Planet Earth's energy imbalance
Jim Hansen knows about the atmosphere from top to bottom. He began his career as an atmospheric physicist, studying under James van Allen, after whom the Van Allen Belts of the upper atmosphere are named. He published papers on the Venusian atmosphere before he moved on to our own. So when Hansen stops talking about degrees of temperature and starts counting how many watts of energy reach Earth's atmosphere and how many leave it, I recognize that we are getting down to the nitty-gritty of what sets Earth's thermostat.
I know about watts. I have a 6o-watt bulb in the lamp over my desk. At school almost forty years ago, my physics teacher had a stock line for any lesson on electricity. "It's the watts what kill," he said, meaning that they are what matters. When Hansen says the sunlight reaching the surface of Earth in recent centuries has been about 240 watts for every 10.8 square feet, I can visualize that. It is four 60-watt bulbs shining on a surface area the size of my desk. That figure ever changes only slightly, because the sun itself is largely unchanging. If the sun were to grow stronger, more radiation would reach Earth, and we would warm up. But only so much. A warmed surface also releases more energy, so eventually a new equilibrium would be reached. Similarly, as additional greenhouse gases trap more solar energy, Earth warms until a new equilibrium is reached, with as much energy leaving as arriving. Put another way, Earth's temperature is whatever is required to send back into space the same amount of energy that the planet absorbs.
So what is happening today? Thanks to our addition of greenhouse gases to the atmosphere, the planet is suffering what Hansen calls "a large and growing energy imbalance" that "has no known precedent." The planet is warming, but it has not yet reached a new equilibrium.
The net warming effect of man-made pollutants is about 1.8 watts per 10.8 square feet. Most of this goes into heating either the lower atmosphere or the oceans. Ocean surfaces and the atmosphere share heat fairly freely, constantly exchanging energy. Because the oceans have a greater heat capacity than the atmosphere, they take the lion's share of the extra energy. But there are time lags in this exchange system. It takes some time to heat the oceans to their full depth. The warming of recent decades has created a pulse of heat that so far has gone as deep as 2,500 feet into the oceans in some places. As this pulse progresses, the oceans are draining more heat out of the atmosphere than they will once they return to a long- term balance with the atmosphere. It is rather like using a central heating system to warm a house. We have to heat all the water in all the radiators before the full effect of heating air in the house is felt. Likewise, the full impact of global warming will be felt in Earth's atmosphere only after the oceans have been warmed.
The best guess is that about 1°F—representing about 0.8 watts per 10.8 square feet—is currently lopped off the temperature of the atmosphere by the task of warming the oceans. That is warming "in the pipeline," says Hansen. Whenever we manage to stabilize greenhouse gases in the atmosphere, there will still be that extra degree to come. Half of it, Hansen reckons, will happen within thirty to forty years of stabilization, and the rest over subsequent decades or perhaps centuries.
While most of the extra heat being trapped by greenhouse gases is currently going into heating the oceans and the atmosphere, there is a third outlet: the energy required to melt ice. At present, no more than 2 percent is involved in this task. But Hansen believes that percentage is likely to rise substantially. Recent surging glaciers and disintegrating ice shelves in Greenland and Antarctica suggest that it may already be increasing. Melting could in future become "explosively rapid," Hansen says, especially as icebergs begin to crash into the oceans in ever-greater numbers.
There would be a short-term trade-off. Extra energy going into melting would raise sea levels faster but leave less energy for raising temperatures. But in the longer term, that would be of no help. For as more ice melts, it will expose ocean water, tundra, or forest. Those darker surfaces will be able to absorb more solar energy than the ice they replace. So we may get accelerated melting and more warming.
The critical term here is "albedo," the measure of the reflectivity of the planet's surface. Anything that changes Earth's albedo—whether melting ice or more clouds or pollution itself—will affect Earth's ability to hold on to solar energy just as surely as will changes in greenhouse gases. On average, the planet's albedo is 30 percent—which means that 30 percent of the sunlight reaching the surface is reflected back into space, and 70 percent is absorbed. But that is just an average. In the Arctic, the albedo can rise above 90 percent, while over cloudless oceans, it can be less than 20 percent.
During the last ice age, when ice sheets covered a third of the Northern Hemisphere, the vast expanses of white were enough to increase the planet's albedo from 30 to 33 percent. And that was enough to reduce solar heating of Earth's surface by an average of 4 watts per 10.8 square feet. It was responsible for two thirds of the cooling that created the glaciation itself. And just as more ice raised Earth's albedo and cooled the planet back then, so less ice will lower its albedo and warm the planet today.
According to the albedo expert Veerabhadran Ramanathan, of the Scripps Institution of Oceanography, if the planet's albedo dropped by just a tenth from today's level, to 27 percent, the effect would be comparable to a fivefold increase in atmospheric concentrations of carbon dioxide." To underline the importance of the issue, Ramanathan is organizing a Global Albedo Project to probe the albedo of the planet's clouds and aerosols. Lightweight robotic aircraft began flying from the Maldives, in the Indian Ocean, in early 2006. The project could prove as important as Charles Keeling's measurements of carbon dioxide in the air.
The prognosis for albedo cannot be good. We have already seen how the exposure of oceans in the Arctic is triggering runaway local warming and ice loss that can only amplify global warming. The same is also happening on land. Right around the Arctic, spring is coming earlier. And such is the power of the warming feedbacks that it is coming with ever-greater speed. As lakes crack open, rivers reawaken, and the ice and snow disappear, the landscape is suddenly able to trap heat. The "cold trap" of reflective white ice is sprung, and temperatures can rise by i8°F in a single day. No sooner have the snowsuits come off than travelers are sweltering in shirtsleeves.
Stuart Chapin, of the Institute of Arctic Biology, in Fairbanks, says that the extra ice-free days of a typical Alaskan summer have so far been enough to add 3 watts per 10.8 square feet to the average annual warming there. As a result, he says, the Arctic is already absorbing three times as much extra heat as most of the rest of the planet. And there are other positive feedbacks at work in the Arctic tundra. In many places, trees and shrubs are advancing north, taking advantage of warmer air and less icy soils. Trees are darker than tundra plants. And because snow usually falls swiftly off their branches, they provide a dark surface to the sun earlier than does the treeless tundra. Chapin estimates that where trees replace tundra, they absorb and transfer to the atmosphere about an extra 5 watts per 10.8 square feet.
This creates a surprising problem for policymakers trying to combat climate change. Under the Kyoto Protocol, there are incentives for countries to plant trees to soak up carbon dioxide from the atmosphere. They can earn "carbon credits" equivalent to the carbon taken up as the trees grow, and use these credits to offset their emissions from power stations, car exhausts, and the like. The idea is to promote cost-effective ways to remove greenhouse gases from the atmosphere—the presumption being that that will cool the planet. But in Arctic regions, the effect will usually be the reverse, because although new trees will indeed absorb carbon dioxide, they will also warm the planet by absorbing more solar radiation than the tundra they replace.
Clearly there is a balance between cooling and warming. But Richard Betts, of Britain's Hadley Centre, says that in most places in the Arctic, the warming will win. In northern Canada, he estimates, the warming effect of a darker landscape will be more than twice the cooling effect from the absorption of carbon dioxide. And in the frozen wastes of eastern Siberia, where trees grow even more slowly, the warming effect will be five times as great. Every tree planted will hasten the spring, hasten the Arctic thaw, and hasten global warming.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.