New analyses suggest the coldest parts of the Little Ice Age—a period so cold the Thames River in the United Kingdom froze thick enough to host wintertime “frost fairs” (above)—didn’t occur at the same time worldwide.

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Ancient global climate events rippled unevenly across the globe

In the past 2000 years, Earth has drifted in and out of extended periods of warmer- and cooler-than-normal climate, including the so-called Medieval Warm Period and the Little Ice Age. Scientists long thought that during these hot and cold spells, temperatures rose and fell in sync across the globe. In fact, Earth warmed and cooled unevenly, with different regions reaching peak high and low temperatures at different times, two new studies suggest. The one exception: Since the mid–19th century, warming trends have covered some 98% of the globe.

Widespread networks of weather stations that could accurately record local temperatures didn’t exist until the last half of the 1800s. But scientists can estimate past temperatures using a variety of natural proxies. These “paleothermometers” include the widths of tree rings and the proportions of isotopes—forms of atoms such as oxygen that contain different numbers of neutrons—in glacial ice, corals, clam shells, cave deposits, and even lake sediments.

These natural chronicles can shed light on aspects of climate extending back hundreds or even thousands of years, says Raphael Neukom, a climate scientist at the University of Bern. So he and colleagues used seven statistical techniques to analyze almost 700 sets of natural climate records from 1 C.E. to 2000 C.E. Some of their analyses—which looked at tree rings, ice cores, and cave formations, among other sources—used simple correlations. Others used more complex relationships.

Despite those differences, all of the methods yielded global average temperature trends that rose and fell in sync with each other throughout the two millennia, the team reports today in Nature Geoscience. In the preindustrial era, volcanic activity—such as the 1815 eruption of Indonesia’s Mount Tambora—seemed to be a major cause of cooling events. Throughout that time, however, changes in the output of solar radiation didn’t seem to influence global temperatures very much, Neukom notes.

But when he and another group of colleagues looked at regional temperature trends, they found surprising variation. For example, the Little Ice Age (which most scientists say began between 1350 and 1450 before being overtaken by warming in the 1800s) was undoubtedly the longest and deepest cool spell of the past 2 millennia. Most scientists had presumed that the Little Ice Age unfolded pretty much the same everywhere, Neukom says.

But his team’s new analyses, reported in Nature, reveal that’s not the case. In the central and eastern Pacific Ocean, the coldest decades of the Little Ice Age fell during the 15th century. In northwestern Europe and the southeastern United States, the deepest cold occurred during the 17th century. For the rest of the world, the strongest chill didn’t occur until the mid–19th century, almost at the very end of this colder-than-normal interval.

The researchers found the same pattern of asynchrony when they looked at lesser-known events like the Roman Warm Period, which toasted the first few centuries C.E.; the Dark Ages Cold Period, which cast a chill from 400 to 800; and the Medieval Warm Period, which defrosted Earth from around 800 to 1200. As in the Little Ice Age, the warmest and coolest decades within those intervals didn’t occur everywhere in the world at the same time.

In most of these events, less than half the globe experienced the most extreme temperatures at the same time, Neukom says. For example, at the height of the Medieval Warm Period, only 40% of Earth’s surface reached peak temperatures simultaneously. That’s a stark contrast to the warming trends seen since the 1850s, he notes: For more than 98% of the globe, the warmest decades in the 2000-year-long interval the team studied occurred during the late 20th century.

The teams’ analyses didn’t specifically address the causes of the recent global trend, but Neukom notes that “natural causes are not sufficient to explain current warming.”

The results show the current global warming is unusual not only in magnitude, but also in terms of its geography, says Scott St. George, a paleoclimatologist at the University of Minnesota in Minneapolis who was not involved in either work. “No matter where you go, you can’t avoid the dramatic march toward warmer temperatures.”