On thin ice

Guess the graph competition answer
Last Wednesday, I posted this graph and asked for guesses as to what it represented. Aside from a few humorous suggestions, most answers were in the right ballpark, suggesting it had something to do with our melting cryosphere. This is perhaps one of the best known effects of climate change (or that part of climate change known as global warming), yet there is widespread confusion about the details.

The correct answer is that the graph represents the Arctic sea ice volume over the last few decades. The worrying downward trend is accelerating, but, unlike the graphs for Greenland or Antarctica, which are also heading down, the number on the y-axis are absolute figures. That is, while Greenland and Antarctica are losing increasing amounts of ice, compared with their total volumes, the amounts currently being lost are miniscule. For them to completely melt would be catastrophic, raising sea level tens of metres, but this is likely to take centuries, if not longer. However, the Arctic Ocean is getting seriously close to "ice-free" in summer,* an event most of us are likely to see within our lifetimes, and which we may well witness this or next decade, according to some experts. Certainly, extrapolating those trend lines points to an early grave for our planet's white top. The lines are unlikely to simply follow that curve, for various reasons, but scientists can identify no reasons to think the trend will reverse anytime soon.
*As long as Greenland still has significant amounts of ice, a residual amount of sea ice is likely to survive. "Ice-free" is usually qualified as largely ice-free. This is different again to an ice-free North Pole, which simply means that there is no sea ice cover at the North Pole, while there might still be some polar ice cap remaining.

What is Arctic sea ice?
It is important to highlight that we are talking about sea ice, that is ice that floats on top of the Arctic Ocean and which expands in winter and contracts in summer. It is generally not nearly as thick as people imagine (averaging just a metre or two) and any given piece of ice is unlikely to be more than a few years old, since it is constantly in motion due to wind and ocean currents and each summer much of it melts. Nonetheless, there has been permanent sea ice cover on the Arctic Ocean for at least somewhere between the last 700,000 and 4 million years, allowing the evolution of unique and endemic species (i.e. not found elsewhere). Since the ice is floating, the concern is not that melting will directly raise sea levels, both because the actual volume of ice is pretty miniscule compared with Greenland and (especially) Antarctica (for comparison, while Arctic sea ice is generally at most a few metres thick, Greenland's ice sheet is generally more than 2 km thick, and over 3 km at points. Antarctica is about ten times greater in volume again) and because floating ice displaces an almost identical volume of water to that contained in the ice (melting ice in a glass of water doesn't cause it to overflow).

Why are people worried?
Concern about the loss of Arctic sea ice is eightfold.

First, it is a canary in the goldmine: a visually dramatic sign of temperature changes that is relatively easy for the public to grasp.

Second, it threatens the unique and endemic Arctic biota (of which polar bears are of course the poster child).

Third, this in turn undermines the way of life of various indigenous groups in the Arctic, who rely on hunting and on the existence of sea ice for their livelihood.

Fourth, less floating white ice means more exposed dark water, which absorbs more solar radiation, increasing the total heat budget of the planet, and specifically of the Arctic Ocean.

Fifth, a warming Arctic Ocean and atmosphere speeds the melt of permafrost in Canada, Siberia and Alaska, not only threatening infrastructure built on it, but also releasing stored methane, a highly potent greenhouse gas that degrades into carbon dioxide, making it both a short term climate nasty and a long term headache.

Sixth, and perhaps of even greater concern, warmer waters increase the rate at which vast submarine deposits of methane clathrates destabilise and are released to the atmosphere, giving a further kick to warming. There is some debate about whether this process is likely to be slow and gradual or whether it might occur relatively suddenly, a process somethings colloquially called a "clathrate gun".

Seventh, the warmer the Arctic Ocean gets, the warmer Greenland is likely to get, and the faster its glaciers slide and melt into the sea. No one is entirely sure how long this will take, but it is a process that once it is underway in earnest, is likely to have a momentum of its own, meaning that our descendants will be committed to ever rising sea levels for centuries to come. Altogether, there is enough frozen water in Greenland to raise global sea levels by more than seven metres.

Eighth, an increasingly ice-free Arctic opens up a geopolitical minefield as nations scramble to take advantage of the resources previously locked away under the ice. The starter's gun for this race has well and truly fired (see video below).

Area vs volume
Another crucial distinction to keep in mind (apart from the difference between wafer-thin and highly vulnerable floating sea ice and gigantic land-based ice sheets that are both more stable and yet ultimately of greater direct threat) is between sea ice area/extent on the one hand and volume on the other. Area/extent is the easiest metric to measure with a satellite image (there is a slight technical difference between these two terms, but they are both basically concerned with a two dimensional account of how much of the Arctic ocean is covered with floating sea ice). Extent has been dropping at a slower rate than volume, which means that the remaining ice is getting thinner. Those only looking at the numbers for area or extent might be fooled into thinking the decline is only worrying, rather than alarming. While summer minimum extent has dropped by about a third over the last thirty years, volume is down by more than three quarters. And human activities are largely to blame.

The road ahead
The Arctic is one of the places where the climate change is already hitting the road. The transformation of the landscape is not merely a computer projection, but observable today. Its consequences are already negative, but the trajectory is even worse. What kind of path are we walking? And where will we find the courage and humility to turn around if we don't like where it is going?
The video at the top of the post is from a recently broadcast BBC programme, covering some of the implications of this story, which also appeared on the BBC news site.