Evidence for Global Warming

	Recent observations of warming support the theory that greenhouse gases are warming the world. Over the last century, the planet has experienced the largest increase in surface temperature in 1,300 years. The average surface temperature of the Earth rose 0.6 to 0.9 degrees Celsius (1.08°F to 1.62°F) between 1906 and 2006, and the rate of temperature increase nearly doubled in the last 50 years. Worldwide measurements of sea level show a rise of about 0.17 meters (0.56 feet) during the twentieth century. The world’s glaciers have steadily receded, and Arctic sea ice extent has steadily shrunk by 2.7 percent per decade since 1978.
	Even if greenhouse gas concentrations stabilized today, the planet would continue to warm by about 0.6°C over the next century because it takes years for Earth to fully react to increases in greenhouse gases. As Earth has warmed, much of the excess energy has gone into heating the upper layers of the ocean. Scientists suspect that currents have transported some of this excess heat from surface waters down deep, removing it from the surface of our planet. Once the lower layers of the ocean have warmed, the excess heat in the upper layers will no longer be drawn down, and Earth will warm about 0.6°C (1° F). But how do scientists know global warming is caused by humans and that the observed warming isn’t a natural variation in Earth’s climate? Scientists use three closely connected methods to understand changes in Earth’s climate. They look at records of Earth’s past climates to see how and why climate changed in the past, they build computer models that allow them to see how the climate works, and they closely monitor Earth’s current vital signs with an array of instruments ranging from space-based satellites to deep sea thermometers. Records of past climate change reveal the natural events—such as volcanic eruptions and solar activity—that influenced climate throughout Earth’s history. Today, scientists monitor those same natural events as well as human-released greenhouse gases and use computer models to determine how each influences Earth’s climate.		Spilling from the Columbia Icefield in the Rocky Mountains of western Canada, the Athabasca Glacier has been shrinking by about 15 meters per year. In the past 125 years, the glacier has lost half its volume and has retreated more than 1.5 kilometers. Athabasca is just one of Earth’s many glaciers that are dwindling as global temperatures climb. Since 1960, glaciers around the world have lost an estimated 8,000 cubic kilometers of ice. (Photograph ©2005 Hugh Saxby. Glacier graph adapted from Dyurgerov and Meier, 2005.)
	Reconstructing Past Climate Change Like detectives at a crime scene, scientists reconstruct past climate changes by looking for evidence left in things like glacial ice, ocean sediments, rocks, and trees. For example, glacial ice traps tiny samples of Earth’s atmosphere, giving scientists a record of greenhouse gases that stretches back more than 650,000 years, and the chemical make-up of the ice provides clues to the average global temperature. From these and other records, scientists have built a record of Earth’s past climates, or “paleoclimates.” Paleoclimatology allowed scientists to show that climate changes in the past have been triggered by variations in Earth’s orbit, solar variation, volcanic eruptions, and greenhouse gases.		The annual layers of snow packed in glacial ice preserve a record of climate stretching back hundreds of thousands of years. To reconstruct past temperatures and atmospheric conditions, scientists use ice cores, tree rings, ocean sediments, cave rocks, and other natural records that preserve a signature of the climate. By understanding how Earth’s climate has changed in the past, scientists gain insight into how and why it might change in the future. (Photograph ©2005 Reto Stöckli.)