During the 20th century, the earth warmed 0.6 degree Celsius (1 degree Fahrenheit), but that warming has been wiped out in a single year with a drop of 0.63 degree C. (1.13 F.) in 2007. A single year does not constitute a trend reversal, but the magnitude of that temperature drop — equal to 100 years of warming — is noteworthy. Of course, it can also be argued that a mere 0.6 degree warming in a century is so tiny it should never have been considered a cause for alarm in the first place. But then how could the idea of global warming be sold to the public? In any case, global cooling has been evident for more than a single year. Global temperature has declined since 1998. Meanwhile, atmospheric carbon dioxide has gone in the other direction, increasing 15–20%. This divergence casts doubt on the validity of the greenhouse hypothesis, but that hasn't discouraged the global warming advocates. They have long been ignoring far greater evidence that the basic assumption of greenhouse warming from increases in carbon dioxide is false.