John Holdren and U.S. President Barack Obama walk on the White House grounds in March 2014.Joshua Roberts / Reuters

The world’s highest-ranking diplomats are meeting in Paris this week to complete the final version of a new UN agreement on climate change.

They are working on a 48-page draft resolution, prepared during the first week of the talks by lower-ranking climate ministers. One of the questions they’ll take up during the week to come: Should the world’s nations attempt to limit climatic warming to 1.5 degrees Celsius, instead of the previously agreed-upon 2 degrees?

1.5 degrees has become one of the most surprising stories of Paris. Many observers expected the international community to drop the two-degree target at the Paris talks due to its scientific impracticability. Instead, thanks to climate activists and sustained diplomacy from the countries most vulnerable to sea-level rise, it might settle on an even more ambitious target. The United States and China have both signaled tentative support for 1.5, despite Saudi Arabian and Indian opposition.

Yet actually achieving 1.5 degrees will be extraordinarily difficult. Speaking to The Atlantic Monday morning from Paris, President Obama’s top science advisor said that it will be an near impossible target to meet.

“The basic answer is the lower the better, but it will be a big challenge to stay under two [degrees]. And 1.5, I would say most technical experts would dispute that 1.5 is attainable,” said John Holdren, the assistant to the president for science and technology. Holdren, a plasma physicist and aerospace engineer by training, also directs the White House Office of Science and Technology Policy.

“The only way to attain [1.5 degrees] is to get very good at technologies and practices that would actually take emissions negative. It wouldn’t be enough to get emissions to zero, we would actually have to develop the technologies to be pulling carbon out of the atmosphere,” Holdren told me.

A May study in Nature Climate Change agreed that carbon emissions would have to go negative sometime this century to stay below 1.5 degrees of warming by 2100. Global carbon emissions would have to peak in the next few years, then decline rapidly, and the world would have to essentially stop emitting greenhouse gases between 2045 and 2060.

That may seem like a long time, but it will also take a very long time to replace coal and natural-gas power plants with solar, wind, and other carbon-neutral options. (Bill Gates has also emphasized this point in this magazine.)

The “global energy system represents something over $20 trillion in capital investment, which ordinarily would turn over on a timescale of 30 or 40 years. There is no way to rebuild the energy system overnight—it’s too big, too costly to do that,” Holdren told me.

“I think the aspect of climate change that probably most people don’t understand and need to is the long time scales involved,” said Holdren. “That is, it takes the climate a long time to adjust completely to what we add to the atmosphere. And as a result of that, we are not even today experiencing the full consequences of what we’ve already added to the atmosphere. If we stopped adding greenhouse gases today, the global temperature would continue to coast upward to close to 1.5 degrees until it stopped.”

“Both of these points push us in the direction of acting sooner. We need to do more and we need to do it faster than most people think.”

He also emphasized that neither of the temperature targets—1.5 degrees or 2 degrees—were chosen for their safety.

“The original agreement of most of the world’s governments in 2009 and 2010 to try to stay below two degree was based not on the idea that two degrees is a red line below that we’re safe, and above it we’re cooked. It was rather based on the idea that two degrees is perhaps the most prudent target that they felt in 2009 and 2010 was attainable,” he said.

“The fact is, the world today is at about .9 degrees [Celsius] above pre-industrial, and we are already experiencing damaging impacts of climate change,” he said. “So we really can’t say that two degrees is safe. We couldn’t say that 1.5 degrees is safe.”

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.