With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (16 page)

The net warming effect of man-made pollutants is about i.8 watts per io.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 longterm 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 i°F-representing about o.8 watts per io.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 tempera tures. 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 3o 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 9o 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 io.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 18°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 io.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 io.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.

18

CLOUDS FROM BOTH SIDES

Uncovering flaws in the climate models

The graph flashed up on the screen for only a few seconds, but it set alarm bells ringing. Had I read it right? The occasion was a workshop on climate change at the Hadley Centre for Climate Prediction, held in Exeter in mid2004. The room was packed with climate modelers from around the world. Even they raised a collective eyebrow when the graph sank in. If carbon dioxide in the atmosphere doubled from its pre-industrial levels, the graph suggested, global warming would rise far above the widely accepted prediction of 2.7 to 8.1'F. The real warming could be i8'F or even higher. Surely some mistake? Too much wine at lunch? No. This was for real.

Till now, climate modelers have graphed the likely effect of doubling carbon dioxide levels using what is known in the trade as a bell graph: the best estimate-about 5"-falls in the middle, and probabilities fall symmetrically on either side. So the chance that the real warming will be 8. 1°, for instance, is the same as that it will be 2.7'. But the graph of likely warming that James Murphy, of the Hadley Centre, was displaying on an overhead screen that morning looked very different. The middle point of the prediction was much the same as everybody else's. But rather than being bell-shaped, the graph was highly skewed, with a long "tail" at the top end of the temperature range. It showed a very real chance that warming from a doubling of carbon dioxide would reach io, 14, i8, or even 2 i°F.

Carbon dioxide is widely expected to reach double its pre-industrial levels within a century if we carry on burning coal and oil in what economists call a business-as-usual scenario. But nobody has seriously tried to work out what i8 degrees of extra warming would mean for the planet or for human civilization. It would certainly be cataclysmic.

Let's be clear. Murphy was not making a firm prediction of climatic Armageddon. But neither was this a Hollywood movie. The high temperatures on the display, he said, "may not be the most likely, but they cannot be discounted." Nor was Murphy alone with his tail. The meeting also saw a projection by David Stainforth, of Oxford University, that suggested a plausible warming of 2i°F. Six months later, this new generation of scarily skewed distributions started turning up in the scientific journals. Unless the editors take fright, these figures will probably become part of the official wisdom, incorporated into the next report of the IPCC.

So what is going on? For one thing, modelers have for the first time been systematically checking their models for the full range of uncertainty about the sensitivity of the climate system to feedbacks that might be triggered by greenhouse gases. Assessing those efforts for the IPCC was the main task of the Exeter meeting. And what has emerged very strongly is that clouds, which have always been seen as one of the weakest links in the models, are even more of a wild card than anyone had imagined. The old presumption that clouds will not change very much as the world warms is being turned on its head. There may be more clouds. Or fewer. And their climatic impact could alter. It is far from clear whether more clouds would damp down the greenhouse effect, as previously thought, or intensify it. Being mostly of an age to remember 197os Joni Mitchell songs, the climate scientists in Exeter mused over coffee that they had "looked at clouds from both sides now." And they didn't like what they saw.

An assessment of the sensitivity of global temperatures to outside forcing -whether changes in sunlight or the addition of greenhouse gases-has been central to climate modeling ever since Svante Arrhenius began his calculations back in the r89os. This assessment mostly revolves around disentangling the main feedbacks.

The three biggest feedbacks in the climate models are ice, water vapor, and clouds. We have already looked at the effect of melting ice on the planet's albedo. It explains why the Arctic is warming faster than elsewhere and giving an extra push to global warming. Water vapor, like carbon dioxide, is a potent greenhouse gas, without which our planet would freeze. The story of what will happen to water vapor is a little less clear cut. A warmer world will certainly evaporate more water from soils and oceans, and this process is already increasing the amount of water vapor in the atmosphere, amplifying warming. In the standard climate models, extra water vapor in the air at least doubles the direct warming effect of carbon dioxide. But it's when we come to clouds that the calculations get sticky.

A lot of water vapor in the air eventually forms clouds. At first guess, you might say that clouds would have the opposite effect of water vapor, shading us from the sun's rays and keeping air temperatures down. They do that on a summer's day, of course. But at night they generally keep us warm, acting like a blanket that traps heat. Globally, these two effectsor, rather, their absence-are most pronounced in deserts. Where there are no clouds, the days are boiling, but the nights can get extremely cold, even in the tropics.

The temperature effects of clouds turn out also to depend on the nature of the clouds. Their height, depth, color, and density can be vital, because different clouds have different optical properties. The wispy cirrus clouds that form in the upper atmosphere heat the air beneath, because they are good at absorbing the sun's rays and re-radiating the heat downward, whereas the low, flat stratus clouds of a dreary summer's day are good at keeping the air below cool.

Researchers still know surprisingly little about how many and what sort of clouds are above our heads. For instance, it has only recently emerged that there may be many more cirrus clouds than anyone had thought. Many are almost invisible to the naked eye, but nonetheless seem to be highly effective at trapping heat. Some studies suggest that, taken globally, the cooling and warming effects of clouds currently largely cancel each other out, with perhaps a slight overall cooling effect. But nobody is sure. And even small changes in cloudiness could affect planetary albedo substantially. If a warmer world tipped clouds into causing greater warming, the effects could be considerable.

So what is the prognosis? Again, a first guess is that extra evaporation will make more clouds, because a lot of the water vapor will eventually turn into cloud droplets. But even that may not be so simple. Evaporation doesn't just lift water vapor into the air to create more clouds; it also burns off clouds, leaving behind blue skies. And greater evaporation can also make clouds form faster, so that they fill with moisture faster, make raindrops faster, and dissipate faster. So, in a greenhouse world, fluffy cumulus clouds that we are used to seeing scudding across the sky all day could instead boil up into dark cumulonimbus clouds and rain out, leaving behind more blue skies.

Bruce Wielicki has been trying to figure out the answer to such questions during more than twenty years of cloud-watching at NASA's Langley Research Center, in Hampton, Virginia. He says that satellite data suggest that clouds probably still have an overall cooling effect on the planet; but, especially in the tropics, there is a trend toward clearer skies. Since the mid- r 98os, the great tropical convection processes that cause air to rise where the sun is at its fiercest have intensified. As a result, storm clouds are forming and growing more quickly in those areas. This may be increasing the intensity of hurricanes across the tropics. Less obvious is Wielicki's discovery that the storm clouds not only form more quickly but also rain out more quickly. That leaves the tropics drier and less cloudy as a whole.