Delays raise emission halt urgency

The inventors of the idea of using wedges to represent efforts that will grow to prevent a billion tonnes of CO2 emissions per year over 50 years made a game out of it. However, University of California, Irvine’s Steve Davis underlines that our ongoing failure to properly put the idea into action is very serious. Credit: Carbon Mitigation Initiative/Princeton University

We need to work towards completely cutting greenhouse gas emissions from energy generation for the goal of keeping global warming below 2°C to be realistic. So says Steve Davis at University of California, Irvine, who has updated a powerful method to make it easier to see what emission cuts are needed. That method originally just worked towards stopping emissions increasing, but Davis says that’s not enough today. “If you’ve been gaining weight and want to lose it, you don’t set a goal of eating just enough sweets to hold your weight constant for a few years, you set a goal of no sweets and set about losing weight right away,” Steve told me. “Why shouldn’t the same thing be true for carbon emissions?”

It’s hard to imagine how we can slow and stop emissions. In 2004, that drove scientists Stephen Pacala and Robert Socolow at Princeton University in New Jersey to come up with the idea of breaking the cuts needed into wedges. Each wedge is a unit of effort in stopping CO2 production, starting from a small beginning and becoming a massive impact. Over 50 years, each wedge of effort grows steadily from zero to a billion tons, or gigaton, of carbon emissions avoided per year. We can use the idea to decide how to cut out each wedge, assigning them to particular reduction efforts and technologies. Since their invention, wedges have been used to teach and make decisions about fighting climate change.

In their original study, the Princeton scientists just used the wedges  to bring our emissions to a stable level. At the time they found this would stop levels of the greenhouse gas in the air going above 500 parts per million (ppm). Reducing emissions from that level to zero would then have put a limit on climate change. However, accelerating emissions since their study have changed the starting point.

Wedge opportunity squandered

Temperature rise in the scenario used by the UN Intergovernmental Panel on Climate Change (IPCC) A2 scenario, and when nine wedges are taken away from the emissions it forecasts (W9). The bars either side of the line show the possible temperature range within two “standard deviation” measures from the main prediction. Even though nine wedges enacted in 2010 would be needed to keep CO2 emissions stable, the world would still near 2°C of warming from pre-industrial temperatures by 2060. Credit: IOP Publishing (See citation at end of entry)

“In 2004, Pacala and Socolow concluded that ‘the choice today is between action and delay’,” Steve and his team-mates wrote in a perspective published in Environmental Research Letters on Wednesday. “After eight years of mostly delay, the action now required is significantly greater.” Yet wedges remain “a powerful concept for dividing up the climate-energy challenge”, Steve said. “For this reason, we thought it was important to show that since 2004, that challenge has become more difficult and to update the wedge concept to reflect that fact,” he added.

The 2004 study called for seven wedges. Steve’s team therefore first took those wedges from the emissions seen in a scenario of the future used by the UN Intergovernmental Panel on Climate Change (IPCC). The US scientists chose a scenario that’s at the high end of the emissions the IPCC predicted – but below the actual emissions the world’s seen over the last decade. To compare the two scenarios, they simulated them in a model linking climate to how carbon cycles through our environment that ran from 2010 to 2060.

With just seven wedges, the model reached 2°C of warming compared to the time before countries started industrialisation by 2052. That rings alarm bells, as one of the few climate agreements our governments have been able to make is to keep warming below that limit. CO2 levels in the air exceeded 500 ppm in 2042, up from 394 ppm today, and reached 567 ppm in 2060. To halt rising CO2 emissions in 2010 the scientists then ran the models with nine wedges instead of seven. Though the nine wedges kept the world around 0.5°C cooler than just seven by 2060, total warming had still reached 1.9°C at this point. CO2 concentrations passed 500 ppm in 2049 with the extra two wedges.

A new climate action vision

Completely halting CO2 emissions could take up to 31 wedges; twelve that are likely to happen if we continue to reduce and make better use of fossil fuels as we have in the past, nine to bring CO2 emissions to a stable level – as Steve and his team-mates showed in their model, and another ten to phase them out entirely. Credit: University of California, Irvine

“This makes clear that stabilising emissions is not the same as stabilizing the climate,” Steve said. “But it also shows that we no longer have the luxury of waiting 50 years to begin reducing emissions. Emissions need to peak and decline much sooner if we still hope to avoid 2°C of warming.” But to do this could take up to 31 wedges, the scientists write. Twelve of these are “hidden wedges”. Their name refers to our ongoing improvements in how efficiently we use fossil fuels. If we continue to do this as rapidly as we have in the past, they’ll happen without the extra effort other wedges need. Nine are the stabilisation wedges that Steve’s team modelled. The final ten are what’s needed to completely eliminate fossil fuels, and therefore CO2 emissions, from generating energy.

“Whether or not it is possible to achieve a complete phase out of emissions in 50 years, it is a defensible aspiration and one that reflects the urgency of the energy transition implied by current climate targets like 2°C of warming,” Steve says. “It’s possible that pursuing such an ambitious transition could result in unacceptable economic or environmental trade-offs, but it’s certain that the transition won’t happen if we set our goal as anything less.”

These results back up previous research from Steve and his team-mates showing that we must now strive to cut our energy generation emissions all the way down to zero. Though the wedges do help map out how to get there, the researchers stress that this will be hard to do, and will demand a shift in our thinking. “Everyone who stands to benefit from cheap fossil energy and/or suffer the effects of a warming planet needs to reconsider their ideas of what a solution to climate change ultimately looks like,” Steve underlined. “Climate targets like 2°C of warming are out of step with the idea that we have decades to emit at current levels while we innovate affordable, carbon-free energy technologies.”

Journal Reference:

Davis, S., Cao, L., Caldeira, K., & Hoffert, M. (2013). Rethinking wedges Environmental Research Letters, 8 (1) DOI: 10.1088/1748-9326/8/1/011001