What appears in these studies is the glacial speed of climate change
itself within the constraints of the Holocene. There is also a sense
here and I may be wrong on this conjecture, that the energy balance
from year to year is pretty constant but on decadal and even century
long trend lines. The modern world speaks to clear decadal trends
against a longer recovery trend from the Little Ice age.
I have long argued that this has been sufficient to explain present
warmer conditions and that we have not broken free of the Holocene
temperature channel.
The short lesson is that looking at a season can be terribly
misleading.
I would like to see a global drive to target bogs for local tree ring
histories. It will not be accurate as to the annual situation but it
will provide signals across a wide geographic region. Claims on
climate conditions require that form of sample depth.
Researchers able to
better pinpoint history of droughts through exploration of tree rings
by Staff Writers
Pittsburgh PA (SPX) Jul 05, 2012
Through
an exploration of tree rings and oxygen isotopes,
researchers at the University of Pittsburgh are now able to better
pinpoint the history of droughts in the arid and semiarid areas of
the American West.
A paper published in
the online issue of the Proceedings of the National Academy of
Sciences explores the Medieval Climate Anomaly, a particularly warm
period occurring in the northern hemisphere of the American West
around 950 to 1250 C.E. While this time period is known as being a
"dry period," the Pitt researchers have discovered an
unexpected complexity to the patterns of drought.
"East of the
Cascade Mountains, the Pacific Northwest is now dry and hot in the
summer and wet in the winter," said Byron A. Steinman, principal
investigator on the project who earned his PhD in geology from Pitt
in 2011 and is now a postdoctoral researcher at Penn State
University. "We've found that it may not have been dry in the
winter in the Pacific Northwest during the Medieval Climate Anomaly."
Steinman,
who worked with Pitt professor of geology and planetary science Mark
B. Abbott, began by studying tree rings, which often can
record past precipitation and temperatures. However, tree rings are
more accurate at recording this information during the spring and
summer months, when the tree is growing and not lying dormant. To
determine the validity of the tree-ring data, the researchers decided
to undertake a study of oxygen isotopes for comparison.
They explored
isotopes found in nearly 1,500 years of bottom-of-lake sediments from
two bodies of water in Washington state: Castor Lake and Lime Lake.
The isotopic composition of these sediments, says Steinman, can
reflect the amount of water entering a lake, especially during the
wet season.
The researchers paid
particular attention to the calcium carbonate in the water (shown in
the form of calcite), as the oxygen in this mineral relates directly
to the isotope ratio of lake water. Castor Lake is on a plateau, and
the water inflow comes only from precipitation and groundwater.
Therefore, no water is lost through evaporation.
However, Lime Lake
loses the majority of its water through a permanent outflow stream.
By comparing the two lakes, the researchers could determine the water
balance between evaporation and precipitation.
To pinpoint the time
of the drought, the researchers looked at two stable isotopes of
oxygen-oxygen 16 and oxygen 18-in the sediments. Oxygen 16 is lighter
than oxygen 18, and so during evaporation more of it is released-the
calcite in the sediments containing more of the oxygen 18. If the
lakes are full of water, however, there will be more oxygen 16 in the
calcite.
The layers of
sediments that are laid down each year can be dated either using
carbon 14 dating of organic material or by locating layers of tephra
(volcanic ash).
In the end, however,
what they found was a mismatch of data.
"The tree ring
and isotope data matched up on a short-term, decadal scale,"
said Steinman. "However, on a longer-term, century scale, the
records diverged. The tree-ring data suggests dry conditions during
the Medieval Climate Anomaly summers while the isotope data suggest
wetter-than-expected winters."
In
the paper, the researchers suggest a strong centennial relationship
over the past 1,500 years between winter precipitation and
the climate variability patterns that shift about every 20
to 30 years in the Pacific (known as Pacific Decadal
Oscillation-PDO). PDO is linked to the El Nino Southern Oscillation,
a tropical phenomenon that influences global weather patterns.
"Before and
during the Medieval Climate Anomaly, the North Pacific Ocean was
warmer, and Washington had a greater precipitation than during the
Little Ice Age, which occurred from 1450 to about 1850 C.E., when
there was less precipitation," said Steinman.
Steinman hopes to
continue this study, producing additional quantitative precipitation
records with different lake systems, to better understand these
climate phenomena.
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