Arguments
What makes ice sheets grow and shrink?
Posted on 13 August 2013 by dana1981
This is a re-post of a Carbon Brief article by Freya Roberts
A new look at how the earth freezes and thaws suggests that while colder glacial periods in earth's history are triggered by changes in the planet's orbit, it takes climate feedbacks to give a full picture of why the planet freezes and thaws every hundred millennia.
The last million years
Earth has cycled between glacial and interglacial periods roughly every 100,000 years out of the past million. When the climate cools, vast ice sheets grow slowly from the north pole towards the equator, burying much of North America, Europe and Asia. When earth warms again, these continent-sized ice sheets melt quickly and retreat towards the poles.
Scientists previously thought that this cycle is caused by variations in the amount of solar energy reaching earth. A scientist named Milankovitch came up with the idea in 1941. He suggested that changes in earth's rotation, tilt and orbit all affect the distance between the sun and the planet, changing how warm it is in a small but significant way, on a timescale of tens of thousand to hundreds of thousand years.
But according to the authors of a new study in Nature, these cycles alone aren't enough to explain how vast ice sheets grow and shrink from glacial to interglacial.
Climate feedbacks
The Nature authors suggest that to understand why vast ice sheets grow and shrink, you need to look at climate feedbacks - natural processes that interact and amplify the changes in the amount of solar energy reaching earth.
Their theory is that while changes in sunlight might be enough to tip the climate system into a glacial period, once ice sheets start to grow, they alter earth's climate in a way means they carry on growing.
One example of such positive feedbacks is changes in how reflective the planet is. The bigger ice sheets are, the more sunlight they reflect back into space. So as ice sheets grow, this helps make the climate colder - and more conducive to ice sheet growth.
Other feedbacks are at play too, the authors suggest. As the water cycle pumps more moisture out of the oceans and into ice sheets, sea levels fall. This alters ocean circulations, and since the ocean and atmosphere are closely tied, the entire climate system is affected.
Ocean circulations currently redistribute heat all around the world - warm surface waters are shown in red and deeper cool waters are show in blue.
The new study is the first time scientists have factored such major feedbacks into model simulations of glacial cycles across the entire northern hemisphere. It's probably the most detailed explanation anyone's come to for why earth goes in and out of glacial periods every 100,000 years.
Prof Heinz Blatter, a co-author on the paper, explained:
"Milankovitch's idea that insolation [the amount of solar energy reaching earth] determines the ice ages was right in principle. However, science soon recognised that additional feedback effects in the climate system were necessary to explain ice ages. We are now able to name and identify these effects accurately."
Where are we at today?
Earth's climate is currently in a warm spell between glacial periods. The last ice age ended about 11,000 years ago. Since then, temperatures and sea levels have risen, and ice caps have retreated back to the poles.
Of course, on top of these epic natural cycles manmade carbon emissions are having an effect on the climate. Over short timescales (geologically speaking) of centuries and millennia, greenhouse gas emissions from human activities are the main cause of climate change.
In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) said it is very likely human activity is responsible for most of the observed temperature increase since the 1950s, mainly because of burning fossil fuels. The report also shows that natural factors, like the amount of energy being given off by the sun, are having a much smaller effect on climate than we humans.
At the moment, the three Milankovitch cycles are out of sync, meaning there's no threat of earth-sun changes affecting the climate significantly any time soon. Rather, by adding greenhouse gas emissions into the mix, mean temperatures and sea levels are set to carry on rising - bringing with them a host of climate impacts.
Abe-Ouchi et al. (2013) Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume. Nature. DOI: 10.1038/nature12374
Excellent piece. One minor nitpick though: Some things are implied to be new results from this study, but they aren't all that new. For instance, the fact that Milankovitch cycles alone aren't enough to explain the ice age cycle has been known pretty much since the beginning of that theory. Scientists have been looking at the necessary feedbacks for decades.
For a general history of the science of the ice ages, see http://www.aip.org/history/climate/cycles.htm [warning, massive wall of text :-)]
This video explains how volcanoes too can melt the cryosphere.
Chris,
What is your point? In your linked video Hansen says that volcanoes affect CO2 and climate over eons of time. Current CO2 changes are human caused and are 10,000 times faster than volcanoes.
No-one disputes that in the past volcanoes have affected climate. Current climate change is caused by humans.
The linked video content is directly relevant to the question "What makes ice sheets grow and shrink?".
Presumably the ice sheet albedo change is relatively straightforward to calculate, and we have data on greenhouse gas changes from ice cores, so is there anything new or substantially different in this study which would lead to different conclusions about fast feedback climate sensitivity - i.e. lower or higher than the generally accepted figure of around 0.75°C/W/m², or 3°C for a doubling of atmospheric CO₂ from 280 to 560ppm?
Dana1981,
Is the section entitled "Where We Are Today" directly from the study itself, or an add-on from Freya Roberts? The paper is paywalled unless you have another link available.
Chris@2,4,
You should be aware that the Hansen video you've linked to and the paper - subject to this post - are talking about different timescales. Hansen talks about 65Ma cenozoic timescale (and by implication of that context, he does not bother to consider the time granularity less than some 1Ma), while the subject paper talks about the pleistocene glacial periods of last 400Ka-1Ma.
Different factors influence average climate over those timescales. In particular, 100ka long Milankovic cycles considered by the paper override the Hansen's tectonic forcings that act on 500ka+ timescales. Therefore, although "the melting cryosphere" appears on your video, I would argue that this video has nothing to do with the article at hand.
It is very important to distinguish the timescale of changes, e.g. while debunking common "climate has changed before" myth.
- rock weathering/volcanoes with tectonic forcings: 500ky+ up to several Ma
- Ice ages with Milankovic forcings: 10ka - 100ky
- AGW: 100y (at least 100 times faster than any natural forcings)
We must make clear distinction about timescales involved, otherwise we fall into the same trap the ignorant "skeptics" have fell with the myth I recalled above.
I have a technical question that I cannot figure out because I don't have access to full text.
I'm interested in knowing some more details: how they ran their simulation and what parameters did they establish to be the "tipping points" moving the climate in and out of glaciation.
In particular I'm interested how their model differ from Archer's CLIMBER model, that argues the minimum solar insolation at 65N be the triggering factor for gradual continental icesheet buildup.
chriskoz #8, fair enough, Though i thought the video was also good, since it points out rate of changes (natural vs human attributed forcing) also explains why there cannot be a runaway snowball earth.
I want Skeptical Science to do an article on this newly published research:
http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1898.html
[JH] See: Greenland Ice Is Melting -- Even from Below: Heat Flow from the Mantle Contributes to the Ice Melt, ScienceDaily, Aug 11, 2013.
p4gs - Looks like references to this paper are already showing up on the denial sites, in the apparent hopes of yet another claim that "it's not us".
Those claims would only make sense if there had been recent changes in the heat from the lithosphere - long standing heat patterns would be part of pre-Industrial conditions as well, and not causes of recent melt acceleration. Not to mention that observed changes in GHGs already account for the Greenland changes we've seen, and such "not us" claims would have to somehow explain those away...
p4gs @11, I would also be interested in such an article.
Just from looking at the abstract, it is a very interesting paper. Evidently, the weight of continental ice sheets causes a thining of the crust under the ice sheet, allowing enhanced geothermal heat flow. The effect can be seen in the upper right of the diagram below, shown with increased detail in the inset. As can be seen, heat flow reaches a minimum (green colours) during interglacials, and a maximum during glacials when the ice is thickest. As an aside, that suggests a very significant melt back of the greenland ice sheet during interglacials, to relieve the burden of ice mass and allow thickening of the lithosphere beneath the Greenland Ice Sheet.
WUWT appears to want to beat this up into a major factor, claiming:
They assert his despite their cutaway view of the Greenland Ice Sheet showing no temperatures above freezing. However, the diagram below should put paid to any such denier fantasies. The range of geothermal heat flux shown is from 0.043 to 0.061 W/m^2. That compares to a global average of 0.087 W/m^2, and is relatively small even for continental plates as can be seen below:
(Units are mW/m^2)
The peak rate, is therefore less than a 10th of the TOA energy imbalance caused by AGW, and the change in heat flow between glacial and interglacial is about a sixth of that again.
I must admit that having actually seen the figure shown, I am surprised that the abstract states:
It would have been rather more accurate to state:
Tom @ 13,
It is true that geothermal heat is trivial at the ice-air interface, but not so at the ice-land interface. I think 'surface' here is meant to be the surface of the lithosphere and not the surface of the ice; with this interpretation the abstract is correct.
I think Freya Roberts post fails to mention that the most important feedback according to the research paper is geological - the delayed isostatic rebound. This keeps the ice elevation low and therefore ice loss remains high while the ice sheet retreats. It is this delay which is responsible for the 100 kyear cycle. Other feed backs, dust feedback, oceanfeedback are less important.
The 100 kyear cycle can be reproduced in models even if CO2 levels are kept constant, although CO2 does influence the size of the ice sheet. If the rebound is instantaneous the 100 kyear cycle is no longer dominant.
IanC @14, yes. I realized essentially the same point last night just before going to sleep. The key point is that given the rate at which heat propogates by conduction, the current warming will not yet have impacted basal ice except by means of surface melt water carrying warmth to the base of the ice sheet (where it is able to do so) or at the edges of the ice sheet.
From what's available on the nature website: "This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14, 15, 16, which is the lithosphere–asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles." Emphasis mine.
From the article above: "Of course, on top of these epic natural cycles manmade carbon emissions are having an effect on the climate. Over short timescales (geologically speaking) of centuries and millennia, greenhouse gas emissions from human activities are the main cause of climate change." (-snip-).
I asked, but did not get a reply ( -snip-), about the final section of this post "Where Are We At Today". It appears to be an add-on from the article author and not part of the paper referenced.
If in fact it is not part of the paper represented in the post, then we are back to the "inaccurate" topic. If it is part of the paper, then I would like to see it and I will retract my statement.
[DB] Argument from personal incredulity snipped. Inflammatory snipped.
Terranova:
There is substantial evidence that Humans have been causing climate change for 8,000 years. Primarily due to farming and deforrestation. The last that I recall, scientists were coming to a consensus that this explaination was the best way to model the recent climate changes. The rate of human caused climate change has substantially increased in the last 150 years. Your cries of "inaccuracy" would be a lot more convincing if you did your homework. Please read up on the past 4-8 millenia and retract your comment above.
Michael Sweet @ 18
There is a difference between contributing and causing. Read your link again. There is nothing there that implies that humans are the main cause of climate change over the party centuries or millenia. Furthermore, the query about the statement being a part of the referenced paper is still not answered.
Terranova@17,19
Can you explain in non-inflamatory terms (so that your post is not snipped) the point you're trying to make?
Your emphasis about CO2 being "not determinative" to glacial cycles does not add anything new to the discussion and your focus on "manmade CO2 emissions" does not mean anything in this context. We already know that CO2 was the feedback rather than causality of pleistocene glacial cycles. And it's also obvious to us that manmade CO2 emissions have overriden the glacial cycles, because said emissions are 100 times faster. So to me, there is no logical value nor point in your post.
Your question about the "Where Are We At Today" section wherabouts can only be answered by Dana. But to me, the section is truthful and valuable, irrespective where it came from: it explains to the possibly unfamiliar reader the context and rate of current climate changes in relation to the changes the original study focussed on.
Terranova,
Since the Milankovich forcing has been cooling for the past 5,000 years, what has warmed besides human interventions? Your claim of "contributing" not "causing" is splitting hairs. Humans have changed the climate for 8,000 years. The question is how much. You are being inaccurate with your pedantic hair splitting. Stop complaining that others have a minor issue until you no longer have the same problem, or worse.
There was a recent SKS article talking about the PDO and ocean gyres and how with a La Nina surface ocean currents flow from east to west across the Pacific, which slows or stops with an El Nino. I don't understand how these surface flows (eg La Nina and the gyres) relate to the warm surface flows shown in the diagram in this article. Anyone help?
In the first 1.5m years of the present ice age, the glacial-interglacial cycle was about 41,000 years, in sinc with obliquity (one of the Milankovitch cycles). It is still in sinc but only every third or so nudge melts the ice. Clathrates sequestered under the ice sheets plus the fact that the deeper the ice sheet, the more unstable it is, may, at least, partially explain this.
http://mtkass.blogspot.co.nz/2011/09/continental-glacier-meltdown.html
William, if clathrates beneath the ice sheets are a significant part of the feedback, then this would imply a few things:
1/ Should see more than CO2 is atmosphere than is explainable from other earth system feedback.
2/ If straight methane is important, then should see enough in ice bubbles to match the supposed forcing.
3/ The methane in ice bubbles should show a d13 signature consistant with significant clathrates.
What does observation say?
1/ is not easy to constrain, but it will be interesting to see AR5 models on this.
2/ Methane rises in ice core, but not to levels of significant forcing
3/ What's been looked at so far (eg Petrenko at al 2009) suggest swamp methane is far more important than clathrates.
BC @ 22 - The graphic above is of the Thermohaline Circulation, whereas the SkS post, A Looming Climate Shift: Will Ocean Heat Come Back to Haunt us?, was focused on the wind-driven ocean circulation. The two circulations are not really separate, but a distinction between the two is necessary for the purpose of explaining the main features of the ocean circulation. SkS has some upcoming posts explaining the wind-driven ocean circulation in more detail, and why the deep oceans are warming.