Study finds sudden shift in "forcing" led to demise of Laurentide ice sheet

June 23, 2015
A study of the demise of the Laurentide Ice Sheet that once covered Canada may help scientists better understand shrinking ice fields today - like this melting ice margin in Greenland. Credit: David Ullman, Oregon State University

A new study has found that the massive Laurentide ice sheet that covered Canada during the last ice age initially began shrinking through calving of icebergs, and then abruptly shifted into a new regime where melting on the continent took precedence, ultimately leading to the sheet's demise.

Researchers say a shift in "radiative forcing" began prior to 9,000 years ago and kicked the deglaciation into overdrive. The results are important, scientists say, because they may provide a clue to how ice sheets on Greenland and Antarctica may respond to a warming climate.

Results of the study, which was funded by the National Science Foundation with support from the National Aeronautics and Space Administration (NASA), are being published this week in Nature Geoscience.

David Ullman, a postdoctoral researcher at Oregon State University and lead author on the study, said there are two mechanisms through which ice sheets diminish – dynamically, from the jettisoning of icebergs at the fringes, or by a negative "surface ," which compares the amount of snow accumulation relative to melting. When more snow accumulates than melts, the surface mass balance is positive.

When melting outpaces , as happened after the last glacial maximum, the surface mass balance is negative.

"What we found was that during most of the deglaciation, the surface mass balance of the Laurentide Ice Sheet was generally positive," Ullman said. "We know that the ice sheet was disappearing, so the cause must have been dynamic. But there was a shift before 9,000 years ago and the deck became stacked, as sunlight levels were high because of the Earth's orbit and CO2 increased.

"There was a switch to a new state, and the ice sheet began to melt away," he added. "Coincidentally, when melting took off, the ice sheet began pulling back from the coast and the calving of icebergs diminished. The ice sheet got hammered by surface melt, and that's what drove final deglaciation."

Ullman said the level of CO2 that helped trigger the melting of the Laurentide ice sheet was near the top of pre-industrial measurements – though much less than it is today. The solar intensity then was higher than today, he added.

"What is most interesting is that there are big shifts in the surface mass balance that occur from only very small changes in ," said Ullman, who is in OSU's College of Earth, Ocean, and Atmospheric Sciences. "It shows just how sensitive the system is to forcing, whether it might be solar radiation or greenhouse gases."

Scientists have examined ice cores dating back some 800,000 years and have documented numerous times when increases in summer insolation took place, but not all of them resulted in deglaciation to present-day ice volumes. The reason, they say, is that there likely is a climatic threshold at which severe surface is triggered.

"It just might be that the needed an added kick from something like elevated CO2 levels to get things going," Ullman said.

Explore further: New method relates Greenland ice sheet changes to sea-level rise

More information: Laurentide ice-sheet instability during the last deglaciation, DOI: 10.1038/ngeo2463

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denglish
1.9 / 5 (12) Jun 23, 2015
De-glaciation happened 9,000 years ago. Clearly, industrial humans were to blame.

"It just might be that the ice sheet needed an added kick from something like elevated CO2 levels to get things going," Ullman said.

No article is complete without an appeal to terror and taxation.
OdinsAcolyte
2.6 / 5 (5) Jun 23, 2015
Conjecture is acceptable but is in no way fact nor should it be confused as such.
MR166
2.1 / 5 (7) Jun 23, 2015
"But there was a shift before 9,000 years ago and the deck became stacked, as sunlight levels were high because of the Earth's orbit and CO2 increased."

Well here it says, plain as day, sunlight levels changed because of earth's orbit. Yet, 97% of the scientists agree that ( as of the 90's ) that changes in solar radiation hitting the earth are very small and could not possibly account for today's changes in the earths temperature. The Ministry of Propaganda is having a hard time keeping it's facts straight. BTW temperature changes preceded changes in Co2 levels.
denglish
2.3 / 5 (3) Jun 23, 2015
It would be interesting to know what the amount of water vapor was in the air too.
MR166
2 / 5 (4) Jun 23, 2015
Here comes the cold.

http://www.expres...activity

If it wasn't for the fact that it is cold we would all be dying from global warming.
Captain Stumpy
5 / 5 (5) Jun 25, 2015
If it wasn't for the fact that it is cold we would all be dying from global warming.
@mr
you are mistaking weather for climate again...

also, please re-read the links i gave you from Francis et al...
there are at least TWO that are very relevant which predicted the warming effects which destabilized the jet stream.... she has been arguing this effect for more than a decade and the observations are now exactly matching her specific predictions... i would say that is substantial validation of her studies (and i won't even mention the rest, because you didn't read it then- obviously- and are likely to ignore it now)

so yeah... you can have very cold *weather* because of global warming. had you not been so keen on ignoring the science, you would have learned that already

the studies are free - Francis also has video's that explain the study and include mapping/visual aids that demonstrate what we see today to make it easier for you to understand if you want
jerryd
5 / 5 (4) Jun 28, 2015
Yes MR166 the earth's orbit changes in long term cycles changing the sunlight closer to the poles. But total earth sunlight doesn't change.
And many other cycles too that warm, cool.
But CO2,GHG,GW is on top of those cycles and faster,larger.
Everyone in science takes that for granted because it is a fact as normal, proven.
On snowpack, glaciers once they start melting, a dark heat absorbing surface from dirt, pollution, etc in the ice gets left behind causing faster melting might be what is happening.
I live in Fla and in my 55 yrs watching the coast, islands disappear and good size areas under salt water, over 200sq miles.
At first they would just disappear, then come back or move. Then about 1970 they stopped coming back at all and the shoreline moved landward 100 yards about everywhere and some places miles. 10-20miles in the most SE.
Also GW isn't the only problem, ocean acidification is really starting to hurt and if it screws up plankton, we are really screwed.
jerryd
4.7 / 5 (3) Jun 28, 2015
The sad thing is clean power, fuels can more easily, cheaply supply the power, fuels we need even with present tech.
Home, building, factory, etc made energy can easily beat FF energy in cost on a level playing field. Even on this massively rigged for FF's clean energy once able to get mass production like solar, wind and many,many other techs most already cost less.
In 70% of the US wind is kicking coal, nuke butt wholesale even with all their/coal-nukes subsidies. Already forced 2 nukes closed because even completely paid for they couldn't compete with wind, hydro or NG.
Fact is not using FF's or utility corporate costs it really shouldn't be surprising home, building, etc made clean power cost less.
denglish
1 / 5 (4) Jun 29, 2015
also, please re-read the links i gave you from Francis et al...

Stop posting and referring to what you don't understand. It is ridiculous.
Captain Stumpy
5 / 5 (6) Jun 30, 2015
also, please re-read the links i gave you from Francis et al...

Stop posting and referring to what you don't understand. It is ridiculous.
by all means, dung, please show where the studies are not legitimate, or that i don't understand what they mean... please!

in fact, please demonstrate where my referral is irrelevant or incorrect as well. teach us all your wonderful logic and show us how your religious views work!
THANKS
Bloodyorphan
2 / 5 (4) Jun 30, 2015
Of course the major cities around the world growing at an unprecedented rate and consuming vast amounts of fresh water, hogging it from the surrounding environments has nothing to do with the lack of ice.

Most countries are now encountering drought conditions on a regular basis, the three rivers dam was completed 2 years ago and low and behold North Korea is now facing the worst drought conditions ever.

Too hard ??, no no no it's CO2 I tell you !!

Pull your heads out and realize we have much greater problems facing us, we've been installing catalytic converters on cars and chimneys for decades now.

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