2. Global Warming

How hot will it get in the future, and how Fast?

There is almost no doubt that the heat is on, and that the climate is changing. The remaining scientific debates are over why the climate is changing, how fast it will change in the future, and the likely consequences of the change. Sophisticated computer models, known as general circulation models, try to answer these questions by analyzing the climate system and forecasting its future.

To assess the effects of global warming, one assumes that the dominant cause of climate change during the next century will be the release of greenhouse gases into the atmosphere by humans, and that the increasing rate of atmospheric build up of the waste gases will continue unabated. The supercomputer models then evaluate all the factors that push and pull the climate, setting the global thermostat.

The models tell of possible futures when atmospheric carbon dioxide levels are about double the pre-industrial concentrations. Although we are almost one third of the way toward reaching this state, it is going to take another 100 years to get there. And since the climate is very complex, the scientific pronouncements about the future climate vary widely. Some of the climate forecasts for the next century are scary and others are not. No one knows which one is true! And it’s far enough in the future that none of the climate experts will be around to check the reliability of their forecasts.

A prestigious group of international scientists has been working on the problem for years. Known as the Intergovernmental Panel on Climate Change, or IPCC for short, it includes hundreds of climate experts. They evaluate the supercomputer model results and develop a coordinated assessment of future global warming under the auspices of the United Nations, including extensive review by both individual scientists and governments. In their report, entitled Climate Change 2001: Impacts, Adaptation, and Vulnerability, the group concluded that the burning of fossil fuels and other human activities are responsible for most of the rise in global temperatures during the last half of the 20th century.

If present trends in the emission of greenhouse gases continue for 100 years, the group concludes, then resultant human-induced global warming will raise the Earth’s average surface temperature between 1.4 and 5.8 degrees Celsius (2.5 and 10.4 degrees Fahrenheit).

The worst-case scenario predicts a huge temperature increase a century from now, comparable to the temperature rise since the last Ice Age, and there may also be dire consequences if the warming takes a middle course. Even a 2-degree Celsius warming over the next century, near the bottom of the predicted range, will probably be the fastest warming in the history of civilization. As we shall see, some scientists predict apocalyptic consequences if it should occur, but others say these are exaggerated fears.

A vocal minority argues that no computer can adequately simulate the complexities of the real world. The computer models, they say, are flawed by massive uncertainties, making their predictions of future global warming about as reliable as a crystal ball or an ouija board. After all, the long-range forecasts of your local weather station are not all that reliable. Even with daily satellite images of weather patterns, we cannot reliably predict the local weather beyond a few days in advance. In Boston, for example, major snowstorms often fail to occur when predicted, and threatening hurricanes often veer off the expected course.

All of the models predict that the globe will warm as the result of the unrestrained emission of heat trapping gases, but different temperatures are obtained under the same conditions and both modest and catastrophic climate changes are foreseen. The reason is that scientists don’t understand the precise role of oceans, water vapor and clouds. They can all amplify the future global warming or cool it. So the experts carefully wrap their pronouncements in statistical properties, scientific uncertainties, and a range of possibilities. The most plausible outcome lies somewhere in between the most extreme projections, between the “end of the world” scenarios and the “good for you” ones.

The mighty oceans are one of the biggest uncertainties. Current estimates suggest that their waters are now soaking up about half the carbon dioxide released into the atmosphere by human activity. Water absorbs less carbon dioxide when it is warmer, which is why you should always keep a carbonated beverage cold. So you might expect that the oceans will amplify the greenhouse warming, absorbing less of the waste gas when it gets warmer and leaving more of it in the air to warm the Earth. Yet, a precise knowledge of how carbon dioxide is buried deep within the oceans, and how the gas is released from them, is not available.

Scientists also do not fully comprehend how water behaves in a warming atmosphere and the amount by which it enhances the warming. Increased temperatures will evaporate more water from the oceans, and the additional moisture should increase the greenhouse warming. Water vapor, in fact, is the main heat-trapping gas in the lower atmosphere. Its greenhouse-amplifying effect is built into the supercomputer climate models, and their predicted average global temperature increase would be substantially less without it. Some critics argue that future warming may even dry out he upper atmosphere, tempering the warming effect of water vapor.

Our understanding of future global warming is additionally hampered by an uncertain knowledge of how clouds affect the Earth’s temperature. At any given moment, clouds now cover at least half the area of the planet, and increased warming should produce even more clouds along with increasing water vapor. The highflying clouds can cool the planet by reflecting more incident sunlight back into space. You may have noticed this cooling effect when a large cloud passed overhead. Clouds also produce a warming effect by absorbing some of the infrared heat radiation emitted by the ground. This heat is reradiated downward, keeping the planet warm. That accounts for warmer nights on a cloudy day. The net temperature effect of clouds depends on which effect dominates and how strong it is, but that is the difficulty, for no one seems to know for sure.

To add to the confusion, the existing supercomputer models offer only a blurred, myopic vision of the world, which isn’t focused enough to resolve most clouds. Although they provide a plausible range of warming forecasts for a century from now, the calculations are reliable only on the broadest scales, such as average temperatures or seasonal changes across the entire world. No computer can possibly evaluate climatic changes everywhere on our planet and in the atmosphere over the next 100 years.

The finite capacity and speed of even the most advanced supercomputer limits its climate calculations to widely separated points, typically no better than 100 or 200 thousand meters between adjacent points. This relatively crude resolution can blur distinctions between land and sea or mountains and plains. It also means that the models cannot zero in and pinpoint localized weather sources, such as clouds or even hurricanes, and they blur distinctions between land and sea or mountains and plains. Thus, even the best global warming predictions represent a stripped-down version of the Earth’s real climate, capable of approximating average conditions over entire countries a long time from now, but too crude to reliably forecast conditions within localized regions of the countries.

In addition, many of the predicted temperature changes from human-induced global warming pale in comparison to natural variations, from the annual seasons to the ponderous ice ages. The climate is naturally changing all the time, warming and cooling regardless of what people do. A prudent society should therefore examine if a hotter globe can be solely attributed to natural processes, as distinct from human ones, and determine how much of the recent rise in temperature is attributable to human activity and how much to natural causes.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University