The Next COP Needs to Confront ‘Overshoot’

For the second year in a row, the United Nations climate conference ended without a consensus declaration that tackling global warming requires transitioning away from fossil fuels. The final agreement at COP30 did, however, touch on another uncomfortable subject: Countries resolved to limit “the magnitude and duration of any temperature overshoot.”

In the 2015 Paris Agreement, 197 nations pledged to try to prevent average temperature rise of more than 1.5 degrees Celsius above pre-industrial temperatures. Now 10 years later, scientists say that exceeding that level has become inevitable. It may be possible to turn the thermostat back down after this “overshoot” occurs, though — a possibility this year’s COP agreement appears to endorse.

The idea demands a far meatier discussion than world leaders have had to date, according to Oliver Geden, a senior fellow at the German Institute for International and Security Affairs, and a key contributor to the Intergovernmental Panel on Climate Change’s scientific reports. If limiting warming to 1.5 degrees now requires surpassing that level and coming back to it later, and if this is something that countries actually want to attempt, there are a lot of implications to think through.

Geden and Andy Reisinger, an associate professor at Australian National University and another IPCC author, published an article last week spelling out what it would mean for policymakers to take this concept of “temporary overshoot” seriously. For example, the final agreement from COP30 encourages Parties to align their nationally determined contributions towards global net zero by or around mid-century.” Net zero, in this case, means cutting CO2 emissions as far as possible, and then cancelling out any residuals with efforts to remove carbon from the atmosphere.

Scientists now estimate that if the world achieves that balance by 2050, we’ll pass 1.5 and bring warming to a peak of about 1.7 degrees above pre-industrial levels. At that point, the planet will not begin to cool on its own. Ensuring that an “overshoot” of 1.5 degrees is temporary, then, requires removing even more carbon from the atmosphere than is being emitted — it requires achieving “net-negative” emissions.

Suffice it to say, you will not find the words “net negative” in any COP agreements. “If 1.5 degrees C is to remain the core temperature goal, then net zero can no longer be seen as an end point but only as a transition point in climate policy,” Geden and Reisinger wrote. The two stress that this wouldn’t prevent all of the harms of going past that level of warming, but it would reduce risk, depending on the magnitude and duration of the overshoot.

I spoke to Geden on Thursday, while the UN climate conference was still underway in Belém, Brazil, about what policymakers are missing about overshoot and the 1.5 degree goal. Our conversation has been lightly edited for clarity.

I’ve had scientists tell me they don’t like the term “overshoot” because the 1.5 degree boundary is arbitrary. How do you think about it?

You can apply the concept of overshoot to any level. You could also apply it to 2 degrees or 1.6 or 1.7. It’s just saying that there is a defined level you care about, and it’s about exceeding that level and returning to it later. That is the basic concept, and then 1.5 is the logical application right now in terms of where climate policy is. That return idea is not very well represented, but that’s how it has been used in the IPCC for quite some time — exceedance and return.

What was the impetus for writing the article with Reisinger and what was your main message?

We wanted to explain the concept of overshoot because it seems that it’s now being discussed more. The UN secretary general started using it in a speech to the World Meteorological Association two weeks before Belém, and now has continuously done so. It also led to some irritation because people interpret it as, He just called 1.5 off, although he usually says, “Science tells us you can come back to it.”

These overshoot trajectories and pathways for 1.5 degrees have been around since at least the Special Report on 1.5 Degrees in 2018, and then increasingly dominated the modeling of 1.5. But we feel that the broader climate policy community never quite got the point that it is baked into these trajectories whenever scientists say 1.5 is still possible. But then this element of, what does this now mean? Who has to do what? How is it possible to get temperature down? That’s even more obscure, in a way, in the political debate, because it means net-zero CO2 is not enough. Net-zero CO2 would halt temperature increase. To get it down, you need to go net negative. And then the obvious question, politically, would be, who’s going to do that?

In the paper, you write that the amount of net-negative emissions required to reduce global average temperatures by just 0.1 degrees is about equal to five years of current annual emissions, or 100x our current annual carbon removal, which is mostly from planting trees. Given that, is it realistic to talk about reversing warming?

That’s not for me to say. If you think about the trajectory — how would, let’s say, a temperature trajectory in the 21st century look? What you would get now is a peak warming level above 1.5. Then really the question is, what happens afterwards? If everybody only talks about going to net-zero CO2 then we should assume it’s that new peak temperature level, and then we just stay there. But if you want to say the world needs to go back down to 1.5 by the end of the century then we have to talk about net-negative levels, and we still may find out that it’s not realistic.

This kind of circumvents the conversation of how good we look on getting to net zero. We all assume that’s doable. I also assume that’s doable. But you cannot forget the fact that right now, our emissions are still rising.

One of the policy implications you write about in the piece is that if Europe were to set a target to go net negative, its carbon pricing scheme could go from a source of income to a financial burden. Can you explain that?

If you have carbon pricing and you have emitters, you can finance carbon dioxide removal through the revenues from carbon pricing. But if you want to go net negative, you need more removals than you have emissions. The question is, who’s going to pay for it? You would always have residual emitters, but if you want to go deeply into net negative, you will run out of revenue sources to finance these removals.

One of the big problems is, conceptually, a government can say, Okay, your factory does not have a license to produce anymore, and you can force it to close down. But you cannot force any entity to remove CO2 for you. So how can a government guarantee that these removals are really going to happen? Would the acceleration of this carbon dioxide removal actually work? Which methods do we prefer? Do we have enough geological storage? It’s all unresolved. This paper is not a call to Europe to say hey, just make a promise. [It’s saying,] can you please really think about it? Can we please stop assuming somebody is going to organize all this to go net negative and then it magically happens? You need to make a serious plan. And you may find out that it’s too hard to do.

Another question is, how will other actors react? I think that’s part of the reluctance to talk about going net negative. The mental model right now of being a frontrunner is going down to the net zero line and then waiting there for the others to come. But if you enter net negative territory, it becomes basically bottomless. So every developing country could, reasonably so, demand ever higher levels of you. In the European Union, where you have 27 member states, even there, you would get into distributional challenges because some member states may ask others to go net negative because they are disadvantaged.

Also, which sectors would be forced to go net negative, which ones can stay net positive? Agriculture, at least as long as you have livestock, will be net positive. Then you have a country like Ireland, with 30% of the emissions coming from agriculture. They will stay a net-positive country, probably, and then others would have to go net negative. So you can imagine what kind of tensions you would get in.

I know you’re not in Belém, but from what you’ve read and from what you’re hearing, do you think that overshoot and all of these questions that you raise are being discussed more there? Do you get the sense that they are making their way into the conversation more?

A bit. The talk you hear is only just about 1.5 and 1.5-aligned, and it makes you wonder what governments or NGOs think, how this is going to happen. In the text presented by the Brazilian government, overshoot is mentioned, and “limiting or minimizing magnitude and duration of overshoot.” But it does not talk about what that actually means.

The whole 1.5 conversation, I think it’s hard for governments to understand. At the same they’re getting told, “if you just look at the pledges, you will end up at 2.6 or 2.7 or 2.8 by the end of the century, you have to do more.” Of course they all have to do more, but to really get to 1.5 they have to do more than they can imagine. If the world does not want to cross 1.5, never ever, it would need to be at net-zero CO2 in 2030, between 2030 and 2035. And if you go later, then you have to go net negative. It’s actually quite easy, but it seems to be uncomfortable knowledge. And then the way we communicate the challenge — governments, scientists, media — it’s not very straightforward.

All these temperature targets are special in the sense that they set an absolute target. Usually policymakers, governments, set relative targets, like 0.7% of national GDP for overseas development aid — you can miss that every year, but then you can say, next year we’re going to meet it. That logic does not apply here. Once you are there, you are there. Then it’s not enough to say that next year we are going to put more effort into it. You just then can limit the extra damage.