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Melt lakes and calibrating the color blue

by Guest Blogger

August 19, 2005

Out on the Greenland ice sheet, the first melt lake we chose for

measuring is about 4,600 feet (1,400m) wide, and drains via a shallow

channel ending in a waterfall down a deep hole a bit bigger than John

and Jason’s small gray dinghy. The ice sheet is more than half a mile

(1000m) thick here, and the water probably goes all the way to the base

of it. Anything going in that hole is on a one-way trip, which

explained the enthusiasm that John and Jason put into their paddling.

Despite their efforts, though, the wind and current took them closer and

closer to the mouth of the fast moving outlet stream. Eventually, John

gave up. He stepped out of the boat into the ankle high water, and

dragged it back to shore. This was not going to be a day for dry feet.

John and Jason relaunched the boat farther from the mouth of the

draining stream, and where the slope is a bit deeper, started its four

horse power motor and got on with their job.

What you might not know about glaciers and ice sheets

This is the thing that a lot of people don’t know about ice sheets and

glaciers: the ice actually moves. Where I was standing yesterday, at

the melt lake, the ice is flowing towards the coast at about a foot

(30cm) per day. If I was very very patient, and long lived, I could

have stood in the same spot on the ice and in only a few thousand years

I’d have reached the sea. That is, of course, assuming the ice sheet

and outlet glaciers still exist by then – a prospect that global warming

is making increasingly unlikely.

It’s long been known that melt water production (surface melting)

coincides with valley glaciers speeding up. As far as anyone knows, the

melt water from the surface finds it way down to where it lubricates the

base of the glaciers – causing the ice to slide faster downhill. In

2002, research was published showing that this same acceleration occurs

inland with the ice sheet itself.

The ice here tends to eventually make its way out to sea, but is also

replenished by winter snow. The snow builds up, compacting into hard

ice. Ironically, heating the atmosphere is both increasing melting, and

causing more snowfall (because warmer air holds more moisture). On the

balance though, it is clear that the ice sheet is losing mass overall.

Satellite images show that it is disintegrating at its edges. And

Jason’s research has show that surface melt water production has

increased by 30 percent over the last 17 years.

How we got here

One of the big questions remaining is how much all of this additional

melt water will speed up the ice draining out to sea. For this, more

needs to be known about surface melt water production, and melt lakes.

Under the ice sheet are ridges, valleys and mountains. These

topographical contours are mirrored by the ice above (only smoothed out

a bit). The melt lakes that form in these depressions cover the

spectrum of blue – from light blue at shallow edges to dark blue chasms

– and actually stay in place as the ice flows past and under than.

Leigh, one of the University of Maine scientists on board for the first

half of our Greenland tour, emailed us with the idea that if we could

measure the depth of a melt lake, maybe the color of the water could be

matched with its depth.

Jason, who has spent much of his career studying melt lakes, took to the

idea. Working with the crew, he developed how to implement it. Our

first plan was to dangle a depth finder from the helicopter, but that

proved impractical for the precision and number of measurements needed.

So we rented an inflatable dinghy small enough to be deflated and

carried in the helicopter.

On site, the little boat was inflated, and survival gear stashed on

shore. While John drove the dinghy, Jason matched positions from a

handheld GPS to measurements taken with the depth sounder. To do one

lake this way took all day, but they were able to get over 300

measurements.

At the end of the day, they deflated the boat and stashed it in a small

ice cave. This morning, Jason went back out with Hettie, our second

mate. Eric (deckhand/explorer) also went along to provide logistics

support (specifically – he inflated the boat and helped get the gear

positioned for the second lake).

After a second hard day of work, Jason feels he has enough data to

create a rough model translating melt lake color into depth. Using this

model with satellite images he hopes to estimate the amount of melt

water contained in lakes on the entire ice sheet. This new

observational methodology will help him, and other glaciologists,

further their research into how surface melt water production is

changing over time, and what the implications are for the fate of

Greenland’s ice sheet.

We’ll keep doing our part by bringing you news from the frontlines of

climate change, but we need you to join us in action. Everyone needs to

pitch in, but if you’re in the U.S. (the world’s biggest global warming

polluter per capita) your help is especially needed.

– Andrew

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