Major new paleoclimatology study shows global warming has upended 6,500
years of cooling
Comprehensive compilation of pre-historic temperature records shows that global warming is reaching levels not seen for at least 6,000 years

Date:
June 30, 2020
Source:
Northern Arizona University
Summary:
Over the past 150 years, global warming has more than undone
the global cooling that occurred over the past six millennia,
according to a major new study.



FULL STORY ========================================================================== [Glacier collapse (stock | Credit: (c) volki / stock.adobe.com] Glacier collapse (stock image).

Credit: (c) volki / stock.adobe.com [Glacier collapse (stock | Credit:
(c) volki / stock.adobe.com] Glacier collapse (stock image).

Credit: (c) volki / stock.adobe.com Close Over the past 150 years, global warming has more than undone the global cooling that occurred over the
past six millennia, according to a major study published June 30 in Nature Research's Scientific Data, "Holocene global mean surface temperature,
a multi-method reconstruction approach." The findings show that the millennial-scale global cooling began approximately 6,500 years ago when
the long-term average global temperature topped out at around 0.7DEGC
warmer than the mid-19th century. Since then, accelerating greenhouse
gas emissions have contributed to global average temperatures that are
now surpassing 1DEGC above the mid-19th century.


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Four researchers of Northern Arizona University's School of Earth and Sustainability (SES) led the study, with Regents' professor Darrell
Kaufman as lead author and associate professor Nicholas McKay as
co-author, along with assistant research professors Cody Routson and
Michael Erb. The team worked in collaboration with scientists from
research institutions all over the world to reconstruct the global average temperature over the Holocene Epoch -- the period following the Ice Age
and beginning about 12,000 years ago.

"Before global warming, there was global cooling," said Kaufman. "Previous
work has shown convincingly that the world naturally and slowly cooled
for at least 1,000 years prior to the middle of the 19th century, when
the global average temperature reversed course along with the build-up
of greenhouse gases. This study, based on a major new compilation of
previously published paleoclimate data, combined with new statistical
analyses, shows more confidently than ever that the millennial-scale
global cooling began approximately 6,500 years ago." Earlier this year,
an international group of 93 paleoclimate scientists from 23 countries
-- also led by Kaufman, McKay, Routson and Erb -- published the most comprehensive set of paleoclimate data ever compiled for the past 12,000
years, compressing 1,319 data records based on samples taken from 679
sites globally.

At each site, researchers analyzed ecological, geochemical and biophysical evidence from both marine and terrestrial archives, such as lake deposits, marine sediments, peat and glacier ice, to infer past temperature changes.

Countless scientists working around the world over many decades conducted
the basic research contributing to the global database.

"The rate of cooling that followed the peak warmth was subtle, only
around 0.1DEGC per 1,000 years. This cooling seems to be driven by slow
cycles in the Earth's orbit, which reduced the amount of summer sunlight
in the Northern Hemisphere, culminating in the 'Little Ice Age' of recent centuries," said Erb, who analyzed the temperature reconstructions.

Since the mid-19th century, global warming has climbed to about 1DEGC, suggesting that the global average temperature of the last decade
(2010-2019) was warmer than anytime during the present post-glacial
period.

McKay, who developed some of the statistical approaches to synthesizing
data from around the world, notes that individual decades are not
resolved in the 12,000-year-long temperature reconstruction, making
it difficult to compare it with any recent decade. "On the other hand,
this past decade was likely cooler than what the average temperatures
will be for the rest of this century and beyond, which are very likely to continue to exceed 1DEGC above pre-industrial temperatures," McKay said.

"It's possible," Kaufman said, "that the last time the sustained average
global temperature was 1DEGC above the 19th century was prior to the
last Ice Age, back around 125,000 years ago when sea level was around
20 feet higher than today." "Investigating the patterns of natural
temperature changes over space and time helps us understand and quantify
the processes that cause climate to change, which is important as we
prepare for the full range of future climate changes due to both human
and natural causes," said Routson. He used an earlier version of the
database to link Arctic warming to a reduction in precipitation at mid latitudes (see related article).

"Our future climate will largely depend on the influence of human factors, especially the build-up of greenhouse gases. However, future climate will
also be influenced by natural factors, and it will be complicated by
the natural variability within the climate system. Future projections
of climate change will be improved by better accounting for both
anthropogenic and natural factors," he said.

The reconstruction of past global temperature is the outgrowth of several
NAU research projects aimed at understanding the causes and effects of
natural climate variability, work that was funded through more than $1.2 million in grants from the National Science Foundation. The team was
recently awarded another $678,000 in grants from the NSF for related
work extending through 2023.


========================================================================== Story Source: Materials provided by Northern_Arizona_University. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Darrell Kaufman, Nicholas McKay, Cody Routson, Michael Erb,
Christoph
Da"twyler, Philipp S. Sommer, Oliver Heiri, Basil Davis. Holocene
global mean surface temperature, a multi-method reconstruction
approach.

Scientific Data, 2020; 7 (1) DOI: 10.1038/s41597-020-0530-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200630072044.htm

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