Thirty-five years ago, NASA climate scientist James Hansen stood in front of Congress with a bold declaration: humans are causing an increase in greenhouse gas emissions, and it’s changing our climate. Some scoffed, but, in coming decades, people saw how dire this warning was.

On Thursday, Hansen and colleagues across the world released a new study with another serious, if not controversial, finding. Climate change will catapult global temperatures into crisis territory earlier than previously thought, the scientists said, warning that Earth could warm more than 1.5 degrees Celsius this decade. Their alarming prediction — that the pace of Earth’s warming is accelerating — stirred some disagreement within the climate community

“The 1.5 degree limit is deader than a doornail,” Hansen, now a director at the Earth Institute at Columbia University, said in a press call with reporters on Thursday. “In the next several months, we’re going to go well above 1.5C [Celsius] on a 12-month average ….For the rest of this decade, the average is going to be at least 1.5.”

Since the preindustrial era, Earth has warmed around 1.2 degrees Celsius on average. But at times, temperatures have spiked beyond that. Some summer months in 2023 have registered global average temperatures between 1.5 to 1.6 degrees hotter than average before the widespread use of fossil fuels.

While 1.5 degrees isn’t a magical tipping point for Earth’s demise, the United Nations has warned of severe and potentially irreversible consequences above that level. Many staple crops wouldn’t be able to grow in such warmth. Even the best water conservation practices wouldn’t combat the projected droughts.

Scientists have long disagreed on exactly how much global temperature will rise with additional atmospheric carbon dioxide. An early study in 1979 estimated that doubling carbon dioxide in the air would cause global increases between 1.5 and 4.5 degrees Celsius. More recently, the United Nations Intergovernmental Panel on Climate Change calculated the Earth could warm by 3 degrees with a doubling of CO2.

But those may be underestimations, the new study found. Hansen and his colleagues analyzed paleoclimate data and the Earth’s energy imbalance to estimate that doubling carbon dioxide could lead to a whopping 4.8 degrees of warming compared to the preindustrial era.

Under the current trajectory of greenhouse gas emissions, they predicted the 1.5 degrees benchmark will be passed in the 2020s, and 2 degrees of warming will be passed before 2050 — a markedly faster rate compared to the prognosis from other scientists. In its most recent landmark climate report, the United Nations stated global temperatures would reach 1.5 degrees in theearly 2030s.

Hansen and his co-authors attribute the rapid warming pace partly to a reduction in aerosols — or particles of pollution in the atmosphere. Some types of pollution reflect the sun’s rays, cooling the planet; as countries clean up their energy systems, cutting down on that pollution can counterintuitively create a warming effect. The new paper suggests that cutting pollution from marine shipping may be causing the Earth to absorb more solar radiation.

The team estimated a global warming rate of 0.18 degrees per decade from 1970 to 2010, but they say the pace will increase to at least 0.27 degrees per decade during the few decades after 2010.

“The two-degree limit can only be rescued with the help of purposeful actions to affect Earth’s energy balance,” said Hansen at the news conference. “We will need to cool off Earth to save our coastlines, coastal cities worldwide, and lowlands while also addressing the other problems caused by global warming.”

Not everyone agrees with the new study. Michael Mann, a professor of earth science at the University of Pennsylvania, posted a lengthy critique of the paper on his personal website.

“The standard is high when you’re challenging scientific understanding,” Mann wrote. “And I don’t think they’ve met that standard, by a longshot.”

Mann argued the ocean’s heat content is growing steadily, but — in contrast to Hansen and his co-authors — is not accelerating. Mann also cited data showing that there does not appear to be a sudden shift in pollution from aerosols over the last few years. Other research have found that a decline in aerosol pollution from cleaning up shipping would only shift global temperatures by 0.05 or 0.06 Celsius.

“While I hold James Hansen to be one of the most (if not the most) important contributors to our modern scientific understanding of human-caused climate change, I feel that this latest contribution from Jim and his co-authors is at best unconvincing,” Mann wrote.

The new study also suggests a path forward for policy — an unusual move for most scientific papers. For decades, scientists have avoided providing any policy prescriptions for dealing with the problem of climate change, preferring to stick to science and data. But in recent years, that has begun to change.

Hansen and colleagues call for a rising price or tax on carbon emissions, subsidies for renewables and nuclear power, and global cooperation on climate goals. They also suggest further research into solar geoengineering, a technique that could cool the planet by injecting particles into the atmosphere to reflect the sun’s light.

In the press call, Hansen also called for further political action from young people and others galvanized by the overheating planet.

“I believe a political party that takes no money from special interests is probably an essential part of the solution,” he said. “Young people should not underestimate their political power.”