evidence: We know the world is warming because people have been recording daily high and low temperatures at thousands of weather stations worldwide, over land and ocean, for many decades and, in some locations, for more than a century. When different teams of climate scientists in different agencies (e.g., NOAA and NASA) and in other countries (e.g., the U.K.’s Hadley Centre) average these data together, they all find essentially the same result: Earth’s average surface temperature has risen by about 1.8°F (1.0°C) since 1880. In addition to our surface station data, we have many different lines of evidence that Earth is warming (learn more). Birds are migrating earlier, and their migration patterns are changing. Lobsters and other marine species are moving north. Plants are blooming earlier in the spring. Mountain glaciers are melting worldwide, and snow cover is declining in the Northern Hemisphere (Learn more here and here). Greenland’s ice sheet—which holds about 8 percent of Earth’s fresh water—is melting at an accelerating rate (learn more). Mean global sea level is rising (learn more). Arctic sea ice is declining rapidly in both thickness and extent (learn more). [...] NASA JPL: "Consensus: 97% of climate scientists agree." A website at NASA's Jet Propulsion Laboratory (climate.nasa.gov/scientific-consensus). (Accessed July 2013.)