There’s a few words and phrases that become commonplace in both sides of the global warming debate. On the pro-Anthropogenic Global Warming side, we often hear of “positive feedback.” The general idea to the feedback argument is that when something bad happens, e.g. melting of ice, it initiates some other process that then melts more ice. Or, it is used in the overall global warming argument as well, because if things get warm, water vapor evaporates, which further acts as a greenhouse gas, making things warmer, and evaporating more water.
It all sounds sensible, but despite the complexity of many of these feedback models and theories, the models just have not done a good job of anticipating these kinds of things (some will tell you different, but making a model fit past data is a far cry from anticipating future occurrences). From my laymen’s perspective, the real answer is clear, even if one doesn’t fully understand all the details. The real answer is that our climate is so incredibly complex (and may I add, well-designed), that there are a plethora of series’ of feedbacks, both positive and negative, that generally take place in response to something happening in nature. In the end, the new equilibrium state is not all that different from the previous one, though there are natural forcings that can present both short-term and long-term trends.
So, what does this have to do with Greenland? Well, we’ve all heard about how the ice is melting in Greenland, and how we’re all underwater if the whole darn thing falls into the ocean. The concern has been driven by this idea of feedback.
You see, in the summer, it warms up. Even in Greenland. As water melts on these glaciers, it finds channels in the ice and falls through. The more water there is, the more pressure there is being thrust down these channels. The water plunges through and starts to lift the glacier a bit, allowing water to actually seep underneath. This “lubrication” speeds up the movement of the glacier towards the ocean. Scientists have thought that, due to different feedback, the whole process would encourage more water seeping from the mountains, more water plunging, more lubrication, and pretty soon we have a glacier racing into the ocean causing death and devastation to us all.
The theory makes sense. But there’s one major problem with it: it hasn’t happened. In fact, the rate of the movement of glaciers in Greenland has actually slowed about 10% in the last 17 years.
Many fear a positive feedback loop, whereby the accelerating flow will bring more ice down out of the mountains and toward warmer temperatures near sea level. Roderik Van De Waal and colleagues at Utrecht University in the Netherlands now say there is no evidence this will happen.
They looked at how meltwater has correlated with the speed of ice flow at the western edge of the sheet, just north of the Arctic Circle, since 1991. They found that meltwater pouring down holes in the ice – called “moulins” – did indeed cause ice velocities to skyrocket, from their typical 100m per year to up to 400m per year, within days or weeks.
But the acceleration was short-lived, and ice velocities usually returned to normal within a week after the waters began draining. Over the course of the 17 years, the flow of the ice sheet actually decreased slightly, in some parts by as much as 10%.
“For some time, glaciologists believed that more meltwater equaled higher ice speeds,” Van de Waal says. “This would be kind of disastrous, but apparently it is not happening.”
Van de Waal believes that the channels that carry the meltwater out to sea freeze up during the winter months. In summer, pulses of water rushing down the moulins to the bedrock overwhelm the narrowed channels, and the increased pressure lifts the ice sheet off the rock, enabling it to move faster.
However, after a few days the channels are forced open by the water, and it drains away from the glacier. As a result, the ice grinds back down against the bedrock and the lubricant effect is lost.
Van De Waal says this indicates that, overall, meltwater has a negligible effect on the rate at which the ice sheet moves.
Basically, the point is that in all this worry about positive feedback, they seemed to overlook what should have been a relatively expected effect of warmer water running through holes in the ice: that the holes would get bigger, thus counteracting the effect of the water-jets lubricating Mr. Glacier. It seems fairly obvious now, but it’s just another example of how one simple balancing act of nature can play havoc with those feedback loops.
In the meantime, just keep this in mind when hearing about all these feedbacks that drive the models. The actual data trends do not show what we’ve been told we will see. Obviously, the future us unknown, and I suppose the models could be right. Don’t count on it. But at what point will they admit that things just aren’t as extreme as they once seemed?