The global reach of human-caused climate change, and the breakneck speed at which it’s happening, is unparalleled within the last 2,000 years, according to new research.
Climate change as we observe it today cannot be compared to other climatic epochs of the past 2,000 years, according to two new studies led by paleoclimatologist Raphael Neukom from the University of Bern in Switzerland. Climate science deniers will find this research a tough pill to swallow, as the new papers will make it increasingly difficult for them to brush off contemporary climate change as a normal thing that happens on a regular basis.
To be clear, the past 2,000 years, also known as the Common Era, has experienced conspicuous climate shifts. Notable examples include the Little Ice Age, a cooling period that lasted from the 15th century through to the late 19th century, and the Medieval Climate Anomaly, a warm, dry period lasting from 950 to 1250 CE.
But as the new research shows, these climatic epochs were smaller than is conventionally assumed, weren’t planet-encompassing events. Moreover, these historic warming and cooling periods were primarily triggered by volcanic activity, highlighting their transient nature. The new papers should heighten concerns about anthropogenic climate change, as there’s no precedent within the last 2,000 years for what our planet is currently experiencing.
In the first of the two new papers, published in Nature, Neukom and his colleagues reconstructed temperature trends of the Common Era. It’s a fairly straightforward task to collect temperature readings from the past 150 years, but beyond that it gets considerably more difficult; thermometer readings of surface temperatures prior to 1850 are sparse to non-existent.
In its place, the researchers used “paleothermometers,” such as measurements of tree ring width, the size of coral reefs, the depth of lake sediment, the size and extent of glaciers, among other natural indicators of warming and cooling trends. In all, the researchers studied nearly 700 of these “proxy records” for their analysis.
Mercifully, the researchers didn’t have to scour the planet themselves in search of this information. These proxy records were accessed in a community-sourced archive known as PAGES 2k. The data within this archive was dense enough that the scientists could construct a year-by-year breakdown of global temperature for the past 2,000 years.
Looking at the data, Neukom and his colleagues found “No evidence for globally coherent warm and cold periods over the preindustrial Common Era,” as the title of the new study reads. Subsequently, the research quashes the commonly held notion that climate epochs happened at the same time around the planet.
For example, extended cold spats of the Common Era were documented in the eastern Pacific regions in the 15th century, in northwestern Europe and southeastern North America in the 17th century, and elsewhere in the 19th century. During the Medieval Climate Anomaly, only 40 percent of the planet’s surface was affected. These figures run in stark contrast to observations made in the final decades of the 20th century, in which climate change was shown to impact 98 percent of the globe, according to the new study.
“This provides strong evidence that anthropogenic global warming is not only unparalleled in terms of absolute temperatures, but also unprecedented in spatial consistency within the context of the past 2,000 years,” the authors wrote.
In an email to Gizmodo, paleoclimatologist Michael Mann from Pennsylvania State University, who wasn’t involved with the new research, said it’s “reassuring and gratifying” that, nearly two decades after his team’s original work—the highly influential “Hockey Stick” studies (here and here)—that “independent, international teams of scientists using entirely different approaches, and more widespread now-available paleoclimate data, have come to virtually identical conclusions to those we offered in our original work,” Mann said.
Like the new study, Mann and his colleagues used reconstructed historical climate trends to highlight the unprecedented nature of anthropogenic climate change.
In the second paper, published in Nature Geoscience, Neukom and his colleagues considered the possible causes of the observed climate anomalies of the Common Era, finding that volcanic activity was the primary culprit.
From 1300 to 1800, for example, the major volcanic eruptions were cited as main factors for historic climate swings, in which eruptions cooled the climate. Notable eruptions included the Samalas volcano in 1257 and Mount Tambora in 1815, both of which occurred Indonesia. At the same time, other factors, such as changes in the Sun’s radiative output, were not enough to produce the observed climate swings.
“These papers result from an ambitious and laborious effort that makes use of the results of scores of individual paleoclimate studies over the years,” Ron Amundson, a professor at the Department of Environmental Science, Policy, and Management at UC Berkeley, told Gizmodo. “Once assembled, they show that the well-known warm and cold periods that humanity has experienced over the past 2,000 years were unlikely global in scale, were caused by natural climate factors, particularly volcanic eruptions and associate dust and ash. Most importantly, they pale in magnitude and global extent to the 20th and 21st century warming that the Earth is now undergoing due to human impacts on the climate system,” said Amundson, who wasn’t involved with the new research.
An accompanying News and Views article written by Scott St. George, a researcher at the University of Minnesota, addressed one potential shortcoming of the new paper: the use of proxy records.
“Tree-ring records, the most frequently used proxy archive in the PAGES 2k database, are sometimes unreliable in registering slow climate changes over several centuries or longer,” wrote St. George. “Moreover, some other proxies—particularly records from marine and lake sediments—exaggerate variations at multidecadal or centennial timescales. It is still an open question how well we can compare global temperatures across this entire 2,000-year span.”
No doubt, there’s still room for improvement as far as paleoclimatology is concerned, but as Mann aptly pointed out, the methods used are increasingly improving, and the data is still telling us the same thing: What’s happening now ain’t normal.
“That’s how science works. If you’re wrong, other researchers will eventually show you to be wrong,” Mann told Gizmodo. “If you’re right, researchers will continue to reaffirm and even extend your findings.”