Our best chance to prevent the extinction of life has passed

Holding the rise in the planet’s average temperature to 1.5 degrees Celsius compared to pre-Industrial levels (1850)—the most ambitious target in the Paris Agreement—is probably already unattainable, with the adverb “probably” merely covering the off chance something unexpected happens. The most optimistic possibility right now is that warming above 1.5C might later be reversed, cooling the Earth back down by the end of this century and returning it to a balance more compatible with the preservation of life. Even this would require a much bigger effort by human societies, especially the most powerful political and economic sectors, to cut greenhouse gas (GHG) emissions and conserve the biomes that absorb carbon dioxide from the atmosphere. And nothing suggests such an effort is underway.

The scenario in which the planet heats up more than 1.5C and then cools down after a time—perhaps decades—is called “overshoot.” Although the word literally means “exceed” in English, the Intergovernmental Panel on Climate Change, or IPCC, a scientific body of the United Nations, applies the term to scenarios where an increase above 1.5C is later reversed. With the current path of GHG emissions indicating that global heating cannot be held below 1.5C degrees, the word has become ubiquitous in debates among climate scientists. “Nobody is arguing that the best scenario is a 1.5C rise accompanied by overshoot, but it has become the best possible scenario, because not exceeding 1.5C just isn’t possible anymore,” says Roberto Schaeffer, professor of energy economics at the Federal University of Rio de Janeiro.

Schaeffer was on one of the teams that compiled the last three of the six assessment reports released by the IPCC since its establishment in the late 1980s. Comprising hundreds of scientists from around the world, the IPCC analyzes the most up-to-date scientific scholarship on climate change, including scenarios of the planet’s temperature increases based on mathematic models reflecting different projections of GHG emissions. The purpose is to guide countries in their policymaking and subsidize debates at climate conferences like COP30, set to take place this November in Belém.

The sixth and latest IPCC report, published in parts between 2021 and early 2023, had already deemed it tough to hold global heating to 1.5C, but it still didn’t discard the possibility outright. To meet the target, peak emissions should have been reached by the beginning of this decade; in other words, emissions would have had to reach their highest point and then begun declining. This did not happen. In addition, emissions would have to drop by 43% by 2030 compared to 2019, but they continue to climb. The IPCC’s official verdict regarding possible new global heating scenarios will only come out in the seventh assessment report, slated for release between 2028 and 2029.

In addition to Schaeffer, SUMAÚMA spoke with German anthropologist Oliver Geden, from the German Institute for International and Security Affairs, and French climatologist Robert Vautard, from the National Centre for Scientific Research at the Pierre-Simon Laplace Institute, about today’s prospects for containing climate change. Both were among the authors of the sixth IPCC report and on the team that will prepare the seventh.

Geden explained that climate science has been talking about the concept of overshoot since around 2010, but the idea has been poorly conveyed. “Scientists said that [holding the rise to] 1.5C was still possible, but without explaining the logic very well. Then people hear we are approaching 1.5C, and they ask themselves why we’re saying it’s still possible.” Now, according to Geden, when “it’s highly likely that we will cross 1.5C within the next 10 years,” the concept has become more visible. “You want to go back to 1.5C, because you think you can not only limit damage, but reverse it. But the question is, will that really happen?” Geden says. “So we say overshooting [reversing the exceeded limit] is the second-best option, but we have to limit the magnitude and duration of the exceedance.”

One of the questions haunting the scenario in which global heating is reversed is that much uncertainty reigns about the consequences of rising temperatures. Every tenth of a degree makes a difference. Depending on the level reached—1.7C, 2C, 3C, or higher—an increasing level of damage may be irreversible. This could occur in biomes such as the Amazon as well as in other systems vital to climate regulation, such as the circulation of the Atlantic currents that carry cold water from the Northern hemisphere to the Southern and warm water northward. In this case, even if the Earth’s average temperature drops again, the point of equilibrium may no longer be the same or might not even exist anymore. Furthermore, along the way extreme weather events will cost human and more-than-human lives that can never be recovered. It’s like in our own lives: you can smoke for 20 years and then quit, and it’s likely that some of the damage to your body will be reversed, but even so you might end up with lung cancer. When a balance is messed with, there are knock-on effects.

“There are irreversible changes that could occur in an overshoot that will not disappear, even if we return to the target temperature [1.5C, as stipulated in the Paris Agreement]. [In this case, we’ll have] a different outcome even if the global temperature is the same [in the end],” explains Robert Vautard. These possible impacts will be addressed in a chapter of the IPCC’s Seventh Assessment Report. “We know that, for instance, if you warm [the planet] more, and even if it cools after, there will be more melting of glaciers, and sea level rise will be greater,” the French scientist says. The sixth IPCC report, for example, estimated that 50% of the world’s coastal wetlands—sea marshes, mangroves,  and seagrass beds—have vanished in the last 100 years due to the combined effects of sea level rise, extreme weather events, and pressure from urbanization.

Schaeffer also underscores the uncertainty involved: “No one knows what it means when a world’s temperature surpasses a certain value and then returns to its previous level. We already know the overshoot strategy [reversing the temperature rise] is risky, but it is still believed, or hoped, that a world 1.5 degrees hotter at the end of the century, with overshoot, will nonetheless be better than a world 2 degrees hotter at the end of the century. That’s the question,” he says.

Vautard points out that the 1.5C limit set in the Paris Agreement, signed in 2015, is a “political threshold.” As we saw with extreme weather events in 2024—such as the floods in Rio Grande do Sul, Brazil, and Valencia, Spain, and the extreme droughts in the Amazon—any temperature rise has an impact, resulting in hundreds of deaths, infrastructure and crop losses, and heightened vulnerability to fire in tropical rainforests.

“We’ve always said that 1.5 is better than 1.6, and 1.6 is better than 1.7. Impacts increase as temperature rises. There is not a single threshold above which everything falls apart,” the French scientist explains. “Anything, even 1.1 degrees, is too much when we talk about impact, because we have billions of people already affected today. The point is that every tenth of a degree matters because impacts are more pronounced at 1.6 than at 1.5.”

In 2024, a report by the United Nations Environment Programme estimated that if countries maintain their current emission reduction policies, the Earth’s temperature will climb 3.1C this century. If all commitments announced by governments were achieved, it could reduce this rise but only to 2.6C. Likewise in 2024, for the first time ever the planet’s average temperature exceeded 1.5C over the average for the reference period of 1850-1900. This increase was due largely to the concentration of GHG released into the atmosphere by humans but was also influenced by El Niño, a natural phenomenon characterized by the warming of Pacific Ocean waters.

What scientists have shown is that if everything continues as it is, by the end of the twenty-first century, when many of today’s children will still be alive, we will reach 3.1C and face a planet hostile to humans and many other species. This is why COP30, in Belém—the first in the Amazon—is crucial to life. It may be our last chance.

Scientists need an average of at least ten years before they consider a temperature consolidated. At the mid-year Climate Meeting, in Bonn, Germany, a group of 61 scientists led by Piers Forster, from the United Kingdom, presented a study that uses IPCC methodology to assess what stage we are at in global heating. The study estimates that from 2015 to 2024, global mean temperatures were 1.24C above the pre-Industrial level, of which 1.22C—that is, almost all of it—was directly associated with human activities. To have an idea of the rate of heating, the IPCC’s Sixth Assessment Report estimated that the rise had been 1.1C through 2020.

The study led by Forster notes a slowdown in GHG emissions in the last decade, although they have continued to climb. According to calculations by the study’s authors, the concentration of carbon dioxide in the atmosphere was 422.8 parts per million (ppm) in 2024, compared to 410.1 ppm in 2019, representing an increase of 48% over 1850, when the concentration was 285.5 ppm. This data is important because oceans and terrestrial biomes like Forests still absorb some 50% of the carbon dioxide emitted by humans. However, whatever isn’t absorbed remains in the atmosphere indefinitely. This is not what happens with other GHGs, like methane, mainly emitted by the digestive systems of such animals as Cattle, fossil fuel production, and waste degradation. Methane stays in the atmosphere for about 12 years.

At a press conference, Forster and other team members spoke of “unprecedented changes in the climate system” and acknowledged that in some cases there are no complete explanations as yet. They showed, for example, that glacier melting and sea level rise are happening at a much faster rate than the IPCC predicted four years ago. They warned that the “carbon budget”—everything the world can emit before the carbon dioxide accumulated in the atmosphere pushes temperatures above the 1.5C threshold—will be exhausted in just three years. In other words, three years’ worth of emissions as measured in 2024. This doesn’t mean the 1.5C threshold will be breached immediately, because there is a delayed effect to carbon accumulation. However, the study estimates heating of 1.5C could be reached or surpassed in five years, in other words, around 2030.

“This shows that countries’ ambitions weren’t enough to hold temperatures below 1.5C. We’ll have to see a radical change in ambition in the next NDCs to try to return to a more optimistic picture in coming years,” Forster said. He was referring to Nationally Determined Contributions, which are the emission reduction targets that countries must submit every five years under the Paris Agreement. This year, governments must submit their emission reduction plans for 2035, but so far only 28 of the more than 190 signatories have done so. Among those yet to comply are such major emitters as the European Union, China, and India. The new NDC for the United States—historically, the biggest emitter and currently outranked by China—was submitted under the previous administration, of Joe Biden, but Donald Trump has withdrawn from the Paris Agreement.

In the IPCC’s Sixth Assessment Report, Roberto Schaeffer of Brazil and Keywan Riahi of Austria chaired the chapter that analyzed some 1,200 scenarios for global temperatures through the end of the century, based on different pathways of GHG emissions. In the three most optimistic scenarios, or category C1, the maximum temperature increase by the year 2100 would be 1.5C with no or limited overshoot (0.1C at most). C2 scenarios predicted a return to heating of 1.5C after a higher overshoot, of 0.1C to 0.3C. In scenarios C3 to C6, the world would reach the end of the century with the temperature increase stabilized between 2C—the highest threshold in the Paris Agreement—and 3C. In the worst-case scenarios, C7 and C8, heating would not be held even to 4C. “We’ve missed a golden opportunity. The first category, not overshooting 1.5C—which the IPCC said was still feasible in 2022—isn’t feasible anymore,” Schaeffer said.

To stabilize the planet at any temperature, even above 1.5C, it is essential for the world to achieve what is called net zero. In other words, the obvious: remaining emissions cannot, in theory, exceed what Nature is capable of absorbing. However, since any carbon already emitted remains in the atmosphere, it would need to be removed to lower temperatures again, reducing the concentration that prompted an imbalance. In scientific terms, this is called “carbon negative” or “negative emissions.”

The best-known, cheapest, and most efficient way to pull carbon from the atmosphere is reforestation and, in general terms, the recovery of biomes, including mangroves and forests that have been turned into degraded pastureland. But this has to be done now, because rising temperatures tend to worsen droughts, make native vegetation more flammable, and hamper tree growth. “It’s not clear when these natural carbon sinks, be they Forests or oceans, will saturate and/or start releasing what they had previously been storing,” Schaeffer says. He estimates the world still has about two or three decades to make this shift, but it needs to start now. “A tree has to be planted today to reach peak carbon capture in 15 or 20 years. All investments must be made in the next four or five years to get a medium- or long-term return,” he says. “But that’s not how politicians think; they only think in terms of four years [referring to the election cycle].”

Schaeffer says it would be cheaper for the world to invest today in the recovery of natural carbon sinks in countries rich in Nature, like Brazil, rather than having to invest in costlier technologies to remove carbon from the atmosphere. The IPCC’s Seventh Assessment Report will take a more in-depth look at the efficacy and feasibility of the direct carbon capture technologies currently under design, primarily in financially wealthy countries. Known as Direct Air Capture with Carbon Storage, or DACCS, these consist basically of large machines that suck out carbon dioxide and store it in geological formations. The problem is the technology consumes huge amounts of energy and, in some cases, a lot of water, which would exacerbate other serious problems.

DACCS is different from the better-known CCS, which stands for Carbon Capture and Storage. The latter is already being used in oil production, for example, but it does not offset the carbon dioxide released during the burning of fossil fuels. There is also Bioenergy with Carbon Capture and Storage, or BECCS, in which the carbon released during the production of biomass energy—from plants such as sugarcane and agricultural waste material like bark and straw—is also captured or stored in the soil. This bioenergy would be considered “carbon negative” since plants also absorb carbon from the atmosphere when they grow.

Oliver Geden recognizes the “moral hazard” involved in the high-tech options for removing carbon, because they tend to favor countries with more financial resources, which in many cases are also the countries principally responsible for the emissions that cause global heating, such as the United States, Japan, and European nations. “You know or believe you can remove it later, so you continue to emit a little more,” said Geden. “That’s why many working in this field are saying we need to make clear how much emission reduction contributes and how much removal contributes. And we should avoid treating them the same when it comes to political prioritization.”

Vautard echoed this logic: “Regardless of the amount of carbon dioxide we manage to remove, we will also need to have a very strong reduction in emissions if we want to stabilize and potentially reverse the increase in temperature.”

Geden pointed out another obstacle, this time geopolitical, to a scenario of overshoot: no country, apart from Denmark, has so far committed to becoming emission negative. “We can only get to net negative [carbon] globally if we say that’s what we want to do and which countries will be first,” Geden said. “Talking about overshooting in the sense of bending the curve [of global temperatures] will only be credible once we have reached the peak of emissions. Continuing to go up in emissions and talking about getting to carbon negative at some point—there’s a disconnect.”

Countries including Brazil, South Africa, and India have proposed that nations classified as “developed,” who are the historical polluters, re-set carbon neutrality goals to earlier than 2050 and commit to achieving negative emissions after that. This demand was part of one of the joint statements from the last BRICS summit, which took place in Rio de Janeiro in July. The group of 11 countries, among them Brazil, South Africa, India, and China, called on the richest nations to achieve “net-zero GHG emissions significantly ahead of 2050, preferably by 2030, and net-negative GHG emissions immediately thereafter.”

The issue is countries like Brazil, India, and China, which are now among the largest emitters, also need to do their part. Counting on later removal of carbon—whether by natural or technological methods— is “quite a gamble for the future,” as Geden put it, because the full impact of increasing temperatures is still not clear even to scientists.

Unlike before, Roberto Schaeffer didn’t apply to be part of the team putting together the seventh assessment report for the IPCC. He says he has already done his part for the UN team. But Schaeffer does participate in groups of international scientists who are developing models, based on seven different scenarios of emissions trajectories, to be submitted to the IPCC. Using these models, the UN panel will produce temperature projections and predict their consequences for natural systems. One of the seven scenarios, nicknamed the “Trump scenario,” begins with very high emissions because, under the climate-change-denier president, the United States has already begun veering away from any effort to curb the climate emergency. “Hopefully, soon, there will be a more reasonable government that will do everything it can to mend the damage and reverse the situation,” Schaeffer said. 

Robert Vautard says the seventh IPCC report will have a chapter on what are called “tipping points,” the points of no return where a system or biome fundamental for climate regulation collapses and impacts the whole Earth. There is a lot of scientific literature today on the topic, but also many different predictions for when exactly these collapses could occur. “There’s a lot of use of this term, but it’s not homogeneous. People talk about tipping points for everything. So, we have to be very rational about the definition. What exactly are we calling a tipping point? What are the critical thresholds? What are the types of events we might witness where the scale is so large it will affect basically the whole planet?” said Vautard of the questions the IPCC will try to answer.
In a time of so many uncertainties, there is still no date for the release of the seventh IPCC report. Generally, these documents, with their many chapters and thousands of pages, are released incrementally every five to seven years. This means the publication of the next installment would only be wrapped up in 2030. Many countries, including Brazil, are calling for the publication of the full report to be brought forward to 2028, in time for it to be used in the second Global Stocktake of the Paris Agreement, also scheduled for 2028. The first Global Stocktake, in 2023, informed the decision made at COP28 the same year, promising an end to deforestation by 2030 and the “transition away from fossil fuels.”

Meanwhile, countries like India, China, and Saudi Arabia are against bringing the deadline forward. They contend the studies on emission reduction scenarios underpinning the sixth report placed undue responsibility on countries classified by the UN as “developing.” They cite a study by Indian scientists, led by Tejal Kanitkar, which shows all of the IPCC scenarios maintain current income, energy consumption, and emission inequalities between financially rich nations and the rest through 2050—that is, energy use, emissions, and per capita income would continue to be higher in countries that are historical emitters. These countries call for more time so the IPCC assessment can incorporate more scientific research done outside of rich countries.

This tug-of-war echoes disputes between groups of countries that block progress at climate conferences. In some cases, petrostates use seemingly reasonable arguments to obscure their interests. But, as with the controversies involving carbon removal technologies, these disagreements show how difficult it is to achieve climate justice even when the cost of inaction is the risk of human extinction.