The World in 2050_ Four Forces Shaping Civilization's Northern Future - Laurence C. Smith [121]
Frankly, we don’t understand the physics of sliding glaciers and ice sheet collapses well enough yet to model the futures of Greenland and Antarctica with confidence. Many things affect the speed and dynamics of that long slide that are hard to measure or see. They include the interplay between the sliding ice and its bed, the heat and lubrication added by meltwater percolating to the bed from the surface, the importance of buttressing ice shelves (which help dam ice up on the land), the ocean water temperature at the ice edge, and others.507 Computer models and field studies—like the one Jason and I were conducting in Greenland—are in their infancy. Scientists are still discovering new things and debating what may or may not be important. This is why the likelihood of accelerated sea-level rise was kept out of the last IPCC assessment, and may be kept out of the next one as well. Might the ice sheets start slipping faster, with higher sea levels right behind? Perhaps—but without well-constrained models, we don’t yet know how likely that is.
Genie in the Ground
Digging into a permafrost landscape usually goes something like this: After cutting through a thick living mat of vegetation, the spade turns over a dark, organic-rich soil, almost like the mulch that one buys to spread in a garden. Usually there are bits and pieces of old dead plants poking out of it. Then, anywhere from several to tens of inches down, the blade goes chunk and will bite no farther. But it’s not a stone. At the bottom of the hole, there is just more of the same organic-rich goop but it is frozen hard as cement, often with a little black ice peeking through. Going any deeper is a major job, requiring a big drill and lots of manpower.
Why on Earth would anybody go all the way to the Arctic to drill holes into frozen black muck? The reason is organic carbon, and we now know that frozen northern soils hold more of it than any other landscape on Earth. In fact, the more we study these soils the more carbon we find. As of 2010 the latest estimate is 1,672 billion tons (gigatons) of pure organic carbon frozen in the ground.508 That’s roughly half of the world’s total soil carbon crammed into just 12% of its land area.
The reason there’s so much carbon there is because this is a place too cold and damp for living things to fully rot away when they die. Live plants draw down fresh carbon from the atmosphere and store it in their tissues. When they die, decomposing microbes chow down, pumping the carbon back to the atmosphere in the form of carbon dioxide (CO2) or methane (CH4) greenhouse gases. But while plants and trees can still grow in cold places, even on top of permafrost, the microbes are hard pressed to finish off their remains because their metabolisms are strongly temperature-dependent (just as stored food decomposes more slowly in a refrigerator than at room temperature). Very often a mulch-like layer of peat will accumulate, building up the ground elevation over time as successive generations of plants root into the semirotted remains of their ancestors. Some decomposition continues underground, but once permafrost sets in, even that halts, and the stuff becomes cryogenically preserved. Since the end of the last ice age, this excess of plant production over plant decomposition has slowly accumulated one of the biggest stockpiles of organic carbon on Earth.
To put that earlier 1,672 gigatons (Gt) of carbon estimate into greater perspective, all of the world’s living