https://www.cbc.ca/news/technology/climate-change-2000-years-1.5222258
[links and images in online article]
Recent warming over the past 100 years is not part of a natural process,
studies find
Periods of warming and cooling were regional and not global
Nicole Mortillaro · CBC News · Posted: Jul 24, 2019 1:00 PM ET
Earth's natural cycles can't account for the recent warming seen over
the past 100 years, new research suggests.
In one of three new studies published in the journals Nature and Nature
Geoscience, researchers found that previous periods of climate change
such as the Little Ice Age and the Medieval Warming Period were regional
and not a global phenomenon.
In contrast, the warming that has occurred over the past century has
been far-reaching and global in nature.
"In this paper, what we do is look at climate over the past 2,000 years
— and traditionally the understanding of climate over this period is
that there were globally coherent periods of climate variability," said
Nathan Steiger, co-author of the paper and an associate researcher at
the Lamont-Doherty Earth Observatory at Columbia University. "But what
we show is that these periods are not globally coherent, as previously
thought."
In order to obtain a robust sampling of data around the world, the
researchers used 700 records obtained from trees, ice, coral, sediment
and more.
In the case of the Little Ice Age, the researchers found that different
parts of the planet experienced changes at different times.
The central and eastern Pacific regions experienced their coldest
temperatures in 2,000 years during the 15th century. But in northwestern
Europe and southeastern North America, the coldest temperatures occurred
during the 17th century. For everywhere else, it occurred during the
19th century.
In contrast, the warming we are seeing today spans 98 per cent of the
planet.
Their study only went as far as the year 2000, but nine of the 10
warmest years on record have occurred since then, with the past five
years being the warmest.
Those who question or disagree with scientific evidence pointing to
anthropogenic, or human-caused, climate change often cite Earth's
natural cycles as one of the reasons for the planet's recent warming.
The Little Ice Age is most often presented as evidence of part of this
natural cycle.
But, Steiger said, "internal variability [doesn't] explain the coherence
that we see for the contemporary warming."
Agreement
In a second study, researchers examined Earth's rate of surface warming
— the global mean temperature — and its drivers. They found that the
rate of warming over periods of at least 20 years was fastest during the
late 20th century.
"We find that at pre-industrial times … major volcanic eruptions were
the major drivers of temperature fluctuations," said co-author Raphael
Neukom, a scientist at the Oeschger Centre for Climate Change Research
at the University of Bern. But external forces such as variations in the
sun's output did not have "a significant influence" on temperatures.
The sun is also often used by climate skeptics as an external driving
force responsible for climate change.
The third paper also concluded that volcanoes played a role in climatic
upheaval in the past.
It's this agreement between the three separate studies that the teams of
researchers believe is an important indicator of the climate state the
planet is currently experiencing.
"The basic conclusion is that what's happening today is anomalous, and
we understand why it's anomalous — it's not a mystery," said Gavin
Schmidt, a climatologist and director of NASA's Goddard Institute for
Space Studies, who was not involved with the study.
But, he said, while the findings may not be entirely new, it's
encouraging to have the new research to support other studies, and to
offer it up to people who are "grasping at straws to avoid" dealing with
climate change.
"This is really like a triple underlining of the fact that what's
happening now is unusual in a multi-centennial context and maybe in a
much larger context," Schmidt said. "And people have to deal with that."
Steiger said his team expected to find warming across the globe but were
surprised at the extent.
"There's a lot of evidence for humans causing the contemporary warming.
From our perspective, what we were most interested in is the past
explaining the past, and how we understand the past, and then contrast
it with the [present]," Steiger said. "There's so much evidence out
there anyway. We didn't need to put the nail in the coffin."
[More climate change headlines and links at the 10n10.ca blog
https://www.10n10.ca/e/CCC-Blog.shtml ;]
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