Greenland ice is melting - even from below

Heat flow from the mantle contributes to the ice melt.

© A. Petrunin/GFZ

Modeled basal ice temperatures of the present-day Greenland Ice Shield across the Summit region, GRIP and GISP2 indicate borehole locations

The Greenland ice sheet is melting from below, caused by a high heat flow from the mantle into the lithosphere. This influence is very variable spatially and has its origin in an exceptionally thin lithosphere. Consequently, there is an increased heat flow from the mantle and a complex interplay between this geothermal heating and the Greenland ice sheet. The international research initiative IceGeoHeat led by the GFZ German Research Centre for Geosciences establishes in the current online issue of Nature Geosciences (Vol 6, August 11, 2013) that this effect cannot be neglected when modeling the ice sheet as part of a climate study.

The continental ice sheets play a central role in climate. Interactions and feedback processes between ice and temperature rise are complex and still a current research topic. The Greenland ice sheet loses about 227 gigatonnes of ice per year and contributes about 0.7 millimeters to the currently observed mean sea level change of about 3 mm per year. Existing model calculations, however, were based on a consideration of the ice cap and considered the effect of the lithosphere, i.e. the earth's crust and upper mantle, too simplistic and primarily mechanical: the ice presses the crust down due to its weight. GFZ scientists Alexey Petrunin and Irina Rogozhina have now coupled an ice/climate model with a thermo-mechanical model for the Greenland lithosphere. "We have run the model over a simulated period of three million years, and taken into account measurements from ice cores and independent magnetic and seismic data", says Petrunin. "Our model calculations are in good agreement with the measurements. Both the thickness of the ice sheet as well as the temperature at its base are depicted very accurately. "

Floating icebergs from Greenland’s largest glacier, Jakobshavn Isbrae. The smooth side indicates part of the iceberg that has already tipped over as sub-marine melting brought it out of balance. 

© I.Sasgen, GFZ 

The model can even explain the difference in temperature measured at two adjacent drill holes: the thickness of the Greenland lithosphere and thus the geothermal heat flow varies greatly in narrow confines.

What does this mean for climate modeling? "The temperature at the base of the ice, and therefore the current dynamics of the Greenland ice sheet is the result of the interaction between the heat flow from the earth's interior and the temperature changes associated with glacial cycles," explains corresponding author Irina Rogozhina (GFZ) who initiated IceGeoHeat. "We found areas where the ice melts at the base next to other areas where the base is extremely cold."

 Floating icebergs from Greenland’s largest glacier, Jakobshavn Isbrae.

© I.Sasgen, GFZ 

The current climate is influenced by processes that go far back into the history of Earth: the Greenland lithosphere is 2.8 to 1.7 billion years old and is only about 70 to 80 kilometers thick under Central Greenland. It remains to be explored why it is so exceptionally thin. It turns out, however, that the coupling of models of ice dynamics with thermo-mechanical models of the solid earth allows a more accurate view of the processes that are melting the Greenland ice. 

Article Source : Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences.

By Science and Universe

Why an Ice Age Occurs Every 100,000 Years

Why an Ice Age Occurs Every 100,000 Years: Climate and Feedback Effects Explained

Glacier Perito Moreno National Park, Argentina Patagonia. (Credit: iStockphoto)

Science struggled to explain fully why an ice age occurs every 100,000 years. As researchers now demonstrate based on a computer simulation, not only do variations in insolation play a key role, but also the mutual influence of glaciated continents and climate. 

Ice ages and warm periods have alternated fairly regularly in the Earth’s history: the Earth’s climate cools roughly every 100,000 years, with vast areas of North America, Europe and Asia being buried under thick ice sheets. Eventually, the pendulum swings back: it gets warmer and the ice masses melt. While geologists and climate physicists found solid evidence of this 100,000-year cycle in glacial moraines, marine sediments and arctic ice, until now they were unable to find a plausible explanation for it.

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.

“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 with a 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 (see box).

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.

During the last ice age northern regions of America, Europe and Asia were covered under thick ice sheets. (Illustration: Ittiz / Wikipedia) (large view) 

Simulating the ice and climate

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.

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.”

Article Source : ETH Zurich.

By Science and Universe