People who dismiss climate change often claim that the earth's warm-up is simply part of “natural climate variability.” A paper published in July in Nature puts that argument to rest. The authors show that warm and cold years were regularly interspersed during the past 2,000 years A and that even the warmest and coldest periods were experienced only by isolated regions at a given time—never across the entire globe simultaneously B. For example, the so-called Little Ice Age occurred in the 1400s across the central Pacific Ocean, in the 1600s across northwestern Europe and in the mid-1800s in other places. The warm Medieval Climate Anomaly occurred in the Pacific in the 900s, in North America in the 1000s and in central South America in the 1200s. But the current warm-up has taken place across 98 percent of the globe at the same time, from about 1900 through today. “It's completely different,” states lead researcher Raphael Neukom of the University of Bern in Switzerland. All regions have heated up relentlessly, in unison.