Perhaps the most visible sign that Earth's climate is warming is the gradual shrinking of its glaciers. In North America, the most visited glacier is the Athabasca Glacier, one of six glaciers that spill down the Canadian Rockies from the Columbia Icefield in western Canada. Visitors who return to the glacier a few years after their first visit will notice the change wrought by warming temperatures.
In the past 125 years, the Athabasca Glacier has lost half of its volume and receded more than 1.5 kilometers (0.93 miles), leaving hills of rock in its place. Its retreat is visible in this photo, where the glacier's front edge looms several meters behind the tombstone-like marker that indicates the edge of the ice in 1992. The Athabasca Glacier is not alone in its retreat: Since 1960, glaciers around the world have lost an estimated 8,000 cubic kilometers (1,900 cubic miles) of ice. That is approximately enough ice to cover a two-kilometer-wide (1.2 mile-wide) swath of land between New York and Los Angeles with an ice sheet that is one kilometer (0.62 miles) tall.
Melting glaciers, dwindling sea ice, rising global temperatures, and rising sea levels. Little by little the evidence is adding up to show that Earth is getting hotter, and scientists are almost certain that people are to blame. A number of activities from burning fossil fuels to farming pump heat-trapping gasesâ€”greenhouse gasesâ€”into the atmosphere. Once in the atmosphere, these gases stay there for thousands of years, absorbing the heat that comes from the Earth and re-radiating it back to the surface, enhancing Earth's natural greenhouse effect.
Between 1906 and 2006, the average surface temperature of the Earth rose 0.6 to 0.9 degrees Celsius (1.08 to 1.62 degrees Fahrenheit), while greenhouse gas concentrations reached their highest levels in at least the past 650,000 years. Most climate scientists believe that there is a connection and warn that if greenhouse gas emissions continue, temperatures are likely to go up 2 to 6 degrees Celsius (3.6 to 10.8 degrees F) by the end of the 21st century.
While this might seem like a small change, it will probably lead to big changes in the environment. Warming temperatures will likely lead to more frequent heat waves, bigger storms, including more intense tropical cyclones (hurricanes), and more widespread drought. Since water expands as it heats, and melting glaciers and ice caps have dumped more fresh water into the world's oceans, sea levels have already started to rise. Higher sea levels lead to more erosion and greater storm damage in coastal areas, many of which are densely populated. As much as 10 percent of the world's population lives in vulnerable coastal regions that have an elevation less than 10 meters (32 feet) above sea level.
To understand what global warming means for humanity, it is necessary to understand what global warming is, how scientists know it's happening, and how they predict future climate. These ideas are explored in the Earth Observatory's newly updated fact sheet on global warming. We invite you to read Global Warming, and then send us your questions with â€œglobal warmingâ€ in the subject line. We won't be able to answer every question that is sent to us, but we will address some of the most frequently asked questions in a follow-up article.