Last Sunday, the extent of sea ice in the Arctic reached its lowest point since such things have been tracked. This is the second of a series of posts looking at the science and the significance of this decline. Today, we’re asking what the “lowest extent of Arctic sea ice” really means.

Here’s the context

Since the late 1970s, the scientific community has used satellites to measure the extent of sea ice covering the Arctic Ocean and surrounding areas. Anything over 15 per cent coverage is included as part of the sea ice extent.

While the actual extent of sea ice going back thousands of years is difficult to determine, the paleoclimate record (mostly from sea floor and ice cores) provides clues as to when the Arctic may have been warmer than it is now. One thing we know for sure is that around 65 million years ago, forests flourished in parts of the Canadian High Arctic. What happened between then and now is less certain. Some studies suggest that two periods – one 5,500 years ago, and one 125,000 – had less summer ice than we do now. It is possible that that the earlier time was completely ice-free in the summer.

The cause of those warming periods appears to be mostly due to the Earth’s orbit and the tilt of its axis (called orbital forcing), which cause the Earth to be exposed to more of the sun’s radiation every 20,000 years or so.

The warming that our planet has been experiencing since the 19th century is caused by very different factors and is happening much more quickly. In fact, the Earth’s orbit today, under similar circumstances, would be causing our planet to cool. There are several reasons that circumstances are different this time around, but the strongest factor appears to be the extremely high levels of carbon dioxide in our atmosphere over the last 100 years or so. That blanket of planet-warming gas appears to be overwhelming the cooling effect that might otherwise be caused by orbital forcing.

Ice thickness matters

Some ice in the Arctic is five or more years old. Having survived several summer melts, it is thicker and denser than newly-formed ice. The Arctic is rapidly losing this thicker ice now, meaning that less ice survives the summer (30 per cent less over the last 30 years). New ice that forms in the winter is thin and will melt away in just one season if summer temperatures are not cold enough – and, for the most part, they aren’t anymore. Most new ice does not survive its first summer.

Thicker sea ice is more likely to survive the summer melt. This ice has survived to the end of August and will likely make it until it starts freezing up in September.

We will continue to see winter ice form in the Arctic for quite some time, but the days of ice remaining through the summer are rapidly disappearing – much faster than any climate models had predicted – and the Arctic could be entirely ice free in the summer as soon as 2030 and certainly by 2100.

A patch of sea ice that has survived most of the summer off Ellesemere Island in the Eastern High Arctic.

The ice keeps the freezer cold

One thing that appears to help keep the overall temperature down in the Arctic is the ice itself. The ice tends to insulate the ocean from the direct heat of the sun.  The shiny white ice reflects the sun’s rays back up like a mirror, whereas exposed dark seawater tends to absorb the sun’s rays and warm up.


The dark exposed water absorbs the sun’s rays while the reflective white ice reflects much of it back into the atmosphere.

More ice-free oceans mean more heat absorbed, warming the sea surface even more. This also means that the Arctic may freeze later in the year than usual because the ocean needs to cool down before ice can form in the fall. And where does all that extra heat go when the ocean releases it? Back into the atmosphere, where warm pockets of air in late fall or early winter can trigger a whole different set of chain reactions. We’ll talk about that in a future post.

In the next few posts, we’ll look at the significance of the decreased ice extent and thickness of Arctic sea ice from a variety of perspectives.



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