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Why 2 degrees Celsius is climate change’s magic number

December 2, 2015 at 6:25 PM EDT
We hear all the time that we need to stop the planet from warming an additional two degrees Celsius above pre-industrial levels. Why is that specific number so important though? And what happens if we exceed that limit? William Brangham offers some background on that climate science target.

GWEN IFILL: The international climate talks continue in Paris, where over 150 countries are trying to reach an agreement to limit the carbon emissions that the vast majority of scientists say drive global climate change.

William Brangham helps us understand why, almost more than anything, one little number matters.

WILLIAM BRANGHAM: For several years now, the stated goal of international climate talks has been to stop the planet from warming an additional two degrees Celsius.

You hear this target mentioned all the time:

MAN: People just talk about two degrees.

MAN: Two degrees.

WOMAN: Two degrees.

MAN: Two degrees.

WOMAN: Two degrees Celsius.

WILLIAM BRANGHAM: But how realistic is that goal? And why is a two degree Celsius target considered important? And let’s say we fail. What does two or three or four degrees of additional warming actually mean?

A bit of background: For the last 10,000 years, the Earth’s temperature has been fairly steady, fluctuating by only about one degree Celsius. Yes, it’s risen and fallen, but all of human existence, everything we have ever done as a species has happened in this narrow temperature range.

Richard Alley is a climate scientist at Penn State University.

RICHARD ALLEY, Penn State University: We have had 10,000 fairly warm, fairly boring years, with little wiggles caused by the sun getting brighter or dimmer, and wiggles caused by volcanoes exploding and blocking the sun with dust for a couple years.

At the end of this 10,000 years of sort of boredom, we are pushing very hard, and we are pushing very hard in a number of ways, but the biggest one of those is putting CO2 in the air to cause more warming.

WILLIAM BRANGHAM: This chart shows the historical amount of carbon dioxide in the Earth’s atmosphere. It too has gone up and down through time. Here’s where humans came in; here’s where we started burning oil and gas and coal; and here’s where we are today.

All that carbon sitting up in the atmosphere traps the sun’s radiation and slowly drives up Earth’s temperature. Now, for the first time in our history, we have pushed above our historical temperature range. The U.N.’s Meteorological Agency says that by the end of this year, the planet will have warmed an additional one degree Celsius since the late 1800s. That’s halfway to the two degree Celsius limit that global leaders in Paris are trying to avoid.

Michael Oppenheimer is a climate scientist at Princeton University.

MICHAEL OPPENHEIMER, Princeton University: We are entering a climate space now which is entirely different than anything that’s existed in the history of humanity, and way out of the range that has existed for the history of civilization.

WILLIAM BRANGHAM: Over many decades, scientists have been asked: How much warming can humanity tolerate, before experiencing the most destructive and dangerous effects of climate change?

This is where the threshold of two degrees Celsius, or about 3.6 degrees Fahrenheit, came about.

Scott Barrett of Columbia University served on the U.N.’s Climate Panel and now studies global climate treaties.

SCOTT BARRETT, Columbia University: I think that the two degree target was chosen more for political reasons than for true scientific reasons. The idea was to — if countries could agree on a collective target, that that would mobilize the action needed to get the whole world to act together.

WILLIAM BRANGHAM: While there’s some uncertainty about how much of a problem two degrees of additional warming will be and how we will be able to adapt to it, scientists say we will likely see longer droughts and more intense heat waves, which could cause big disruptions to the world’s food supply.

At two degrees, sea levels could rise several feet, which would flood many coastal communities in the U.S. and potentially cause large migrations of people from countries like Bangladesh and India and Vietnam.

And according to the most recent data, 2015 is now going to be the hottest year on record.

MICHAEL OPPENHEIMER: If we don’t start with rapid emissions reductions and substantial emissions reductions, that we will pass a danger point, beyond which the consequences for many people and countries on Earth will simply become unacceptable and eventually disastrous.

SCOTT BARRETT: This issue has attracted more diplomatic attention than any issue in human history, and what we have seen for 25 years is all these little tweaks, these modifications that have been tried. They don’t change the fundamental result: Global emissions keep rising, and they’re going in the wrong direction.

WILLIAM BRANGHAM: Last year, NASA released this animation showing a year’s worth of global carbon emissions compressed into a few minutes. And you can see the three main culprits right there: the U.S., Europe, and the new top emitter, China.

In advance of these Paris talks, many of the world’s biggest emitters, including the top three, have made voluntary pledges to cut back their emissions. They’re considered the most ambitious targets ever pledged, but will they be enough to stay below two degrees?

To answer that, a group of researchers created what’s called a carbon budget. It’s an estimate of how much carbon energy we can continue to burn while still staying under the two degree threshold. And, as you can see, fairly soon, a matter of decades, global carbon emissions will have to drastically go down to keep the warming in check.

But there’s another problem. Just taking those pledges made by the U.S., the E.U., and China alone, by 2030, those three will account for nearly all the budgeted emissions, leaving barely anything for the remaining five billion people on Earth.

That includes entire continents like Africa and South America. It includes the subcontinent of India, which will inevitably emit more and more carbon as its 1.3 billion people buy more cars and ship more goods as its economy grows.

This is the challenge facing policy-makers in Paris: How does the world accommodate billions of people, people with growing energy needs that scientists say the planet simply can’t tolerate? Most scientists see climate change as the biggest, most complicated long-term challenge the world has faced.

But, for some, there’s optimism.

RICHARD ALLEY: We’re not going to stabilize the composition of the atmosphere today. There are a lot of people who drove to work this morning who are going to drive home this evening.

Changing the energy system is a 30-year task or longer. You could look at this and say, wow, we’re in trouble. You also could look at this and say a journey of 1,000 miles starts with a single step. And by starting on the path that will get there, we will generate the knowledge, we will generate the technologies and the will to do more.

WILLIAM BRANGHAM: President Obama before he left Paris yesterday echoed this optimism.

PRESIDENT BARACK OBAMA: I think we’re going to solve it. I think the issue is just going to be the pace and how much damage is done before we are able to fully apply the brakes.

WILLIAM BRANGHAM: Others are dubious. They argue that the Paris talks, which are based on voluntary pledges, simply won’t demand enough to keep the planet below the two degree threshold.

SCOTT BARRETT: It’s somewhat, I think, deceptive to think that this is a success. There’s no enforcement mechanism at all in this agreement. It’s easy to agree to something when you announce the pledge yourself and when you know you’re not really going to be held accountable as to whether you meet the pledge or not.

WILLIAM BRANGHAM: Most scientists believe that, even if every country followed through 100 percent on their voluntary pledges, there’s already enough CO2 in the atmosphere to warm the planet by two degrees.

Scientists and world leaders in Paris hope that, even if this threshold is breached, nations will not just follow through on their pledges, but will agree to dial back emissions even more in the future.

For the PBS NewsHour, I’m William Brangham in Washington, D.C.