The 1.5C challenge: How close are we to overshooting, triggering critical climate tipping points, and needing to go beyond Net Zero?


As world leaders prepare to take stock of global progress on limiting global warming to 1.5C at COP28, the Director of our Net Zero Intelligence Unit, Simon Retallack, met with the Head of Climate Impacts Research at the Met Office Hadley Centre and University of Exeter and a lead author of three IPCC Assessment Reports, Professor Richard Betts MBE, to discuss why this target should remain a vital focal point for international efforts to tackle climate change. 

Key takeaways

  • 1.5C global warming is not a cut off between safe and dangerous; it is a marker beyond which climate change impacts become unacceptably severe, which makes it a useful target.
  • Human activity is undoubtedly responsible for the 1.2C global warming over the past 100 years, which is already intensifying extreme weather events.
  • To have any chance of limiting warming to below 1.5C we have to bring emissions to zero or Net Zero by the middle of the century at the latest.
  • At current levels of global emissions, we risk reaching 1.5C in the next decade.
  • Phasing out fossil fuels is essential to limit warming to 1.5C.
  • Carbon removal technologies will be necessary to reach Net Zero and can help bring temperatures back down if we overshoot 1.5. However, this becomes more difficult the higher the warming and the longer we stay above 1.5, and many of these technologies are not yet proven at scale.
  • Protecting natural carbon sinks like forests and oceans is essential for human health and limiting global warming.
  • At COP28, countries must make stronger commitments to reducing emissions urgently; developed countries must also provide support to help climate-vulnerable countries avoid and adapt to climate change.

The importance of 1.5C and the risk of triggering critical tipping points

SR: Richard, thank you very much for taking the time to speak to me. 1.5C has become an iconic objective for the global community in our efforts to avoid dangerous climate change. Can you remind us why it's so important?

RB: 1.5C is now regarded as the level at which climate change risks really start to become potentially unacceptably severe. This is all in the context that climate change impacts are already happening - we're already seeing several extreme, horrendous weather events, even just recently. An important point is that 1.5°C of global warming is not a hard cut off between safe and dangerous, but it's a kind of marker as to where we become increasingly concerned. You often need some kind of number as a target to work towards, like a speed limit on the road. If you’re on the motorway, the speed limit might be 70 miles an hour. That doesn't mean that 69 miles an hour is safe and 71 is dangerous. But 70 miles an hour is the judgment as to what is acceptable, and that’s a useful analogy for the 1.5C target.

It used to be 2°C of course, in the Copenhagen Accord, but it was realised that this did not really account for the risks of things like long term irreversible loss of the Greenland ice sheet. The melting of the Greenland ice sheet is one of the large-scale climate tipping points which could be passed if warming exceeded 1.5C and would likely be irreversible. Because of this concern over sea level rise, it was the small island developing states in particular that pushed for the more ambitious limit of 1.5C warming rather than 2C. 

Could we explore that a bit more? The sea level rise associated with the collapse of the Greenland ice sheet is five to six metres or something of that order, is that right?

Yes, six metres. If we lost all the ice. It would take centuries to millennia for most of that to happen, but that’s still not very reassuring for low-lying countries and islands who are worried about their very existence.

That would obviously be a huge concern. What are the other tipping points that risk being triggered if we allow temperatures to rise by more than 1.5C above pre-industrial levels?

There's a whole set of tipping points. The level at which they could be initiated varies quite a lot. One iconic one is the loss or dieback of the Amazon Forest. The level of global warming at which that could happen is very uncertain, but it's somewhere between about 2C and 6C above pre-industrial levels. Probably more towards the lower end. One of the reasons why that's a tipping point is the forest itself helps to maintain its own climate in a wetter and cooler state. As you lose a large enough part of the forest, the climate gets dryer, so the forest wouldn't grow back again.

Another tipping point would be loss of ice from Antarctica. There's phenomena called marine ice cliff instability and marine ice sheet instability, where the edges of the ice sheets could suddenly lose large amounts of ice. That could rapidly increase sea level rise.

There are also the shifts in the Atlantic Meridional Overturning Circulation, this rather technical sounding term, which is basically to do with the way in which the oceans are bringing warmer waters to higher latitudes, especially in the North Atlantic. The slowing down of this is linked to a warmer climate: if you get more freshwater and cold water from the melting of the Greenland ice sheet, that could actually stop the sinking motion which drives this circulation. If it were to collapse completely, that would have enormous impacts for Western Europe. Basically, we would have much colder winters, as cold as Siberia, and the summers would also be very dry which would have catastrophic impacts for agriculture in the UK, for example.

In the Met Office, we think that’s unlikely to happen this century, but that's not much comfort, because you need to be looking further ahead anyway, don't you?

Is it the case that the impacts of the tipping points you might get over 1.5C are mostly irreversible?

Generally speaking, one of the definitions of tipping points is that you move to a different state which is irreversible, at least on the time scales that we would experience. For instance, the loss of the Amazon Forest may be permanent or take decades or centuries to return.

And yet there are a lot of people who think surely the impacts won’t really be as bad as the models predict. What would you say to them?

The question is, can we afford to take the risk? We may be uncertain, but uncertainty works both ways. I would prefer to rule out really dangerous risks rather than be certain that they're there. I would like to keep the chances of severe and catastrophic risk as low as possible.

Emissions need to be reduced incredibly fast to limit global warming to 1.5C

Could we look now at what it would take to stay below 1.5C above pre-industrial levels? To start, could you remind us of the link between emissions of greenhouse gases, their concentration in the atmosphere and temperature rise?

Yeah, sure. Carbon dioxide is naturally occurring in the atmosphere and without it and other greenhouse gases the world would be frozen solid, so we do need these gases to keep the earth habitable. There are natural flows of carbon between the atmosphere and the land ecosystems such as plants taking up carbon and soils and microbes and animals releasing carbon back to the atmosphere, and the same in the ocean, as the ocean waters both absorb and emit carbon, so we've got the carbon cycle. But we've been disrupting that cycle by putting more carbon into the atmosphere through emissions from burning fossil fuels – coal, oil and gas – and also from deforestation. The amount we're putting in every year is about 10 billion tons, which is quite substantial.

Even though these natural flows exist between the atmosphere, ocean and land, and they are large, the natural difference between the inputs and outputs is small. The emissions that we are putting into the atmosphere is large compared to the natural balance. And as a result, atmospheric CO2 concentrations are going up. There is more than enough carbon being emitted by human activity by burning fossil fuels and deforestation to account for the rise in CO2 in the atmosphere. In fact, if it wasn't for natural sinks, the atmospheric CO2 rise, the concentration in the atmosphere, would have been about double what it has been.

We are benefiting from this kind of free ecosystem service. The CO2 rise is entirely human-caused, along with others like methane and nitrous oxide and water vapour as well. As these build up in the atmosphere, they inevitably cause an extra warming due to the greenhouse effect. The warming that we've seen over the last 100 years, which is about 1.2C so far, is confidently linked to the build-up of greenhouse gases. There's no other explanation for that build-up of greenhouse gases: it is certainly linked to our emissions. So, we are definitely causing the warming.

On atmospheric concentrations of CO2, they are now at about 417 parts per million, is that right?

It’s actually going to be about 420ppm on average across this year.

The highest on record since human civilization began. Is that right?

Yes. So, before we started burning fossil fuels in the industrial revolution, atmospheric CO2 was about 278-280 parts per million, so we've increased that by 50%.

And how immediate is the effect of increasing concentrations of CO2 on the global temperature?

It has a large effect straight away, but it also has further effects that kick in further down the line, especially in the warming of the deeper oceans, which is a major contribution to sea level rise. So there are immediate and long term effects.

OK, so based on past emissions, have we already locked in a certain temperature rise?

No, if we were to immediately reduce all emissions we could avoid going beyond 1.5C global warming. So the question is can we reduce emissions fast enough? Emissions would need to be reduced incredibly fast to avoid going past 1.5C degrees. There is an element of uncertainty in this, as with all future projections, around exactly how much warming you get as a result of a certain level of greenhouse gas build up. So the question of whether it is even possible to prevent warming of no more than 1.5C is all down to the odds, really. Even if we knew how much we were going to reduce emissions by, it would still be a numbers game in terms of the probability of staying below it.

Could you talk us through those probabilities? What is the highest concentration level of greenhouse gases we can reach to have a particular probability of staying below 1.5C?

To have any kind of chance of limiting warming to below 1.5C we have to bring emissions to zero or Net Zero by the middle of the century at the latest. It's certainly not 100% chance of limiting warming to 1.5C if we reach Net Zero by the middle of the century – more like 50%. 

And what atmospheric concentration of greenhouse gases is that associated with?

Again there’s a range, but somewhere around 440-450 parts per million. So not that much more above where we are at the moment.

That gives us a really small carbon budget remaining.

It does.

At current levels of emissions, how many more years do we have before we hit 450?

Just over a decade. We’ll probably see individual years at 1.5C above pre-industrial temperatures before then. That could actually happen at any time now because the year-by-year warming isn't smooth. But when we talk about going past 1.5C degrees warming, this is actually the long-term average. However, that could happen in around the next ten years or so if we carry on at current rates.

By longer term, you mean how long? How many decades of data to suggest we’d breached it?

Well, I think the IPCC would look at around 20-30 year average. What I'm saying is if we went past 1.5C in about ten years’ time, that would be the middle of a 20-year period. So of course, we wouldn't know for certain, under that definition, until ten years later, but of course it's when you get there that really matters, isn't it?

Overshooting 1.5C should not mean giving up

Let us turn to overshoot scenarios. There’s a lot of concern that our chances of keeping warming to below 1.5C are now very poor, and some people say we need to be realistic and we shouldn't fixate on that as a target. As a climatologist, how do you respond to those sorts of comments?

Well, if we were to overshoot, which as I say, is looking increasingly likely, that doesn't mean we should give up. Every fraction of a degree of warming is worth fighting for. So even if we do overshoot, we should still be aiming to reduce emissions urgently to limit warming to 1.6C or 1.7C, if we don't make 1.5C. That is still possible, and it doesn't seem likely that run away feedbacks would be initiated when we go past 1.5C. Returning from an overshoot would be possible, but increasingly difficult the more you overshoot because carbon sinks will get weaker and so on. But having some kind of target to aim for makes sense. It's just about limiting warming to as low levels as we can and being aware that we're approaching this 1.5C warming level.

And is it fair to say that every year we delay in limiting emissions decreases the time available to limit warming to 1.5C degrees?

Yes, it does. The situation is incredibly urgent. I just mentioned that once you've overshot 1.5C, it's still worth fighting to limit warming. But that doesn't mean you shouldn't still aim for 1.5C now.

Do you think it's still possible for us to achieve Net Zero in time?

That's the million dollar question, isn't it? I really don't know, to be honest. It is technically possible. The IPCC has shown that there are technologies at our disposal that we can use to at least halve emissions: it just becomes a matter of political will and people getting on with it, really. There are no particular barriers to reducing by this first half, which is a large part of getting to Net Zero. Whether we reduce the next half depends on whether we can develop things like carbon removal and put in place larger measures to decarbonise. So personally, I think it's hard to say, but I think we should keep aiming for it.

Fossil fuels must be phased out, but carbon dioxide removals will still be needed

Whilst we are on the subject of removals, in overshoot scenarios they become more and more relevant. We're seeing increasing emphasis from fossil fuel companies on the role of carbon capture and storage (CCS) in achieving Net Zero, but in practice, a lot of CCS technologies haven't been demonstrated at scale. How useful do you think focusing on the role of carbon capture and storage is?

I think CCS shouldn't be viewed as an alternative to emissions cuts. It’s going to be an additional solution. At some point we are going to have to be actively removing carbon from the atmosphere, especially if we do overshoot 1.5C warming. But even if we don't overshoot, I think there's always going to be some level of emissions. Even if it's not carbon, it could be methane coming from food production. Rice, for example, a huge, staple food crop produces loads of methane, so actual zero emissions seems highly unlikely. Therefore, we are going to need some level of greenhouse gas removal. That's why it's worth pursuing CCS, but it shouldn't be at the expense of reducing emissions. And that's the concern of course. A lot of people are worried that removal technologies will distract from what is really needed, which is emissions cuts.

What's your view on the need, therefore, for fossil fuels to be phased out?

They need to be phased out. It’s basically putting carbon into the system that has not been there for millions and millions of years. Phasing them out is something that needs to happen.

That's clear. You mentioned that if we do overshoot 1.5C, we could use these removals technologies to return to 1.5C? How could we do that? Would that involve us having to go carbon negative?

Yes, exactly. It's going beyond Net Zero. It's actually negative emissions, so the removals have to outweigh the emissions. There's technological means of removing carbon and then natural means like woodland creation, reforestation and so on. However, the technological processes aren't yet proven at scale.

With the natural processes, you remain vulnerable to climate change. For instance, you might plant a forest which would take carbon out for a while, but then if you had a huge fire that would return the carbon to the atmosphere. But it would serve a purpose in the intervening years; having taken some carbon out even temporarily can help buy time, of course. But it's not necessarily a permanent solution. It's not a permanent alternative to just not emitting it in the first place.

How quickly could a return to 1.5C happen in an overshoot scenario?

The speed of the return would depend on how deeply negative the emissions were, but it would likely be decades to centuries really, for what you might consider plausible scenarios. This is because there's physical limits to how much you can reforest, there will be limits for how much energy is available for technological removals.

Protecting nature is essential for human health and reaching Net Zero

You have spent a lot of your career looking at terrestrial carbon sinks. Can you tell us, at a high level to start with, how important protecting nature is for tackling climate change?

It is crucial because, as I was saying right at the beginning of the conversation, we've actually had this free ecosystem service of natural carbon uptake from land ecosystems and the oceans, which have meant the build-up of CO2 in the atmosphere has been about half of what it should have been based on our emissions alone. Therefore, protecting nature as a carbon sink is crucial. There are many other reasons, of course, as well, like valuing biodiversity for its own sake and the effects on local climates. The Amazon keeping itself cooler and moister by recycling rainfall and so on, other places will do that as well. So yes, looking after nature and ecosystems is an absolutely crucial part of this.

Thinking specifically about the Amazon, what does your latest research suggest in terms of the threats to it and the impact that losing it would have on the regional climate and globally?

Losing the forest, as well as accelerating global warming by increasing the build-up of CO2, would also cause extra warming of a couple of degrees or more in the Amazon region and the surrounding areas. It would also dry out the area, so reduced rainfall and therefore reducing river flows impacting on hydropower, for example, impacting water resources and increasing the risk of drought.

There are some parts of Brazil dependent on water carried in the winds from over the Amazon for some of their rainfall supplies. The increase in temperature could be significant enough to increase the risk of heat stress for people over large parts of that area. Because it's a humid region, heat stress risks are particularly high because when it's humid, you can't cool your body by sweating. So it's not just about temperature itself, if you’ve got high temperatures combined with high humidity, that's when you get really severe risks to health and even the risk of death. Those high levels of temperature and humidity could be reached if the forest was lost.

We’ve seen recent data from Brazil suggesting that rates of deforestation are declining under the new administration. Does that give you some hope that there's still time for the Amazon?

Yes, it does. The new government in Brazil has very clearly stated its ambitions on climate and looking after the Amazon. So yes, we're not at the Amazon tipping point yet and I think it could be avoided.

Climate change impacts are being felt already, but the international community can still course correct

Looking at the state of global progress on tackling climate change, do you think there's anything the scientific community could have done differently or should now do differently in terms of communication on the need to reduce greenhouse gas emissions?

I'm not sure what else we could have done differently. I think we've been very, very clear for decades what the issues are - and things are playing out as predicted. We said this would happen. I've been working in this area for 30 years, and the predictions we were making in the early 90s are now playing out. Even the predictions made in the 1970s in the very early climate models have been shown to be correct as well. I think we do need to keep making sure the world is aware that the risks are real.

We are now in a state when our impact on the climate is clearly playing out to have very negative impacts already. We're already seeing the impact of climate change on weather systems and we can now look at individual extreme weather events and say yes, they've been made more intense or more likely by climate change. And many of these have had dreadful impacts in terms of large numbers of deaths and massive economic impacts.

Making clear that it is happening now, I guess that's one thing that we need to start doing more often because it's a current thing, not just a future thing. I'm finding that a lot of what I’m writing now is what I would have written 20 or 30 years ago but I’m putting it in the present tense rather than the future tense now. 

So the idea that 1.5C degrees warming is somehow safe is to be dispelled, because it’s already unsafe.

Yes, exactly. It's a marker where what is already unsafe becomes unacceptably unsafe.

Does last year’s Kunming Montreal agreement give you some hope that world leaders are seeing the connection between protecting nature and tackling the climate crisis and tackling them in two-step?

Yes. It does. Each time you get these kinds of agreements, it does offer hope that people are listening and trying to do something. Of course, governments have to be held to account against their promises. But at least if they’ve actually said they will do something, that is something they can then be held to account on.

I do think there is hope. I don't hold with the kind of ‘doomism’ view that all is now lost. We can still get out of this. Alongside reducing emissions though we have to be adapting to climate change. We do need to live with the changes that have already happened and that we have already now committed to as well. So adaptation is as urgent as mitigation in emissions reductions.

We have COP28 in just a couple of months - what do you think are some of the most important steps that should be taken there?

It's probably worth saying that a lot of people are expecting COP28 to be not hugely ambitious. But hopefully more countries make more and stronger commitments to emissions reductions, and more commitments to supporting each other on adaptation, especially the developed world helping the developing world. The global South needs a lot of help because that's where a lot of the most severe impacts are, of course. And they've contributed the least to the issue. So I would hope to see more progress on that.

A very good answer to end with Richard. Thank you so much for your time. It has been a fascinating discussion.


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