If anyone knows climate change, it's James Hansen.

The former NASA scientist gained credit for forging wide awareness of the issue when, in 1988, he told Congress "the evidence is pretty strong" that human-caused global warming is — well — a thing.

Hansen testifies before the Senate Foreign Relations Committee in March 2014. Photo by Chip Somodevilla/Getty Images.

Hansen, who now works at Columbia University's Earth Institute, has a history of being ahead of the curve when it comes to climate science. So you can imagine why a new paper he's set to publish this week alongside 16 other researchers is causing quite the stir.

Hansen's new research claims sea levels could rise about 7 feet higher than other estimates throughout the next 85 years.

As we know, higher temperatures caused by increased global greenhouse gas emissions is melting ice near the Earth's poles. That means higher sea levels.

In his new paper, which will be published in the peer-reviewed Atmospheric Physics and Chemistry journal, Hansen claims that — unless humans start emitting way fewer greenhouse gases, like, now — ice at the poles will melt at exponentially faster rates than currently predicted.

The IPCC, the U.N.'s panel focused on climate change, predicts sea levels to rise about 3 feet by 2100. That figure's already alarming scientists. Hansen's new research, however, predicts it'll be more like 10 feet.

A sea level 10 feet higher would make coastal cities like New York, London, and Shanghai completely uninhabitable.

Aerial photo of New York City by Bruce Bennett/Getty Images.

It's sort of difficult to overstate how big of a deal that is. Not only would this affect millions of people, several trillion dollars worth of infrastructure could be destroyed.

“Parts of [coastal cities] would still be sticking above the water," Hansen said. “But you couldn't live there."

The year 2100 ... some of us could still be alive!

The discrepancy between Hansen's figures and the U.N.'s largely comes down to melting freshwater ice as opposed to melting saltwater ice.

Hansen's research claims that when freshwater ice on land (and not saltwater icebergs) melts into the oceans near places like Greenland and Antarctica, the colder freshwater traps the warmer, saltier water below. This trapped, warmer water causes further melting below the surface — a process that only increases the amount of additional water being added to our oceans as more freshwater ice melts into the sea.

Scientists have claimed that limiting global climate change to 2 degrees Celsius (about 3.6 degrees Fahrenheit) could protect the Earth from devastatingly high sea levels. But the trapped warmer seawater is increasing the melting process quicker than expected, according to Hansen, so that 2 degrees Celsius figure won't cut it.

The IPCC doesn't consider this freshwater effect in its projections. Hansen says there's evidence it's already happening.

Greenland's disappearing glaciers have become alarming symbols of a warming planet. Photo by Uriel Sinai/Getty Images.

But before you panic! Don't worry. There are reasons to be hopeful we can prevent the worst that global warming has in store.

After all, combating climate change is no longer on the world's back burner (global warming pun unintended).

Ahead of the the United Nations' climate summit in New York City back in September 2014, more than 400,000 people took to the streets to show world leaders that curbing climate change needs to be a priority.

And government bodies are listening. The U.S. and China, for example, reached a historic agreement just months after the climate march that will cut each country's coal consumption way back in the years to come. So far, China is taking the commitment seriously.

This will result in significantly less greenhouse gas emitted by two of the largest nations contributing to global warming.

Photo by Andrew Burton/Getty Images.

Hansen agrees: We still hold the fate of our planet in our hands when it comes to climate change.

He said we “could actually come in well under 2 degrees if we make the price of fossil fuels honest." That means taxing fossil fuel-burning businesses and governments, which will encourage eco-friendlier practices.

Even with his latest research painting a dire future ahead, if Hansen believes we can make a meaningful difference in the decades to come, we all should.

But we have to act big, and we have to act now.