It's a popular recipe for reversing global warming: Break the problem into manageable chunks, known as "wedges," and use technology we already have to reduce greenhouse gas emissions in the decades ahead.
But a new study out of UC Irvine says that comforting recipe is wrong. The original prescription of seven wedges – each involving changes in our sources of energy, or the capturing and storing of carbon dioxide, the most troublesome greenhouse gas – must grow to at least 19, and perhaps as many as 31, the study says.
And far from relying on existing technology, blunting the coming climate change will require "a fundamental and disruptive overhaul of the global energy system," according to the study, "Rethinking Wedges," published last week in Environmental Research Letters.
"Seven wedges aren't nearly enough to, quote unquote, solve the problem," said Steve Davis, a UC Irvine climate scientist and the lead author of the study.
The idea of concentrating on wedges – such as wind or solar power, nuclear power or increased fuel efficiency – comes from a 2004 paper that became a landmark in the climate science community.
The paper argued for a two-phase attack: working to slow and eventually stop the growth of greenhouse gas emissions for the first half of the 21st century, then, in the second half, actually reducing emissions.
It presented 15 options for wedges to stabilize emissions over the ensuing 50 years, and said a combination of any seven could accomplish the task.
One of the authors of the paper, Robert Socolow of Princeton, agrees that times have changed, and that inaction since 2004 requires more wedges – nine, in fact, he now says.
But he questions Davis' assertion that cutting greenhouse gas emissions nearly to zero by midcentury might be necessary, instead of simply holding them steady at present-day levels.
And Davis' paper calls for a new crop of wedges, called "phaseout" wedges. The suggestion that these would require new, disruptive technology might create an artificial distinction, Socolow said.
The paper seems to suggest "that somehow, you need a different class of technology, you need entirely new technological options, to cross a line between stabilization and phaseout wedges," he said. "There is no line there. The line is being created for rhetorical purposes."
The difference of opinion is not just a matter of academic scientists arguing over abstractions. The discussion relies on computer modeling.
Policymakers who wrestle with such questions often talk of a goal for Earth's climate: preventing an average temperature rise greater than two degrees Celsius above that of the pre-industrial era.
The two-degree target is usually linked with cutting emissions in half by midcentury, splitting the difference between flattening emissions and a full phaseout.
The disagreement comes over how to keep carbon dioxide levels in the atmosphere below, say, 450 parts per million, a benchmark roughly connected with the two-degree limit. The present level is 394 parts per million.
The debate is explored in the UCI paper using a computer model that projects changes in Earth's climate over decades based on plugged-in assumptions.
Those, in turn, are based in part on measurements of present-day emissions of carbon dioxide and other greenhouse gases.
Model runs by Davis and his co-authors show that seven or even nine wedges, each cutting emissions by about 25 billion tons over 50 years, would have only a modest effect on surface temperatures by midcentury.
The nine-wedge approach, for instance, would allow warming close to two degrees by 2060, with a 500 parts per million threshold surpassed by 2049.
Far more radical changes are needed, Davis says, to prevent a potentially devastating rise in warming in the decades ahead.
"I really think we need to transform the existing energy system from one that uses the atmosphere as a waste dump for CO2 (carbon dioxide), to one that produces the same amount of energy without emitting any CO2 into the atmosphere," Davis said. "I don't think anyone would argue that that isn't a tremendous revolution in the way we produce and consume energy."
But Socolow says Davis's study fails to specify what those revolutionary changes might be.
And he still thinks more modest goals, such as flattening greenhouse gas emissions by midcentury instead of radically reducing them, are well worth pursuing.
In fact, he fears that trying to force too rapid a change on the world's energy systems could backfire.
"When I think about the world committing to a very tough target, I think we'd be doing a foolish thing," he said, because of "so much mischief on the part of world leaders in pursuit of some abstract goal. If you unleash a lot of global leaders with this task, it could be a very sorry outcome."
The two scientists do agree that achieving any substantial reductions in carbon dioxide emissions in a world that has done comparatively little since 2004 is a major challenge.
"He and I think the two-degree target is really difficult," Davis said. "The question is really hard to answer about the political will for this kind of transition to come about. I like the idea of not relaxing the target to something more plausible, but keeping it at two degrees. It's right to have one, no matter how difficult or plausible it might be."
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