Three strikes and you’re out is a pretty good rule. And the politicians and negotiators attending the Paris climate summit, “COP21”, in December 2015 were facing their third strike. Their first and second attempts to bind the world into a meaningful pact that would control greenhouse-gas emissions—in Kyoto in 1997 and in Copenhagen in 2009—had failed. If on their third time at bat they could do no better, the world was cooked.
There was thus immense pressure on all at the conference to achieve a robust outcome. And a group of politicians and policymakers representing some of the world’s poorest countries had a very specific and controversial requirement for what it should contain. James Fletcher, of St Lucia, recalls that he and his fellow representatives of Caribbean states were “very clear in our minds that 1.5°C was a red-line item. It was one of the things that we said kind of silently: that we would be prepared to walk away from the negotiations if there was a sign we would not be getting a reference to 1.5°C in the Paris agreement.”
Many island states had the same red line. Their reasoning was simple. For a country like the Maldives, with more than 80% of its land rising less than one metre above sea level, more than 1.5°C (2.7°F) of global warming would see most of its sovereign territory disappear. Some continental countries which felt themselves at particular risk, or felt a particularly strong sense of solidarity, embraced the cause too. Third-strike make-or-break Paris was the perfect place to take a stand.
In the years since they originally signed the UN Framework Convention on Climate Change (UNFCCC), which was negotiated in 1992, the countries of the world had not committed themselves to a temperature target. Part of what mattered about Paris was that they were finally going to do so. The limit most countries, including all the big emitters, had in mind was 2°C. It had become accepted, without any compelling evidence, as a boundary below which global warming, while regrettable, did not constitute “dangerous anthropogenic interference with the climate system”—the thing that the unfccc’s signatories were pledged to avoid. It was also much better than what then seemed on the cards if the world did not act; business-as-usual projections showed temperatures rising 3.5°C or more above the pre-industrial baseline.
Given the predisposition against it by all the large countries, the 1.5 brigade’s tough stance managed to get their ideas further than most observers had expected. The text gavelled into history after two weeks of negotiations went beyond a simple 2°C goal, speaking instead of “Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C.” Cue cheers and hugging. “It was one of the rare victories of the poor, vulnerable countries in this arena,” says Saleemul Huq, a veteran of climate negotiations from Bangladesh.
Wishing they were there
In the years since Paris, the 1.5°C target went from something to be pursued to something totemised. A stretch goal has been widely treated as a paramount one.
The process was strengthened by a report published by the Intergovernmental Panel on Climate Change (IPCC) in 2018 which compared what the world might expect at 1.5°C with what 2°C would look like. Even as temperatures crept up by just half a degree, impacts and risks became worse in every possible domain, from fisheries to floods, droughts to decimated ecosystems. In a 2°C world, some 420m additional people would be exposed to record heat, millions more people would have their livelihoods wiped away by higher seas. An ice-free Arctic would be expected once a decade rather than once a century.
As well as looking at impacts, the 2018 report also weighed in on emission pathways. Its conclusions formalised the idea that, in order for the 1.5°C target to be met, net emissions needed to zero out around the middle of the century. The “Net-zero by 2050” mantra galvanised politicians and businesses as well as activists.
In 2019 the Science Based Targets initiative, a non-profit project that provides the corporate and financial sectors with guidance and technical assistance on their plans for climate action, launched the “Business ambition for 1.5°C” campaign with 28 early adopters. At last check, 1,558 companies had joined. In 2019 16% of the global economy was covered by net-zero pledges; by 2021 net-zero-by-2050 pledges covered 70%. “The mobilisation of finance and business is very much driven by the 1.5-degree target,” says Stephanie Maier of Climate Action 100+, an investor-engagement group with 700 members holding nearly $70trn-worth of assets.
The urgency engendered by the 1.5°C target may be one of the reasons why, in the years since Paris, the peak temperatures seen on projections of what will happen if countries honour their pledges have steadily dropped. According to the un Environment Programme (unep) the range of temperatures by 2100 is around 2.8°C under current policies, and 2.4°C if countries live up to all the commitments about future policy made to the unfccc in Paris and since. That is real progress.
At the same time, seeing the target treated as attainable has led many to believe that added political will and increasingly fervent denunciations of fossil fuels can get the range of the possible all the way down to a warming of just 1.5°C. Thus, before the COP26 climate summit it hosted in Glasgow last year, the British government framed its goals for progress in terms of an aim to “keep 1.5 alive”. Two weeks later, it deemed its modest achievements to have provided the life support necessary.
That was, to put it mildly, misleading. This year, as the climate world meets in Sharm el-Sheikh on the Red Sea for COP27, hosted by Egypt, it would be far better to acknowledge that 1.5 is dead.
An emissions pathway with a 50/50 chance of meeting the 1.5°C goal was only just credible at the time of Paris. Seven intervening years of rising emissions mean such pathways are now firmly in the realm of the incredible. The collapse of civilisation might bring it about; so might a comet strike or some other highly unlikely and horrific natural perturbation. Emissions-reduction policies will not, however bravely intended.
Most in the field know this to be true; those who do not, should. Very few say it in public, or on the record. An activist movement based on galvanising enthusiasm is hard put to admit defeat on its chosen goal. Doing so can also feel, to those who care, like giving up on the poorest, who will suffer more than any others after the threshold is breached.
But the truth needs to be faced, and its implications explored. What does the certainty of a post-1.5°C world mean for the planet? Can a world which warms significantly more find its way back? And what will missing a totemic target mean for the credibility and sustainability of continued efforts to limit climate change?
Welcome to the machine
To see why 1.5°C is dead, and also to understand how it contrived to remain plausible for as long as it did, look at what is called the carbon budget: the amount of cumulative carbon-dioxide emissions associated with a specific amount of warming. Such budgets can be estimated pretty well from climate models; they are among their more robust products and among the most useful for policy.
With a sense of the budget in question, other modellers can try and produce emission pathways that deliver what the budget requires, using computer models which couple the climate to the economy which aim to be consistent with the science of both. These don’t allow emission cuts to increase arbitrarily, but only at rates consistent with possible investment and other constraints such as maintaining reasonable supplies of energy.
According to the ipcc the budget for a 50% chance of avoiding more than 1.5°C of warming is 2,890bn tonnes of carbon dioxide. Some 2,390bn of this had already been emitted by 2019. That left a pre-pandemic carbon budget of 500bn tonnes. Since then, a further 40bn tonnes has been emitted each year, roughly, leaving less than 400bn tonnes in the budget.
What sort of scenarios can be imagined for spending such a sum of smoke?
As a reductio ad absurdum, ten years of emissions at today’s rates would be enough to burn through the entire 1.5°C budget; after that everything dependent on combustion would have to be turned off for good. A plateau in emissions is clearly possible; an instantaneous cutoff is not.
If instead you imagine the world as a whole immediately beginning to cut emissions at once things look a little more practical. If it makes half the cuts in ten years, it has another ten years to make the other half.
But none of the models can produce a pathway with cuts steep enough to get to zero anything like that quickly. And if the start of the cuts is delayed, as is currently happening, they have to be steeper still.
There is every likelihood, then, that the world will overspend its budget, and push past its 1.5°C target. However there is a loophole.
If the world commits itself to substantial “negative emissions”—to pulling carbon dioxide back out of the atmosphere—the space for positive emissions is expanded. In a world with a 400bn tonne budget, for example, 600bn tonnes can be emitted if 200bn tonnes are quickly removed.
That could provide a net zero trajectory with emissions reduction kept at the rates the economic models allow.
The prospect of negative emissions justified the 1.5°C language in the Paris agreement. It has since become common currency as the conceptual basis of all “net zero” policies.
But if negative emissions help produce plausible pathways, they also represent a dangerous lure. Delay the start of reductions, reduce the steepness of their decline, and you can still balance the budget by adding to the negative emissions later on. Thus pathways which give a decent chance of meeting the 2°C limit now use a lot more negative emissions than similar pathways did back in the days of Paris; the cuts not made in the past seven years have been transmuted into negative emissions decades hence.
The not-yet-final cut
This still works for 2°C pathways. But for 1.5°C pathways the jig is up. It is just about possible to twist models far enough that they will produce a crash-course 1.5°C trajectory. But they have some very obvious defects.
First, they require a huge new carbon-removal industry to be built more or less from scratch in just a couple of decades. One such trajectory has a billion tonnes of carbon dioxide removed by 2030 and mid-century negative-emissions of carbon dioxide at 6bn tonnes a year. That requires a level of drawdown considerably greater than today’s rate of natural-gas production (around 3.2bn tonnes a year).
Second, such trajectories require cuts in fossil-fuel use which go beyond the extraordinary, with emissions reduced by 43% or more as early as 2030. “Who believes that we can halve global emissions by 2030?” asks Daniel Schrag, an Earth scientist at Harvard who was a White House scientific adviser during Barack Obama’s presidency. “It is so completely outside the realm of the technology and economics and politics of the world. Is it technically feasible? I guess. But it’s so far from reality that it’s kind of absurd.”
And, third, even such extreme and implausible trajectories do not, for the most part, actually keep the temperature rise below 1.5°C; they overshoot it a bit and count on negative emissions to then bring the temperature back down. The huge assessment report the IPCC started publishing last year has 97 hypothetical scenarios in its “low-or-zero-overshoot” category. Only six have no overshoot.
In April, shortly after the relevant part of the report’s publication, Glen Peters of Norway’s Centre for International Climate Research wrote that “There exists no scenario in the [IPCC] assessment that peaks in 2025, and then reaches 1.5°C.” He should know: he was one of the lead modellers involved. “Maybe it is possible,” he says, “but it is really clutching at straws.”
Such modelling confirms and reinforces what one can see by looking at a range of indicators of progress on the mitigation of climate change, as the World Resources Institute did in a recent report (see table). A lot are pointed in the right direction. None are at the level they would need to be to meet the 1.5°C target. But models and the expertise behind them go beyond noting shortfalls. They can also sketch time frames. Emissions are not just going to push the world beyond the 1.5°C limit. They will probably do so pretty soon.
Global average temperatures are currently 1.0-1.3°C above the pre-industrial. According to Britain’s Met Office and the World Meteorological Organisation, there is a 48% chance that global average temperatures will be 1.5°C higher than pre-industrial in at least one of the next five years. Dr Huq speculates that it will be passed before the IPCC brings out the next of its monumental assessments, expected at the end of this decade; Dr Peters takes much the same view. The latest report “is the last IPCC assessment that warns us of what will happen,” says Dr Huq. “The next...will just chronicle more losses and damages that have [already] happened.”
For climate scientists, a single year above 1.5°C is not quite the point; they like to work with averages that smooth out fluctuations from year to year. That would require a decade or two of data. The public seems less likely to make the distinction, and rightly so. Once one year reaches 1.5°C, the odds that the average will soon rise to that level, and stay above it until after emissions fall to zero, are very high.
With that little time, even thinking outside the box offers very little hope. The box, here, is treating the whole issue as a matter of carbon budgets. There is more to climate than that. Some argue that acting really decisively on other warming factors, such as methane and soot, might change the picture. Such cuts are worth making, and indeed they sometimes pay for themselves. But the most aggressive pathways already factor in reductions in methane far steeper than any achieved to date.
Dark side of the Sun
But there is also a more radical non-carbon-dioxide based option. Solar geoengineering (also known as solar radiation management or modification) would try to cool the world off by cutting down the amount of sunlight that reaches the Earth’s surface; less sunlight, less warming. The most discussed method for achieving this involves putting particles into the stratosphere to bounce a little of the incoming sunlight straight back out into space. Such cooling is seen in action after very large volcanic eruptions; the huge amounts of sulphur they squirt into the stratosphere create tiny reflective particles of sulphate “aerosols”. Geoengineering would be much less spasmodic. A steady stream of sulphur would be sprayed into the stratosphere for decades, or even centuries.
According to the latest projections by UNEP, which are roughly in line with those made by others, if countries were to live up to all their most recent emissions-reduction pledges and, beyond that, those with notional net-zero targets actually hit them, warming should peak at about 1.8°C above the pre-industrial. Katharine Ricke, a researcher at the University of California San Diego who has done a lot of work on solar geoengineering, estimates that, if such a scheme were to be based on sulphate particles, the 0.3°C of cooling needed to bring a 1.8°C world down to a 1.5°C world would require something like 3m tonnes of sulphur a year delivered to the stratosphere. It would also need a new class of very high-flying planes to get the sulphur up there, a system for monitoring what exactly it was doing to the stratosphere to be set up, a world-girdling set of air bases and some chunky new supply chains.
Careful with that axe
Above and beyond those practical requirements, if such a scheme were not to be a massive source of political conflict and public concern it—and, crucially, who controls it—would need a significant degree of approval, or at least acquiescence, from all around the world (see Science & technology section). That would be a big turnaround. Solar geoengineering has been treated as the bête noire of climate policy and science for well over a decade. Researchers and environmental groups worry that engaging with the topic will start humanity down a slippery slope towards a hyper-engineered planet where greenhouse gases continue to be emitted with impunity and the underlying climate becomes ever more unbalanced.
To avoid going over 1.5°C would require getting such a scheme going in a decade or so. Politics aside, which they would not and should not be, that is highly unrealistic. One recent study suggested that it would take 15 years for the capability to be set up. And before a decision to do so could be rationally made, more would need to be known about the possible effects on stratospheric circulation and chemistry. Field tests to that end would doubtless be dogged by controversy. And models of the possible consequences in terms of regional temperatures and rainfall patterns, water security, agricultural yields, tropical storms and human health would need to be much better than those available today. “We do need at least five years to produce that new set of studies,” says Dr Ricke.
If a solar-geoengineering programme would be slow to start, though, it would be much slower still to stop. Glitter in the sky would mask some of the temperature effects of higher greenhouse-gas levels, but it would not lower the levels themselves. The warming power of the gases remains the same. That means the only way to end a solar-geoengineering programme without precipitating a jump in temperatures is to bring the underlying greenhouse-gas levels down first.
For 0.3°C of overshoot that would mean removing tens of billions of tonnes of carbon dioxide. If the world treated solar geoengineering as an excuse for easing off on its current pledges, the amount needing removal would rise accordingly. At best, solar geoengineering merely delays the challenge of carbon removal. At worst it hugely increases it.
But knowing that the 1.5°C milestone will fairly soon be in the rear-view mirror is leading people to take the idea more seriously than they have in the past. The Climate Overshoot Commission, assembled under the auspices of the Paris Peace Forum, is a group of 15 former heads of government, senior politicians and others chaired by Pascal Lamy, a one-time head of the World Trade Organisation. Taking the idea that the world will heat beyond 1.5°C as its starting point the commission is looking at greatly enhanced adaptation, carbon removal and solar geoengineering. It is acutely aware of the risks. “One country, or one operator, does this: barrum!” Mr Lamy says, making the noise of an explosion. “This has inevitable consequences on the rest. We don’t know these consequences, and we have to look at that.”
Oliver Geden, a climate-policy analyst at the German Institute for International and Security Affairs, sees the need to open such questions up as one of the reasons policymakers have not been frank about the 1.5°C limit. “If you say 1.5 is not going to happen you create a problem,” he says, “And there’s no obvious solution to that problem.” A “1.5°C at any cost” framing would imply solar geoengineering come what may. Trying to change the target to 1.7°C or 1.8°C would allow critics to argue that “wolf” had been cried. It would also bolster justified calls from poor countries for rich ones to provide much more generous support for adaptation (see our special report), and strengthen the case for “loss and damage” payments to those harmed by climate change, something of which the rich world is very chary. “Both of these are politically quite unattractive,” says Mr Geden. “It’s more attractive to stick to a keep-1.5-within-reach framing.”
The demise of 1.5°C does not mean that the fundamental policy implication of the Paris agreement is changed. The world needs to stabilise atmospheric greenhouse-gas levels by massively reducing its emissions and by gaining the ability to reabsorb those emissions that it cannot abate. And doing so more quickly is better. For some, a global temperature target never made sense in the first place. Dr Schrag at Harvard points out that the climate system as a whole mostly operates on a sliding scale, where higher global temperatures bring greater impacts and risks. “1.5°C is not safe and 2.2°C is not the end of the world,” he says.
Scientists do know, though, as the IPCC showed in 2018, that the less the temperature rises, the better. 1.6°C is better than 1.7°C: 1.7°C is better than 1.8°C. As a new mantra has it, “every fraction of a degree matters”. To Dr Schrag, it is never too late. “It is always the case that reducing the severity of climate change is a worthy investment. If we were at four degrees, keeping it from going to six is a noble thing to do.”
Set the controls...
Politically, such meliorism could weaken calls for drastic climate actions. Having an absolute goal strengthens people’s rhetoric; admitting that things are on a sliding scale opens the way to trade-offs. But here, at least, reality is in the process of trumping rhetoric. And if a new realism sees pressure for impossible levels of emissions reduction give way to fierce advocacy for adaptation measures that are both plausible and vital, some at least would be well served.
As to the 1.5°C target, it may yet have a role to play. Stabilising the global temperature by achieving a net-zero world opens the possibility of a net-negative one in which that temperature could be lowered. What level of negative emissions, and possibly solar geoengineering, such a world might employ would depend on its experience and its ambition. At that point 1.5°C might become an appealing target again—but this time approached from the other, sorrier and perhaps wiser direction.
