Greenland’s ice sheet is not simply a giant ice cube, inert but for gradual erosion from climate change. It’s a dynamic, shifting landscape, a place of delicate balance between the forces that create ice and those that destroy it. Under its own immense weight, ice flows continuously towards the coasts and is lost whenever icebergs break off into the sea, or when the surface is warm enough to melt or sublimate. Ice is constantly replaced as falling snow compacts, or rain and meltwater freeze. These incomes and outgoings add up to an ice budget that changes with altitude (lower down the air is warmer, so there is more melting), the seasons, and long-term climate change. Future sea level depends on whether or not Greenland can balance its books or stays in the red.
But it’s not only the surface and edges of the ice sheet we need to watch: it’s the bottom too. Ice at the base is under immense pressure from the hundreds of metres of ice above it, which allows it to melt at temperatures below zero degrees Celsius. The meltwater flows out to sea, sandwiched between the ice sheet and the rock on which it sits, creating a waterslide that helps the ice flow more quickly than it would under gravity alone.
And meltwater on the surface doesn’t behave as we might expect. We call the meltwater that hasn’t refrozen ‘runoff’, but it doesn’t simply run downhill like water off a duck’s back. Most of it carves paths straight down through the ice, helped by crevasses on the surface. In the height of summer these ‘moulins’ become dangerous chutes of fast flowing water. The water reaches the bottom and adds to the waterslide effect. It’s been known since the 1970s that ice in mountain glaciers and parts of Greenland speeds up during the summer melt season.
Ten years ago, Jay Zwally and others put forward an idea that had profound implications for sea level. They suggested acceleration of the waterslide – ‘enhanced basal lubrication’, a.k.a. the ‘Zwally effect’ – could be an alarming mechanism for rapid sea level rise. As the climate warmed, the melting would increase, and the ice would speed up towards the coast. Ice would then be lost more quickly, not only by creating more icebergs, but also by melting more rapidly at these lower altitudes. The extra meltwater would feed the cycle, resulting in faster and faster ice loss.
We’re the first to test this idea using models of the Greenland ice sheet, comparing the effect on sea level with the usual baseline projections that don’t include the Zwally effect. What we found surprised us – it made very little difference. Even in our worst case scenario, enhanced basal lubrication by surface meltwater added only 8mm to sea level over two hundred years, less than five percent of the baseline sea level rise. In some of our tests, it even reduced it.
Our study has three parts: finding out how ice speed is affected by surface meltwater using observations, predicting climate change and its effect on surface melting using climate models, and predicting how these would affect ice flow and sea level using ice sheet models.
A speed limit for ice
Does ice speed up indefinitely as melting increases, or is there a natural speed limit? We’d like to measure this relationship in many different places and over many years, but unfortunately we only have ice speed observations for a single place (11 sites, along a line perpendicular to the coast in southwestern Greenland) and a few years (4 sites 2006-2010; 7 sites 2009-2010):
And not all of these have observations of runoff. So because we don’t have complete data, and because we want to make predictions with a climate model, we actually quantify how ice speed in the real world relates to runoff in the climate model. GPS receivers are left in the ice all year to measure ice speed. We convert this to “speed-up”: the ratio of the average speed throughout the year to the lower, winter speed. The faster ice moves in the summer relative to winter, the larger the speed-up. The meltwater runoff is simulated with the “MAR” regional climate model (covering only the area over Greenland, so that we could use higher resolution). Here is measured speed-up (vertical axis) plotted against simulated runoff (horizontal axis):
On the left-hand side there is a fairly straightforward relationship: more runoff means more speed-up. But past a certain point things are not so clear. Does the speed-up plateau, or even decrease? And either case, does this actually mean the Zwally effect is self-limiting? Clearly we don’t have enough data to make definitive statements.
So we make the loosest, most flexible statements we can. The graph below – sorry for the typos and watermark – shows the huge range of possibilities we consider. In the worst case scenario (“Max”), speed-up keeps increasing with runoff. But in the best case scenario (“Min”), it declines.
Zwally was concerned about something like the worst case scenario, where more meltwater always means faster ice. But how realistic is the best case scenario? Could the lubrication effect actually decrease with more meltwater?
Recently both observations and theory have indeed supported this kind of decline. It involves a massive change in plumbing. When there is not much runoff, the water forces its way to the coast through an inefficient web of pockets (‘cavities’) linked by narrow channels. This creates high water pressure, lifting the ice from the rock below it and making it slide. When runoff increases past a critical threshold, the extra water carves wide pipes (‘channels’) through the ice which are much more efficient at flushing the water out to sea. The water pressure drops: the waterslide slows down.
The important point about our study is that we do not assume either is more likely. We simply predict both their effects on sea level.
We predict changes in climate and meltwater runoff for the SRES A1B emissions scenario using the same regional climate model as above (MAR), which takes inputs from a global climate model (ECHAM5). The prediction is about a tripling of the amount and area of meltwater runoff between 2000 and 2100. If the Zwally effect were important, we might expect such a large increase in runoff to lead to a large speed-up and therefore a large sea level rise.
Future ice flow and sea level
We use the climate change and runoff, and our relationship between runoff and speed-up, as inputs to four different models of the Greenland ice sheet, to predict changes in ice flow and sea level. (We want to make projections over two hundred years, so we assume runoff is constant from the year 2100 to 2200.) Across the board, the effects are very small.
The baseline sea level rise between 2000 and 2200 ranges from 163mm to 173mm for the four ice sheet models. Enhanced basal lubrication adds just 4-8mm to sea level in the worst case scenario, and between a 1mm rise and a 1mm lowering (relative to the baseline) in the best case scenario. We test this with another global climate model, and another emissions scenario, but the largest effect is always less than five percent.
Why so small? No matter whether we use the best or worst case scenario, enhanced basal lubrication mainly changes the ice speed and distribution, rather than the amount lost to sea.
The ice speeds up inland, which makes it thinner. This could make it more vulnerable to melting, through the lower altitude. But we found the increase in melting was small, and in any case was compensated by the ice thickening at the edge from the faster flow.
In the best case scenario the ice is slower at the edge relative to the baseline, because of the decline in speed-up at higher runoff. This can mean that less ice is lost at the coast, so the sea level contribution is smaller: but the thicker ice at the edge partly, or totally, compensates, depending on the ice sheet model. This is why the best case scenario can either lower or raise sea level relative to the baseline.
In the worst case scenario, the ice is faster everywhere. But the effect on sea level is still small relative to the baseline sea level rise.
This is the first attempt to study whether meltwater lubrication of Greenland’s waterslides is important to future sea level. We find it is not. Instead we should focus on understanding changes in melting and snowfall and (to a lesser extent) icebergs.