Saturday, August 10, 2013

Two Tales of Sunshine and Ice

Sou | 8:13 PM Feel free to comment!

In the past couple of days, HotWhopper has had some visits by someone who seems to think the only thing that has a marked effect on climate is the sun.  There was much talk of ice ages and insolation.  So when I came across a couple of stories about ice and sunshine today, it seemed fortuitous.

Garwood Valley, Antarctica:- Tenfold increase of ancient melt rates

The first one is about Antarctica, specifically Garwood Valley near McMurdo Bay.

Source: LA Times

The valley is below freezing for most of the year.  It's in a zone of continuous permafrost and has remnants of the Ross Sea Ice Sheet.  It's the sun that is melting the ice more quickly.  From the LA Times (my bold):
The tenfold increase from ancient melt rates evident in a dry valley near McMurdo Bay over little more than a decade comes despite a local two-decade cooling trend.
Cliff-face measurements of the buried ice in the four-mile-long Garwood Valley revealed melt rates that shifted from a creeping annual rate of about 40,000 cubic feet per year over six milleniums, to more than 402,000 cubic feet last year alone, according to the report published Wednesday in the journal Nature Scientific Reports. (That’s a leap from the capacity of about eight standard railroad boxcars to 77.)
“We think what we’re seeing here is sort of a crystal ball of what coastal Antarctica is going to experience,” said geologist Joseph Levy, of the University of Texas, lead author of the study. “When you start warming buried ice and other permafrost in the dry valley, it’s going to start to melt and it’s going to start melting in a style that’s consistent with permafrost thaw in the Arctic.”
Read more here in the LA Times or go direct to the paper in Nature Scientific Reports

Levy et al (2013)  Accelerated thermokarst formation in the McMurdo Dry Valleys, Antarctica, Scientific Reports 3, Article number: 2269 doi:10.1038/srep02269

The mechanics of ice formation and retreat in an ice age

This second paper explains more about the ice ages - glaciation and retreat every 100,000 years or so.

Insolation + glaciated continents + climate

The ice ages that occur every 100,000 years or so are the product not just of variations in insolation, but also the mutual influence of glaciated continents and climate....

...Using computer simulations, a Japanese, Swiss and American team including Heinz Blatter, an emeritus professor of physical climatology at ETH Zurich, has now managed to demonstrate that the ice-age/warm-period interchange depends heavily on the alternating influence of continental ice sheets and climate.

The topography is completely different under ice

“If an entire continent is covered in a layer of ice that is 2,000 to 3,000 metres thick, the topography is completely different,” says Blatter, explaining this feedback effect. “This and the different albedo of glacial ice compared to ice-free earth lead to considerable changes in the surface temperature and the air circulation in the atmosphere.” Moreover, large-scale glaciation also alters the sea level and therefore the ocean currents, which also affects the climate.

Weak effect of insolation + feedback effects = strong impact

As the scientists from Tokyo University, ETH Zurich and Columbia University demonstrated in their paper published in the journal Nature, these feedback effects between the Earth and the climate occur on top of other known mechanisms. It has long been clear that the climate is greatly influenced by insolation on long-term time scales. Because the Earth’s rotation and its orbit around the sun periodically change slightly, the insolation also varies. If you examine this variation in detail, different overlapping cycles of around 20,000, 40,000 and 100,000 years are recognisable.

Given the fact that the 100,000-year insolation cycle is comparatively weak, scientists could not easily explain the prominent 100,000-year-cycle of the ice ages with this information alone. With the aid of the feedback effects, however, this is now possible.

Astronomical parameters, ground topography, climate and feedback effects

The researchers obtained their results from a comprehensive computer model, where they combined an ice-sheet simulation with an existing climate model, which enabled them to calculate the glaciation of the northern hemisphere for the last 400,000 years. The model not only takes the astronomical parameter values, ground topography and the physical flow properties of glacial ice into account but also especially the climate and feedback effects. “It’s the first time that the glaciation of the entire northern hemisphere has been simulated with a climate model that includes all the major aspects,” says Blatter.

Why ice ages start slowly and end quickly

Using the model, the researchers were also able to explain why ice ages always begin slowly and end relatively quickly. The ice-age ice masses accumulate over tens of thousands of years and recede within the space of a few thousand years. Now we know why: it is not only the surface temperature and precipitation that determine whether an ice sheet grows or shrinks. Due to the aforementioned feedback effects, its fate also depends on its size. “The larger the ice sheet, the colder the climate has to be to preserve it,” says Blatter. In the case of smaller continental ice sheets that are still forming, periods with a warmer climate are less likely to melt them. It is a different story with a large ice sheet that stretches into lower geographic latitudes: a comparatively brief warm spell of a few thousand years can be enough to cause an ice sheet to melt and herald the end of an ice age.

The Milankovitch cycles

The explanation for the cyclical alternation of ice and warm periods stems from Serbian mathematician Milutin Milankovitch (1879-1958), who calculated the changes in the Earth’s orbit and the resulting insolation on Earth, thus becoming the first to describe that the cyclical changes in insolation are the result of an overlapping of a whole series of cycles: the tilt of the Earth’s axis fluctuates by around two degrees in a 41,000-year cycle. Moreover, the Earth’s axis gyrates in a cycle of 26,000 years, much like a spinning top. Finally, the Earth’s elliptical orbit around the sun changes in a cycle of around 100,000 years in two respects: on the one hand, it changes from a weaker elliptical (circular) form into a stronger one. On the other hand, the axis of this ellipsis turns in the plane of the Earth’s orbit. The spinning of the Earth’s axis and the elliptical rotation of the axes cause the day on which the Earth is closest to the sun (perihelion) to migrate through the calendar year in a cycle of around 20,000 years: currently, it is at the beginning of January; in around 10,000 years, however, it will be at the beginning of July.

Based on his calculations, in 1941 Milankovitch postulated that insolation in the summer characterises the ice and warm periods at sixty-five degrees north, a theory that was rejected by the science community during his lifetime. From the 1970s, however, it gradually became clearer that it essentially coincides with the climate archives in marine sediments and ice cores. Nowadays, Milankovitch’s theory is widely accepted.

“Milankovitch’s idea that insolation determines the ice ages was right in principle,” says Blatter. “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.”

Adapted from Fabio Bergamin at ETH Life

Abe-Ouchi et al. (2013) Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume. Nature. DOI: 10.1038/nature12374

Sunshine came softly through my window today 
...after a few days of rain it was very welcome - and blew my little mind, if you know what I mean... :)

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