Climate Controversy: Does the 2 Degree Goal Need to Go?

Global Warming
(Image credit: David Carillet |

It's time to ditch the goal of keeping Earth's warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit), researchers argue in a new opinion piece — a suggestion likely to receive pushback from many in the climate science and policy community.

In 2009, at the United Nations Climate Change Conference, international negotiators drew a line in the sand: Humanity must not let the planet get hotter, on average, than 2 degrees Celsius above preindustrial levels. This number has been the focal point of international agreements and negotiations ever since, but it has also been controversial.

Former NASA climate scientist James Hansen, who retired from the agency in order to pursue climate change activism, has long argued that 2 degrees of warming is too much, and will still lead to catastrophic consequences. [8 Ways Global Warming is Already Changing the World]

Now, David Victor, a professor of international relations at the University of California, San Diego, and Charles Kennel, an atmospheric scientist at the Scripps Institution of Oceanography, in La Jolla, California, want to get rid of the 2-degree goal, too, but for different reasons. The goal is unobtainable and scientifically oversimiplified, Victor and Kennel wrote in an opinion piece published today (Oct. 1) in the journal Nature.

"This has been part of this delusion that has happened in the climate policy community for the last 10 to 15 years, where people have been able to pretend that what they are doing is going to make an impact," Victor told Live Science.

2 degrees

The argument against the 2-degree goal is twofold. First, Victor said, "It's become very clear to me over the last few years that this goal is not achievable."

Victor isn't alone in this view. In February, for example, researchers from the Union of Concerned Scientists wrote in the journal Nature Climate Change that the time has come to acknowledge that the world won't manage to keep warming below 2 degrees.

Meeting the goal is technically possible, Victor said, but would require a massive, coordinated international effort, plus the immediate availability of technology that isn't yet ready for prime time.

The goal also oversimplifies the complexity of the climate system, Victor and Kennel argue. There is no question that carbon dioxide (CO2) warms the atmosphere, trapping heat near the surface of Earth in the same way a glass windshield bakes the interior of a truck parked in the sun. But the climate is complicated. Some heat gets absorbed by the ocean. Natural variability means that the rise in temperatures is not even, but occurs in fits and starts. High latitudes are more sensitive than the rest of the planet, which explains why the Arctic is melting rapidly even as the average global temperature has remained fairly stable since 1998, rising less quickly than in the years before.

"Most of the energy is going into the oceans, so it is not being measured as temperature in the air," Victor said. [Top 10 Surprising Results of Global Warming]

To better capture this complexity — and to create more effective policies to deal with ongoing carbon emissions — Victor and Kennel suggest a switch from the single 2-degree goal to a set of Earth "vital signs" that will better assess the planet's health.

Among these, they recommend considering concentrations of CO2 in the atmosphere, which recently hit 400 parts per million (ppm); ocean temperatures, which are measured with a network of automatic submersibles; high-latitude temperatures; and extreme events. Most of these are already being monitored, Victor said, with the exception of extreme events. It can be difficult to link particular weather events to overall climate change, because of natural variability, but more researchers are working to do so.

A controversial plan

A shift away from 2 degrees would be complex, Victor said, but multiple measures are the norm in policy circles. The U.N.'s Millennium Development goals, for example, relied on eight economic indicators.

"The public does this all the time in other areas, whether it is crime policy or urban development or infrastructure spending," Victor said. But he acknowledged that the suggestion is likely to be controversial.

Interviews with experts contacted in advance of the paper's publication suggest he's right.

"I wish I could say more supportive things about the piece, but I'm afraid it is rather misguided." The comment to Live Science came from Michael Mann, a climate scientist at Pennsylvania State University, famous for his "hockey stick" graph showing rising global temperatures.

Mann argued that the "pause" in global temperature increases over the past 16 years is a manifestation of short-term variability, a problem that would plague any measurement of the climate system, including Victor and Kennel's alternatives. Other complexities make the vital-signs index too simplistic, Mann said. Small particles called aerosols can cool the atmosphere, while other greenhouse gases, such as methane, can warm it more than carbon dioxide. [Earth in the Balance: 7 Crucial Tipping Points]

"These are critical components in any assessment of future climate change," Mann said. "There are fundamental reasons that experts in this field haven’t proposed such a simplistic metric for climate stabilization."

Most of all, the suggestion is "dangerous," Mann said, because it would give the public and policymakers an excuse to kick the can down the road on climate change.

"It’s possible that we will fail to stabilize temperatures below 2 [degrees] C warming even given concerted efforts to lower our carbon emissions, but simply discarding this goal would make failure almost certain," he said.

The members of the public are also unlikely to connect with Earth's "vital signs" any more than they do to the notion of 2 degrees of warming, said Anthony Leiserowitz, director of the Yale Project on Climate Change Communication, who was also not involved in the original op-ed.

"Most of the public don't know or understand the 2-degree goal or target," Leiserowitz said. "Most of them have never heard of it and don't know anything about it."

Nor are people any more informed on metrics such as the concentration of carbon dioxide in the atmosphere. In a recent study, Leiserowitz and his colleagues surveyed Americans on carbon dioxide concentrations. Only 7 percent knew that it was 390 ppm at the time (preindustrial levels were 280 ppm), and "I would guess a fair number of those were guessing," Leiserowitz said.

Leiserowitz said he agrees that measurements of climate change need to be more varied, particularly because the 2-degree goal gives the impression that everything will be fine until then, after which the planet becomes a complete disaster zone. In fact, he said, the change is gradual, with more and more impacts for every fraction of a degree of warming. But to engage the public, scientific measurements aren't likely to do the trick.

"Lives lost, illnesses and injuries caused, dollars lost to a variety of impacts that climate change is going to have, species lost at least in part due to climate change — those are the indicators that most people in the general public are going to be the most interested to know about, not the concentrations," Leiserowitz said.

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Stephanie Pappas
Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.