Arctic sea-ice melt: A comparison of March (left) and September (right) sea ice extent between 1979 (white plus blue) and 2014 (white). Click to enlarge. (Kennedy Elliott).

In 2013, the National Academy of Sciences released a report on “abrupt” climate change — basically, the chance of dramatic, planetary-scale changes, like the loss of ice sheets or the shutdown of the overturning circulation of the oceans.

Most of these concerns involve the future, not the present. But the study found that at least one abrupt change is already upon us — the loss of sea ice cover over the oceans of the Arctic. “This rapid reduction in Arctic sea ice already qualifies as an abrupt change with substantial decreases in ice extent occurring within the past several decades,” the study noted.

The downward trend usually draws the most attention in September, because that’s when overall ice extent reaches its annual low, and the lows have been getting lower and lower. But in a new visualization, the Post’s Kennedy Elliott uses data from the National Snow and Ice Data Center to show that in their records going back to 1979, there’s actually been a plunge, over time, for ice extent for each individual month of the year:

Source: National Snow and Ice Data Center.

Note: This does not mean that every month had less ice than the previous one for 35 years straight. Rather, it means that Januarys today have less ice than they did in prior decades, Februarys today have less ice than they did in prior decades … and so on.

The loss of Arctic sea ice has myriad consequences, including threatening habitat for species like polar bears and undermining traditional means of hunting practiced for generations by native communities.

But one of the most troubling effects involves the overall impact of this change on the climate system as a whole. It’s known as a “positive feedback”: If there’s less Arctic sea ice, then there’s more exposed ocean water, and water is much darker than ice (in scientific parlance, it has a lower “albedo“). This means that less sunlight is reflected away from the planet, and more is absorbed — causing still more warming, and more ice melting, and more exposed ocean.

Scientists have known this would happen for many decades. Indeed, in a much-cited 1980 study that’s a kind of classic of climate change research, Syukuro Manabe and Ronald Stouffer used a climate model to examine the effects of much higher levels of carbon dioxide in the atmosphere, and found the following:

….In summer, the absorption of solar radiation by the Arctic Ocean surface in the 4 x CO2 experiment is much larger than the corresponding absorption in the 1 x CO2 experiment. This difference results from the reduction of surface albedo caused by the disappearance of sea ice or the formation of water puddles over the sea ice….This additional energy of both solar and terrestrial radiation is transferred directly into downward oceanic heat flux…and it enhances the melting of sea ice and increases the warming of mixed layer ocean.

That was published 35 years ago — or, right around when the trends shown above began.