A group of climate scientists has reached a surprising conclusion about Earth’s past eras of naturally-driven, global warming and cooling—they weren’t global after all.
The authors of new studies in Nature and Nature Geoscience used evidence of ancient climates gathered around the world, from tree rings to coral reefs, to examine the pace and extent of well-known episodes of warming or cooling over the past 2,000 years. They report that events like the Little Ice Age and Mediaeval Warm Period, driven by natural variability, were actually more regional than global in scope.
In fact, the only time in the past 2,000 years that nearly all of the Earth has undergone significant warming or cooling is the present period of change that began in the 20th century, according to the research of Nathan Steiger, an atmospheric scientist at the Lamont-Doherty Earth Observatory of Columbia University, and colleagues. The rate of warming was also higher during the second half of the 20th century than during any similar period of the past 2,000 years, the studies found.
“It was surprising to us that the coherence of the climate, prior to the industrial revolution, was much more regional,” Steiger says. “There were regional periods of cold or warmth, but it’s only during the contemporary period where there’s a global warm period that’s very different from what we see in the past. On one hand it isn’t all that surprising that the climate now is fundamentally different, but this provides a really nice long-term context where can clearly see that contrast.”
Previous studies of past climates have identified well-known periods when the Earth warmed or cooled abruptly during the past 2,000 years. Standout eras include the Little Ice Age (1300s to 1800s), the Mediaeval Warm Period (800 to 1200), the Dark Ages Cold Period (400 to 800), and the Roman Warm Period, which occurred during the first few centuries A.D.
“I think that back in the past the assumption was that these must have been global events, and that if you have a record from a tree ring or an ice core somewhere on the planet you should see evidence of the Medieval Warm Period or you should see evidence of the Little Ice Age,” says Scott St. George, who studies environmental variability at the University of Minnesota and wasn’t involved in the research.
But the new studies suggest otherwise. For example, the coldest period of the Little Ice Age varied widely depending on the region of the planet. The coldest temperatures of the past 2,000 years occurred in the 15th century in the Pacific Ocean, the 17th century in northwestern Europe and southeastern North America, and elsewhere not until the mid-19th century.
Finding the warmest period of the past 2,000 years is far simpler. For 98 percent of the globe, the warmest temperatures in the last two millennia occurred in the late 20th century, the authors report.
Kevin Anchukaitis, a paleoclimatologist at the University of Arizona not involved in the research, says the idea that the Medieval period and Little Ice Age weren’t eras of truly global change has been discussed in previous studies, and the authors’ recent conclusions support that earlier work. “They were broad warm and cold periods, within which different regions of the globe had their coldest or warmest periods at different times. For the Little Ice Age, we know this is linked to volcanism,” Anchukaitis says.
One of the studies also found that rates of warming during the second half of the 20th century were the fastest of the 2,000-year period, based on global average temperatures over timespans of two decades or more. “We looked at the warming rate, how fast it was warming or cooling over the globe over the last 2,000 years, and we found that the most drastic warming over the past 2,000 years occurred during the second half of the 20th century, which highlights the extraordinary character of current climate change,” coauthor and paleolimnologist Raphael Neukom of the University of Bern said in a media briefing. Neukom also noted that the team’s various past climate reconstructions largely agreed with the predictions of climate models on the scale of one to three decades, suggesting that those models’ future climate forecasts may also be accurate over the next few decades.
“What struck me is how robust the earlier reconstructions are,” says Kim Cobb, a climate scientist at the Georgia Institute of Technology unaffiliated with the research, agrees. “This vastly enriched dataset of new paleoclimate records, combined with state-of-the-art modeling, tends to confirm earlier efforts of climate scientists going back 20 years or more. … So the idea that 20th century climate change is very unusual, and outside the rage of natural variability, is certainly being reinforced with an exclamation point now with these new efforts.”
Both studies’ global temperature reconstructions used multiple methodologies, created with the ever-growing repository of ancient climate data maintained in the Past Global Changes or PAGES 2k. Dozens of scientists from countries around the world have contributed nearly 700 records to the open-access database, adding details about ancient climate that were uncovered in glacial ice, ocean sediments, tree rings, corals and other sources. The resource allows scientists to recreate wide snapshots of global climate that would have been extremely difficult just a few short years ago.
“Each one of those records requires an enormous amount of work in the field, and then in the lab,” St. George says. “When you think of ocean corals, a lot of those are recovered using an underwater drill by people in scuba suits. It’s hard to find a 1,000-year-old tree that can reflect changes in temperature or find sediments in an undisturbed lake. So it’s a real challenge sometimes, and there’s a lot of effort that goes into each one of the data points that was used as the foundation for these climate maps.”
Despite the fact that more data is available to paleoclimatologists than ever before, Anchukaitis believes that significantly more work needs to be done if scientists are to gather a truly global picture of past climate. “To make progress in understanding the climate of the [past 2,000 years], we should move beyond applying a smorgasbord of different statistical methods,” he says via email. Instead, scientists need a renewed effort to gather paleoclimate records from places and times that are underrepresented in compilations like PAGES 2k.
“The proxy network is largely Northern Hemisphere tree-rings, tropical records (corals) decline rapidly by 1600, and there are relatively few Southern Hemisphere records outside of the Antarctic ice cores,” Anchukaitis says. “So claims about global spatial patterns prior to about 1600, particularly for the tropics and southern hemisphere, must be viewed cautiously.”
Neukom and colleagues’ study also found that huge volcanic eruptions were a major driver of temperature fluctuations at timescales of two or three decades, while other natural factors, like solar output, didn’t seem to have a significant influence. A third, related study by Stefan Brönnimann and colleagues focused exclusively on the role that five massive volcanic eruptions, including the 1815 Tambora episode, had on shaping climate at the end of the Little Ice Age. The eruptions created a cooling effect, weakened monsoons in Africa causing droughts, and shifted storm tracks over Europe that resulted in increased snowfall and glacier growth in the Alps.
This volcanically active time period, unusual in the past 2,000 years, coincides with the start of industrialization. The overlap makes teasing out which factors impacted climate at that time both difficult and extremely important.
“It does sort of mask the effect of industrial processes, where they are starting to omit more CO2, because they counteract each other,” Steiger says. “So volcanoes could cool, and humans would warm by the release of greenhouse gasses. It’s tricky to parse out what’s what.”
Taken together, the findings of these three studies help better our understanding of Earth’s past climate history and highlight how contemporary climate change is unique over the past 2,000 years.
“They’ve shown that not only is the warming that we’ve experienced in the last few decades larger in magnitude than the kinds of changes we’ve seen due to natural factors in the past, [but] it’s affecting almost the entire planet in the same way at the same time,” St. George says. “That’s really different than earlier prolonged climate changes due to natural factors which sometimes affected a large part of the planet but nothing close to 100 percent. The current warming that we’re going through is almost everywhere, and that’s what really makes it distinct from earlier climatic events due to natural causes.”