We see the weather change every day; weather is the condition of the atmosphere over relatively short periods of time, from seconds to a few years. Less apparent are changes in climate: Climate is the average and range of weather events over longer periods, from decades to centuries—or longer.
Climate is what we expect, weather is what we get.
–Mark Twain, American writer (1835–1910)
In fact, Earth’s climate has been changing for more than a billion years. Slight shifts in Earth’s orbit, the activity of the Sun (such as observed in sunspot cycles, a factor in the Almanac’s weather forecasts) and volcanoes, the composition of the atmosphere, and even the frequency and magnitude of collisions with extraterrestrial objects have resulted in dramatic changes in Earth’s climate. These include several Ice Ages, when much of the planet was frozen, and several warm periods, during which tropical temperatures reached all the way to the poles.
Earth’s atmosphere acts to keep temperatures—and therefore, our weather—relatively steady: Our day-to-night temperature range is between about 10 and 30 degrees F. Compare this to the atmosphere-less Moon, where the temperature normally varies more than 500 degrees F between day and night! Lunar temperatures range from about 260°F during the day to minus 280°F at night.
Our planet’s climate and temperature stability occur because Earth’s atmosphere works in much the same way as the glass in a greenhouse. The glass allows sunlight to enter the greenhouse unimpeded, where it heats the air and ground within, yet the glass prevents the heat from leaving. So it is that sunlight heats Earth’s atmosphere and ground, yet the atmosphere prohibits a lot of the heat from escaping back into space.
A diagram of the greenhouse effect, showing the flow of energy in watts per square meter. (14°C = 57°F) Illustration by Robert A. Rohde.
The gases in Earth’s atmosphere, our greenhouse, play a key role in maintaining this stability. Nearly 99 percent of our atmospheric gases consists of nitrogen and oxygen. However, it’s the other gases—ozone, water vapor, methane, and carbon dioxide—that cause the greenhouse effect (the capture of heat in the atmosphere) in varying ways.
Ozone is not a significant concern with regard to climate change. Scientists generally agree that it has stabilized in the atmosphere. Water vapor produces clouds, and their presence can increase temperatures (by blocking heat from escaping into space) or decrease temperatures (by blocking sunlight from reaching Earth), depending upon the altitude and thickness of the clouds. Some believe that methane released into the atmosphere by permafrost melt may, in the coming decades, cause an acceleration of warming; this would be significant.
Carbon dioxide (CO2) is another story: It has received the most attention with regard to its effect in changing Earth’s climate.
Historically, the amount of CO2 in the atmosphere has varied from about 180 parts per million during Ice Ages to nearly 300 parts per million during the interglacial periods. However, since the late 1700s, which marked the beginning of the Industrial Revolution, the amount of carbon dioxide in the atmosphere has increased; today, it is more than 400 parts per million, or about 0.04 percent of the atmosphere.
What Changes Earth’s Climate
A substantial majority of scientists conclude the increase in carbon dioxide to be the driving force behind recent climate change. In particular, there is the fact that the 10 warmest years in millennia, averaged across the entire Earth, have all occurred since 1998, when CO2 presence has been at its highest.
However, some scientists believe the increased carbon dioxide to be only a trace amount—a quantity too small to change Earth’s climate.
Many of the scientists find that the primary cause of the escalation of CO2 is the burning of fossil fuels such as coal, oil, and gasoline. What’s more, this majority believes that as carbon dioxide continues to increase, Earth’s temperature will continue to grow warmer, on average, and fundamental weather patterns will change, resulting in both more flooding and more droughts.
Detractors disagree, and often point to other relatively warm periods in history, such as the 1930s in the United States, as an indication that recent “warmest years” are part of Earth’s natural cycles.
Whichever view is correct, we can be certain that weather and climate will continue to change, and we will all experience the effects.
Did You Know?
About 66 million years ago, a comet or asteroid hit Earth near Mexico’s Yucatán Peninsula. The impact, which had a force with more than a billion times the energy of an atomic bomb, sent so much soot into the atmosphere that it substantially reduced the amount of sunlight reaching Earth’s surface, thereby causing the average temperature to drop by nearly 50 degrees F for the next 30 years.
This dramatic cooling, combined with the diminished sunlight—which lessened the ability of plants and plankton to carry out photosynthesis—is believed to have caused the extinction of about three-quarters of all of the plant and animal species then on Earth, including nearly all of the dinosaurs.
However, the situation also provided opportunities. Mammals were the primary winners, evolving into new forms, including horses, whales, bats, and primates, with apes and then humans eventually becoming the dominant species.
The first ongoing, real-time measurements of carbon dioxide (CO2) in the atmosphere were undertaken by Charles David Keeling at Mauna Loa observatory in Hawaii in 1958. He continued taking measurements until he died in 2005. At this time, supervision of the project was taken over by his son, Ralph, a professor of geochemistry at Scripps Institution of Oceanography in California.
These records represent the best known and most widely accepted atmospheric CO2 measurements. (Daily and historic records can be seen at www.CO2.earth/daily-co2.)
Today, measurements are also made at many other sites around the world, as well as by satellite. Historical CO2 levels from the past 800,000 years are estimated by measuring bubbles of air trapped in the Arctic and Antarctic ice sheets.