For centuries before 1800, CO2 levels fluctuated narrowly at close to 270 parts per million (ppm)—that is, 0.027 percent by volume. By 1900 they rose slightly to 290 ppm, a century later they were nearly 375 ppm, and in the summer of 2020 they rose above 420 ppm, more than a 50 percent increase above the late 18th-century level.[37] Preindustrial methane levels were three orders of magnitude lower—less than 800 parts per billion (ppb)—but they have more than doubled, to nearly 1,900 ppb by 2020, while nitrous oxide concentrations rose from about 270 ppb to more than 300 ppb.[38]
