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Wednesday, July 18, 2012
Past Rapid Sea Rise
I have no doubt that the 8200 year event or pulse was caused by the
draining of Lake Agassiz. This is actually quite recent and it
clears up the time frame question for me. It is my conjecture that
ice Cap decay began with the Pleistocene Nonconformity which I place
around 13,000 years ago. The earlier date proposed here most likely
reflects the level of uncertainty. In the event a pulse of melt
water would have initiated the process as unstable ice was melted
out. Thereafter the ice loss was slower and took centuries. Even
8200 years ago the flushing of Lake Agassiz was only the beginning of
the last phase. This implies that remnant ice resided in the
Canadian North far into historical times. From 13000 BP to say even
5000 BP it took 9000 years to eliminate the Ice Cap. None of this is
properly fitted into climate modeling efforts I am sure and
represents a significant unknown in terms of historical understanding
of the climate.
Since the total loss was approximately 300 feet for the entire period and pulse losses amount to less that fifteen feet, it turns out that the actual rate of change experienced was around one meter per century in terms of sea level. I suspect that the cultural record likely supports that.
This also informs us that the assumption that permafrost is a stable
phenomena is possibly misplaced. More properly the Northern forests
are slowly changing in the face of the stable Holocene climate and
are possibly undergoing evolutionary transformation.
The only thing that protects the stability of the Arctic Ice is the
existence of the Greenland Ice Cap Remnant. Yet its loss may be in
the cards although that seems unlikely.
cause of rapid sea level rise in the past has been found by
scientists at the University of Bristol using climate and ice sheet
models. The process, named 'saddle-collapse', was found to be the
cause of two rapid sea level rise events: the Meltwater pulse 1a
(MWP1a) around 14,600 years ago and the '8,200 year' event. The
research was published in Nature this week.
a climate model, Dr Lauren Gregoire of Bristol's School of
Geographical Sciences and colleagues unearthed the series of events
that led to saddle-collapse in which domes of ice over North America
became separated, leading to rapid melting and the opening of an ice
Evidence of these
events has been recorded in ocean cores and fossil coral reefs;
however, to date the reason behind the events was unclear and widely
domes up to 3 km thick (three times the height of Snowdon), formed in
regions of high snowfall and higher topography, such as the Rocky
Mountains. Together with the saddles - lower valleys of ice between
the domes - these made up the ice sheet.
Towards the end of the
last ice age, at the time of mammoths and primitive humans, the
climate naturally warmed. This started to melt ice at increasingly
high elevations, eventually reaching and melting the saddle area
between the ice domes.
This triggered a
vicious circle in which the melting saddle would lower, reach warmer
altitudes and melt even more rapidly until the saddle had completely
melted. In just 500 years, the saddles disappeared and only the ice
The melted ice flowed
into the oceans leading to rapid sea level rises of 9 m in 500 years
during the Meltwater pulse 1a event 14,600 years ago and 2.5 m in the
second event, 8,200 years ago.
Dr Gregoire, lead
author of the study, said: "We didn't expect our model to
produce such a rapid sea level rise. We got really excited when we
realised that the events we simulated corresponded to real events!"
In the model, Dr
Gregoire found that saddle-collapse could explain a significant
amount of the sea level rise observed: "The meltwater pulse
produced by the saddle-collapse can explain more than half of the sea
level jump observed around 14,600 years ago. The rest probably came
from the progressive melting of ice sheets in Europe and Antarctica."
This research not only
identifies the process which caused the melting of the North American
ice sheet and the trigger for rapid sea level rises in the past, but
also increases our understanding of the nature of ice sheets and
climate change, allowing further questions to be posed and, with more
Research like this
allows climate and ice sheet models to be tested against evidence
from the real world.
If climate models are
able to reflect patterns observed in natural records our confidence
in them increases. This is particularly relevant where the models are
also used to investigate the effect of climate change on ice sheets
in the future.
study was funded by the NICE Marie Curie Research Training Network
and the NaturalEnvironment Research Council (NERC),
and the numerical model simulations were carried out using the
facilities of the Advanced Computing Research Centre (ACRC) at the
University of Bristol.