[I was at an event the other night bringing together people from the economic-policy and climate activism worlds. I was asked to talk about the macroeconomic case for a Green New Deal, and the question of “how do we pay for it?” Here is a somewhat extended and edited version of my remarks.]

Most of the Democratic candidates now have plans for major public investment programs to deal with the challenge of climate change. These involve spending on the order of 2 percent of GDP on average, ranging from half a percent for Beto O’Rourke up to 4 percent for Bernie Sanders.

A question that will get asked about any of these plans is, how do we pay for it? Can we afford it?

We might simply reject the question, on the grounds that what we cannot afford is to continue dumping carbon into the atmosphere. Any plan to substantially reduce carbon emissions will pass any reasonable cost-benefit test.

But I think we should answer the “pay for it” question. It has a good answer!

The question is really two questions:

– How can federal government finance it? – what new money coming in will match the new money going out?

– Are the real resources available, or will we have to sacrifice production in other areas?

On first question, given that low interest rates now seem to be a permanent feature of our world, it is very hard to make a an argument that additional borrowing on the scale of 2, 3 or even 4 percent of GDP would be economically costly. When interest rates are below GDP growth rates, the debt-to-GDP ratio stabilzies on its own, even if you run deficits forever. Unlike in the 1980s and 1990s, when interest rates were higher, today it is impossible for public debt to snowball out of control.

If it has no effect on growth, additional debt-financed spending of 2 percent of GDP would bring the debt-GDP ratio to about 105 percent in 2030. Debt-financed spending of 4 percent of GDP would bring the ratio to 125 percent. Looking around the world, or at history, there is just no evidence that debt at that level has any economic costs. The US ended World War II with a debt ratio of about 120 percent of GDP, the UK over 200 percent. Japan today has a debt ratio of 250 percent of GDP, while France and Belgium have debt ratios around 100 percent. None of these countries have seen any of the negative consequences – spiking interest rates, rising inflation, a collapsing exchange rate — that are supposed to follow from excessive government debt. Quite the opposite, in fact.

And if there is any boost to growth from additional spending – any role for what economists call called hysteresis — then the debt ratio would be even smaller. If we take a standard estimate of the multiplier — the boost to GDP from an additional dollar of public spending — of 1.5, and assume half of that effect is permanent, then debt-financed public spending can actually leave the debt ratio lower than it would be otherwise. In which case the new spending would fully pay for itself, even without any new revenue. Of course there is a lot of uncertainty around these questions – I wouldn’t promise an effect on growth that large. But it doesn’t seem crazy to think that a program public investment could substantially raise the economy’s productive potential.

If we do want to raise revenue, there is also plenty of space for taxes on very high incomes and wealth, or a carbon tax, or other taxes that are socially desirable for their own sake, to finance some substantial portion of a decarbonization program. A recent very thorough study of the space for high-end income and wealth taxes by a couple of professors at NYU identified taxes that could raise over 2 percent of GDP on a very targeted base of the highest incomes. A wealth tax, again targeted at the very richest households, could raise another 2 percent or so. These are taxes we would like to raise anyway, because great concentrations of income and wealth are bad for our democracy and for our society. (There’s even evidence they are bad for our health.) So if we can finance decarbonization this way, we shouldn’t see it as a cost.

We often hear that it’s a fantasy to say that decarbonization will be economically costless, that it isn’t realistic to talk about spending on this scale without broad-based tax increases, without sacrifices by the middle class households. But this is crackpot realism. Of course there will be costs in particular carbon-intensive sectors of the economy. But the notion that investing in decarbonization necessarily requires sacrifices by working people in general, or painful choices about the federal budget, is just not borne out by the numbers.

On the real resources side, the critical point is that by any measure, the US economy has operated below potential for the large majority of the time in recent decades. Taking official statistics at face value, since 1980 there have been 192 months when the unemployment rate was more than one point above the NAIRU – the unemployment rate targeted by the Fed. There have been only 18 months when it was more than one point below. It took a full seven years after the last downturn for output to return to official estimate of potential. The total shortfall equaled 25 percent of GDP.

Even the most ambitious climate plan would have been barely enough spending to fill that gap.

And there are lots of reasons to think that these official measure understates economic potential. GDP today is more than 10 percent below what was forecast a decade ago. Labor force participation still significantly down from a decade, even among those 25-54 – prime working-age adults. Inflation is still below target. Wage growth is still slow. Almost any alternative measure you can think of suggests that the economy is running well below potential even today, and that there is enough slack for a substantial program of public investment without the need to reduce production of anything else.

Even if we think the economy is operating at normal capacity today, there are major social benefits to letting demand push up against capacity – to running the economy hot. There is strong evidence that the only way you get a rise in the wage share and especially a rise in wages at the lower end, is with sustained very low unemployment – what people call a high-pressure economy. Consistent with that, we’ve begun to see some recovery in wages at the bottom of the distribution in the past couple years. This is welcome, but it’s nowhere enough to make up for losses in previous years. For that, we still need more spending, stronger demand.

And that’s today. In a few years, we are likely to want more spending much more.

Today many people are talking about the possibility of recession within next year or so. Nobody except for a few cranks is talking about a sudden surge of inflation, or sudden takeoff of wages.

If there is a recession, the ability of the Fed and other central banks to offset a fall in demand is gong to be even more limited this time than it was in the last recession. In past recessions, the Fed has typically reduced rates by 5 points, and this has still not been enough to stabilize demand. Now we will be starting from a federal funds rate of only 2 percent, giving room for only 2 points of cuts. And there is good reason to think that the economy is less sensitive to changes in the policy rate than we used to believe. Central bankers themselves are quite clear that we will need more public spending in a recession. When Fed chair Jay Powell testified before Congress earlier this summer, Alexandria Ocasio-Cortez asked him what he would do in the event of another deep recession. He said monetary policy would not be enough, that the Fed would need help from fiscal policy – from the federal government spending more. Christine Lagarde, in her first public comments after being appointed head of the European Central Bank, said the same thing, that governments in the eurozone needed to spend more to boost demand. The central bank can’t be “the only game in town,” she said.

One of the big lessons of the stimulus debates in the last recession is that it is very hard to ramp up public spending in a hurry. There are not a lot of “shovel ready” projects out there waiting for someone to just start writing checks. So if we think we are going to need a big boost to public spending in the near future, we had better begin ramping it up now.

Many discussions of the cost of responding to climate change start from the idea that we are fully using our resources. If this were true, we’d have to ask how much consumption is worth giving up today in order to maintain a habitable planet in the future. Obviously the answer should be: A lot! But we don’t have to ask the question, because it isn’t true. We are living in a world where we are not using all our real resources, because of a lack of demand. Some people call this secular stagnation. We are living in a world where the central macroeconomic problem is that there is too little spending to fully utilize the economy’s productive potential – not just occasionally in recessions, but all the time, or at least on average.

Some people will say to this: Ok, we agree that the economy is running below capacity. We agree there is space to add more federal debt, and to raise taxes on the rich. Still, you could use that space for anything. It’s not a case for Green New Deal specifically.

This sounds superficially reasonable, but I don’t think it’s right. Because the evidence of recent history suggests that we won’t use that space.

Almost everyone today agrees that stimulus in last recession was too small — and that even if it might have been big enough in the abstract, it was offset by massive anti-stimulus at the state-local level. The situation in Europe is even worse, with deep austerity almost everywhere, with the result that countries like Italy have lower GDP than a decade ago. Even when mainstream economists say there is actually a case for deficit spending and not to worry about balanced budgets, it turns out to be very hard to get the political system to listen.

If we don’t use our productive capacity and our financial capacity for a Green New Deal, it’s very likely we won’t use it for anything.

The discussion of public budgets, among economists and much of the media and policy world, has not caught up to reality. We still talk about governments being subject to deficit bias – that’s a term of art in the macroeconomics literature, used to justify all kinds of rules to restrict government spending. We have this idea that without some sort of hard external constraint, elected officials are going to declare it’s Christmas every day and shovel money out the door on anything popular. We assume that you need some sort of disciplining device to force policymakers to make hard choices, or else they will just try to spend without limit. But governments today don’t suffer from deficit bias. On the contrary: The problem is austerity bias. For whatever reason governments refuse to spend even when the economic case for it is overwhelming.

This isn’t just a wasted opportunity for all sorts of valuable public spending. It imposes real costs in slower growth, fewer jobs, lower wages. And slow growth and low employment and wages have political costs too, as we know.

In this environment, it’s wrong to think about tradeoffs and making hard choices. This may sound strange coming from an economists, but it’s wrong to think about opportunity costs. The question is not, why should we do this rather than that? The question is, how do we break through the logjam that stops us from doing anything at all?

One of the unique things about climate change is that it may be a crisis urgent enough to overcome the entrenched austerity bias of governments, and to push public spending up toward the level needed to get true full employment. It may be the only thing urgent enough other than a major war — which we certainly do not want.

So when we look at the cost of the climate proposals out there against today’s macroeconomic background, the question should not be, are they too expensive? The question should be: Are they expensive enough?