2. Global Warming
Using past records to separate the warming effects of the Sun and humans
Changes in the amount or distribution of the sunlight illuminating the Earth can produce substantial variations in our climate, and we have proxy records of its variable output over past centuries. This data can be compared to global temperatures during the same period, to determine if the Sun is responsible for most of the warming and cooling of our Earth. Although natural variations in the solar output can explain most of the temperature variations over the past centuries, it appears that global warming by heat-trapping gases, emitted by human activity, is required to explain the sharp rise in global temperatures during the 1990s.
The Sun is, after all, the driving force for all climate and weather on Earth. The annual seasons are, caused by a change in the amount of incident sunlight, due to the tilt of the Earth’s rotational axis. During the Earth’s yearly orbit, a given hemisphere is tilted toward the Sun, producing summer, then away resulting in winter.
The word “climate” comes, in fact, from the Greek word klima, for tilt. Nowadays climate denotes long-term changes, on time scales of years, decades and centuries, while weather usually refers to short times of hours, days, weeks or months.
The annual change in solar radiation produces large seasonal temperature fluctuations in the Northern Hemisphere where most of the world’s land is now found. The difference in the average surface temperature between northern winter and summer is an enormous 15 degrees Celsius (27 degrees Fahrenheit). In the tropics, the temperature changes little year round, since the amount of incoming solar radiation in those latitudes is least affected by the Earth’s tilt. The tropical regions also receive the greatest amount of heat because the Sun’s vertical rays travel to the ground through the least amount of intervening air.
Although the sunlight that illuminates our days provides a seemingly reliable beacon, the Sun’s visible luminosity varies in tandem with the Sun's 11-year magnetic activity cycle and these changes could affect our climate. The colored sunlight passes right through to the ground, providing a direct warming or cooling of the lower atmosphere.
Stable detectors placed aboard satellites above the Earth’s atmosphere have been precisely monitoring the Sun’s total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant. It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years, and probably longer. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.
Comparisons with other stars, that resemble the Sun in mass and age, indicate that the Sun could undergo more substantial variations in brightness than those observed by satellites so far.
In order to say that the climate is getting warmer, we cannot just extrapolate from recent measurements. The case for global warming must instead be based on century-long temperature records over a large part of the globe. We can then determine if the recent rise in temperature is a significant departure from long-term trend, and evaluate how much of the warming is attributable to human activity and how much to natural causes.
Scientists have therefore reconstructed variations in the climate of the past, comparing them to the Sun’s changing output. Data extracted from tree rings and Antarctica ice cores indicate that solar activity has indeed fallen to unusually low levels at least three times during the past one thousand years, each drop corresponding to a long, cold spell of roughly a century in duration. As an example, sunspots virtually disappeared from the face of the Sun for the 70-year period between 1645 and 1715, when Europe experienced one of the coldest periods of the Little Ice Age. During that time, alpine glaciers expanded, the river Thames, England, and the canals of Venice, Italy regularly froze over, and painters depicted unusually harsh winters in Europe. The Sun was then about 0.25 percent dimmer, and the reduction in solar brightness produced an estimated drop of about 0.5 degrees Celsius (0.9 degrees Fahrenheit) in the global mean temperature.
It is very difficult to distinguish any human influence in the observed temperature record of past centuries. The natural ups and downs of the Sun’s brightness or the cooling effect of volcanoes can explain almost all of it. Fine particles created during powerful volcanic eruptions, such as Mount Tambora in 1815, Mount Krakatau in 1883, and Mount Pinatubo in 1991; can spread out high above the ground, forming an invisible, umbrella-like shield that blocks some of the incoming solar radiation and causes temporary global cooling. Similar sulfate aerosols arise from fossil fuels burnt in power plants, factories and automobiles, partially offsetting the full warming effect of their carbon-dioxide emission.
The warning signal of human-induced global warming only rose above the confusing noise of the Sun and other natural effects in the 1990s, when the Earth became exceptionally hot. There is absolutely no evidence for such a decade in the historical temperature records going back 1,000 years (Fig. GW.2). This sharp, unprecedented rise in the average global temperature during the last decade of the 20th century cannot be explained as a temporary swing produced by natural causes alone, and its is very likely that heat-trapping waste gases are at least partly responsible for it. The available evidence suggests that greenhouse gases emitted by industrial economies mainly cause this warming.
Humans have conquered the land, moved mountains, and redirected rivers. Airplanes have provided easy access to almost anywhere in the world, and communications satellites have connected us all in an electronic web. The globe has shrunk, and we might now be altering the entire atmosphere.
For most of history, we believed that climate and weather are governed by outside forces, beyond the influence of humans, but today we are no longer so sure. As the result of rapid, unprecedented population and industrial growth over the past century, humans can collectively alter the course of nature.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University