Paris Climate Talks

Top U.S. Scientist: World Must Act Now to Reverse Climate Change

“The world needs ultimately to completely decarbonize,” says John Holdren, Obama’s science advisor. Here’s what he says about why we need to do it—and whether we can.


Populations in an area outside of Manila have extended onto coastal mudflats and waterways that are susceptible to flooding and rising sea levels. Top U.S. science advisor John Holdren says this global problem has been amplified by climate change.

As global leaders open their final week of negotiations, the White House's top scientist spoke with National Geographic to offer his thoughts on why it's urgent for countries to tackle the climate crisis now—and why he's optimistic about the future for his grandchildren.

Here's our conversation with John Holdren, President Obama’s science and technology advisor. It has been lightly edited and condensed. (Read more about climate change.)

You are in Paris for the final week of the United Nations' climate talks, where more than 195 countries are scrambling to draw up a pact to lower global greenhouse gas emissions. What is your role this week? Why are you here?

One of my own fields for many decades now has been the causes, consequences, and remedies of climate change. So I am here, in part, so that if questions arise in negotiations that depend on details of science, I can provide those details.

But I am also here amplifying the Obama Administration’s messages about why climate change requires an international solution in which everybody participates, both with respect to reducing the emissions that are driving climate change and with respect to increasing preparedness and resilience for the changes in climate that are already ongoing and that we will not be able to stop overnight.

How important is it that the Paris talks produce a global agreement?

Well, you know, the basic scientific understanding of how climate is changing and why climate is changing tells us that it is urgent that the nations of the world act now, both to reduce their emissions and to increase their preparedness and resilience against ongoing climate change.

It is absolutely essential, if we want to avoid catastrophic impacts of climate change, that we turn this problem around starting now.
John Holdren

We know without any doubt that the climate is already changing in ways that are not explainable by natural influences and that are precisely explainable as a consequence of the heat trapping gases that we have added to the atmosphere by fossil fuel burning and deforestation. We know that damaging impacts are already occurring all around the world. In some parts of the world, we’re seeing drastic increases in heat waves; we’re seeing in other parts of the world increases in the power of the strongest storms, more torrential downpours and associated flooding, melting of permafrost, increased coastal erosion. All of these things are occurring in patterns and with fingerprints that tie them directly to the changes in climate that humans are causing. We know further that these changes cannot be stopped overnight. There’s tremendous momentum in the climate system, and there is tremendous inertia in the energy system, the agricultural system, the forestry system, the practices that are driving these changes. And therefore, it is absolutely essential, if we want to avoid catastrophic impacts of climate change, that we turn this problem around starting now.

Some scientists have expressed concerns in recent years about the possibility of "feedback loops," such as the thawing of Arctic permafrost, which releases more carbon dioxide and methane, which then leads to more thawing of permafrost, and on and on. Which of these loops, if any, do you consider a serious threat? Is there one that worries you the most?

Well, you’ve already mentioned one of the most worrisome ones. There is two to three times as much carbon stored in the soils of the far north, in tundra and permafrost, as is already in the atmosphere. And we know that heating of the atmosphere is leading to the melting of the permafrost; there is evidence that is increasing the emissions of carbon dioxide and methane already. There is also a lot of methane tied up in methane hydrates—ice crystals that contain methane in shallow sediments in northern oceans. If substantial quantities of that methane were released as a result of warming it would accelerate the whole global process of climate change, which we’re already experiencing. That is certainly one of the most worrisome feedbacks.

We know that damaging impacts are already occurring all around the world.
John Holdren

There are other feedbacks that are worrisome in terms of passing a tipping point in terms of the irreversible melting and disintegration of the Greenland ice sheet and the Antarctic ice sheet. The Greenland ice sheet alone contains enough water to raise sea level by about 23 feet. Just the West Antarctic ice sheet, the most vulnerable part of the Antarctic, contains enough water to raise sea level another 17 or 18 feet. No one thinks that can happen overnight, it may well take centuries, but we are in danger, if not already passing the point, where the eventual loss of those ice sheets becomes inevitable.

There has been a lot of discussion in Paris about whether we should try to limit global temperature rise to 1.5 degrees or 2 degrees. How different might the world look at 1.5 degrees versus 2 degrees?

I think the reason most of the nations of the world, most of the governments of the world, agreed in 2009 and 2010 to a target of 2 degrees Celsius was not that that level was safe but that it is, at least was, their view that it is the most prudent target that is still within reach. The origins of the 2-degree figure go back several decades, but are not solid from the scientific point of view. That is, there’s no reason to think that below 2 degrees is safe and above 2 degrees is dangerous. In my view, we are already experiencing dangerous climate change today, and we are at about nine-tenths of a degree above pre-industrial levels. The point is simply that the higher you go, the greater the chances of the kinds of feedbacks and tipping points we were talking about a few moments ago. And from the standpoint of the science, certainly 1.5 degrees would be better than 2. One degree would be better than 1.5, but it’s already going to be extremely challenging to stabilize the atmosphere at a level corresponding to 2 degrees and it would be obviously then even more challenging to achieve a target lower than that. But from a standpoint of desirability, a lower target would be better. The question is feasibility.


John Holdren, President Obama’s top science advisor, says climate change has brought “drastic increases in heat waves, increases in the power of the strongest storms, more torrential downpours and associated flooding, melting of permafrost, increased coastal erosion.”

National Geographic: There's so much about the complexities of climate change that we don't understand—the impact on ocean food webs, the complex interplay between rising temperatures and human disease. What are the most important scientific questions for which we do not yet have clear answers?

Well, again, you’ve touched on some of the main ones, the vulnerability of the ocean food webs, the vulnerability of the corals. We have a lot of reason to believe that the combination of the warming of the surface layers of the ocean, the acidification of the ocean, the depletion of oxygen—all of these stresses together are putting marine food webs at risk. But our detailed understanding of when and where the most serious impacts will materialize is still lacking. Same thing is true of tropical forests. There’s a lot of reason to believe continuing climate change is resulting in drying in the Amazon. The models mostly show an increase of drying in the Amazon, and there is a very real possibility that even at 2 degrees C the character of the Amazon would fundamentally change. It would no longer be a tropical forest. One of the ironies of this whole situation is that the two biggest reservoirs of biodiversity on the planet are the tropical forests and the coral reefs, and both are apparently particularly threatened by global climate change.

Does the United States ultimately need to completely decarbonize? Is it possible and how fast could it be done?

The world as a whole needs ultimately to completely decarbonize. The studies of where we need to get show that as a world we need carbon emissions to be in the vicinity of 50 percent below those of 2000 by 2050. They need to get close to zero by the end of the century we’re now in, close to zero by 2100, and ultimately all the way to zero. Is it feasible? Yes, it is certainly technically feasible. The real question is can we make the needed changes rapidly enough to get there as quickly as we need to. That is not a challenge of scientific or technological feasibility, it is a challenge of economic and social practicality.

One of the limitations of renewable energy sources such as solar and wind power is the difficulty of storing that energy at night or when the wind is not blowing. A lot of people are working on advances in battery and storage technology. Where are we with this technology? Are there major breakthroughs just around the corner?

I think we’re making great progress in battery technology for one. Battery technology has been under study, of course, for many decades and for a long time progress was slow. But new developments in materials sciences and fundamental understanding are really advancing the technology of batteries very rapidly. There are other approaches to energy storage, including compressed air storage, including superconducting storage, that are also being vigorously pursued, and I am optimistic about the prospects for improving storage over time.

Can you recall an interaction with President Obama that truly surprised you?

[Laughs.] That’s one I’m not going to answer. One of the conditions of my job is I do not discuss the content of my discussions with the President.

You've testified before very hostile congressional committees about climate change. For many people outside the Beltway, the inner workings of Washington D.C., are a mystery.  What do you think people might not understand about the relationship between the White House science team Congress?

Well, of course, the really key question, and I think most people understand this, is that in the end the Congress controls the purse strings. The appropriations of the Congress determine the budgets that are available for the activities of the federal government, and these include research and development on clean energy technologies. They include research and development on approaches to building preparedness, resilience, and adaptation to climate change. They include the budgets for fundamental research, which ultimately will be the source for the kinds of insights we will need in the second part of this century and beyond to grapple with this question. And, no matter what the views of a President and his or her Administration, in the end we depend on the elected representatives in Congress to make sensible choices about the investments the federal government needs to make to meet these challenges, and about other policies, including tax policy, including immigration policy, that ultimately affect the environment for innovation.

It’s up to us, and after us, our children and grandchildren, to make sensible choices.
John Holdren

Presidents can propose, in the end Congress disposes. There are things we can do with executive action without the Congress. The President’s Climate Action Plan, which he announced in June 2013, contains a great many measures that we are pursuing and can pursue with only modest cooperation from the Congress in the budget department. But it would be beneficial for addressing this challenge if we could get a greater degree of cooperation from the Congress. And I think all of us in the Administration continue to hope that either in this Congress or the next one, a more positive attitude toward addressing the climate change challenge will materialize.

You are the top science advisor to the most powerful office in the world. You also are a grandfather. How optimistic are you about your grandchildren's future?

You know, if I were not at least cautiously optimistic I would have retired by now and be smelling the flowers and going fishing. The reason I am still doing this work at the age of 71 is I believe it is possible for us to master this challenge. I believe it’s possible for us to bring climate change under control at a level where the consequences, while not negligible, will be manageable. And therefore, it continues to be very much worth doing everything we can to make that happen.

Can you tell us about how different you think the world might look for your grandchildren when they're your age?

Neither I nor anyone else has a clear crystal ball on what the world will look like 50 to 60 years from now. My grandchildren range in age from 10 to 24. It’s a long time until they’ll be the age that I am now. I hope that they will be living in a peaceful world, a sustainable world. A world where the impacts of climate change have been stabilized and brought under control. But again, that is up to us. We don’t predict the future, we make the future by the choices that we make. And it’s up to us, and after us, our children and grandchildren, to make sensible choices.

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