Earth has warmed and cooled many times since its formation about 4.6 billion years ago. Global climate changes were due to many factors, including massive volcanic eruptions, which increased carbon dioxide in the atmosphere; changes in the intensity of energy emitted by the Sun; and variations in Earth’s position relative to the Sun, both in its orbit and in the inclination of its spin axis.
Variations in Earth’s position, known as Milankovitch cycles, combine to produce cyclical changes in the global climate. These cycles are believed to be responsible for the repeated advance and retreat of glaciers and ice sheets during the Pleistocene Epoch (1.8 million to 11,500 years before present), when Earth went through fairly regular cycles of colder “glacial” periods (also known as ice ages) and warmer “interglacial” periods. Glacial periods occurred at roughly 100,000-year intervals.
An interglacial period began about 10,000 years ago, when the last ice age came to an end. Prior to that ice age, an interglacial period occurred about 125,000 years ago. During interglacial periods, greenhouse gases such as carbon dioxide and methane naturally increase in the atmosphere from increased plant and animal life. But since 1750 greenhouse gases have increased dramatically to levels not seen in hundreds of thousands of years, due to the rapid growth of the human population combined with developments in technology and agriculture. Human activities now are a powerful factor influencing Earth’s dynamic climate.
The ice of the polar regions furnishes clues to the makeup of Earth’s ancient atmosphere. Ice cores that scientists have bored from the ice sheets of Greenland and Antarctica provide natural records of both temperature and atmospheric greenhouse gases going back hundreds of thousands of years. Layers in these ice cores created by seasonal snowfall patterns allow scientists to determine the age of the ice in each core. By measuring tiny air bubbles trapped in the ice and properties of the ice itself, scientists can estimate the temperature and amount of greenhouse gases in Earth’s past atmosphere at the time each layer formed. Based on this data, scientists know that greenhouse gases have now risen to levels higher than at any time in the last 650,000 years.
Greenhouse gases are rising, and temperatures are following. Before the late 1800s, the average surface temperature of Earth was almost 15°C (59°F). Over the past 100 years, the average surface temperature has risen by about 0.7 Celsius degrees (1.3 Fahrenheit degrees), with most of the increase occurring since the 1970s. Scientists have linked even this amount of warming to numerous changes taking place around the world, including melting mountain glaciers and polar ice, rising sea level, more intense and longer droughts, more intense storms, more frequent heat waves, and changes in the life cycles of many plants and animals. Warming has been most dramatic in the Arctic, where temperatures have risen almost twice as much as the global average.
GLOBAL WARMING IN THE FUTURE
Scientists project global warming to continue at a rate that is unprecedented in hundreds of thousands or even millions of years of Earth’s history. They predict considerably more warming in the 21st century, depending on the level of future greenhouse gas emissions. For a scenario (possible situation) assuming higher emissions-in which emissions continue to increase significantly during the century-scientists project further warming of 2.4 to 6.4 Celsius degrees (4.3 to 11.5 Fahrenheit degrees) by the year 2100. For a scenario assuming lower emissions-in which emissions grow slowly, peak around the year 2050, and then fall-scientists project further warming of 1.1 to 2.9 Celsius degrees (1.9 to 5.2 Fahrenheit degrees) by the year 2100.
Melting polar ice and glaciers, as well as warming of the oceans, expands ocean volume and raises sea level, which will eventually flood some coastal regions and even entire islands. Patterns of rainfall are expected to change, with higher latitudes (closer to the poles) projected to receive more rainfall, and subtropical areas (such as the Mediterranean and southern Africa) projected to receive considerably less. Changes in temperature and precipitation patterns may damage food crops, disrupting food production in some parts of the world. Plant and animal species will shift their ranges toward the poles or to higher elevations seeking cooler temperatures, and species that cannot do so may become extinct. Increasing levels of carbon dioxide in the atmosphere also leads to increased ocean acidity, damaging ocean ecosystems.
Human beings face global warming with a huge population at risk. The potential consequences are so great that many of the world’s leading scientists-and increasingly, politicians, business leaders, and other citizens-are calling for international cooperation and immediate action to counteract the problem.
