When scientists refer to climate change, they typically mean the warming period kicked off by the Industrial Revolution in the 1800s. Climate change didn't "start" then—the climate has always experienced changes—but the climate change that began in the late 19th and early 20th centuries is roughly eight times faster than any other warming period known to scientists. Warming trends are accelerating; the climate warmed twice as quickly in the past 40 years as it did in the early 20th century.