'Past a point of no return': Reducing greenhouse gas emissions to zero still won't stop global warming, study says

Even if human-caused greenhouse gas emissions can be reduced to zero, global temperatures may continue to rise for centuries afterward, according to a scientific study published Thursday.

"The world is already past a point of no return for global warming," the study authors report in the British journal Scientific Reports. The only way to stop the warming, they say, is that "enormous amounts of carbon dioxide have to be extracted from the atmosphere."

The burning of fossil fuels such as oil, coal and gas release greenhouse gases such as carbon dioxide and methane into the atmosphere, causing global temperatures to increase and sea levels to rise.

The scientists modeled the effect of greenhouse gas emission reductions on changes in the Earth's climate from 1850 to 2500 and created projections of global temperature and sea level rises.

"According to our models, humanity is beyond the point of no return when it comes to halting the melting of permafrost using greenhouse gas cuts as the single tool," lead author Jorgen Randers, a professor emeritus of climate strategy at the BI Norwegian Business School, told AFP.

"If we want to stop this melting process we must do something in addition – for example, suck carbon dioxide out of the atmosphere and store it underground, and make Earth's surface brighter," Randers said.

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The study said that by the year 2500, the planet's temperatures will be about 5.4 degrees Fahrenheit warmer than they were in 1850. And sea levels will be roughly 8 feet higher.

The authors suggest that global temperatures could continue to increase after human-caused greenhouse gas emissions have been reduced as the continued melting of Arctic ice and carbon-containing permafrost increase water vapor, methane and carbon dioxide in the atmosphere.

The melting of Arctic ice and permafrost also would reduce the area of ice reflecting heat and light from the sun.

According to the study, to prevent the authors' projected temperature and sea-level rises, all human-caused greenhouse gas emissions would have had to be reduced to zero between 1960 and 1970.

To prevent global temperature and sea level rises after greenhouse gas emissions have ceased, and to limit the potentially catastrophic effects on Earth's ecosystems and human society, at least 33 gigatons of carbon dioxide would need to be removed from the atmosphere each year from 2020 onward through carbon capture and storage methods, according to the authors.

That's roughly the total amount of carbon dioxide the global fossil fuel industry emitted in 2018, according to Business Insider.

One expert, Penn State University meteorologist Michael Mann, told USA TODAY that he was skeptical of the computer model used in the study: "The climate model they have used is a very low complexity model. It doesn’t realistically represent large-scale atmospheric circulation patterns, such as ocean circulation, etc.

"While such models can be useful for conceptual inferences, their predictions have to be taken with great skepticism. Far more realistic climate models that do resolve the large-scale dynamics of the ocean, atmosphere and carbon cycle, do NOT produce the dramatic changes these authors argue for based on their very simplified model.

"It must be taken not just with a grain of salt, but a whole salt-shaker worth of salt," Mann said.

Another expert, Mark Maslin, a professor of climatology at University College London, also pointed to shortcomings in the model, telling AFP that the study was a "thought experiment."

"What the study does draw attention to is that reducing global carbon emissions to zero by 2050 is just the start of our actions to deal with climate change," Maslin said.

The study authors urge other scientists to follow up on their work: "We encourage other model builders to explore our discovery in their (bigger) models, and report on their findings."

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This article originally appeared on USA TODAY: Climate change study: Reducing greenhouse gases won't stop warming