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luni, 16 iulie 2012

Following the Ice: Is this Global Warming?

From the mess tent, we can hear huge boulders crashing through rapids half a kilometer away. The boulders sometimes sound like approaching footsteps, and as we’re all just a tiny bit nervous about an unlikely polar bear visit, conversations trail off and we listen.

In the four years our camp has existed on this glacial river, more meltwater is spilling out from beneath Leverett Glacier than we’ve ever seen. What’s more, the river has spilled over its banks and is now eroding a glacial moraine near our camp that was likely pushed there in the 1700’s during the Little Ice Age. It’s only June and the river is still rising.

The Greenland Ice Sheet is the most impressive thing I’ve ever seen. Looking out over its seemingly endless expanse of white, grey, and black textures of crevasses and rolling hills of ice, one feels close to infinity. On my last trail run, I ran to the top of a small mountain surrounded on three sides by the ice sheet. I was wearing running shorts and a tee shirt; the sun was bright and a steady wind coming off the ice kept the mosquitoes away. I sat down on a slab of granitic gneiss and leaned against a warm boulder. The wind was surprisingly balmy and humid, despite having just crossed the Greenland Ice Sheet. I closed my eyes and soaked in the heat and sun. Later that day I reformatted my air temperature graphs from last year’s season to fit the data collected this June. The y-axis had to be expanded by 10 degrees.

Can we say this year’s warm weather is because of global warming? It’s not for certain, and it’s important not to ascribe one especially warm season or year to global warming. It’s probably more important not to write off global warming as a hoax when it snows in Washington, D.C. This is particularly true in the high latitudes (the Arctic and Antarctic) where temperature variability between years is higher than anywhere else. In the tropics, there is very little variation between years (or even seasons) compared to the much larger variations nearer the poles. However, while average temperatures have climbed in both the tropics and the high latitudes, the arctic and Antarctic have warmed significantly more. In general, the closer to the poles, the greater the increase in average temperature but also the greater the variation between years. Last year we had snow in the middle of June and only a handful of days with bad mosquitoes. This year in early June, it was often unbearably hot in our tents by 5:00 a.m. while outside, millions of mosquitoes swarmed around our tent doors, waiting for us to give into the heat and step outside.

Global warming refers to increases in average temperature over the entire globe over a period of decades, not in one location or over one year. I don’t think this point can be made often enough. While we cannot say one warm season is the result of global warming, I think the fact that our ice-melt-fed river is spilling out of its banks and eroding things that have clearly been there for centuries seems worth noting.

Finally, although scientists have been trying to beat the public over the head with this concept for at least two decades, it’s probably worth pointing out again: There is widespread, broad consensus among scientists, not just about global warming, but also about what’s causing it: burning fossil fuels.

I wouldn’t be surprised if someone reading this decides to disagree in the comments. There is a very vocal and surprisingly incensed group of people who think the climate is not changing or that it’s all just natural variation. All I can say is that the so-called climate change deniers have views completely at odds with what the mainstream scientific community agrees on. Rest assured, the global warming controversy exists outside of the scientific community.

When I’m not collecting samples and keeping equipment running in camp, I’m cramming for my qualifying exams for my chemical oceanography Ph.D. in the MIT/WHOI Joint Program. Much of our exam focuses on the carbon cycle and CO2. Because the ocean is the largest sink for CO2 produced from burning fossil fuels (on time scales that affect people), a large part of my exams are focused on tracking fluxes of CO2 in the ocean and then determining its fate (What happens to CO2 once in the ocean and how long will it stay there?). For many years the faculty at MIT and Woods Hole have written qualifying exams that focus on making sure we understand (in excruciating detail I might add) the central role the carbon cycle plays in marine chemistry and Earth’s climate. Because most of us who come out of the program will spend our careers as scientists, the exam is at least partly designed to ensure we are prepared to research and understand the profound way people are changing our planet by adding CO2.

Climate Change Feedback Loops

Doing some “back of the envelope calculations,” we’ve estimated that the river discharging from the glacier is carrying about 400,000 metric tons of sediment per day to the ocean (Ten grams of sediment/liter with a discharge rate of 470 cubic meters/second: feel free to stop reading and try the math!).

That’s a lot, but what’s really interesting about glacial meltwater is that this sediment load is composed of extremely fine-grained, freshly broken rock material that is primed to consume CO2 through chemical weathering reactions. Although rock weathering is thought to ultimately control Earth’s CO2 budget on time scales of millions of years, photosynthesis—something probably much more familiar to everyone—is the most important sink for CO2 on year-to-year time scales.

Surprisingly, glaciers may also play an important role in driving photosynthesis in the world’s oceans. Glacial meltwater, icebergs, and wind-blown dust from glaciated landscapes can all carry essential nutrients (especially iron) to nutrient-starved regions of the ocean (Raiswell et al., 2006). These glacial nutrient sources can fertilize the ocean, stimulate photosynthesis and ultimately cause the consumption of atmospheric CO2. If you are scratching your head thinking, “So more glaciers can lead to more CO2 taken out of the atmosphere which would cool the planet, grow glaciers cause more icebergs, more CO2 consumed, more cooling,” then you’re on the right track and have identified something called a feedback loop.

Scientists are wondering if, during the ice ages, glacial-fed phytoplankton blooms in the Southern Ocean (the ocean surrounding Antarctica) helped keep CO2 levels at a minimum, which helped keep the world in a freezer. Basically, the glaciers themselves may have stimulated photosynthesis and kept global CO2 levels low, which then grew more glaciers and so on, until some external force threw off the balance. This external force would’ve been episodic changes in the amount of sunlight reaching Earth’s surface (called the Milankovitch Cycles). Once the timing and intensity of the sun’s heat changes (or humans release too much heat trapping CO2), another feedback loop can take over.

Consider permafrost. Permafrost is a thick layer of subsurface soil in polar regions that remains frozen year-round. While you may have heard of permafrost you may not have realized that permafrost contains vast quantities of carbon that was frozen in place thousands of years ago. As the planet warms, and permafrost melts, this ancient carbon becomes food for microbes that transform that carbon into CO2 gas and methane (CH4), which warms the planet, melts more permafrost, producing more CO2 and CH4 (see Frey and Smith, 2005). Ice cores taken from the Antarctic Ice Sheet give us a datable record of the atmospheric CO2 concentrations through time. These records tell us that ice ages are often ended by sharp, fast upswings in CO2. This upswing could very well be the release of CO2 from melting permafrost.

For many scientists, the questions are more immediate. How are glaciers affecting climate today? Will the world’s shrinking glaciers produce more or less icebergs, meltwater, and windblown dust and will this help or hinder photosynthesis in the ocean? One interesting hypothesis is that as Antarctica’s ice sheet collapses, a temporary increase in icebergs in the Southern Ocean could help fertilize marine phytoplankton and slow global warming as the blooms consume CO2 through photosynthesis. Or not. For now, I think the jury is still out regarding the role the world’s ice will play in the changing climate.

There are many of CO2 feedback loops, and glaciers and permafrost are just part of a few of them. Serious accounting skills are required to understand the delicate balance between all of the world’s CO2 feedback loops, as well as all the sources and sinks for CO2. All we know for sure is that much like the natural variations in the sun’s intensity, burning fossil fuels is heating the planet and could set off a CO2 feedback loop with unforeseen consequences.

For what it’s worth, over the last four years in our camp in Greenland, all the nearby ponds have rapidly shrunk and we suspect melting permafrost beneath the water is causing them to drain (think of their bottoms falling out). Meanwhile, as noted in the above post, we may be on our way to a record-breaking melt year that may be part of a larger trend.


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