21-1 Past Climate Change

How Have the Earth’s Temperature and Climate Changed in the Past? Climate Change Is Not New

Over the past 4.7 billion years it has shifted due to volcanic emissions, changes in solar input, continents moving as a result of shifting tectonic plates, strikes by large meteorites, and other factors. At some times, the troposphere’s average temperature has changed gradually and at other times fairly quickly. Over the past 900 000 years, the average temperature of the troposphere has under- gone prolonged periods of global cooling and global warming. These alternating cycles of freezing and thawing are known as glacial and inter- glacial (between ice ages) periods.

How Do Scientists Study Climate Change? Drill Holes and Make Measurements

Scientific clues about the Earth’s past temperatures and climate are found deep within its glaciers and ice caps, such as those in Greenland and Antarctica. Scientists drill into these museums of atmospheric history and extract long cores of ice. In 2004, data from cores drilled in Antarctic ice indicated that the current interglacial period could last for another 15 000 years before a new ice age occurs—unless our activities seriously alter the Earth’s climate.

Scientists analyze air bubbles trapped in different segments of these ice cores to uncover information about past tropospheric composition, temperature trends, greenhouse gas concentrations, solar activity, snowfall, and forest fire frequency.

Scientists also study past climates by drilling cores into the bottoms of lakes, ponds, and swamps. Then they analyze different zones of the sediment for pollen, fossils, and other clues about what types of plants lived in the past and trends in plant life over time. For those who like detective work, finding out about the Earth’s climate history is a fascinating activity.

Scientists also make direct measurements to get current information about tropospheric temperature, composition, and trends. They measure temperatures using thermometers on land and at sea and on weather balloons at various altitudes.

Finally, scientists collect air samples at different locations and altitudes and analyze them to detect changes in the chemical composition of the troposphere. For example, since 1958 environmental chemist Charles Keeling has analyzed CO2 levels in the troposphere at the Mauna Loa observatory in Hawaii.