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Earth’s Average Temperature Lower Now Than It Was 5,000 Years Ago
March 7, 2013, 1:31 pm
Filed under: Uncategorized

Earth’s Average Temperature Lower Now Than It Was 5,000 Years Ago:

Getting hotter fast.That’s the conclusion of a new study, “A
Reconstruction of Regional and Global Temperature for the past
11,300 Years,” being published in the journal Science
today. But before drawing in a sigh of relief about the future of
global warming, the researchers also point out that the rapid
warming over the last century has essentially cancelled out 2,000
years of gradual cooling.
The researchers from Oregon State University and Harvard
University came to their results by combining 73 different proxy
climate records (assembled into what they call stacks) spanning the
past 11,500 years. They report:

Our results indicate that global mean temperature for the decade
2000–2009 has not yet exceeded the warmest temperatures of the
early Holocene (5000 to 10,000 yr B.P.). These temperatures are,
however, warmer than 82% of the Holocene distribution as
represented by the Standard5×5 stack, or 72% after making plausible
corrections for inherent smoothing of the high frequencies in the
stack. In contrast, the decadal mean global temperature of the
early 20th century (1900–1909) was cooler than >95% of the
Holocene distribution under both the Standard 5×5 and
high-frequency corrected scenarios. Global temperature, therefore,
has risen from near the coldest to the warmest levels of the
Holocene within the past century, reversing the long-term cooling
trend that began ~5000 yr B.P.

From the abstract:

Surface temperature reconstructions of the past 1500 years
suggest that recent warming is unprecedented in that time. Here we
provide a broader perspective by reconstructing regional and global
temperature anomalies for the past 11,300 years from 73 globally
distributed records. Early Holocene (10,000 to 5000 years ago)
warmth is followed by ~0.7°C cooling through the middle to late
Holocene (<5000 years ago), culminating in the coolest
temperatures of the Holocene during the Little Ice Age, about 200
years ago. This cooling is largely associated with ~2°C change in
the North Atlantic. Current global temperatures of the past decade
have not yet exceeded peak interglacial values but are warmer than
during ~75% of the Holocene temperature history. Intergovernmental
Panel on Climate Change model projections for 2100 exceed the full
distribution of Holocene temperature under all plausible greenhouse
gas emission scenarios.

The new study finds that changes in the amount of summertime
sunlight striking the Northern Hemisphere due to changes in the
Earth’s orbital orientation toward the sun is chiefly responsible
for the recent alternation between Ice Ages and warmer periods like
the one we’re currently in. From the study’s press release:

“During the warmest period of the Holocene, the Earth was
positioned such that Northern Hemisphere summers warmed more,”
[Shaun] Marcott, [lead author from OSU] said. “As the Earth’s
orientation changed, Northern Hemisphere summers became cooler, and
we should now be near the bottom of this long-term cooling trend –
but obviously, we are not.”

So how do recent changes in global average temperature compare
to the past record of climate? Again from the press release:

“The last century stands out as the anomaly in this record of
global temperature since the end of the last ice age,” said Candace
Major, program director in the National Science Foundation’s
Division of Ocean Sciences, which co-funded the research with NSF’s
Division of Atmospheric and Geospace Sciences. “This research shows
that we’ve experienced almost the same range of temperature change
since the beginning of the industrial revolution as over the
previous 11,000 years of Earth history – but this change happened a
lot more quickly.”

Taken at face value, the new study does suggest, to paraphrase
the old investing caveat, that we should worry about what past
performance may have to say about future results.


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