There was a time when it was convenient to think about global warming and the destructive consequences it poses for life on Earth as a possible, but not necessarily probable, event happening in a distant future. Those days are over, according to a major new United Nations report. The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report, the most authoritative summary of climate science in the world, shows that for the next generation, increasing heat waves and wildfires, hurricanes and floods, drought and sea level rise are now inevitable. And the consequences could be dire—potentially leading to millions of people displaced, thousands of lives lost, and billions in economic damage.

Due to all the human-caused emissions already in the atmosphere, halting the climate crisis in the near term is no longer possible. But actions taken now can still mitigate the worst effects of global warming for generations to come—though time is running out to act. 

“The scientific community in this report is saying, ‘These are the metrics. This is the physical science. And now it’s up to civil society and the world to grapple with this stark reality,’” Brenda Ekwurzel, director of climate science for the Climate and Energy Program at the Union of Concerned Scientists and a coauthor of the fourth National Climate Assessment for the United States, told Sierra. “This is the report card for human society and how much we’re changing that climate.”

In unusually direct and dire language, that assessment starkly concludes that human industry has caused a dramatic change in the global climate system through the persistent burning of fossil fuels such as coal, oil, and gas. When those fuels are burned, they release gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N20), which gather in the atmosphere and trap heat, creating a “greenhouse effect.” That heating melts glaciers, leading to sea level rise; heats up oceans, making them more acidic; and amplifies weather events such as hurricanes and wildfires, making them more destructive. This cycle leads to the potential for climate tipping points. Less Arctic ice means a diminished mirror for reflecting the sun’s rays; excess heat leads to melting permafrost, which then releases more heat-trapping methane leading to more excess heat; deforestation releases carbon leading to the same cycle. 

The past five years have been the hottest on record, with 2020 the warmest ever recorded and 2021 on track to topple that record. Each of the past four decades has been hotter than any decade that proceeded it since 1850, according to the report. Ocean levels have risen eight inches over the past century, and that rate of sea level rise is accelerating. Sea level is rising faster than it has in 3,000 years.

What is the cause for this dramatic change in climate? The report is unequivocal: human activities. “Human influence has warmed the climate at a rate that is unprecedented in at least the last 2,000 years,” according to the report’s Summary for Policymakers. “In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years (high confidence), and concentrations of CH4 and N2O were higher than at any time in at least 800,000 years (very high confidence).” 

Without action, and with every degree of warming, more climate catastrophes—massive wildfires incinerating entire towns, hurricanes taking out coastal cities, coral reefs dead or dying, species going extinct—become more likely. 

A total of at least 2.7 degrees Fahrenheit (or 1.5 degrees Celsius) is already locked into the climate for the next 30 years, according to the report, guaranteeing that the planet will continue getting hotter regardless of any action nations take today. That rise could level off if nations aggressively phase out fossil fuels starting now until approximately 2050. 

“Emissions today have a profound influence in dialing up the oven temperature of Earth,” Ekwurzel says. “There is nothing you can do to stop that from past emissions. It’s up to us to decide what the future of our emissions, and therefore the future of the planet, is going to be.” 

Since 1988, the IPCC has released a summary assessment of the latest climate science every seven years. The last assessment was issued in 2013. The Sixth Assessment Report was coauthored by 234 scientists from 195 member countries who synthesized over 14,000 research studies. For these IPCC reports, scientists don’t produce independent scholarship; they collect and evaluate thousands of existing peer-reviewed studies to present a unified consensus on the state of global climate today. 

“This is the most powerful institution of global knowledge ever convened,” Jesse Keenan, a member of the IPCC Working Group III and a professor and social scientist at Tulane University, told Sierra. “It shows us that we have tremendous collective power in understanding the nature of these problems, but also understanding what the solutions are. This has set forth what those challenges really are—it’s not saying how to do it; it’s not getting into the politics of it. It maintains a certain objectivity that’s critically important for understanding the trade-offs and policy decisions.”

“What we’re seeing with this report is what’s at stake,” Keenan says. “Every successive IPCC report that we see is demonstrating with greater clarity exactly what is at stake.” 

While the IPCC does not prescribe specific policies, the report makes clear that whatever measures governments decide to take, they must involve the rapid and immediate cessation of the burning of fossil fuels for energy. “This report must sound a death knell for coal and fossil fuels before they destroy our planet,” United Nations Secretary-General António Guterres said in a statement released with the report, calling it a "code red for humanity." “There must be no new coal plants built after 2021. OECD countries must phase out existing coal by 2030, with all others following suit by 2040. Countries should also end all new fossil fuel exploration and production and shift fossil fuel subsidies into renewable energy. By 2030, solar and wind capacity should quadruple and renewable energy investments should triple to maintain a net zero trajectory by mid-century.”

The report poses five different “possible climate futures” in which scientists evaluate what the world will be like given how little or how much action world powers take to limit greenhouse gas emissions, from the wholesale cessation of burning fossil fuels for energy by 2050 to a scenario in which nations completely fail to act, including variables for such things as population growth and international partnership. In the scenario in which nations fail to act, average global temperatures could potentially rise by anywhere from 3.6 degrees to 7.2 degrees Fahrenheit (or 2 to 4 degrees Celsius), leading to an apocalyptic scenario. However, the authors found that if countries immediately lower emissions, the planet will begin to cool around the middle of the century. 

The report also breaks new ground in scientists’ capacity to connect individual extreme weather events to global warming—what’s called “attribution science.” Extreme weather like the heat dome that plagued the Pacific Northwest this summer, heavy rain events like the one that caused catastrophic flooding in Germany, and monster hurricanes like the one that devastated Puerto Rico would be unlikely without human-induced climate change. Typically, the IPCC reports focus on global phenomena. In the case of the Sixth Assessment, a greater scrutiny is taken to regional events.  

The IPCC Sixth Assessment will take center stage in November, when national negotiators from around the world gather (some in person, some remotely) for the United Nations–sponsored climate summit in Glasgow, Scotland. All eyes will be on the world’s biggest emitters to take action in response to the report’s findings and follow through on commitments made in 2015, when the Paris Agreement was forged.  

“In the past, a lot of the focus has been, what can we as individuals do to help with this problem,” Ekwurzel says. “But as individuals, we can only do so much. The global pandemic showed that starkly. Many of us stopped flying or driving as much or taking a personal vehicle to get to their jobs, and it barely registered as an emissions reduction. That tells you that this is not an individual-scale, individual-family-scale problem. This is a problem that has to deal with the core structural reasons why we got into this trouble in the first place. That means we have to look hard at what sources we use for energy. We need a just transition for workers that are in industries making harmful products, and we need to transition energy companies away from fossil fuels into renewables. This is a human-driven problem. Therefore, we are all part of the solution.”