Earth is unique in the solar system for many reasons: Not only is it the only planet with abundant water, but it is the only one whose water exists in all three states: solid, liquid, and gas. Earth is the only planet with an abundance of life (or, as far as scientists know, with any life).

Earth is also unique because of its climate. Mercury and Venus, both close to the Sun, are too hot. Mars and the outer planets, all far from the Sun, are too cold. Even the Moon, which is the same distance from the Sun as Earth, has an inhospitable climate because it has no atmosphere to insulate it. Earth, therefore, is sometimes called the "Goldilocks Planet" because its climate is, as the old story goes, not too hot and not too cold, but "just right." Earth's climate is so hospitable because of the greenhouse gases in the atmosphere. These gases allow sunlight through but trap some of the heat that reradiates from the planet's surface, helping to create a temperate climate that has allowed the proliferation of an enormous number and variety of living organisms.

While Earth's climate is hospitable for life, it can vary tremendously from place to place, as a comparison of the temperature and precipitation patterns in the Arctic with those of a tropical rain forest will quickly reveal. Climate also varies through time: Throughout Earth's 4.55 billion-year history, its climate has varied enormously. During much of that time, conditions were hot and moist; but sometimes the air was frigid, with ice coating the polar regions and mountains. Even in the past millennium, average temperatures have been variable. For instance, during the Medieval Warm Period (A.D. 1000 to A.D. 1300),

they were relatively high, while during the Little Ice Age (A.D. 1550 to A.D. 1850) they were comparatively cold. Despite these two anomalies, average global temperatures have only varied within a range of 1.8°F (1°C) since the end of the Pleistocene Ice Ages about 10,000 years ago, when human civilization began. Throughout Earth's history, temperatures have correlated with the levels of greenhouse gases in the atmosphere. When the planet is warm, greenhouse gases are high. When the planet is cool, greenhouse gas levels are low.

That Earth's climate is naturally variable is unquestionable, and it is certainly true that temperatures have generally risen since the end of the Pleistocene. But what now alarms climatologists is that global temperatures are rising more and at a higher rate then at any time in human history. Around 1990, global temperatures began to rise at a rate unseen in the past 2,000 years, and the warmest years of the past millennium have been in the past two decades. Climatologists almost universally agree that human activities are to blame for a large portion of the temperature gains. Activities such as burning fossil fuels or forests release greenhouse gases into the atmosphere. Rising greenhouse gas levels trap more of the planet's reradiated heat and help to raise global temperatures. The escalating temperatures of the past few decades are referred to as "global warming."

When the potential for increased temperatures due to human activities was first discussed several decades ago, nearly all scientists were skeptical. While humans had undoubtedly had an impact on the planet—for example, through the creation of pollution—the thought that human civilization could affect a system as large and complex as climate was hard to accept. Sound scientific evidence gathered since that time has turned nearly all of these climate skeptics around. The vast majority of them now agree that global warming is under way and that human activities are largely to blame.

The Intergovernmental Panel on Climate Change (IPCC), established by the United Nations (UN) in 1988, is the main international body charged with evaluating the state of climate science. The more than 300 participants of the IPCC consist mostly of government and academic scientists who evaluate the peer-reviewed papers and scientific information available and issue recommendations for informed action. The first panel included many skeptics; its first report, published in 1990, stated that added greenhouse gases were likely the cause of some of the warming that had been seen but that the range of temperature increase was within what could be expected with natural climate variation. The second report, in 1995, increased the blame for rising temperatures on human activities, stating, "The balance of evidence suggests a discernible human influence on global climate." By the 2001 report, many skeptics had changed their opinion: "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities." The scientists who compiled the fourth report, in 2007, called global warming "unequivocal" and say with over 90% certainty that the warming taking place since 1950 is being caused by human activities. The scientists on the fourth report overwhelmingly agree that recent changes in climate are altering physical and biological systems on every continent, and blame human-generated greenhouse gas emissions for climate change. During the past decade or so, many other scientific organizations in the United States and other nations have issued similar scientific studies.

Why is global warming a problem? Climate has been much warmer in Earth's past, and the temperatures predicted for the next few centuries are low compared with the temperatures during many earlier periods. There are several reasons that humans should not want the globe to become too warm: For one, many animals and plants will likely go extinct, starting with polar organisms but eventually including organisms in other climate zones. People depend on many of these wild plants and animals for such resources as food, building materials, and even the chemical compounds included in many pharmaceuticals. Another reason involves human systems. Modern agriculture and human settlement patterns, among many other features of human civilization, depend on very small climate variations. A drastic change in climate, even on a smaller scale than those that have taken place earlier in Earth history, could destabilize human civilization.

The effects of global warming are already being seen. Glaciers and polar ice caps are melting. Winters are shorter and, as a result, some plants and animals are changing their seasonal behaviors: Flowers are blooming earlier, and birds are migrating to higher latitude locations. Coral reefs and forests are dying around the world. In the case of forests, their demise is often due to the invasion of insects from warmer climates. The weather is becoming more extreme: Catastrophic floods, record-breaking heat waves, and intense hurricanes are now more "normal" than they were a few decades ago. Even ocean currents appear to be changing, putting established climate patterns even more at risk. According to climate model predictions, this is just the beginning.

Some of the world's political leaders are beginning to recognize the dangers of this new warmer world. In the forward to a 2005 conference report developed by Great Britain's Meteorological Office, Tony Blair, then prime minister of the United Kingdom, said, "It is now plain that the emission of greenhouse gases, associated with industrialization and economic growth from a world population that has increased six-fold in 200 years, is causing global warming at a rate that is unsustainable." While many other world leaders have gotten on board, some extremely important leaders, most notably in the United States, remain unconvinced.

Without a global consensus, the plan to reduce greenhouse gas emissions is a mishmash of promises without any real action. To reduce greenhouse gas emissions, as climatologists say is necessary, the nations of the world must come up with viable plans for increasing energy efficiency, for developing new technologies, and possibly even for removing greenhouse gases to reservoirs outside the atmosphere. The sooner these actions are taken, the less extreme future changes in human behavior will need to be. While these plans are being made, and technologies are being developed, Earth will continue to warm. Therefore, local, regional, and global entities will need to prepare for the changes to the climate system that are already inevitable.

This volume of the OUR FRAGILE PLANET series explores climate change throughout Earth history, but especially during the past few decades. Part One describes how Earth's climate system works. It also focuses on climate change: what causes it, how scientists learn about it, what patterns it has had in Earth history, and how it is happening now. Part Two looks at the effects of climate change already being seen in the atmosphere, hydrosphere, and biosphere. Predictions of what a warmer world will be like are discussed in Part Three. Finally, Part Four describes the ways people can approach the problem of climate change: from alterations that can be made to lessen its impacts, to adaptations that must be made to warming that is already inevitable.



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