Myth No. 4: There have been big climate changes in the past, such as the Little Ice Age and the Medieval Warm Period, so why can't recent climate changes just be explained by natural variability?
People who dispute evidence of recent global warming sometimes point to two episodes in the past 1,000 years called the Little Ice Age and the Medieval Warm Period -- times when northern hemisphere temperatures were higher or lower than average for decades or even centuries -- as examples of internal variability, a kind of natural randomness in the climate system that can't be explained by any specific forcing. If true, perhaps internal variability could explain the current rapid global warming, skeptics argue. In other words, maybe our current warming is just an unlucky roll of the dice, a blip rather than a long term trend.
Climate scientists now understand that the Medieval Warm Period was caused by an increase in solar radiation and a decrease in volcanic activity, which both promote warming. Other evidence suggests ocean circulation patterns shifted to bring warmer seawater into the North Atlantic. As we'll see in the next section, those kinds of natural changes have not been detected in the past few decades. Charles Jackson noted that when computer models take into account paleoclimatologists' reconstructions of solar irradiance and volcanoes for the past 1,000 years, the models reproduce the Little Ice Age and Medieval Warm Period. Those events turn out to not be random noise after all.
Check back tomorrow for myth No. 5 in this series.
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