The Planet is Heating Up—and Fast Glaciers are melting, sea levels are rising, cloud forests are drying, and wildlife is scrambling to keep pace. It's becoming clear that humans have caused most of the past century's warming by releasing heat-trapping gases as we power our modern lives. Called greenhouse gases, their levels are higher now than in the last 650,000 years. We call the result global warming, but it is causing a set of changes to the Earth's climate, or long-term weather patterns, that varies from place to place. As the Earth spins each day, the new heat swirls with it, picking up moisture over the oceans, rising here, settling there. It's changing the rhythms of climate that all living things have come to rely upon. What will we do to slow this warming? How will we cope with the changes we've already set into motion? While we struggle to figure it all out, the face of the Earth as we know it—coasts, forests, farms and snow-capped mountains—hangs in the balance. Greenhouse effect The "greenhouse effect" is the warming that happens when certain gases in Earth's atmosphere trap heat. These gases let in light but keep heat from escaping, like the glass walls of a greenhouse. First, sunlight shines onto the Earth's surface, where it is absorbed and then radiates back into the atmosphere as heat. In the atmosphere, “greenhouse” gases trap some of this heat, and the rest escapes into space. The more greenhouse gases are in the atmosphere, the more heat gets trapped. Scientists have known about the greenhouse effect since 1824, when Joseph Fourier calculated that the Earth would be much colder if it had no atmosphere. This greenhouse effect is what keeps the Earth's climate livable. Without it, the Earth's surface would be an average of about 60 degrees Fahrenheit cooler. In 1895, the Swedish chemist Svante Arrhenius discovered that humans could enhance the greenhouse effect by making carbon dioxide, a greenhouse gas. He kicked off 100 years of climate research that has given us a sophisticated understanding of global warming. Levels of greenhouse gases (GHGs) have gone up and down over the Earth's history, but they have been fairly constant for the past few thousand years. Global average temperatures have stayed fairly constant over that time as well, until recently. Through the burning of fossil fuels and other GHG emissions, humans are enhancing the greenhouse effect and warming Earth. Scientists often use the term "climate change" instead of global warming. This is because as the Earth's average temperature climbs, winds and ocean currents move heat around the globe in ways that can cool some areas, warm others, and change the amount of rain and snow falling. As a result, the climate changes differently in different areas. Aren’t temperature changes natural? The average global temperature and concentrations of carbon dioxide (one of the major greenhouse gases) have fluctuated on a cycle of hundreds of thousands of years as the Earth's position relative to the sun has varied. As a result, ice ages have come and gone. However, for thousands of years now, emissions of GHGs to the atmosphere have been balanced out by GHGs that are naturally absorbed. As a result, GHG concentrations and temperature have been fairly stable. This stability has allowed human civilization to develop within a consistent climate. Occasionally, other factors briefly influence global temperatures. Volcanic eruptions, for example, emit particles that temporarily cool the Earth's surface. But these have no lasting effect beyond a few years. Other cycles, such as El Niño, also work on fairly short and predictable cycles. Now, humans have increased the amount of carbon dioxide in the atmosphere by more than a third since the industrial revolution. Changes this large have historically taken thousands of years, but are now happening over the course of decades. Why is this a concern? The rapid rise in greenhouse gases is a problem because it is changing the climate faster than some living things may be able to adapt. Also, a new and more unpredictable climate poses unique challenges to all life. Historically, Earth's climate has regularly shifted back and forth between temperatures like those we see today and temperatures cold enough that large sheets of ice covered much of North America and Europe. The difference between average global temperatures today and during those ice ages is only about 5 degrees Celsius (9 degrees Fahrenheit), and these swings happen slowly, over hundreds of thousands of years. Now, with concentrations of greenhouse gases rising, Earth's remaining ice sheets (such as Greenland and Antarctica) are starting to melt too. The extra water could potentially raise sea levels significantly. As the mercury rises, the climate can change in unexpected ways. In addition to sea levels rising, weather can become more extreme. This means more intense major storms, more rain followed by longer and drier droughts (a challenge for growing crops), changes in the ranges in which plants and animals can live, and loss of water supplies that have historically come from glaciers. Scientists are already seeing some of these changes occurring more quickly than they had expected. According to the Intergovernmental Panel on Climate Change, eleven of the twelve hottest years since thermometer readings became available occurred between 1995 and 2006.
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It has been one of the central claims of those who challenge the idea that human activities are to blame for global warming. The planet’s climate has long fluctuated, say the climate sceptics, and current warming is just part of that natural cycle — the result of variation in the sun’s output and not carbon dioxide emissions. But a new analysis of data on the sun’s output in the last 25 years of the 20th century has firmly put the notion to rest. The data shows that even though the sun’s activity has been decreasing since 1985, global temperatures have continued to rise at an accelerating rate. The solar hypothesis was championed publicly in March by the controversial UK TV Channel 4 documentary, The Great Global Warming Swindle. The programme has been heavily criticised for distorting scientific data to fit the sceptic argument and Carl Wunsch, a professor of physical oceanography at MIT who featured in the programme, later said that he was “totally misled” by the film makers and that his comments were “completely misrepresented.” The new analysis is designed to counter the claim that solar activity may be to blame for global warming. “The temperature record is simply not consistent with any of the solar forcings that people are talking about,” said lead author Mike Lockwood at the Rutherford Appleton Laboratory in Oxfordshire. Changed direction “They changed direction in 1985, the climate did not ... [the temperature] increase should be slowing down but in fact it is speeding up.” Global temperatures are going up by 0.2 degrees per decade and the top 10 warmest years on record have happened in the past 12 years. One way that the sun affects the climate is through clouds. The sun’s magnetic field shields the Earth from its high energy particles called cosmic rays. The rays help form clouds that reflect the sun’s energy back into space and cool the planet. So if the sun’s magnetic field is high, there should be a fall-off in cosmic rays, fewer clouds and more warming. But Prof Lockwood’s data, published recently in the Proceedings of the Royal Society, shows the sun’ ;s magnetic field has declined since 1985, even as the world heats up. James Hansen, a Nasa climate scientist who was once gagged by the Bush administration for speaking out on global warming, said:“The reason [this paper] has value is that the proponents of the notion that the sun determines everything come up with various half-baked suggestions that the sun can somehow cause an indirect forcing that is not included in the measurements of radiation coming from the sun,” he said. “These half-baked notions are usually supported by empirical correlations of climate with some solar index in the past. Thus, by showing that these correlations are not consistent with recent climate change, the half-baked notions can be dispensed with.” Rescue attempt Nir Shaviv, an astrophysicist at the Hebrew University of Jerusalem and a proponent of the solar hypothesis, has tried to rescue the idea by invoking a time lag between changes in the sun and its effect on the Earth’s climate. But Prof Lockwood dismissed this as “disingenuous.” “Nobody has invoked that kind of lag before. It’s only been invoked now as a way out,” he said. Even if the lag were 50 years then he believes we would begin to see the rise in global temperatures slowing down. A spokesman for the Royal Society, the U.K.’s leading scientific academy, said: “This is an important contribution to the scientific debate on climate change. At present there is a small minority which is seeking to deliberately confuse the public on the causes of climate change. They are often misrepresenting the science, when the reality is that the evidence is getting stronger every day. We have reached a point where a failure to take action to reduce carbon dioxide and other greenhouse gas emissions would be irresponsible and dangerous.” Channel 4 and Martin Durkin, producer of The Great Global Warming Swindle, declined to comment. GUARDIAN NEWSPAPERS LIMITED 2007 |
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