Any reader not familiar with Milankovitch can read the Wiki write up here.
I’d already considered posting this interesting new Nature paper entitled ‘Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years,’ so when Luke Walker also drew my attention to it, I decided to give it a go.
The first paragraph summarises the paper:
The Milankovitch theory of climate change proposes that glacial–interglacial cycles are driven by changes in summer insolation at high northern latitudes. The timing of climate change in the Southern Hemisphere at glacial–interglacial transitions (which are known as terminations) relative to variations in summer insolation in the Northern Hemisphere is an important test of this hypothesis. So far, it has only been possible to apply this test to the most recent termination because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined. Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. This ratio is a proxy for local summer insolation, and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter. The accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation. Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations.
The paper states, “contrary to hypotheses ascribing the trigger of glacial terminations to CO2, obliquity (axial-tilt), or southern summer insolation, our chronology implicates northern summer insolation as the primary trigger.”
“In summary, the mean phasing of Antarctic climate, as well as the timing of the last four terminations and three post-interglacial coolings, are consistent with the hypothesis that high northern latitude summer insolation is the trigger of glacial–interglacial cycles. The role of CO2 as conveyor and amplifier of the orbital input should be quantified with climate models run using our new timescale; this quantification is important for future climate change predictions. Our timescale should be validated further with new radiometric age markers, as well as by process studies for complete understanding of the physical link between O2/N2 and local insolation. With future O2/N2 measurements, it may be possible to apply this method to the Dome Fuji and Dome C cores for termination V and older terminations, to investigate the phasing of climate and atmospheric composition with respect to orbital forcing further back in time.”
Fortunately, there is a write up here which makes it easier to understand compared with the original article:
“Strong Evidence Points to Earth’s Proximity to Sun as Ice age trigger”
A question unresolved for more than a century may have an answer Scripps Institution of Oceanography/UC San Diego
When do ice ages begin? In June, of course.
Analysis of Antarctic ice cores led by Kenji Kawamura, a visiting scientist at Scripps Institution of Oceanography, UC San Diego, shows that the last four great ice age cycles began when Earth’s distance from the sun during its annual orbit became great enough to prevent summertime melts of glacial ice. The absence of those melts allowed buildups of the ice over periods of time that would become characterized as glacial periods.
Results of the study appear in the Aug. 23 edition of the journal Nature.
Jeff Severinghaus, a Scripps geoscientist and co-author of the paper, said the finding validates a theory formalized in the 1940s but first postulated in the 19th Century. The work also helps clarify the role of carbon dioxide in global warming and cooling episodes past and present, he said.
“This is a significant finding because people have been asking for 100 years the question of why are there ice ages,” Severinghaus said.
A premise advanced in the 1940s known as the Milankovitch theory, named after the Serbian geophysicist Milutin Milankovitch, proposed that ice ages start and end in connection with changes in summer insolation, or exposure to sunlight, in the high latitudes of the Northern Hemisphere. To test it, Kawamura used ice core samples taken thousands of miles to the south in Antarctica at a station known as Dome Fuji.
Scientists studying paleoclimate often use gases trapped in ice cores to reconstruct climatic conditions from hundreds of thousands of years in the past, digging thousands of meters deep into ice sheets. By measuring the ratio of oxygen and nitrogen in the cores, Kawamura’s team was able to show that the ice cores record how much sunlight fell on Antarctica in summers going back 360,000 years. The team’s method enabled the researchers to use precise astronomical calculations to compare the timing of climate change with sunshine intensity at any spot on the planet.
Kawamura, a former postdoctoral researcher at Scripps, used the oxygen-nitrogen ratio data to create a climate timeline that was used to validate the calculations Milankovitch had created decades earlier. The team found a correlation between ice age onsets and terminations, and variations in the season of Earth’s closest approach to the sun. Earth’s closest pass, or perihelion, happens to fall in June about every 23,000 years. When the shape of Earth’s orbit did not allow it to approach as closely to the sun in that month, the relatively cold summer on Earth encouraged the spread of ice sheets on the Northern Hemisphere’s land surface. Periods in which Earth passed relatively close in Northern Hemisphere summer accelerated melt and brought an end to ice ages…………
…..The team found that the changes in Earth’s orbit that terminate ice ages amplify their own effect on climate through a series of steps that leads to more carbon dioxide being released from the oceans into the air. This secondary effect, or feedback, has accounted for as much as 30 percent of the warming seen as ice ages of the past have come to an end…..
Jennifer Marohasy BSc PhD is a critical thinker with expertise in the scientific method.
CO2 feedback accounting for about 30% of warming out of glaciation periods? I see no problem with that. Especially as the relationship is logarithmic, with less and less warming from CO2 as CO2 levels increase.
And where are we now in the cycle?
Where are we now in the cycle ?? – wouldn’t be counting on a regular cycle just because the last few have been regular.
http://www.ncdc.noaa.gov/paleo/milankovitch.html
What does The Milankovitch Theory say about future climate change?
Orbital changes occur over thousands of years, and the climate system may also take thousands of years to respond to orbital forcing. Theory suggests that the primary driver of ice ages is the total summer radiation received in northern latitude zones where major ice sheets have formed in the past, near 65 degrees north. Past ice ages correlate well to 65N summer insolation (Imbrie 1982). Astronomical calculations show that 65N summer insolation should increase gradually over the next 25,000 years, and that no 65N summer insolation declines sufficient to cause an ice age are expected in the next 50,000 – 100,000 years ( Hollan 2000, Berger 2002).
Ian,
You are absolutely right. What’s more, CO2 concentrations are so “high” (relatively speaking for a trace gas) that doubling current levels would have nearly no effect at all.
Or alternatively absolutely wrong in conclusion. 2 X CO2 below seems to give around 3C .. .. hmmmm
So Schillsy – simply tell us where the maths is wrong here:
http://www.realclimate.org/index.php/archives/2007/04/learning-from-a-simple-model/
Radiative forcing – whether from the sun or from greenhouse gases – has pretty much the same effect regardless of how it comes about.
I am impressed with the ability of these scientists to measure summertime insolation 360,000 years ago. Just by looking at ice cores no less!
Imagine my astonishment when I look at the post below and see the difficulty others have measuring TSI today, and all they had to do was look up!
Didn’t even have to get their feet cold.
Well it’s temperature rate chemistry versus sensor drift.
By measuring the ratio of oxygen and nitrogen in the cores, Kawamura’s team was able to show that the ice cores record how much sunlight fell on Antarctica in summers going back 360,000 years.
Makes me wonder why the climate scientists don’t throw away all their thermometers, hygrometers, barometers, and widgets, and instead just take an air sample to tell us how hot it is.
There is one thing I have to thank you for.
Prof Hansen of GISS has taken to preaching the full on fire and brimstone sea level rise.
“All hell is gonna break loose” or something like that (note – Luke it’s not an exact quote, just the flavor).
But here we have testimony that with Milankovitch cycling to it’s extremity, augmented with the utmost CO2 feedback, even then Mother Gaia in the throws of heatflash still has a polar ice cap to deposit samples of air for climate scientists to drill and poke through.
Hard to sell that 20 meter sea rise now.
James – “Makes me wonder why the climate scientists don’t throw away all their thermometers, hygrometers, barometers, and widgets, and instead just take an air sample to tell us how hot it is.”
That’s just stupid. The reason scientist use proxy records is that there were no thermometers then. Why would anyone use proxy records when solid measurements exist.
James did Hansen say “everything” could melt? No. So still plenty of space for ice.
If small glaciers and ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around 0.5 m.
Melting of the Greenland ice sheet would produce 7.2 m of sea level rise, and melting of the Antarctic ice sheet would produce 61.1 m of sea level rise.[3] The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5-6 m.
The IPCC 4AR is less than 0.5m by the end of this century. Hansen has mused a 20m rise in sea level in the Royal Society paper http://www.journals.royalsoc.ac.uk/content/l3h462k7p4068780/pdf/TA071925.pdf
Why – his albedo flip mechanism, ice sheet disintegration instead of straight melting and the paleo record suggesting that that’s what happened last time the world was as warm as a modelled business-as-usual (BAU) scenario.
That time is NOT now.
“Indeed, the palaeoclimate record contains numerous examples of ice sheets yielding sea level rises of several metres per century when forcings were smaller than that of the business-as-usual scenario. For example, about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years.”
Should you believe Hansen – well he hasn’t said trust me – he’s laid out the evidence. At least read his paper and not just the op eds if you’re going to put the boot in.
New Scientist also reviews http://environment.newscientist.com/article/mg19526141.600.html
I used to admire Hansen, but he has reached the stage where the men in white coats are getting ready to take him away (in my opinion). The rubber room beckons.
Now it is “albedo flip mechanism”, the guys a complete tosser.
Luke claims that Hansen didn’t claim all the ice would melt so how is the albedo of the remaining ice going to flip? Especially in a warmer, wetter world with more clouds and greater substitution of cloud albedo for ice albedo.
I can’t believe you’re asking that question. As usual you’ve missed the point. The mechanics of ice sheet disintegration are not well known.
“Indeed, the palaeoclimate record contains numerous examples of ice sheets yielding sea level rises of several metres per century when forcings were smaller than that of the business-as-usual scenario. For example, about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years.”
Those who cannot learn from history are doomed to repeat it. George Santayana
Ender – “Why would anyone use proxy records when solid measurements exist.”
http://tinyurl.com/ywac23
On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes
“An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings.”
///Indeed, the palaeoclimate record contains numerous examples of ice sheets yielding sea level rises of several metres per century when forcings were smaller than that of the business-as-usual scenario. For example, about 14,000 years ago, sea level rose approximately 20 metres in 400 years, or about 1 metre every 20 years///
I am wondering how a climate epoch which would melt a meter of the Antarctic icecap every twenty years could be well represented in ice cores?
Another case for offsets?
Did Hansen get to decide how big the offsets would be?
Who says it melts evenly. The theory is that ice sheets are undermined by meltwater and large chucks slough off – and there’s examples of that.
– and the literature is full of geological history sea level rises and falls – they’re not Hansen’s work.
Check your geological time sea level history. Here’s a version/summary http://en.wikipedia.org/wiki/Sea_level_rise