snow cover on land and sea-ice cover on the ocean will expand, especially at frigid latitudes near and poleward of the Arctic Circle. The greater expanses of these bright white surfaces will reflect much more incoming solar radiation than the dark-green land surfaces and deep-blue ocean they replace. Land and water are heated by the solar radiation they absorb, but snow and ice reflect almost all incoming radiation back to space. In this way, increased expanses of snow and sea ice in the Arctic amplify and deepen any cooling by a "positive feedback." The same positive-feedback process works when climate warms. Warming causes snow and sea ice to retreat, which exposes more of the darker land and ocean, which absorb more solar radiation, which makes the Arctic warmer by an extra amount beyond the initial warming.
Allowing for this poleward amplification of temperature changes, the 0.8°C global-mean warming caused by preindustrial human inputs of greenhouse gases should have been about 2°C (3.6°F) in Arctic regions (fig. 10.1C). This warming is sizeable, but it has been masked by a somewhat larger natural cooling driven by changes in Earth's orbit. As noted in chapter 4, the intensity of summer solar radiation at high latitudes has decreased by about 8 percent since 11,000 years ago. This reduction in radiation has slowly nudged high-latitude climate toward colder summers, even though human-generated greenhouse gases have been pushing in the opposite direction.
Several natural climatic sensors have registered a slow summer cooling trend in the Arctic over the last several thousand years. One is the slow southward retreat of the northern boundary of spruce forest, with frigid Arctic tundra advancing southward and replacing trees. Another example comes from sediments in the Norwegian Sea, where species of plankton that prefer to live under and around sea ice have become more abundant, indicating more prevalent sea ice in recent millennia. Still another line of evidence is recorded in small Arctic ice caps, which contain layers showing significant melting during summers 10,000 years ago, but few such layers in recent millennia.
The northeastern Canadian Arctic is especially interesting to climate scientists because of its role in the long history of ice-age cycles over the last 2.75 million years. This frigid region was the place where the last remnants of the most recent ice sheet melted (see fig. 4.1). A gigantic dome of ice had extended halfway to the equator until 16,000 years ago but then had gradually retreated to the northeast during the next 10,000 years and ended up as small remnants in northeast Canada by 7,000 to 6,000 years ago. In this region today, small and rapidly melting remains of ice caps still lie perched on high terrain from Baffin Island at 65°N north to Ellesmere Island at 83°N, just about the same range of latitudes as the Greenland ice sheet on the opposite side of the Labrador Sea.
Many scientists think that those many dozens of glacial cycles that have occurred during the last 2.75 million years began in this very region. Milankovitch had suggested that snowfields grow into small ice caps and then large ice sheets when summer solar radiation grows too weak (and summers too cold) to melt the snow. Summers are cold in this area for two reasons: in part because it lies so far north, and in part because the northeast coast of Canada is a slightly elevated block. Given the fact that the snow cover normally melts away for just a couple of months in a typical modern summer, this region is obviously close to a state of glaciation right now. This made me wonder: What would it take to push it over that edge? More specifically: Would this region now be glaciated had it not been for the warming from those greenhouse-gas releases caused by farming?
Climate scientist Larry Williams explored this issue in a climate-modeling analysis of this region in the 1970s. He found that a cooling of about 1.5°C (2.7°F) relative to 1970s' temperature levels should be enough to cause permanent snow cover and glaciers to form on the higher terrain along the edges of the Labrador Sea. In effect, the small ice caps that now exist there would have expanded into somewhat larger masses of ice, although still confined to this far northeastern corner of Canada. His study further suggested that an additional cooling of 1—2°C (1.8-3.6°F) would be enough to cause permanent snowfields to form over a larger area beyond Baffin Island.
This modeling study from almost 30 years ago ties in directly with the greenhouse-gas history shown in figure 10.1. Williams's estimates can be used to peel away the warming effects caused by human inputs of greenhouse gases and predict whether or not large glaciers would now exist. Williams estimated the industrial-era warming in this region over the 100 years prior to his study at 1.5°C. He noted that if this recent warming were removed, the highest terrain of Baffin Island should be brought right to the edge of glaciation. In confirmation of this conclusion, the ice caps that now exist on Baffin Island are thought to have formed in the cooler climate just prior to the industrial-era warming of the last century. Because of the subsequent warming, these ice caps are rapidly melting, and many are likely to disappear in the next few decades.
The second greenhouse-gas warming that can be peeled away is the 2°C preindustrial warming caused by humans and the expansion of early agriculture (fig. 10.1C). With this earlier warming effect removed, Williams's results suggest that a broader area of high terrain in northeast Canada would have been brought to a state of permanent snow cover and thus glaciation.
The implication of these model results, combined with the new evidence for early human generation of greenhouse gases, was startling. Part of northeast Canada might well have been glaciated before the industrial era had not our early greenhouse-gas emissions occurred, and ice would exist in the region today were it not for the combined effects of the preindustrial and industrial-era gas emissions. It seemed that human greenhouse gases have stopped a glaciation!
This startling conclusion triggered another recollection from earlier climate studies. In 1980 geologist John Imbrie, a central figure in confirming the Milankovitch ice-age theory (chapter 4), had enlisted his mathematician son to try to devise a simple method to convert the well-known changes in solar radiation through time into an estimate of the history of global ice volume. The logical basis of this exercise was straightforward. For the last several thousand years, the ice sheets have regularly grown and melted at the three major orbital cycles of
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