The new study, just out on Wednesday in the journal Nature, suggests human-caused, or anthropogenic, climate change has been going on for decades longer than existing temperature records indicate. Using paleoclimate records from the past 500 years, the researchers show that sustained warming began to occur in both the tropical oceans and the Northern Hemisphere land masses as far back as the 1830s — and they’re saying industrial-era greenhouse gas emissions were the cause, even back then.
“I don’t think it changes what we know about how the climate has warmed during the 20th century, but it definitely adds to the story,” said Nerilie Abram, an expert in paleoclimatology at Australian National University and the new study’s lead author.
People first started keeping organized, global temperature records starting around the 1880s, and these are the records that many scientists reference when looking back on how the climate has changed over the last century. And it’s clear that it’s been warming — and that human activities are the primary cause. But just looking at records from the 1880s on doesn’t tell the whole story, according to Abram.
“We can see that by only looking from the 1880s on, we don’t have the full picture of how we’ve been changing the climate,” she said.
The new research involved 25 scientists from around the world, including more than a dozen researchers from the PAGES 2k (or Past Global Change 2000 year) Consortium, a group supporting research into Earth’s past in order to gain a better understanding of its climate future. The PAGES team has been involved with creating paleoclimate reconstructions of temperatures over both land and sea. These reconstructions have relied on special analyses of coral, tree rings and ice cores, all of which contain chemical fingerprints that can give scientists insights into what the climate was like hundreds or even thousands of years ago.
The research team used these paleoclimate records to look back at the progression of industrial-era warming across the Earth over the past few hundred years. The industrial era is a period of time loosely beginning around the mid-18th century, when industrial growth around the world led to a sharp increase in the burning of fossil fuels and the emission of greenhouse gases, which contributed to the onset of anthropogenic climate change on Earth.
The team’s reconstructions indicated that significant and sustained warming began in the tropical oceans around the 1830s, about the same time it began over the continental land masses in the Northern Hemisphere. Warming in the Southern Hemisphere was delayed until about 50 years later, the reconstructions suggested — this likely has to do with differences in oceanic and atmospheric circulation there, Abram said.
(This video, which the researchers have put together to help summarize their findings, helps illustrate the temperature changes indicated by the reconstructions over the past 500 years.)
The reconstructions suggest that the current pattern of sustained, long-term warming on Earth began much earlier than existing temperature records might lead us to believe. The findings are “further evidence that the climate has already changed significantly since the pre-industrial period,” said Ed Hawkins, a climate researcher at the University of Reading who was not involved with the new study, in an emailed comment.
The researchers also examined climate models to find out whether climate simulations would match up with their reconstructions, as well as to gain some insight into what exactly was causing the warming effect. They found the models generally agree with their reconstructions for the Northern Hemisphere, although they don’t quite capture the delayed warming effect in the Southern Hemisphere for reasons that remain unclear.
And importantly, the researchers say, the simulations suggest that the influence of greenhouse gases in the atmosphere is the cause behind the early onset of warming.
The researchers suggest that this early, human-influenced onset of warming may mean the climate is more sensitive to the effects of greenhouse gas emissions than scientists previously thought.
The researchers do note that “naturally forced climate cooling may have helped to set the stage for the widespread onset of industrial-era warming in the tropical oceans and over Northern Hemisphere landmasses during the mid-nineteenth century.” In 1815, powerful volcanic eruptions in Indonesia were responsible for a strong cooling effect on Earth, which reversed when the sustained warming pattern began to emerge.
But some experts are saying that the research team should be ascribing more importance to this early 19th-century cooling effect in the context of the warming that came after it. Michael Mann, distinguished professor of atmospheric science at Pennsylvania State University said he is “troubled” by the researchers’ suggestion that the planet may be more sensitive to greenhouse gas emissions than previously thought.
Mann said a lot of the warming observed in the early 1800s was actually just the climate naturally recovering from this unusual cooling effect, and not primarily caused by the influence of greenhouse gas emissions, which would not become a primary driver of warming until decades later.
“There was certainly some anthropogenic warming prior to the late 19th century,” he worte in an email, pointing to some recent research he co-authored that supported this effect. “But the authors overstate how much, and how early, by incorrectly conflating early 1800s warming caused by the recovery from these eruptions with early greenhouse warming.”
But the researchers stand by their interpretation of the reconstructions.
“Our initial reaction to detecting this early onset of warming was the same – that what we were seeing was the climate rebounding from the big eruptions in the early 1800s and that later the effect of greenhouse gases kicked in,” Abram noted in a follow-up email to The Post. “But as we continued to test the data and our methods it became clear that you don’t need these big eruptions in the early 1800s to explain the early warming.”
According to Abram, the findings could have important implications for the ongoing global discussions about the range of temperature increases that should be considered safe for the planet. At last year’s UN climate conference in Paris, participating nations agreed to a goal of keeping global warming within 2 degrees Celsius of their pre-industrial levels at most — and within 1.5 degrees if possible — in order to avoid triggering more dangerous climate consequences.
According to Abram, the study’s results suggest that “we have already warmed the planet more than what we would think we have if we’re basing our assessments just on the period since the 1880s.” And this means we may actually be closer to the kind of dangerous climate consequences many experts have predicted the planet could see if we blow past the 2-degree threshold, she suggested. For instance, some natural ecosystems — coral reefs, for instance, which are already known to be suffering from the effects of global warming — may have experienced much greater climate changes in the past few hundred years, far beyond the conditions they originally evolved to tolerate, than scientists previously thought. These are points that policymakers could take into consideration when considering how much climate warming we should be willing to accept in the future.
In any case, the researchers feel that the climate reconstructions can provide important insights into the Earth’s sensitivity to human activities, which can help inform our understanding of how future climate change might progress. And they point to the importance of creating reconstructions in the first place for making these insights possible.
“Actually finding that humans had a measureable impact on the climate in the mid 19th century was somewhat of a surprise,” Abram said. “It’s a finding that, no matter which way we tested, we kept coming up with that same answer.”
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