Evidence for a changing climate

The evidence for recent climate change comes from several independent methods that agree with each other, which is the key epistemic point of this lesson: no single dataset alone would be fully convincing, but several different instruments, methods, and research teams around the world converging on the same conclusion is what gives scientists confidence -- a lesson worth applying to evaluating any scientific claim, not just this one. Direct thermometer records since the mid-1800s show global average surface temperature has risen by about 1.1 to 1.2 degrees Celsius, with most of that rise occurring since the 1970s.

Ice cores drilled from Antarctica and Greenland trap tiny ancient air bubbles, letting scientists directly measure atmospheric CO2 going back 800,000 years; through natural glacial and warm cycles over that entire span, CO2 oscillated roughly between 180 and 300 ppm, while today's level, above 420 ppm, is higher than at any point in that 800,000-year record. Sea level has risen roughly 21 cm (about 8 inches) since 1880, from a combination of thermal expansion (warmer water takes up more volume) and melting land ice, confirmed independently by tide gauges and, more recently, satellites. Glaciers are retreating on every continent, documented by repeat photography and direct measurement, and phenology records (the timing of natural events like spring leaf-out and lake ice breakup) show many such events shifting measurably earlier over the long term.