The loss of Arctic sea ice is one of the clearest signs of human-caused climate change, and there's been nothing but blinking red alarms coming from the top of the world lately. The past four winters have seen such anemic sea ice growth that they've been the lowest four maximum sea ice extents since 1979.

At the same time, the region’s climate has seen temperatures increase at more than twice the rate of the rest of the world, with record-shattering seasons becoming more common.

During the summer, you can now take a luxury cruise ship through the Northwest Passage, and even in the winter, ice is failing to show up in places where it's normally so thick that it groans.

Now come two new studies that emphasize that unless the global community drastically lowers emissions of planet-warming greenhouse gases to keep warming to the strictest target under the Paris Climate Agreement, sea ice-free summers are going to become common during many of our lifetimes.

Both studies, published in the journal Nature Climate Change on Monday, investigated whether holding global warming to 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above preindustrial levels through 2100, would make much difference for the Arctic ice pack when compared to a 2-degree Celsius, or 3.6-degree Fahrenheit, temperature limit.

Using different climate model simulations and techniques, the studies both show that adhering to the aspirational target of 1.5 degrees Celsius under the Paris Agreement would have a far better chance of keeping the Arctic Ocean ice-covered year-round. Sure, there might be a year or two where natural variability would push sea ice cover low enough to be considered ice-free, but this wouldn't happen year after year.

Specifically, holding global warming to 1.5 degrees Celsius, as opposed to 2 degrees, would cut the probability of an ice-free summer occurring by the year 2100 from 100 percent to 30 percent, according to one of the studies, by Alexandra Jahn, a climate researcher at the University of Colorado at Boulder.

Under a 2-degree scenario, though, that ice-free summer would be far more likely to happen, putting Arctic wildlife like polar bears, walrus, as well as human settlements at risk. Considering that the world is currently on course for at least 3 degrees Celsius, or 5.4 degrees Fahrenheit, of warming by 2100, when compared to preindustrial levels, a 1.5-degree target might seem quaint, or even pointless to study. After all, 3 degrees of warming could yield a permanently ice-free Arctic Ocean in the summer, with all the cascading repercussions throughout ecosystems and weather patterns that would entail.

However, Jahn, who studies Arctic sea ice and ocean circulation in the Far North, said these new findings are worth paying attention to because they help scientists and policy makers see the benefits of stricter greenhouse gas emissions cuts. When the Paris Agreement was written in 2015, Jahn said in an interview, there were few studies that had quantified how much better a 1.5-degree target would be compared to 2 degrees.

Frozen flood ponds on the sea ice, as seen during an Operation IceBridge flight on Mar. 11, 2017. Image: NASA/Jeremy Harbeck

The agreement, which the U.S. intends to leave once it's able to do so in 2020, calls for countries to cut greenhouse gas emissions to limit global warming to "well below 2 degrees Celsius" above preindustrial levels, and "to pursue efforts to limit the temperature increase to 1.5 degrees Celsius" through 2100. Yet at the time it was written and adopted, scientists had not yet fully evaluated the benefits of a 1.5-degree target, Jahn said.

“There were no studies in the climate community that looked at these low warming scenarios,” Jahn said. She thinks her study, as well as the other by researchers in Victoria, Canada, help "fill in the probability space" to include lower carbon scenarios.

Walt Meier, a senior research scientist at the National Snow and Ice Data Center (NSIDC) in Boulder, Colo., said he's not surprised the studies found such a large difference between Arctic sea ice cover at 1.5 degrees Celsius of warming versus 2 degrees.

"Sea ice is quite sensitive to temperature because it’s so thin. And as temperatures warm, it gets thinner. The thinner the ice, the higher the chances that summer melt will be enough to remove the ice," he said in an email. Meier was not involved in the new studies.

"I think that somewhere between 1.5 degrees Celsius and 2.0 degrees, the ice cover gets thin enough over a large enough region of the Arctic for it to completely melt during summer. At the low end, 1.5 degrees, there is probably enough remaining thick ice (e.g., greater than 2 meters) that it’s less likely that all of that thicker ice could melt in a summer," Meier said.

He also said the difference between "ice free" and sea ice-covered may be small, depending how a study defines them.

"Another thing to keep in mind is that when we’re talking “ice-free” we’re looking at a threshold, so there can be a lot of sensitivity right near that threshold," Meier said. "For sea ice, the threshold is generally 1 million square kilometers... So, the threshold may make differences between sea ice covered and ice-free seem larger than the really are," he cautioned.

There's another issue, though, with studies like these that examine the benefits of a 1.5-degree target. Based on emissions trends and projections, such a target is illusory, since the world is on track to blow right past it. Might it make more sense to study the consequences of far more severe warming, given that that's where we're headed?

But Jahn, for one, rejects that line of thinking, reminding us that "our future emissions choices" will determine whether one target or another is feasible.

She says she's an optimist, and technological breakthroughs could come along to enable us to meet the lower targets, even if we exceed them for a period of time.

One interesting caveat in the new Arctic sea ice work is that while ice would grow back if the temperature were to cool again, it's not until carbon dioxide amounts in the air are brought down again that true ice recovery could occur.

This story has been updated to include comments from Walt Meier of the NSIDC.