Last month, the U.N. Intergovernmental Panel on Climate Change presented the gravest assessment yet of global warming’s mounting threat, warning of mass extinction, collapsing ecosystems, widespread starvation and displacement if the world doesn’t take quick action.

“Science has spoken,” U.N. Secretary-General Ban Ki-moon said during the presentation of the report in Copenhagen. “There is no ambiguity in the message.”

The IPCC’s fifth assessment presents a series of proposals for easing or adapting to these “severe, pervasive, and irreversible impacts,” but the big one is the obvious one: Cut carbon dioxide emissions, ideally to zero by the end of the century.

The problem is that’s the same basic message, if stated in more emphatic terms, that policymakers have been ignoring, if not openly ridiculing, for decades.

David Keith, a professor of public policy and applied physics at Harvard University, isn’t counting on dire reports to change that.

Instead, he is seriously exploring the possibility of altering the climate system itself to counteract the escalating dangers, using technology to create our own knob on the Earth’s thermostat.

As carbon dioxide levels from fossil fuels surge ahead of earlier forecasts and the first effects of climate change make themselves known, geoengineering proposals have gained increasing currency within the scientific mainstream.

Two of the most-studied approaches include: Cloud brightening, which entails spraying salt particles toward coastal clouds to make them reflect away more heat; and stratospheric injection, which pulls off a similar trick by lofting particles like sulfur dioxide into the sky.

Climate computer models show that both can bring down average global temperatures. And in the latter case, so does history.

The second-largest volcanic eruption of the last century occurred on the afternoon of June 15, 1991, when the summit of Mount Pinatubo collapsed. The stratovolcano, on an island in the Philippines, blasted 20 million tons of sulfur dioxide into the atmosphere. By reflecting away solar radiation, these particles ultimately lowered global temperatures by nearly a full degree Fahrenheit.

Keith and others have looked closely at ways of reproducing this cooling effect. Most conclude that it can be done — and relatively cheaply, at that.

The critical question is: What are the secondary effects? Some scientists have argued it would have an unequal impact throughout the globe, alter rainfall pattern in potentially dangerous ways and deplete the ozone layer. Others say the climate system is simply too complicated and interconnected for anyone to say with certainty what will happen.

But Keith has explored the use of synthetic particles that could potentially provide just the right amount of reflection in just the right places, while reducing the associated risk of ozone depletion. He is increasingly confident that, in any case, it would be far safer than simply allowing temperatures to soar throughout the century.

Many of his peers exploring geoengineering argue that such technologies, which by definition would have to operate at a global scale, must be options of last resort. Our first obligation, they insist, should be to cut greenhouse gas emissions as rapidly and dramatically as possible.

Keith says this is a “morally indefensible” argument at this stage of history. Sure, we should ratchet down carbon dioxide, he says. But it won’t happen anywhere near the levels necessary in time to avoid dire consequences from climate change, so it’s time to stop pretending.

During an hour-long conversation in his corner office at Harvard’s Pierce Hall, Keith discussed the status of his research, the risks involved in geoengineering, and why it is time to move the work out of the lab and into the field.

The interview that follows has been edited for space and clarity.

Re/code: During your 2007 TED Talk, you described an approach to stratospheric injection that involved levitating particles into the mesosphere that would migrate over the poles — the idea being that you can do the best job by cooling the poles, while minimizing downside impacts on other regions. Is that the approach you’re still focused on — and where does the research stand?

David Keith: That’s a really fine idea, I published on it, and I think there are variations of that idea that might work. But it’s a pretty far out idea, so it wasn’t my focus then and it’s not now.

I think it’s important because it gives people a sense of — if we went down this road as a species, of seriously trying to figure out how to do solar geoengineering carefully, subtly — where it might go in terms of tuning the radiative forcing.

I work across a lot of pieces of research, but I’d say the frontiers are trying to understand how much effectively turning down the sun, solar geoengineering, actually reduces the climate risk that people care about: Crop losses, ice sheets melting, temperature extremes, or what have you.

There’s no question it reduces the global average temperatures; even the people who hate it agree you could reduce average global temperatures. The question is: How does it do on a regional basis?

By far the single most important thing to look at on a region-by-region basis is the impact on rainfall and temperature.

And the answer is, it works a lot better than I expected. It’s really stunning.

A lot of us thought that, in fact, geoengineering would do a lousy job on a regional basis — and there’s lots of talk on the inequalities — but in fact, when you actually look at the climate models, the results show they’re strikingly even.

Now, it’s not perfect and there are some things it won’t do. Turning down the sun does nothing for ocean acidification.

But it looks like it can cut, like, 80 percent of the total variation in climate, which is really stunning.

In some ways we should be singing it from the rooftops. But the scientific community is so painfully scared of talking about it. These papers come out, and people find the best ways to say, well, it sort of works, but it’s really awful.

The fact is, people really appear to have found a way to significantly reduce the climate risk — by more than half, which is a big deal.

If the 2007 approach is a fine idea but a far-out one, what’s the variation of that getting most of your attention today?

The most plausible thing you can do at the beginning, I think — and I think it’s pretty widely believed — is to put sulfuric acid in the stratosphere.

We know how to do this. We hired an aircraft engineer to look at how to do it, and it’s really easy. We’ve thought a lot about how specifically to do it, and we’re developing an experiment for testing that.

Presumably you’d begin ramping it slowly, and for small amounts of sulfuric acid there don’t seem to be very significant side effects. As it gets larger, there would be more.

We are actively working on designs for advanced particles, but what I’ve been working on recently are advanced particles that are less advanced than the ones in that earlier paper. So they’re particles that would have less ozone impact, or even actually have a positive impact on ozone.

Can you describe, in a somewhat basic way, how some of the ideas you were talking about in the TED Talk work, like levitating particles and getting them to move to certain places?

That idea relies on this piece of physics from the early 1900s called photophoresis. So 100-year-old physics, which is very well understood as an astrophysical phenomenon. There’s no question you get these levitations, so that’s not my idea.

The new idea is that you’d make engineered particles that take advantage of this levitation property. Also, you could give them little magnetic and electrical fields that allow you some control over where they went in latitude. The magnetic field tips the particles a little, and that tipping force pushes it one way or another.

That could help you tune, so if you wanted to have more cooling at the poles, which is probably what you want to do.

I think you’ve stated that this is not the right approach — that cutting CO2 as much and as quickly as possible is the right approach, but it’s not happening so …

I’ve actually critiqued people for saying that.

It’s a very tempting thing to say. There are at least three big reports that say that. Everyone wants to say it. The question is, is it actually ethically or morally defensible?

Let me start with a crude analogy: If you have somebody who has cancer, and you’re considering chemotherapy, you could at that point say, “Well, you should have stopped smoking 20 years ago.” But it’s 20 years ago, so you can’t stop smoking 20 years ago.

I’ll say this: If I was a voter and there’s a global referendum about CO2 policies, I would vote for really very stringent controls on CO2. At the same time, I would move forward with solar geoengineering.

What I think I cannot say as a responsible scientist, and a lot of my friends do, is that the right thing to do is — or we must — cut emissions first.

It’s a nice thing to hope, but I don’t think it’s a factual statement.

We are cutting emissions, just not very well. But if solar geoengineering provides substantial reduction in risk, especially to the poor and vulnerable people on the planet and to ecosystems, the fact we haven’t cut emissions yet is not a reason not to do it.

If you have something that reduces the risk of something else, it’s rational then to shift your efforts a little bit. I think it is fair to say that nothing we’ve learned about solar geoengineering gets us out of the fact that eventually we must bring emissions to zero.

We’re now spending much less on cutting emissions than most of us think is actually the right amount, so I’m not saying we should spend less than we’re spending now. I think there’s no question we should spend much more.

Let me go the other direction with this. The title of your book is, “A Case for Climate Engineering.” You’ve written an entire book on the subject, so now I’m going to ask you to summarize it in a minute. But given where we are, given the reality of the science and the reality of policy, how do you articulate why now is the time for us to be doing this research?

First of all, that is the right word, “research.” I’m actually not saying we should do it now.

There’s a real danger of groupthink. A small group of people, of which I’m obviously one, could be wrong. So I think it’s really important to broaden it out in terms of more scientists around the world working on it.

But I’d say the balance of evidence now strongly suggests that doing a moderate amount of it would actually have benefits — in terms of reduced climate risks to ecosystems, to people, etc. — that are much bigger than the direct risk.

Now, there will be some direct risks, for sure. If you put sulfuric acid in the atmosphere, some people could die from the extra air pollution. That’s a serious issue, and not one to take lightly. There’s an ethical aspect to taking action that results in harm.

But it seems clear that the net impacts would be hugely positive. And that seems to me to be true from essentially all climate models. Other people might come to different conclusions.

Most of the climate modeling we’ve done to date assumes that solar geoengineering is being used to return the climate back to a preindustrial state. If you do that, precipitation actually goes down, and you almost certainly, by any measure, are overdoing geoengineering.

But what I — and not just I — am discussing is a much more moderate approach, where you cut the rate of global warming in half. If you do that, precipitation is not cut, you just stop the growth of precipitation, and you have roughly half the rate of global temperature increases.

Cutting emissions is something I spent all my career arguing we must do. But fundamentally the benefits mostly go to later generations. Solar geoengineering has benefits that go mostly to the generation that does it. The generation that does it fields both the risks and the benefits.

I know this is an area where you’ve been misrepresented in the past, so I want to be totally clear: Where are we in terms of lab research versus going out into the field and actually doing this? And what do we have to accomplish before we get to that point, or are we there?

I think we’re there. We have two papers we just submitted (since published here and here) that are really trying to lay out the argument.

There are lots of experiments that will tell us something useful about the risk and efficacy of [geoengineering], and these experiments are so small that their physical risks are negligible.

It’s possible to argue that we shouldn’t do them because of social lock-in: Once we do them, we can’t stop. That argument is rational, but I don’t think it’s actually convincing.

We’re looking at something called “SCoPEx” stratosphere control. We’ve now got it down to less than a kilogram, in practice less than 100 grams, of sulfuric acid per launch. To put it in perspective, that’s like one minute of flight.

I think even the strongest critics would not actually argue that’s an objective risk to the planet. My thinking is that since these experiments can be done safely and will tell us useful things about how well this technology works and especially the risk, that we should do it.

And, sorry, have you?

Have we? No, no.

But I want to be clear. Lynn Russell (of UC San Diego) did do an experiment, which wasn’t labeled as geoengineering, but in practice kind of was.

What kind of buy-in do you think you need before you transition to doing it?

The short answer is normal science. But I think the question is, what else is needed?

First of all, what you certainly want is a completely independent risk assessment, but it’s important to say that’s true of anything one does as a university professor working with federal grants — and it’s true by the law.

So the question is, what else should we do specific to geoengineering? Whose permission should we ask? The simple answer is: Nobody.

We’ll be transparent, we’ll abide by federal laws, we’ll abide by environmental impact laws. So my sense is that the science could just go ahead.

But I would prefer if there was some kind of international collaboration, to work with various international bodies that could coordinate these things and provide some independent review.