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Sunday, November 18, 2012

Climate change connection between Global temperatures, ice volume and sea level

There is a rapid response connection between polar temperature changes, ice volume and sea leavel according to a new method of dating of sea level records and comparing them with temperature data and ice core records for the last 150,000 years.

Previously the dating of ice-volume changes relied on the ‘Red Sea relative sea-level (RSL) record’. However this dating method did not have independent age control which inhibited detailed comparison with other well-dated climate parameters, such as temperature or CO2 records from ice cores from Greenland and Antarctica. The new method of dating the RSL record used Mediterranean data from radiometric (Uranium-series) dating of cave deposits which could be applied to the last glacial cycle reaching back 150,000 years, providing a continuous sea-level record with excellent independent age control.

According to the study sea level rise reached speeds of "at least 1.2 metres per century during all major episodes of ice-volume reduction" in the last 150,000 years.

One of the study authors, Professor Elco Rohling from the National Oceanography Centre Southampton said: "This is the first time that these rates could be measured for any other period than the end-of-ice age 'terminations/deglaciations'. Although it is always hard to step from palaeo reconstructions to future projections, it suggests that when significant ice-volume adjustments happen, they are rarely slow."

Ice sheet response is like the inertia in a heavy freight train

"Ice sheet responses to a change in climate forcing are like the responses of heavy freight trains to firing up the locomotive. They are hard to set in motion (slow to ‘spin up’), but once they are reacting, they will be equally slow to ‘spin down’. So a lag of a few centuries is worrisome, because we have been warming up the climate for 150-160 years now. If the natural relationship (when changes in climate were slower than today) also holds for the very fast changes in climate today, then we are coming into that ‘window’ of time where we may expect to start seeing some unprecedented responses in the large ice sheets. This then may tie in with observations of the past decade or so of large ice-shelf collapses around Antarctica and Greenland, the major melt-area expansion over Greenland, changes in the flow speed of major ice streams (both Antarctica and Greenland), and increasing ice-mass loss over West Antarctica/the Antarctic Peninsula and Greenland." (Emphasis added by me)

The polar regions do react differently with Greenland temperatures and ice volume loss occurring virtually simultaneously, while Temperature changes over Antarctica lead changes in ice volume usually by several centuries says Elco Rohling.

“Our observations suggest that the Greenland temperature changes may largely be regional responses to changes in ice volume (at that time over North America and also Eurasia). In contrast, the Antarctic relationship suggests that temperature changes as recorded in Antarctic ice cores may have been driving the ice-volume changes. Possibly, the Antarctic temperature changes reflect the fundamental underlying global temperature changes that drove the ice-volume changes, while the Greenland temperature record primarily represents a regional temperature response to the changing ice volumes. These are intriguing hints at fundamental aspects of the Earth System’s response to climate change, which merit further investigation.” said Rohling.

Whether this delay in Antarctica stands true for the much faster anthropogenic warming rates we are doing to the planet might be a different matter. A warmer and fresher Southern Ocean and warm currents are already melting Antarctic ice shelves with grounding lines starting to receed. Once ice shelves are in retreat, glaciers and ice streams can accelerate carrying and discharging large volumes of ice sheet from deep within the continent. The West Antarctic Ice Sheet is particularly unstable lying on bedrock that dips well below sea level.

Rapid Ice Sheet Melt and sea level rise

Anyone who has been following research into rates of sea level rise in the past will be aware of research into the meltwater pulse 1A event which occurred at the end of the last glacial period between 13,000 and 14,600 years which showed how fast sea level rise could occur: about 20 metres in less than 500 years.

Ice sheets can collapse relatively rapidly according to research from Penn State University in 2009: "Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years." says Pollard and DeConto (2009) in Nature, referring to modelling the West Antarctic ice sheet growth and collapse through the past five million years. (See Global Warming in Antarctica: Thwaites and Pine Island Glaciers accelerating, West Antarctic Ice Sheet losing mass)

Indeed, James Hansen predicted in an 2007 interview on the ABC 7.30 Report that the earth will pass a tipping point resulting in Sea Level Rise of up to a metre every 20 years.

So, we know there is a connection between increasing polar temperatures, ice volume loss, and sea level rise. We know these changes start relatively slowly because of the immense inertias in the climate system in the oceans and polar regions. This research indicates a lag of two or three centuries from the start of changes to a rapid response in ice volume loss. We have been pumping CO2 into the atmosphere with the industrial revolution for the last two centuries resulting in rapid temperature increases in the polar regions (much more so than the mid-latitudes and tropics), so we are odds on to see some spectacular ice sheet disintegration in the next 100 years. And with the rapid ice sheet loss will come rapid sea level rise.

Some scientists think we may have a 20 metre sea level rise already in the pipeline based upon studying the geological record, the current level of warming (0.8 degrees C) and the warming inertia already built into the system (at least 1 degree C more) without considering our present emissions trajectory. There have been scientific projections done for sea level rise for next 500 years which says that "Most rise is expected after stabilization of forcing, due to the long response time of sea level. For all scenarios the rate of sea level rise would be positive for many centuries, requiring 200–400 years to drop to the 1.8 mm/yr 20th century average."

According to Rohling in the media release from the University of Southampton: “We cannot say whether this proves the case, but at least the time delay of the modern ice-sheet responses relative to climate change would seem to agree with the response timescales we have now found in the palaeo-record.”


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