We are becoming aware of two very important changes in the Arctic that you need to know about. These are separate thing but related, and both are almost certainly the outcome of anthropogenic global warming (AGW). They are:
The sea ice that covers much of the Arctic Sea during the winter is normally reduced during the northern summer, but this year, the reduction has been dramatic. There is less sea ice in the Arctic Circle than recorded in recent history.
The massive continental glacier on Greenland, the largest glacial mass in the Northern Hemisphere, has undergone more melting this summer than observed over recent years. A few weeks ago you may have heard about a great melt in Greenland. This is not that. The July Melting event was interesting and I’ll discuss it below, but the news story breaking today is about something else.
The Arctic Sea Ice
Every summer the large areas of the Arctic Sea’s ice melts away, then it refreezes each winter. The minimum extent of ice is typically reached in about mid September. The extent of ice at this minimum has been getting steadily less over time. Direct and accurate measurements don’t go back far enough to track the effects of AGW over the entire time this has been happening, but we can pretty easily look at the last few decades. Have a look at this graph:
Note that the total amount of sea ice in an average year in the 2000’s decade is about one third less than the total amount of ice in the 1980s, at the minimum period in September. Below the 2000’s line are plotted the three most ice-free years in the dataset; those are the extreme years. The present year, 2012, is tracked through mid-August on this graph.
The present year, 2012, is on track for breaking all records for the Arctic Sea ice minimum.
Here’s an interesting side story. Notice how the red line for 2012 is much straighter in a downward direction than the other lines for the same time period for the last several days of measurement. My understanding is that large storms in the Arctic appeared, covering the sea ice from observation for several days, and when the storms cleared a whole bunch of the ice was gone. This is not that unusual. Storms hasten the disappearance of sea ice. But this was a more dramatic than typical example of this event.
In case you were wondering if it was storminess and not AGW that is causing this year’s ice to be less than the other years, and I’m sure that climate change deniers will make this claim, keep in mind that a) this recent storminess does not explain more than a small amount of the ice reduction compared to overall melting and b) AGW has caused there to be more storminess in the Arctic and more warmth in the Arctic.
The Greenland Melt-off of July
Before getting to the really big news from Greenland, I want to first remind you of an interesting event that happened in July, reported by NASA (I mentioned it here). Every summer, some of the ice melts on the surface of Greenland’s massive glacier, then much of that refreezes. However, the melting is usually spotty…here and there and rarely everywhere. The highlands are too cold to melt at all in some years. But July was very warm and there were a few days when virtually the entire surface of greenland melted. There were slushy puddles everywhere. Then, much of it refroze. This happens now and then. We can assume that widespread melting like this is more common in a warmer world, and will be part of the process of glacial wasting as the Greenland Ice Sheet turns over time into seawater. But this particular event was in and of itself not entirely unheard of.
But there is a neat graph that shows why glacial melting is both more important than one might thing and also more complicated than one might think. Have a look:
Just as sea ice extent in the Arctic reduces every summer, the albedo of the Greenland Ice sheet reduces. The white fresh frozen snow that falls over the winter is highly reflective…has a high albedo…and as it melts and gets slushy and mixes with water is has lower albedo. Plain water has very low albedo compared to snow. This is important because high albedo surfaces reflect a lot of sunlight (which provides heat) back into space while low albedo surfaces absorb sunlight, converting it to heat that adds to the local and ultimately global temperature.
There is a feedback mechanism at work here. Imagine that something happens to make for a late Canadian winter with a widespread heavy snow storms much later than usual. This could be caused by a combination of events happening just right in a given year. So, late in the spring there is a lot more snow cover than normal. This snow will reflect a lot of sunlight away so the beginning of the summer is cooler. If this effect lasts into fall, early snows may cause the subsequent winter to be even colder and snow to fall instead of rain, producing more Albedo. Etc. Conversely, something that reduces albedo in a given year may cause more melting of snow and ice, which means less albedo, and thus more melting. You get the picture.
In the days before we understood Orbital Geometry and before we had a very good ida of how air and sea currents really work, this feedback effect with albedo was considered as a candidate for what causes glacial periods to come on and go away. We now know that albedo related forcing and feedback is not the prime mover in climate change, but it is still an effect.
In the graph above, you see the line for the present year. Note that Greenland albedo is lower for the entire year than for any of the other years plotted. Then, in July, that melting event occurs. Then, the water freezes. Albedo is not like sea ice extent (compare to the two graphs). Sea ice extent is a slowly changing ponderous slow moving variable, while Albedo is al wiggly-wobbly and highly variable. A big snow storm, albedo goes up. A big rain storm, albedo goes down. So, the wobbliness of the line does not mean too much, but it is very cool to see the direct relationship between observed widespread melting and albedo. And, this effect will probably play a role as the Greenland Ice Sheet melts away.
And now, for the big news
The really big new that is coming out today is about greenland. From a press release covering the findings of Mardo Tedesco, professor of Earth and atmospheric sciences at The City College of New York:
Melting over the Greenland ice sheet shattered the seasonal record on August 8 – a full four weeks before the close of the melting season…
This year, cumulative melting in the first week in August had already exceeded the record of 2010, taken over a full season
“With more yet to come in August, this year’s overall melting will fall way above the old records. That’s a goliath year – the greatest melt since satellite recording began in 1979.” …
This spells a change for the face of southern Greenland, he added, with the ice sheet thinning at its edges and lakes on top of glaciers proliferating.
Professor Tedesco noted that these changes jibe with what most of the models predict – the difference is how quickly this seems to be happening.
To quantify the changes, he calculated the duration and extent of melting throughout the season across the whole ice sheet, using data collected by microwave satellite sensors.
This ‘cumulative melting index’ can be seen as a measure of the ‘strength’ of the melting season: the higher the index, the more melting has occurred.
This year, Greenland experienced extreme melting in nearly every region – the west, northwest and northeast of the continent – but especially at high elevations. In most years, the ice and snow at high elevations in southern Greenland melt for a few days at most. This year it has already gone on for two months.
Here’s the graph showing the relative amount of ice melt per year in Greenland for the last few decades.
Stay tuned. And buy knickers.
Professor Tedesco’s Web Site is here.
Source Material (other than the press release):