Twenty-Five Wedges or More

In a perspective just out in Science commenting on a new paper (Davis et al.) that shows another way to explain the decarbonization challenge, Marty Hoffert of NYU explains how the magnitude of the challenge of stabilizing atmospheric concentrations of carbon dioxide at a low level has been underestimated:

Pacala and Socolow (8) analyzed a scenario that envisioned stabilizing atmospheric concentrations of CO2 at 500 ppm within 50 years. They found that reaching that goal required the deployment of seven existing or nearly existing groups of technologies, such as more fuel-efficient vehicles, to remove seven “wedges” of predicted future emissions (the wedge image coming from the shape created by graphing each increment of avoided future emissions). Those seven wedges, each of which represents 25 gigatons of avoided carbon emissions by 2054, are cited by some as sufficient to “solve” climate change for 50 years (9).

Unfortunately, the original wedges approach greatly underestimates needed reductions. In part, that is because Pacala and Socolow built their scenario on a business as usual (BAU) emissions baseline based on assumptions that do not appear to be coming true. For instance, the scenario assumes that a shift in the mix of fossil fuels will reduce the amount of carbon released per unit of energy. This carbon-to-energy ratio did decline during prior shifts from coal to oil, and then from oil to natural gas. Now, however, the ratio is increasing as natural gas and oil approach peak production, coal production rises, and new coal-fi red power plants are built in China, India, and the United States (10).

The enormous challenge of making the transition to carbon-neutral power sources becomes even clearer when emissions-reduction scenarios are based on arguably more realistic baselines, such as the Intergovernmental Panel on Climate Change’s “frozen technology” scenario ( 11, 12). Capturing all alternate energy technologies, including those assumed within this BAU scenario, means that a total of ~18 of Pacala and Socolow’s wedges would be needed to curb emissions (13) (see the figure). And to keep future warming below 2°C, even under the Davis et al. age-out scenario, an additional 7 wedges of emissions reductions would be needed— for a total of 25 wedges (see the figure).

The total is even more than 25 wedges if you want to avoid using the oceans as a store of carbon dioxide or reduce emissions below 2010 levels.. The numbers that Hoffert presents in his perspective are the same as those that I present in The Climate Fix, under a similar analysis. However you do the math, the challenge is huge and making progress will take decades of effort.