures) that much of the world – at least in principal – has agreed to avoid.

That threshold, the equivalent of an atmospheric concentration of 450 parts per million of carbon dioxide equivalent (CO2e), reflects a growing consensus among climate scientists and politicians from across the globe. (It contrasts with the call (pdf) by NASA climate scientist James Hansen that a concentration of 350 ppm is needed to prevent the world from irrevocably altering the climate. Others, however, such as the economist Sir Nicholas Stern, have argued that a 450 ppm target would be unrealistic, given the rapid growth in developing country emissions and slow policy response, and that a more realistic goal is in the range of 500 or 550 ppm.)

To put these numbers in perspective, 350 ppm is roughly equivalent to 1 degree C (1.8 degrees F) global average warming at equilibrium relative to preindustrial temperatures; 450 with 2 degrees C (3.6 degrees F) warming; and 550 with 3 degrees C (5.4 degrees F).

President Bush’s 2025 peaking date without question would exclude reaching a 350 ppm CO2e target, and almost certainly preclude reaching a 450 ppm CO2e target.

The figure below, taken from the 2007 IPCC Fourth Assessment Report, shows the mean peaking date associated with various stabilization targets, with a fifth to ninety-fifth percentile range around that mean. According to this analysis, a 445-490 ppm target would require peaking around 2010, and there is less than a 5 percent chance that peaking after 2018 would allow such a target to be met.

Similarly, a 490-535 ppm target would also require a peak date around 2010, with only a 15 percent chance of a post-2020 peaking date reaching this stabilization target. A 2025 global peaking date for emissions would have a hard time even meeting a 535-590 ppm range, lying on the far end of the fifteenth to eighty-fifth percentile range.

As the IPCC report states, “the uncertainty range [in peaking dates] is relatively small for the more stringent targets, illustrating the reduced flexibility of the emissions path and the requirement for early mitigation.”

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Range of peaking years associated with different stabilization targets.
Figure from IPCC AR4 WGIII Chapter 3 (pdf).

The widely publicized Stern Review on the Economics of Climate Change (pdf) provided a similar analysis, including global emissions reduction rates needed to meet particular stabilization targets given different peaking years. Stern argued that emissions reduction rates of more than 10 percent per year are extremely unlikely, given the sunk costs involved in capital investments, no matter how optimistic one is about technological growth.

Stern’s report shows that reaching a 450 ppm target would be impossible given this 10 percent constraint for peaking years after 2010. Similarly, a 500 ppm target with a peaking date after 2020 would be possible only by overshooting 500 ppm and later reducing atmospheric concentrations, and this pathway would be associated with a global emission reduction rate of more than 5 percent per year. A 550 ppm target would be compatible with a 2025 peaking date in this study, though it would still require a global emissions reduction rate of around 3 percent a year, Stern indicated.

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Peak emissions date and reduction rates associated with different stabilization targets. Figure from the Stern Review (pdf).

Regardless of which study one considers, the President’s 2025 peaking goal for greenhouse gas emissions is fundamentally incompatible with a limiting of the world to less than 2 degrees C warming. Even if the rest of the world were to agree to peak emissions by the same date, it would still be difficult to meet a three degree target, especially given projected reductions in the cooling aerosol emissions.

While calculating the implications of certain “peak year” or emission intensity targets is difficult, journalists may do well to evaluate how compatible various targets are with differing greenhouse gas stabilization scenarios, and what amount of warming they imply. The President’s proposal represents a rejection of the 2 degrees C threshold of “dangerous warming” that much of the rest of the world has agreed, again at least in principal, to avoid.

Zeke Hausfather

Zeke Hausfather, a data scientist with extensive experience with clean technology interests in Silicon Valley, is currently a Senior Researcher with Berkeley Earth. He is a regular contributor to Yale Climate Connections (E-mail: zeke@yaleclimateconnections.org, Twitter: @hausfath).
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