The new report from the United Nations Intergovernmental Panel on Climate Change (IPCC) is the most comprehensive look ever at the state of climate science. And, it’s massive.

The full report is nearly 4,000 pages long, with 234 authors and some 14,000 citations to existing scientific studies. It includes mountains of evidence detailing the scope of human-induced climate change and expectations of what the future might hold if greenhouse gas emissions continue to climb.

U.N. Secretary General António Guterres called the findings “a code red for humanity.” These are five sentences — taken directly from the report — that illuminate the warning.

‘It is unequivocal that human influence has warmed the atmosphere, ocean and land.’

Neither scientists nor the United Nations use terms such as “unequivocal” lightly.

According to Alexander Ruane, a scientist at the NASA Goddard Institute for Space Studies and an author of the latest report, the language is the most powerful yet used to connect human activities to climate change and the warming the world has already seen.

“We are very careful around confidence statements,” he said. “What you’re seeing here is a very strong statement.”

The wording is also representative of how far the science has come over the decades. The 1995 IPCC report, which at the time was groundbreaking, wrote that “the balance of evidence suggests a discernible human influence on global climate.” The 2014 version of the report said that “human influence on the climate system is clear.”

Today, it’s “unequivocal.”

‘The last decade was more likely than not warmer than any multi-centennial period after the Last Interglacial, roughly 125,000 years ago.’

The 2014 report also found that the preceding three decades had warmed more than any other period in the previous 1,400 years. The 2021 report pushes that timeline back much further.

The jump is largely due to better data sets and methods, said Darrell Kaufman, a paleoclimate scientist at Northern Arizona University and another author of the report. It’s also a result of the Earth warming even more since then.

In the past decade, Kaufman said there has been a temperature increase of 0.29 degrees Celsius (0.19 degrees of actual warming and 0.1 degree due to better methods and data) more than was reported in 2014.

The latest version of the report, he said, also addresses the rate of change at a much more gradual level, moving from millennial to decanal time scales. It finds that ″human influence has warmed the climate at a rate that is unprecedented in at least the last 2,000 years.”

Even the lowest emissions scenario, Kaufman noted, estimates that at least 1 degree Celsius of warming — compared to 1850-1900 levels — will persist for centuries. He says that means “there’s no going back.”

‘Human-induced climate change is already affecting many weather and climate extremes in every region across the globe.’

Another major advance from earlier assessments is scientists’ ability to link climate change to weather and climatic systems.

“The IPCC has connected the dots on climate change and the increase in dangerous extreme weather events … far more directly than in past reports” said Michael Mann, professor of atmospheric Science at Pennsylvania State University.

It has also shown that these impacts are going to be felt in nearly every corner of the globe, and will get worse as temperatures increase across higher and higher thresholds.

At 1.5 degrees Celsius of global warming, for instance, parts of North America will already see an uptick in the number of days during which temperatures climb above 95 degrees Fahrenheit. But under a scenario of 4 degrees Celsius of warming, large swaths of the country will see as many as 30 additional days with temperatures above 95 degrees Fahrenheit.

‘With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact-drivers.’

Ruane sees this line as among the most important in the report.

“Impact-drivers,” he explained, refer to the dozens of different types of climate change effects, such as temperature or coastal flooding, that the report examined. The report demonstrates that the impacts will become incrementally worse as emissions, and consequently warming, grow.

“It’s more than just the headline extreme events,” he said, adding that “these largest set of changes start to act in combination.”

The IPCC report found that many changes due to greenhouse gas emissions are already “irreversible” — such as melting glaciers, said Ruane. But there is some hope amid the gloom.

“This is not just a cause for concern but also a cause to recognize that we have agency here,” he said. “If we can stabilize the climate system, we can steer clear of the largest changes.”

‘Global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in carbon dioxide (CO2) and other greenhouse gas emissions occur in the coming decades.’

Discussions around climate change often involve the idea of keeping the planet from warming beyond certain thresholds, such as 1.5 degrees or 2 degrees Celsius — the latter of which scientists and policymakers have identified as a red line if the planet is to avoid catastrophic and irreversible consequences.

The world is already experiencing 1.09 degrees of warming, according to the report. The best-case scenarios it explored would stabilize warming at 1.5 degrees, but that would involve cutting emissions to “net-zero” by 2050.

So far, though, countries are falling short of what is needed to avoid the worst effects of climate change, according to a U.N. analysis from earlier this year. Even if current emissions pledges are realized, they would amount to just a 1 percent reduction in global emissions by 2030, compared to 2010 levels. Scientists say the number needs to be closer to a 50 percent reduction.

“We must act decisively now,” said Guterres, the U.N. chief. “Every fraction of a degree counts.”