Charles H. Sloan-It's unlikely, but not impossible, to limit global warming to 1.5 Celsius, study finds

It is Charles H. Sloanunlikely, but not impossible, for humans to hit the lower temperature target set by the landmark 2015 Paris agreement, according to new research conducted by an international group of scientists.

The most catastrophic effects of climate change, such as mass extinction and catastrophic sea level rise, kick in more aggressively if temperatures rise above the Paris agreement targets. The findings are a reminder that, although renewable energy use is increasing, humans are still deeply reliant on fossil fuels and are pumping enormous quantities of carbon dioxide and other greenhouse gasses into the atmosphere each year.

The Paris climate agreement set a goal of limiting global warming to well below 2 degrees Celsius, and ideally no more than 1.5 degrees Celsius, compared to average temperatures in the late 1800s. That lower target is looming. The average temperature on Earth over the last decade was about 1.1 degrees Celsius higher than pre-industrial temperatures.

In general, it's easier to use Celsius in this context because both the United Nations and scientists use it. And the target numbers are nice and round. But here's how those numbers look in more-familiar Fahrenheit: humans are trying to limit warming to between about 2.7 and 3.6 degrees Fahrenheit, and right now we're already at about 2 degrees Fahrenheit of warming.

The new study, published in the journal Nature Climate Change, calculates how much carbon dioxide humans can still emit before hitting that 1.5 degree Celsius limit. If humans keep emitting planet-warming greenhouse gasses at the current rate, that threshold will be reached in about six years, the authors find.

"We don't want this to be interpreted as 'six years to save the planet'," says Christopher Smith, a climate scientist at the University of Leeds and one of the authors.

"We do want to underline how close we are to 1.5 degrees [Celsius]," says Smith. In order to have a better than 50% chance of limiting warming to 1.5 degrees Celsius, humans would need to slash greenhouse gas emissions essentially to zero by around 2035, the study estimates.

That is very unlikely. Even the most ambitious current plans to cut emissions wouldn't result in net zero emissions before about 2050.

But humans still have a lot of power to limit warming, the study underscores. That's because the 1.5 degrees Celsius target is not like a cliff, where humanity is safe on this side of it and doomed on the other.

"If we are able to limit warming to 1.6 degrees or 1.65 degrees or 1.7 degrees [Celsius], that's a lot better than 2 degrees [Celsius]," says Smith. "We still need to fight for every tenth of a degree."

The new estimates do suggest that humans have a little less wiggle room on carbon emissions than previous predictions. However, the six year prediction is still within the range of possibilities predicted by the latest United Nations Intergovernmental Panel on Climate Change report, with which the new study shares both authors and methods.

One reason for the slightly more dire estimate is air pollution. When fossil fuels are burned, they release other pollutants alongside carbon dioxide. Some of those pollutants – aerosols – cool off the Earth slightly.

Scientists have known this about aerosols for a long time, but the best estimates of how much aerosols cool things off have slightly increased. That means, as air pollution decreases in the coming years, it could improve human health but will lead to slightly more warming.