This story originally appeared in Scientific American and is part of Covering Climate Now, a global journalism collaboration boosting coverage of climate history.

On October 31, world leaders began descending to Glasgow, Scotland, for the United Nations Climate Change Conference, or COP26, in a last-ditch effort to defuse the climate emergency by limiting global warming to less than 1 , 5 degree Celsius. Reaching this level would still lead to severe storms, deep flooding, severe droughts and problematic sea level rise, but it would avoid even more serious consequences. Global temperature has risen by almost 1.1 ° C since the Industrial Revolution.

A clear understanding of how emissions affect temperature shows that there is still time to achieve the political agreements, economic transformations and public buy-in needed to sharply reduce emissions, limit temperature rises and limit heat loss. destruction. Nations can dodge the 1.5 degree cap if they make deep cuts now. As of July 30, the emission reduction commitments of the 191 signatory countries of the 2015 Paris Climate Agreement would allow a warming of 2.7 degrees by 2100, according to a report published in September by the secretariat of the Convention. -United Nations Climate Change Framework, the group that coordinates ongoing commitments to the Paris Agreement. The burden for the COP26 meeting is to close the gap. Here is what needs to happen.

The first step is to get rid of an old idea that the public, media and policy makers are not clear about: the idea that even if humans stop emitting carbon dioxide overnight, l The inertia of the climate system would continue to increase the temperature for many years. Because CO2 can linger in the atmosphere for a century or more, the argument goes, even if the concentration stopped increasing, the temperature would continue to rise because the heat-trapping mechanism is already in place. In other words, some level of future warming is “built in” into the system, so it is too late to avoid the 1.5 degree threshold.

But scientists dismissed this idea at least a decade ago. Climate models consistently show that “engaged” (content) warming does not occur. As soon as CO2 emissions stop increasing, the atmospheric CO2 concentration stabilizes and begins to slowly decline as oceans, soils and vegetation continue to absorb CO2, as they always do. The temperature is no longer rising. It also does not decrease, as atmospheric and oceanic interactions adjust and balance each other. The net effect is that “the temperature neither goes up nor down,” says Joeri Rogelj, research director at the Grantham Institute — Climate Change and the Environment at Imperial College London. The good news is that if countries can reduce their emissions substantially and quickly, warming can be kept below 1.5 degrees.

To avoid this threshold, the world can only emit a defined amount of CO2 in the future. This amount is known as the carbon budget. In 2019, the year before the COVID pandemic depressed the global economy, the world released around 42 gigatonnes of CO2, which is similar to the 2018 level and what happens in 2021. According to the scenario of mid-range of the full report of the Intergovernmental Panel on Climate Change released in August, “Climate Change 2021: The Basis of Physical Science”, an additional 500 gigatonnes of CO2 emissions will increase global temperature by 1, 5 degree. Nations still have about 11 years at current emission rates (2032) before they run out of budget.

This threshold moves further into the future, however, if countries drastically reduce their production very soon. Aggressive policies, adopted now, can create more time and more hope to avert disaster. In a 2018 report, the IPCC said the world needs to reach net zero carbon emissions by 2050 to keep warming at 1.5 degrees. To get down that road, according to the UN’s September report, nations must cut their emissions in half by 2030. Each year of delay brings the world much closer to the brink of the precipice. “We are not trying to meet the temperature targets,” says Rogelj, who is also a senior researcher at the International Institute for Applied Systems Analysis and a key author of the IPCC 2021 report. “We try to stay as far from the edge as possible.”

Degrees of risk

If nations fail and the rise in temperature exceeds 1.5 degrees, it will still be crucial to make immediate and continued reductions to stay below 2.0 degrees of warming, a level at which scientists say impacts become more serious and extremely difficult for companies to manage. To avoid this threshold, the world can only emit another 1,350 gigatonnes of CO2, according to the August IPCC report. At 42 gigatonnes per year, that will happen by 2052. Again, if countries significantly reduce their emissions soon, that date is extended as well.

If countries don’t make significant reductions this decade, the subsequent reductions needed to limit temperature rise to 2.0 degrees will be much more difficult to achieve. “Each year that passes imposes a huge penalty for any future reductions that would be necessary,” says Josep Canadell, chief researcher at CSIRO, Australia’s national science agency, and lead author of the IPCC 2021 report.

It’s also important to understand, says Rogelj, that every tenth of a degree of warming more than 1.5 degrees increases the risk of weather damage, sea level rise and other ailments for further harm. ecosystems and people, especially the most vulnerable. He compares the increasing risk of jumping from a platform that can now be a meter high: healthy adults can touch the ground without hurting themselves, but small children and the elderly will be injured. Each additional tenth of a degree raises the platform. “Two meters away,” Rogelj says, “many more people are likely to be injured. And at a certain height, everyone will be seriously injured.

The IPCC carbon budget analysis includes a measure of uncertainty, about 15% up or down. And the midrange scenario means countries have a 50% chance of keeping warming at 1.5 degrees if they limit future emissions to 500 gigatons. To improve the odds to 83 percent, according to the IPCC, the budget drops to 300 gigatons. The numbers get even tighter if nations continue to burn rainforests because there will be less vegetation pulling CO2 from the atmosphere. Countries must also take into account societal factors, such as ensuring that the economic challenges of reducing emissions are distributed fairly among citizens.

Of course, if the world were to reduce emissions only marginally and never reach net zero, “the concentration of CO2 in the atmosphere will continue to rise and the temperature will continue to rise,” says Susan Solomon, professor of environmental studies and atmospheric chemistry at the Massachusetts Institute of Technology, which has contributed to numerous reports on climate change.

Human lag

The dialogue leading up to COP26, where countries will try to encourage each other to commit to further reducing their emissions, focuses on CO2. But the atmosphere is affected by other greenhouse gases such as methane and nitrous oxide, by climatic feedbacks such as the disappearance of sea ice and by aerosols, small particles of pollution released mainly by the sea. burning of fossil fuels. If CO2 emissions remain at current levels, but methane emissions increase and other feedbacks strengthen, the world will warm 1.5 degrees before 2032 and 2.0 degrees before 2052. The IPCC scenarios include some level of additional warming due to these factors. They do not include the so-called negative emissions from the machines that pull CO2 out of the sky, because the economic viability of these systems is just too uncertain, Canadell says.

The UN report uses a different metric to account for other greenhouse gases, called CO2 equivalents, an amount that represents the warming due to CO2 as well as methane, nitrogen oxides and carbon dioxide. other gases such as hydrofluorocarbons. But its analyzes are parallel to those of the IPCC. As of July 30, according to the UN report, 113 of the 191 countries that have signed the Paris Agreement have pledged to reduce their emissions. According to the latest promises, global emissions by 2030 would actually be 5.0% higher than 2019 – not lower – in the midrange scenario used by the IPCC. The report notes that the emissions of countries that have published revised targets since 2015, as a group, would indeed be lower in 2030 compared to 2019, so the net increase in the world would come from countries that did not. not improved their initial commitments and countries that never committed.

At current emission rates, according to the UN report, the world would use 89% of the remaining 1.5 degree budget by 2030 and 39% of the 2.0 degree budget by 2030. October 25 , a week before the start of COP26, the secretariat was to count all additional country updates made since July 30. All eyes will be on the G20 countries – 19 countries plus the European Union which together account for around 90 percent of gross world product. G20 countries are responsible for around three-quarters of global emissions, according to Taryn Fransen, a senior researcher at the World Resources Institute who studies countries’ long-term climate strategies. She looks forward to hearing how countries will keep their pledges, known as Nationally Determined Contributions, or NDCs. Net zero goals are important, says Fransen, “but every country really has to get there.”

To get there, nations must jump – now. Some scientists are starting to use the old language of climate change to point out what needs to be done. The warming factor that is built in “is human infrastructure,” says Solomon. If countries let current stocks of coal-fired power plants, natural gas plants, transportation systems, industrial complexes and buildings live their natural lifetimes, they are committing to some further warming. There is also a lag in stopping the rise in temperature, she notes, “a lag in human action – the slowness of people’s response to the problem.” The practical question, says Raymond Pierrehumbert, head of the Planetary Climate Dynamics Group at the University of Oxford, is: How quickly will the world be able to remove greenhouse gases from the global economy?

Mark Fischetti is editor-in-chief at Scientific American. It covers all aspects of sustainability.