When billionaire libertarian David Koch died this week following a decades-long battle with prostate cancer, the Arctic was rapidly melting. The Amazon rainforest was on fire. And the Earth had just experienced its hottest month in recorded human history.

These planetary conditions mimicked closely what scientists had tried to warn the public about 30 years ago, when they first sounded the alarm on climate change. They were also the warnings Koch worked most of his career to make sure the American public never accepted, nor did anything about.

At the time of his death, David Koch was one of the richest men in the world, having amassed a fortune of approximately $49 billion through his part-ownership of his oil tycoon father’s business, Koch Industries. Along with his brother Charles Koch, he used that fortune to fund a powerful yet secretive right-wing libertarian movement, known by its critics as the Koch network.

I asked Christopher Leonard, author of Kochland: The Secret History of Koch Industries and Corporate Power in America, to reflect on David Koch’s climate legacy.

It feels weird to be calling you, asking you about all the terrible things this person did now that he is dead. But that is sort of what your book was about, right?

Well, here’s how I would put it. I don’t think David Koch believes what he did was terrible. And my book documents really carefully what David and Charles Koch did politically and specifically regarding the issue of climate change. So I don’t feel bad about it at all today, because I think David and Charles Koch believed deeply in what they were doing.

How would you summarize Charles and David Koch’s effect on the climate change issue?

Koch Industries—that is, David and Charles Koch and their political network—has played an almost unparalleled role in helping to cast doubt on the basic science behind climate change; create doubt in the public mind that climate change is real; and particularly, most importantly, to cast doubt on the idea that government regulation can or should do anything to regulate greenhouse gas emissions.

Tactically, the way their political network did this was by funding a constellation of think tanks, from the Heartland Institute to the Cato Institute to the American Energy Alliance. And they produced study after study predicting terrible economic consequences if the federal government regulated greenhouse gas emissions. At the same time, the Koch network employed campaign donations and its own private force of volunteers and employees called Americans for Prosperity to fuel and organize the Tea Party movement. The Koch political network rode the wave of the Tea Party movement expertly, injecting its own concerns into the Tea Party’s agenda, and turning climate change into a front-burner issue for the movement.

Before your book came out, Americans for Prosperity’s actions were well known. But one of the things your book uncovered was that Charles and David Koch were truly at the forefront of the climate denial movement; that they were in the room, helping to create the movement’s strategy right when the scientific consensus was just starting to form.

Yeah, and this is really important. Back in 1991, during the first Bush presidency, there was true bipartisan consensus that that there was a climate crisis and that something needs to be done.

Right, George H.W. Bush said he was going to fight “the greenhouse effect” with “the White House effect.”

Exactly. And the Bush administration promoted market strategies to fight pollution. They created the so-called cap-and-trade system that reduced acid rain and pollutants from power plants. So they even had this Republican-approved, market-based approach to the problem that they could have implemented.

But at that time, Koch began to fund, well, let’s just be kind and call them climate skeptic groups, who very intentionally just tried to monkey-wrench the scientific consensus of climate change and exploit the weaknesses in how the scientific community talks about certainty. The fundamental science behind climate change is not complicated at all. Carbon is translucent; it traps light and increases heat; we release gigatons of it into the atmosphere every year. This is basic stuff. But scientists caged themselves in with caveats when they talked about predicting exact increases in temperature from this, or the exact degree to which fossil fuel emissions are responsible.

And again, in 1991, through a constellation of think tanks and funded thinkers, Koch exploited that weak point in scientific discourse and started politicizing the issue of climate change. And it’s never gone back.

But it’s not actually a weak point in scientific discourse—that’s what’s crazy about the whole thing to me. Uncertainty is what makes scientific discourse what it is. It’s a basic part of how scientists talk based about theory and hypothesis. They don’t say “here’s how it is, definitely”—they present a range of different outcomes. And I feel like the Koch network played a huge role in politicizing normal scientific language, by taking advantage of the general public’s unfamiliarity with how uncertainty is discussed in the scientific community.

And it’s well proven by Naomi Oreskes and Erik Conway’s fantastic book, Merchants of Doubt, that this is a strategy that tobacco companies pioneered, and that the fossil fuel industry picked up for climate change. And it’s remarkably effective.

So if the Koch brothers had not been pouring money into these groups—both denialist groups and groups that argue that policies to fight climate change would destroy the economy—where would the climate denial movement be today?

It would be far more marginal than it is. There’s no question about it. It would be far more marginal.

It is difficult to organize any political movement. It takes years. It takes money. It takes coordination. Because of Koch’s funding, the climate denial movement had an organizational backbone out of the box, off the shelf, if you will. And that helped promote the voices. It helped mainstream the ideas. And critically, it helped translate climate denial into a political reality.

I think one of the key moments in the whole story was in 2009, when Congress was getting close to passing the cap-and-trade billthe bill that would have put a price on carbon pollution. The Koch network played a vital and unrivaled role in burning down the moderate wing of the Republican Party that acknowledged the reality of climate change. And that forever changed the political discourse, to where now, any credible Republican politician who wants to raise enough money for reelection cannot even acknowledge basic facts of science.

You said earlier in our conversation that you believe that David Koch and Charles Koch really believed that what they were doing was correct. What makes you think that? I mean, how could someone be doing this, in the face of overwhelming scientific consensus, and not know that he was spreading misinformation?

I’m not trying to sound cheesy here; this is the honest truth. This is one of the great questions of history. To me, this is the question, “Why did we really invade Iraq?” And I don’t know. As an outside journalist, I cannot sit here and give you the satisfactory answer. Do they really believe that climate change is as phony or overstated? Do they really believe that market forces are going to come in and solve this problem? Are they just not as concerned with the impact of their actions? It’s difficult.

Charles Koch would not talk to me on the record or answer my questions on this topic; the company would not make him available. But I will tell you, I interviewed senior, senior Koch Industries people who’d been there for decades, who believed in their hearts that climate change is a hoax. So I don’t know how much of this is a belief system that’s reinforced when you live in the oil industry world, and how much of it is intentionally avoiding the scientific evidence. I just don’t know.

How much do we know about what David thought about climate change, personally—separate from the Koch brothers as a duo or entity or organization?

His rhetoric closely tracks with his brother’s. David said, for example, that the Department of Energy and the federal government should not be in the business of picking winners and losers. He said that if the market wants us to have renewable energy sources like wind and solar, the market will respond and create those. He said it’s destructive for the federal government to try to intervene and guide that process. And that is a classic, classic Koch position. That’s the position David Koch took.

Before I called you, I read an article that said some Koch-funded organizations are increasingly participating less in explicit denial—and even that some were recognizing that climate change was real. Have you seen any such shift over the years?

I have seen zero shift in the big picture strategy and tactics around delaying any climate legislation.

Right, because the point has always really been delay.

I mean, I interviewed Koch’s top lobbyist Philip Ellender in 2014 in his office, and he cast doubt on the science of climate change to me on the record in 2014. That’s Koch’s top lobbyist. So I do not buy into the happy, fuzzy narrative that they’re changing course. They have put down a clear marker that no Republican can break orthodoxy and regulate greenhouse gas emissions.

What is the legacy that David Koch will leave for the planet?

There’s no question that the Koch political network has played a vital and even unparalleled role in delaying government regulation of greenhouse gas emissions in the United States. Now, they would claim they have done the world a favor, by reducing the power of the federal government. But critics would definitely say that, due to the lack of action, it is undeniable that the concentration of greenhouse gases in our atmosphere has increased over the last eight years. We’ve passed thresholds that people said were extremely risky ten years ago.

And what does David death’s mean for everything he worked for on climate change? Has his death weakened his crusade in any way?

It’s the opposite. David Koch’s tragic passing will have no impact whatsoever on the political strategies of the Koch network or the operation of the corporation. Charles Koch has always been the center of gravity for that, not David.

The machine will continue to go forward as it has, even without David Koch at the forefront.

This interview has been lightly edited and condensed for clarity.