It's not just humans who track long-term trends in the weather. The history of our planet's climate is recorded by nature herself — written in tree rings and arctic ices, corals and deep ocean sediments.
Each of these records tell only a small piece of the Earth's climate story, but together they form a sprawling and complex novel of the past, helping scientists better understand how and why certain trends emerged and foreshadowing what we might expect the Earth's climate to look like in the future.
In 2017 an international consortium known as PAGES (Past Global Changes) published its largest database yet of climate temperature records, stretching from the present day back 2,000 years, by the start of the Common Era.
The data points were recorded by trees, ice, lake and ocean sediment, and mineral deposits in caves, as well as by humans. They come from 648 locations across the globe, including all the continents and major ocean basins.
Scientists have only just begun to mine the PAGES data for insights into trends of the past, but already they have made some intriguing discoveries.
After analyzing 2,000 years of detailed records kept by both nature and humans, researchers have discovered that the average surface temperature of the Earth has warmed faster in the past few decades than it did in the previous 1,900 years, proving once again that the current warming the planet is experiencing is unprecedented in the past two millenniums. What's more, the data also suggest that the warming in the most recent decades has been uniform across all regions of the planet, a phenomenon that scientists say has not been seen in the past 2000 years.
A third study based on the same data shows that for most of the Common Era volcanic events have been the primary driver of global temperature change. Today, greenhouse gases in the atmosphere play a more dominant role in driving global temperature, the authors said.
Raphael Neukom, a paleoclimatologist at the University of Bern in Switzerland who led two of the three studies said the trio of papers, published Wednesday in the journals Nature and Nature Geoscience, all suggest the same thing: "Climate variability in the pre-industrial period is totally different from what we observe today."
Some of the findings reported Wednesday contradict, or at least complicate previously held truths.
For example, in the Nature paper, Mr. Neukom and his colleagues use the PAGES data to poke holes in the widely accepted theory that periods of cooling and warming over the past 2000 years affected the globe uniformly. Perhaps the most well known of these epochs is what scientists call the Little Ice Age — a cool period that persisted from the 16th century to the 19th century. Similarly, the Medieval Climate Anomaly is known as a warm, dry period that lasted from AD 950 to 1250.
"The traditional understanding was that climate over these periods was globally coherent," said Nathan Steiger, a paleoclimatologist at Columbia University in New York who worked on the paper. "But when we looked at the PAGES data, we found they are not as coherent as we thought."
After analyzing the data using multiple statistical methods, the authors found that what were previously assumed to be global trends in temperature were actually regional trends for all known climate epochs except the one we find ourselves in today.
For example, while the Little Ice Age did represent a global cooling, some parts of the planet were coldest during the mid-19th century, while others had their coldest weather several centuries earlier. At the height of the Medieval Climate Anomaly, only 40% of the Earth experienced peak temperatures at the same time.
But when the authors ran the same analysis for our current climatic epoch, known as the Contemporary Warm Period, they found that peak temperatures have been seen across all regions of the globe except Antarctica within the past 51 years.
This uniformity was unprecedented in the past 2000 years.
"The familiar maxim that climate is always changing is certainly true," wrote Scott St. George, a geographer at the University of Minnesota at Minneapolis who was not involved in the work. "But even when we push our perspective back to the Roman Empire, we cannot discern any event that is remotely equivalent … to the warming over the past few decades."
The authors did not look into what might be causing this unique phenomenon, because, as Mr. Steiger said, it wasn't necessary.
"There is a lot of evidence that the Contemporary Warming Period is human-caused," he said. "We don't need the paleolithic climate data to address that."
In another study, a team of researchers led by Stefan Bronnimann of the University of Bern in Switzerland used the PAGES data to explore the causes of climate fluctuations from 1300 to 1800. This analysis led them to conclude that in this time period a cluster of volcanic eruptions were the primary drivers of global temperature change and that their effects lingered over many decades.
(Large volcanic eruptions can cause wide-scale cooling because their ash gets into the stratosphere and reflects sunlight back into space.)
They argue that at least some of the global temperature increases the Earth experienced starting in the 1830s were probably due to the planet's slow recovery from the volcanically induced cooling. However, the data also suggest that from the late 19th and early 20th century onward, greenhouse gases in the atmosphere dominated the subsequent warming trend.
Today, there is little doubt among scientists as to how those greenhouse gases got into our atmosphere.