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The 2-Degree Warming Limit Is Arbitrary And Beside The Point

On Saturday at the Paris climate talks, negotiators released a draft agreement that outlines, in broad strokes, a plan for curbing climate change. Although that draft represents a step beyond the failed Copenhagen negotiations in 2009, it includes 939 pieces of bracketed text that highlight disagreements yet to be resolved.

One point of contention: how much climate change we are willing to tolerate. How hot does it have to get before the sea rises too far, the droughts come too often, and the glaciers melt too much?

Since 2009, the U.N. Framework Convention on Climate Change’s goal has been to make sure the Earth doesn’t get warmer than 2 degrees Celsius above pre-industrial levels.1 That sounds like a small number, but because it’s a global average, it contains all sorts of fluctuations — for the whole planet to get warmer by that amount means that some places are getting much hotter. There’s disagreement at the talks over whether this new agreement should aim to hold the temperature increase below 1.5 degrees Celsius above pre-industrial levels or whether the limit should remain at 2 degrees.

But these goals are arbitrary. Two degrees became the default goal almost by happenstance. The number originated with Yale economist William Nordhaus, who back in the 1970s published a paper suggesting that a rise in global temperature of more than 2 degrees would represent temperatures hardly seen over the past several hundred thousand years. Nordhaus’s number was based on preliminary intuition, though climate data has confirmed that during the past 100,000 years, global mean temperatures have rarely, if ever, reached much higher than 2 C above those around 1800.

Two degrees isn’t a magic number that will somehow hold catastrophe at bay. We have already nearly arrived at a 1-degree temperature rise, and as Reto Knutti and his colleagues argue in this week’s Nature Geoscience, the 2-degree goal is not a scientific one. Although the 2-degree limit is often referred to as a “guardrail,” Knutti told me, “there’s no scientific research to show that 2 degrees of warming is safe.” And there’s a reason for that — science alone can’t determine what’s an unacceptable level of danger.

The best analogy for this, he said, is speed limits on roads. “You could quantify the risk of dying at certain speeds, but even if you could quantify that perfectly, it would still not tell us what speed limit is appropriate — that’s a judgment call,” he said. Set a universal speed limit of 30 mph, and the roads are safer. But now it takes a long time to get places. On the other hand, if the speed limit is set very high, people can get to their destinations faster — but the roads become more dangerous. Science alone can’t determine the right middle ground between these two extremes.

The 2-degree limit is similarly a compromise between costs, benefits and risks. Set the warming limit too high, and it may not inspire the appropriate urgency. Set it too low, and it may be so strict that countries don’t sign on to the agreement because they’re afraid their economies will become too stunted if they have to stop using fuels that produce emissions. Given the social, political, and economic factors at play, science can’t provide a one-size-fits-all solution. Climate change affects regions and countries differently, and it’s difficult to precisely predict at which temperatures and greenhouse gas emission levels climate change will become unbearable. Residents of the Persian Gulf region may cry uncle sooner than those living in northern climates, and island states will feel the change before landlocked countries.

Determining where to draw the line requires judgment calls as well as science. It’s a question of priorities. For instance, Knutti said, the world could ask itself, “Do you care or not if the polar bear goes extinct?” There are reasons polar bears might matter: They’re icons of the fight against global warming, and their extinction might be a harbinger of a cascade of habitat changes that could ripple down the food chain. But on the other hand, setting a temperature limit to save the polar bear may not allow for a developing country to hit its GDP target. Which matters more to the world at large?

There’s not always agreement about what the priorities should be, obviously. Developed nations made their wealth with fossil fuels, and they aren’t going to give them up if that would require relinquishing the lifestyles they’ve become accustomed to. Even staunch environmentalists tend to go green only as long as it’s convenient.

Meanwhile, poor countries would like to develop their economies and lift their standards of living. But doing so without the fossil fuels that got the rest of the world there is an expensive challenge, especially as they face more extreme weather events because of climate change.

There’s also the question of how much wishful thinking should inform the goal we set. The countries who stand to suffer the greatest harm from rising temperatures and seas have come to France to make the case for lowering the target to 1.5 degrees Celsius, and members of the Alliance of Small Island States have rallied around the slogan “1.5 To Stay Alive.” But the 1.5-degree goal is looking as achievable as cold fusion right now. Before the conference began, attending nations submitted voluntary pledges for reducing greenhouse gas emissions, and the numbers are clear — these plans leave us on track to exceed the 2 C “guardrail” by 2100.

Some scientists argue that it’s folly to pretend that we can meet the 2-degree goal without revolutionary change. In a commentary published in the journal Nature Geoscience in October, energy and climate expert Kevin Anderson, deputy director of the Tyndall Centre for Climate Change Research, concludes that “even a slim chance of ‘keeping below’ a 2°C rise now demands a revolution in how we both consume and produce energy.” The Intergovernmental Panel on Climate Change has proposed 400 scenarios calculated to achieve a 50 percent or better chance of meeting the 2-degree target. But, Anderson writes, most assume either an ability to travel back in time (to prevent emissions that have already happened) or the successful and large-scale adoption of technologies to remove carbon from the atmosphere that do not yet exist in a scalable form. Sure, new tech can do wonders, but not in a 2-degree time frame, he writes. The scenarios that don’t rely on this type of magical thinking assume that global emissions peaked around 2010, a concept hard to square with what we’ve seen thus far.

Some climate experts, including Knutti, say that arguing over the right limit just distracts from the more important problem of the planet burning. While the world gathers for yet another climate conference, emissions continue to accumulate in the atmosphere. In the words of climate scientist Gavin Schmidt, “If you are driving in completely the wrong direction, arguing about where you’ll park if you arrive isn’t your highest priority.”

At the end of the day, this is really a conversation about speed: How quickly are we going to address the problem? No matter what target you choose, greenhouse gas emissions must eventually go to zero if you want to stop temperatures from rising, Knutti said. The longer we wait, the more drastic the change will be. We’ve seen what 1 degree Celsius of warming does (we’re nearly there). We’re almost certain to see what 2 degrees looks like, no matter what goal is set in France. Whatever number negotiators choose, it won’t mean much unless they also take the necessary steps to get us there, and the pledges they’ve made so far leave a serious gap.


  1. Judging by anomalies in the global average surface temperature.

Christie Aschwanden was a lead science writer for FiveThirtyEight. Her book “Good to Go: What the Athlete in All of Us Can Learn from the Strange Science of Recovery” is available here.