Archives for August 2007

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…..

For the first time a globally coherent solar cycle response to the surface temperature has been established. Charles Camp and Ka Kit Tung report in Geophysical Research Letters (DOI: 10.1029/2007GL030207) that global temperatures oscillated by 0.2C during the high and low points of the cycle. The research uses satellite solar radiation and surface temperature gridded data from the last 50 years over four and a half solar cycles.

The authors’ analysis also shows greater warming in the polar regions in common with climate model predictions.

In a yet unpublished but submitted paper, Tung and Camp undertake another analysis, without resorting to climate models, that give a climate sensitivity for doubling CO2 between 2.3 and 4.1C as a 95% confidence interval.

The authors add that due to ocean lag effects these numbers are likely to be underestimates.

The work, bound to be controversial, puts more complexity back into the game coming hot on the heels of the Smith et al internal variability paper.

But reader beware, these papers are serious science not rambling quasi-political anecdotes.