Climate-change sceptics sometimes argue that there is no cause for concern from global warming because in the past 2000 years the world has already gone through several natural cycles of warming and cooling from which it has always rebounded on its own.
However, new research has found that rather than being global events, these natural cycles were actually regional changes that never affected more than part of the globe at any one time.
Furthermore, these warming and cooling patterns didn’t occur anywhere close to simultaneously in the parts of the world they did affect.
The findings are published in a suite of papers in the journals Nature and Nature Geoscience.
“Traditionally, the understanding is that there were globally coherent periods of climate variability,” says Nathan Steiger, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory in New York, US, who was part of a study team led by Raphael Neukom, a climate researcher from the University of Bern, Switzerland.
“There was a cold period called the Little Ice Age [1300-1850 AD], a warm period called the Medieval Warm Period [800-1200 AD], and a couple of other periods as we go back further in time [the Dark Ages Cold Period, 400-800 AD, and the Roman Warm Period, 1-400 AD].
“But what we show is that these periods weren’t globally coherent, as previously thought.”
Modern climate change, on the other hand, is starkly different, not just because of the rapid rate of warming, but because more than 98% of the Earth is warming simultaneously.
“That very much stands out in contrast to the past 2000 years,” Steiger says.
The reason this wasn’t discovered before appears to be that these multi-century warming and cooling epochs were best documented in Europe. So much so, that most of their names are tied to European history: Medieval Warming, Dark Ages Cooling, Roman Warming.
“It was easy to make the assumption that these changes were happening everywhere on the planet at the same time,” says Scott St. George, a paleoclimatologist at the University of Minnesota, US, who was not part of the study team, but wrote a supporting commentary in Nature.
“What this study has demonstrated is that less than half of the planet was being pushed in the same direction – whether warming or cooling – at the same time.”
Now, he says, “no matter where you go, everywhere is being pushed in the same direction at the same time”.
To prove this, Steiger, Neukom and colleagues turned to a newly created global database of climate records called PAGES 2k, which compiled nearly 700 measures of climate over the past 2000 years, based on such measures as tree rings, ice cores, growth patterns in coral reefs, cave deposits, and sediment records, from across the world.
From this, for example, they discovered that the coldest parts of the Little Ice Age hit the central and eastern Pacific in the fifteenth century, but didn’t hit major parts of Europe or North America until the seventeenth century – and were delayed in other parts of the world until the nineteenth.
Not that this means natural processes can’t produce simultaneous climate shifts.
But, unlike the multi-century shifts, these occur over time horizons of a decade or two, says Neukom, who led a separate study that used the same PAGES 2k dataset to look at shorter-term shifts in global climate.
What they found, Neukom says, is that in pre-industrial times these shifts were driven primarily by major volcanic eruptions, which blasted fine particles high into the atmosphere, producing a bright haze that reflects sunlight back into space, reducing the amount of heat reaching the Earth’s surface.
In yet another study, a team led by Stefan Brönnimann, also of the University of Bern, concluded that many of the impacts of even the Little Ice Age are the result of volcanic activity – specifically a cluster of five major eruptions that occurred between 1808 and 1835.
The most famous of these is the 1815 eruption of Indonesia’s Mt. Tambora – one of the most massive volcanic blasts in human history, and cause of what later became known in major parts of the Northern Hemisphere as “The Year Without a Summer”.
These blasts, Brönnimann, says, produced a “gear shift” in many parts of the world’s climate systems, producing, among other things, weak monsoons in Africa and India, and changes in ocean circulation that produced climate effects that reverberated all the way through the 1840s.
Among other things, these caused a southward shift in North Atlantic storms that dumped more snow in the Alps and contributed to advance of their glaciers, years after the last of the eruptions.
St. George adds that it’s also been suggested that the Medieval Warm Period might, in large part, have been created by a “long, quiet period” without major volcanic eruptions. “It sounds plausible,” he says.
As for the cause of the current warming period?
“We didn’t address the question of human causes,” Steiger says. “It’s implied, rather than explicit. There’s a lot of evidence that the contemporary period is human-caused. We don’t have to look at paleoclimate to look at that.”