Over the Next 82 Years, Can Humanity Hold the Line at 1.5°C of Warming?

The old adage that you should "aim for the top and you'll land in the middle" may not apply to climate change. Against a risk no one can predict, the only option is to go all out.

It is Nobel season again. On the morning of October 8, in Incheon, South Korea, the Intergovernmental Panel on Climate Change (IPCC) — winner of the 2007 Nobel Peace Prize — released a report the world had been waiting for. It assesses, from a scientific standpoint, what the target written into the 2015 Paris Agreement — holding the rise in global average temperature to "1.5°C" by the end of this century — actually means, and how it might be reached.

The Paris Agreement sets 2°C as the baseline limit and treats 1.5°C as a higher goal, the kind you have to jump to reach. How close humanity gets to 1.5°C tells us how far we still are from the baseline. And 1.5°C is not a synonym for safety; it only means consequences that are somewhat less severe. If humanity misses 1.5°C and then carelessly blows past 2°C, no one can say whether what awaits us is a "tipping point," after which the planet enters an era of runaway climate change driven by a chain of self-reinforcing greenhouse effects: melting sea ice weakening the Earth's ability to reflect sunlight, thawing permafrost releasing methane (a powerful greenhouse gas), and so on. Holding the 1.5°C line is therefore a question about the future of everyone alive.

Yet human activity has already warmed the planet by 1°C. Over the next 82 years, can we hold the line at 1.5°C?

The United Nations Environment Programme and the World Meteorological Organization set up the IPCC jointly in 1988 to provide expert scientific assessment of climate change. It does no research of its own. Instead, scientists around the world assess the thousands of studies published each year and report what science has established about climate change. In 2007 it shared that year's Nobel Peace Prize with former U.S. vice president Al Gore, in recognition of their work building up and spreading knowledge about human-caused climate change, and laying the groundwork for measures to counter it.

Scientists at the press conference said this Special Report on Global Warming of 1.5ºC is the most important report the IPCC has ever released, and the one it has put the most effort into: for the first time, all three of its working groups worked on a single report, and no earlier IPCC report has covered so many disciplines.

The report found that although 1.5°C and 2°C are only half a degree Celsius apart, the gap between their consequences is out of all proportion. For example:

Compared with capping warming at 2°C, holding it to 1.5°C would cut global sea-level rise by 2100 by 10 centimeters, sparing 10 million people from the threat of rising seas; it would make an ice-free Arctic summer a once-a-century event rather than one that comes at least once a decade; and coral reefs would shrink by 70–90% instead of disappearing almost entirely (more than 99%). A 1.5°C world would also bring fewer extreme weather events, better-preserved biodiversity, smaller losses in corn, rice and wheat yields, half as many people short of water, and hundreds of millions fewer people pushed into poverty.

"Every extra bit of warming matters, especially since warming of 1.5°C or higher increases the risk of long-lasting or irreversible changes, such as the loss of some ecosystems," said Hans-Otto Pörtner, a marine biologist and co-chair of IPCC Working Group II, which assesses the impacts of climate change, adaptation and vulnerability.

That the world's countries broadly agreed on a "2°C" target for the first time in the Paris Agreement was a remarkable political achievement. It is still a sorry target. 1.5°C is the goal worth chasing with everything we have.

But the reality is that the planet has already warmed about 1°C above pre-industrial levels. "More extreme weather, rising sea levels, shrinking Arctic sea ice and other changes have already shown us what 1°C of global warming does," said Zhai Panmao, chief scientist at the Chinese Academy of Meteorological Sciences and co-chair of IPCC Working Group I, which assesses the physical science basis of climate change. At the current rate of warming, the report estimates, 1.5°C will very likely be reached sometime between 2030 and 2052. Data cited in the report show that under the emissions pledges countries have set through 2030 as required by the Paris Agreement (Nationally Determined Contributions, or NDCs), and whatever they pledge after that, human-caused warming could reach 3–4°C by 2100. Even so, news reports say governments around the world are still largely failing to meet the targets they set for themselves under the agreement.

Given all this, keeping warming below 1.5°C in 2100 looks like an impossible task — so much so that an American scientist who worked on the Special Report on Global Warming of 1.5ºC told The Guardian, before it came out, that on reaching 1.5°C we are “nowhere near on track.”

The IPCC still thinks it is possible — but only through "rapid, far-reaching and unprecedented changes in all aspects of society." By the report's calculations, that means cutting global net human-caused carbon dioxide emissions about 45% from 2010 levels by 2030 and reaching "net zero emissions" by 2050, while sharply cutting the other substances that drive warming, chiefly methane (the main component of natural gas). Meeting the 2°C target, by comparison, would take only a 20% cut before 2030, with net zero not due until 2075. "Net zero emissions" means every bit of carbon emitted must be matched by an equal amount removed.

Those "rapid, far-reaching and unprecedented changes" have to happen in land use, energy, industry, buildings, transport, cities and more. They include replacing fossil fuels with renewable energy, using less energy, protecting and planting forests, restoring ecosystems, eating diets that put less strain on resources, and using carbon capture and storage. By 2050, for instance, renewables would need to supply 70–85% of the world's electricity. Of the fossil fuels left, natural gas could account for about 8%, provided carbon capture and storage facilities are installed, while coal use would fall to nearly zero. The report also urges ordinary people to change their behavior, by using less energy and shifting to diets that consume fewer resources.

Global warming could indeed reverse as greenhouse gas concentrations fall. But the report warns that if temperatures are allowed to overshoot the 1.5°C limit in the expectation that lower carbon dioxide concentrations will later undo the warming, humanity will lean harder on carbon dioxide removal (CDR). No one has proved these methods work at large scale, and some pose serious risks to sustainable development. One is “solar radiation management” — injecting gases such as sulfur dioxide into the stratosphere to form aerosols, or releasing particulates directly, to cut the amount of solar radiation reaching the ground. Another is "ocean fertilization" — adding nutrients such as iron, nitrogen and phosphorus to the ocean to build up its biomass, so that surface waters fix more carbon while the deep ocean and seafloor sediments store it. Even the most conventional removal method, planting trees, can disrupt land use and threaten food security if it is done badly.

And the report goes further: if temperatures overshoot by 0.2°C, it may be impossible to deploy enough carbon dioxide removal at all — and even if the temperature rise can be reversed, by then a great deal of irreversible damage, such as the loss of some ecosystems, will probably already have been done.

So the mindset of "develop first, clean up later," of assuming that things will sort themselves out when we get there, will only let the problem run away from us. Moving fast, on the other hand, pays off plainly. The report's Summary for Policymakers says that only if global carbon emissions start falling well before 2030 can the world avoid overshooting the temperature limit and avoid depending on large-scale carbon dioxide removal later. The lower emissions are before 2030, the smaller the challenge of holding warming to 1.5°C after 2030. Delay brings its own risks: rising costs, lock-in to carbon-emitting infrastructure, stranded fossil-fuel assets, and the loss of flexibility in medium- and long-term responses. Some Chinese media turned all this into clickbait headlines like "Global warming leaves humanity just 12 years — the post-90s generation paid its social insurance and housing fund for nothing."

From October 1 to 5, before the press conference, government representatives and scientists from the IPCC's 195 member countries had already gathered in Incheon to discuss the report and settle the text of the Summary for Policymakers. The IPCC works by mobilizing large numbers of scientists worldwide every five to six years for a fresh round of scientific assessment of climate change; this is the sixth round. The previous one was completed in 2014, and its findings fed the discussions at the Paris climate conference at the end of 2015. That assessment analyzed the consequences of 2°C in detail but did not study 1.5°C closely. Although "1.5°C" ended up in the Paris Agreement — as a goal worth striving for alongside 2°C as the baseline — no one was very clear on what scientific basis it had, what effects it would bring, or by what route it might be reached. So the following year the United Nations Framework Convention on Climate Change (UNFCCC), which runs the climate negotiations and pushes implementation of the convention, invited the IPCC to assess 1.5°C, and the report came out in 2018.

Setting a target first and only then going back to work out what it means is an awkward way to proceed. To understand how that happened, you have to start with the history of "1.5°C."

In 1990, more than 40 small island states and low-lying coastal countries formed the Alliance of Small Island States to press the case for how vulnerable they are to climate change. In 2008 one of them, Tuvalu, commissioned a German institute to study what 2°C of warming would do to the country, and found the results catastrophic. It therefore argued that warming should be kept "well below 1.5°C," and that demand became the alliance's demand.

Whether to cap warming at 1.5°C or 2°C became the main dispute at the 2009 Copenhagen climate conference. The result was the non-binding Copenhagen Accord, which made 2°C the primary target while adding that the possibility of 1.5°C would be considered.

After that, the 1.5°C target won wide support. Some scientists argued that 2°C was not enough to give the planet a safe future; others argued that 1.5°C could not be achieved at all.

At the 2015 Paris climate conference, 1.5°C became a focus of bargaining once again, and it ended up as the higher goal on top of the basic one of staying "well below 2°C." But 1.5°C had been put forward as a political demand, and what it meant scientifically was not clear. Only after Paris, then, did the UNFCCC go back and ask the IPCC for a scientific assessment of 1.5°C. That is where the special report of October 8 came from.

Some might say that on 1.5°C, politics has displaced science. But now that humanity is already feeling the effects of climate change, "who should bear how much of the future loss?" is no longer a scientific question. It is a political one.

The 2°C target has a drama of its own. Its origins go back to the 1970s, by which time scientists had already agreed that human activity was warming the planet. But 2°C was not proposed by a natural scientist. It came from an economist — none other than William Nordhaus, the American economist whom the Royal Swedish Academy of Sciences named this year's Nobel laureate in economics seven hours after the IPCC's press conference on October 8. Combining natural laws, technological conditions and socioeconomic factors, he worked out a first estimate that a reasonable ceiling for carbon dioxide concentration would be twice the pre-industrial level, which corresponds to 2°C of warming. In 1990 a Swedish research institute likewise took 2°C as an acceptable warming limit. The figure then moved into politics: the European Union adopted it first, in 1996, and in 2010 it was written for the first time into a binding climate treaty, the Cancún Agreements, in the words "below 2°C." In 2015 it finally entered the Paris Agreement — the broadest, most comprehensive and most politically ambitious climate agreement humanity has reached — as "well below 2°C."

2°C is not a precise figure either, but its value lies in being short and forceful, which turned it into a bugle call for the world's response to climate change.

At the Incheon press conference, questioners in the room and online asked the scientists again and again: how much confidence do you really have that 1.5°C can be achieved? And the scientists answered, again and again, that as scientists they can only answer scientific questions. In terms of the laws of physics and chemistry it is possible, but it also depends on political resolve and on countries' institutions, and those are not questions scientists can answer. They are the questions governments will have to answer at the 24th UN climate conference in Katowice, Poland, this December.

As for how the Chinese government should answer, Jiang Kejun, a researcher at the Energy Research Institute of China's National Development and Reform Commission who helped write the report, told reporters: "China's low-carbon energy companies already lead the world. Global emissions cuts and climate action are especially good for China's technological development, its exports and its economy. Looking ahead, China is expected to pass the United States as the world's largest economy by 2040. If the world is peaceful and suffering fewer disasters by then, that will help China's development too. Addressing climate change serves China's long-term interests. I hope China can set stronger emissions targets."

The scientists may say they deal only with science and technology, but the report is not without a social and political dimension.

The IPCC says in the report that it has tried to look at 1.5°C through the lens of reducing poverty, achieving sustainable development and delivering social justice. Climate change is itself unfair: the people who emit least are often hit hardest. At the same time, dealing with it means huge transfers of money and technology, which means creating many new jobs; and the fossil fuel industry, once highly concentrated, would give way to a far more dispersed industry of wind, solar and other renewables. That could actually make the economy more inclusive — one piece of good news in a mountain of bad.

Asked by reporters about fairness in climate change, Jim Skea, co-chair of IPCC Working Group III and a professor at the Centre for Environmental Policy at Imperial College London, said he had just accepted an invitation from the Scottish government to chair a "Just Transition Commission," set up to answer exactly that question.

"The low-carbon transition doesn't mean fewer jobs, it means more jobs. Many new kinds of work will appear, the workforce will need new skills, and the supply side of the job market will have to be remade. These are things we need to take seriously, to make sure the energy transition happens in a fair and just way," Professor Skea said.

Cover image: iceberg B-15, via Wikipedia