Precisely how the incoming Trump administration will deal with climate change remains uncertain. But Donald Trump’s statements during the campaign and since his election – and also his Cabinet nominations and his immediately purging the whitehouse.gov website of climate science information – signal, at a minimum, that he will not make addressing climate change a priority.

And that the administration likely will move to shelve federal government mitigation efforts.

Throughout his campaign and during the transition leading up to his January 20th inauguration, Trump frequently had been dismissive of the science and bullish on coal and fossil fuels generally. Proponents for aggressive action and many in the climate science research community have expressed increasing concerns.

In recent months, two climate modelers – Ben Sanderson, of the National Center for Atmospheric Research, in Boulder, Colorado, and Reto Knutti of ETH Zurich, Switzerland – examined how Trump administration inaction and actions might influence future planetary warming. They concluded that a four- or eight-year delay in mitigation could lead to substantially exceeding global temperature limits for dangerous levels of emissions and concentrations, perhaps indefinitely.

Background on the scientists’ analysis

Sanderson and Knutti wrote in a December Nature Climate Change commentary that the U.S. accounts for about 18 percent of global greenhouse gas emissions, likely leading some to conclude that U.S. inaction “would only have a small effect.” They said that would be “a naïve assessment.”

They argue, to the contrary, that continued or increasing emissions during that period of U.S. inaction would put the world so far off course that it could not recover before dangerous limits have been bypassed.

“Delay is the worst enemy for any climate target,” they wrote. That’s because the additional emissions accrued during a four- or eight-year period of U.S. mitigation inaction would leave the amount of CO2 in the atmosphere beyond or close to the maximum amount considered by the nearly 200 signatories of the 2015 Paris Climate Agreement to be sufficiently protective.

In their December commentary, the two scientists cautioned against “overinterpreting” their analysis, given uncertainties in how the “economic and ideological shift in U.S. governance will affect greenhouse gas emissions.” Rather, they said their findings help in anticipating the “bounding scenarios that could plausibly happen.”

Though the new administration has not yet articulated a detailed climate policy – and mid-January Senate confirmation hearings on Trump cabinet nominations did little to elaborate – the researchers said there is ample reason to be concerned about the “very different path … for future U.S. climate policy.”

Recognizing all the uncertainties, Sanderson and Knutti assumed that if the U.S. were to drop its commitments to cut back carbon emissions and also stop funding cutbacks by less-developed countries, other big emitters – the European Union, China, and India – might do likewise.

(Many experts conclude that even if China moves to fill a leadership void created by Trump administration decisions, the global agreements represented by the 2015 Paris agreement might well break down, with no clear timetable for a subsequent renewed global accord.)

4- to 8-years of inaction = 15 to 20-year setback

In their analysis, Sanderson and Knutti elaborated on the consequences if U.S. momentum to address climate change stalls, and if global cooperation to deal with climate change doesn’t resume in earnest until 2025.

“Even if emissions were to decrease again after eight years, it could take an additional 15-25 years for emissions to get back to current levels, assuming mitigation rates typical for strong mitigation scenarios,” they wrote.

Sanderson and Knutti conclude that “only immediate, global concerted and effective action can achieve the temperature targets discussed in Paris.” They calculated the chances that, with further delays, the planet might avoid warming by more than 1.5 degrees C or 2 degrees C above pre-industrial temperatures, the goal and stretch goal agreed to in the Paris agreement.

It’s important to understand that the planet already has warmed by about 1 degrees C since the industrial revolution. Climate researchers are in general agreement that even 1 degrees C more would commit the planet to dramatically increased sea-level rise. In addition, they’re concerned that the entire planet would suffer punishing changes from changing storm patterns and intensity, heat waves and droughts. Many climate scientists fear that temperatures more than 2 degrees C above pre-industrial temperatures would be catastrophic in countless ways.

The Sanderson and Knutti commentary, published in Nature Climate Change, was not peer-reviewed. But their commentary was based on a methodology Sanderson and two coauthors presented last summer in a peer-reviewed study published in Geophysical Research Letters, a journal of the American Geophysical Union.

Science Writer Dan Grossman’s Q&A

with NOAA Scientist Ben Sanderson

This Q&A relates to the description above of the Nature Climate Change commentary in which the interviewee and a colleague conclude that a U.S. four- or eight-year halt on reducing greenhouse gases would have serious adverse impacts lasting far beyond the delay itself.

Yale Climate Connections freelance science journalist Daniel Grossman spoke with NOAA scientist Ben Sanderson by telephone about their commentary, in which they argued that a U.S. climate stand-down of four or eight years would set back climate efforts by 15 to 25 years.

A lightly edited transcription of their interview follows:

Dan Grossman: I realize that it makes sense that a problem is solved sooner rather than later. But it’s not obvious that a four- or eight-year delay during a Trump administration would have longer-lasting impacts. Why isn’t that right?

Ben Sanderson: There are two main reasons. First, the climate system has inertia. And second the quantity that matters is the cumulative emissions of carbon, not the amount emitted in any given year.

The critical number to keep in mind is the total amount of carbon that humans can safely emit to stay within either 1.5 or 2 degrees C of the pre-industrial temperature. It’s called the carbon budget.

That total is calculated from the beginning of the industrial revolution until the time we stop burning fossil fuels. We’ve already emitted over 600 billion tons of carbon, out of a total budget of about 1,000 billion tons to remain below 2 degrees C, or about 700 billion tons to remain below 1.5 degrees C. We are using up the budget very quickly right now.

At present rates of global emissions, we have somewhere between four and 10 years before we have committed the Earth to blow past the 1.5-degree goal. For 2 degrees, we have somewhere between 15 and 25 years.

Grossman: You mean that if the incoming administration drops the ball for eight years, we commit the planet to bypassing 1.5 degrees?

Sanderson: Yes. With an eight-year delay in action, it would be virtually impossible to avoid going above 1.5 over pre-industrial. We’re very, very close to these thresholds, so the timeline for achieving them is incredibly short.

Grossman: But barring the reduced effort you posit in your peer-reviewed paper, is it still possible to avoid dangerous warming impacts?

Sanderson: So the background to this and the reason that the delay would render these targets unattainable is that anything short of an unprecedented World War II-level global-scale effort, starting now, would probably fail to prevent 1.5 degrees of warming. It would require global cooperation to completely transform the energy and transport infrastructure of the planet on a timescale of a few decades. To avoid a plus-1.5-degrees world, net emissions from the entire planet needs to be zero by 2040 or just after, just over 20 years from now. After that we’d have to be actively removing CO2 from the atmosphere.

But after an eight-year delay, we get into the realm of it being completely politically infeasible to stay within 1.5 or even 2 degrees C of the pre-industrial temperature.

Grossman: Why couldn’t we just pick up where we stopped after the delay? What makes a short period of delay, four or eight years, so dire?

Sanderson: The basic point is that for four or eight years we’d be putting an additional allotment of carbon into the atmosphere. And even if after the delay ended and we started reducing emissions at the same rate as we might otherwise do, we’d have to first remove that additional carbon in the atmosphere that was emitted during those lost years.

That extra CO2, translates into an increased temperature commitment for the future. Even if, post 2024, we reverted to the emissions reductions anticipated before the delay, it would be nearly impossible to return to a trajectory that would keep us safely below the Paris temperature thresholds. We’d have to pull huge amounts of CO2 directly from the air later in the century.

Some such negative emissions are already assumed. But even more would be needed. Prior scenarios that keep us below 1.5 degrees C already assume that we will max-out our technological capacity for negative emissions in the latter part of the century. In effect, the delay translates to a long-term commitment to a temperature rise beyond what we could have otherwise achieved.

Grossman: And that’s assuming that emissions stay at current rates and don’t actually rise?

Sanderson: Yes. And we can’t be certain that will happen. We also considered the effect of a short-term increase in emissions, due to policies encouraging production and consumption of fossil fuels like coal. That would commit us to an even higher temperature, both because of the extra CO2 that would be released during that period and because the investment in high-carbon infrastructure would lock in increased emissions for decades afterwards.

The scenarios for avoiding the 1.5- or 2-degree temperature limits already push the boundaries of feasibility. Any delay will cause us to miss the targets. In our paper, we calculate that if emissions increase for the next eight years (rather than falling as might otherwise occur), there’s only a 33 percent chance of avoiding warming of 2 degrees C or more.

Grossman: In your paper, you also discussed the effects of reduced research on solar power, wind energy, and other technologies for lowering carbon emissions. What would be the outcome of that?

Sanderson: We considered the possibility that eight years of reduced research in low-carbon infrastructure would hinder our ability to cut emissions post-2024, setting us back even further. If we add this effect, it makes the chance of achieving the two-degree target effectively nil, only about 10 percent. Adding in this factor increases the possibility of warming to a 3 degrees C world.

Grossman: Considering all these possibilities, what’s the bottom line?

Sanderson: The consequences of eight years of pro-fossil fuel policies in the U.S., which in turn trigger a global short-term abandonment of climate policy ambition, could be huge. It would put us it into a completely different scenario for the long-term future. The differences in costs and impacts between two-degree and three-degree worlds in the long term are enormous.

What we do even over the next decade is really important for determining what the world of our children and grandchildren will look like. It could transform the landscape of the world in the second part of this century. This is real and will have consequences for people that we know. The idea that you can kick the ball down the road for another decade is just demonstrably not true. The time is running out to avoid dangerous climate change.

Grossman: You wrote, “It’s not easy to be dispassionate watching an uncertain future unfold.” Should we expect scientists to be dispassionate about these things?

Sanderson: As climate scientists, we spend all of our time thinking about the impact of different emissions scenarios. They’re abstract, but the impacts translate into real problems which will affect people’s lives in real ways. Things like health, mortality, and disease outbreak.

It’s hard not to have an emotional response when the conclusions are so clear that the consequences of unbridled and untamed emissions would be disastrous for humanity and for every ecosystem on the planet. It’s hard not to come to the opinion that it would be better to act to avoid damage beyond what we’re already committed to.

Grossman: The new administration might be antagonistic to government employees or scientists opposed to pro-fossil-fuel policies. Are you worried about your job?

Sanderson: I’m concerned, of course. But my plan, and the plan of everyone I know, is just to carry on doing our jobs. Whatever course humanity takes, climate change will happen to some degree, and understanding how those changes are going to affect our society can give us a head start on mitigating some of the risks.

And so, we’ll carry on doing good science to provide the best information. If the political climate stops us from doing that, it would in my view be disastrous. Some of the best climate science in the world is done within the United States, and if American climate scientists were silenced, there would be dire consequences in terms of mankind’s ability to predict the effects and to respond to climate change.