Introducing Global Warming

Climate change is a reality. Today our world is hotter than it has been in two thousand years. By the end of the century, if current trends continue, the global temperature will likely climb higher than it has at any time in the past two million years. While the end of the 20th century may not necessarily be the warmest time in Earth's history, what is unique is that the warmth is global and cannot be explained by the natural mechanisms that explain previous warm periods. There is a broad scientific consensus that humanity is in large part responsible for this change, and that choices we make today will decide the climate of the future.

But to understand what scientists, politicians and others mean when they speak of "climate change" and "global warming", we need to know some basic background information on how the earth maintains a climate in the first place and what natural processes are at work, which affect the earth's temperature.

Earth's Atmosphere

Earth's atmosphere is a layer of gases that surrounds our planet and is kept in place by the Earth's gravity. This mixture of gases (commonly known as "air") is roughly 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide, and trace amounts of other gases, in addition to water vapor. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night. Other planets such as Mars and Venus also have atmospheres, but of different gaseous compositions that would likely be hostile to human life. The troposphere is the lowest layer of the atmosphere, and very few of us (unless we become astronauts) will ever travel beyond it in our lifetime.

Earth's Climate

Climate is the sum of weather conditions existing over a period of time--a month, a decade, a century or longer--and climate change results from a complex process that scientists are only beginning to understand.

Our climate has been constantly changing since the Earth began, with periods of global warming and global cooling long before human beings (and their various activities) came on the scene. Although modern weather measurements and tools go back only 100 to 150 years, scientists are able to deduce long-term regional temperature trends as well as changes in the atmosphere's chemical makeup by examining natural indicators or "proxies" (including ocean and lake sediments, ice cores, fossils, tree rings, and corals) and by using numerical models run on sophisticated computer software, to estimate and trace the ebb and flow of the Earth's climate over hundreds and even thousands of years. The Earth has experienced natural global warming and cooling many times in the past and this can offer useful insights into present processes.