Limiting global temperature rise to no more than two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels has long been a goal of diplomats, environmentalists, and many scientists. However, two researchers are now calling that effort "wrong-headed." And, with just 15 months left before global leaders meet in Paris to forge an international climate agreement, their commentary is also facing criticism.
David Victor, a professor at the School of International Relations and Pacific Studies, and Charles Kennel, director emeritus at the Scripps Institution of Oceanography, argue that "new goals are needed" in an opinion piece in the current issue of Nature.
"It is time to track an array of planetary vital signs — such as changes in the ocean heat content — that are better rooted in the scientific understanding of climate drivers and risk," they maintain.
The suggestion to scrap the two-degree goal, however, is poorly timed, Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, told VICE News.
"It's provocative," he said. "For people involved in the UN climate negotiations, which does not include me, it's like reopening a wound. Given how long it's been to get people past this target issue and on to actual actions, revisiting this could be a huge distraction."
The two-degree target has framed nearly every recent public policy proposal on climate change and ongoing efforts to implement an international pact on curbing greenhouse gas emissions. But Victor and Kennel call the goal "effectively unachievable."
The average global temperature has already increased 0.85 degrees Celsius (1.4 degree Fahrenheit) above pre-industrial times due to climate change. Even if greenhouse gas emissions stopped overnight, temperatures are expected to rise an additional 0.8 degrees Celsius as gases already emitted continue to warm the atmosphere. That leaves little room between the amount of warming already locked into the climate system and the two-degree threshold, say Victor and Kennel.
"It's certainly an aspirational goal," Princeton University geoscience professor Michael Oppenheimer told VICE News. "But without such a goal, it is unlikely the world's governments would ever take adequate action. I do think it's a good way to frame policy and I do think that two degrees Celsius is within the range of plausible, science-based choices."
Re: 2°C. If you are driving in completely the wrong direction, arguing about where you'll park if you arrive isn't your highest priority
— Gavin Schmidt (@ClimateOfGavin)October 2, 2014
Oppenheimer was lead author on the UN Intergovernmental Panel on Climate Change's 1997 climate study and says even if warming is held below two degrees Celsius some regions of the world will still experience dangerous changes to local conditions.
"The two-degree threshold may not be perfect or even achievable but it stands on scientifically solid ground and it focuses us on a specific goal," Heidi Cullen, chief scientist for Climate Central, a non-profit science journalism organization, told VICE News. "It is just one way to define how burning fossil fuels and clear-cutting forests are doing harm."
Victor and Kennel also say the two-degree goal fails to offer concrete instructions to governments on how they should go about addressing climate change. What good is a goal, in other words, if it's not linked to concrete recommendations on how to act?
The pair point to the UN's eight Millennium Development Goals, which range from eradicating extreme poverty to ensuring environmental sustainability. Each objective includes specific targets and indicators of progress.
"In other areas of international politics, goals have had a big effect when they have been translated into concrete, achievable actions," say the authors.
Limiting the concentration of carbon dioxide in the atmosphere to a certain level or tracking the heat content of the oceans, they suggest, are better indicators for tracking progress on climate change.
Much of the environmental movement has rallied around the number 350 — the parts per million of carbon dioxide in the atmosphere that many scientists have identified as a threshold to avoid catastrophic climate change.
One advocacy group has even adopted the number as its name: 350.org.
The group's spokesperson Jamie Henn told VICE News: "More scientific indicators would clearly be a good thing. It's probably no surprise that we think 350 parts per million is a strong benchmark, both scientifically and metaphorically."
But, Henn added, the two-degree threshold conveys that humans are approaching a dangerous threshold, risking catastrophic melting of land and sea ice, sea level rise, and disruptions to agricultural production.
"More information is always better," said NASA's Schmidt. "We are already monitoring these in the scientific community. But what difference does any of this information make to any action that might be taken? That is where people need to be working and arguing. Whether parts per million or degree Celsius or millimeters of sea level rise is the most important is irrelevant. It just serves to excite the chattering climate class."
"Ultimately, climate change by any other number is still a crisis," said Henn.
Follow Robert S. Eshelman on Twitter: @RobertSEshelman
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