Greenland darkening to continue

A press release from City College of News York: Darkening of the Greenland Ice Sheet is projected to continue as a consequence of continued climate warming, Dr. Marco Tedesco, a City College of New York scientist, said at the European Geosciences Union (EGU) General Assembly in Vienna today.

Tedesco told a press conference in the Austrian capital that the projection is based on a model that only accounts for the effects of warming on snow grain size and melting.

An associate professor in City College’s Division of Science and head of its Cryospheric Processes Laboratory that he founded, Tedesco is an authority on the Greenland Ice Sheet where he has conducted annual research.

He noted that a darkening of the Greenland Ice Sheet associated with increasing temperatures and enhanced melting occurred between 1996 and 2012. It was promoted by:

• Extensively and persistently increased surface snow grain size;
• The expansion and persistency of the areas of exposed bare ice and by the increased surface impurities concentration associated with the appearance of dirty ice;
• Increased impurities concentrations due to consolidation with snowmelt. 

Tedesco, however, added that his research had not found any evidence that points to either increased atmospheric deposition of impurities or to the number of fires over Eurasia and North America as being factors....

NASA photo of Greenland ice flowing around a ridge of bedrock

Dark snow: from the Arctic to the Himalayas, the phenomenon that is accelerating glacier melting

John Vidal in the Guardian (UK): The phenomenon of "dark snow" is being recorded from the Himalayas to the Arctic as increasing amounts of dust from bare soil, soot from fires and ultra-fine particles of "black carbon" from industry and diesel engines are being whipped up and deposited sometimes thousands of miles away. The result, say scientists, is a significant dimming of the brightness of the world's snow and icefields, leading to a longer melt season, which in turn creates feedback where more solar heat is absorbed and the melting accelerates.

In a paper in the journal Nature Geoscience, a team of French government meteorologists has reported that the Arctic ice cap, which is thought to have lost an average of 12.9bn tonnes of ice a year between 1992 and 2010 due to general warming, may be losing an extra 27bn tonnes a year just because of dust, potentially adding several centimetres of sea-level rise by 2100. Satellite measurements, say the authors, show that in the last 10 years the surface of Greenland's ice sheet has considerably darkened during the melt season, which in some areas is now between six and 11 days longer per decade than it was 40 years ago. As glaciers retreat and the snow cover disappears earlier in the year, so larger areas of bare soil are uncovered, which increases the dust erosion, scientists suggest.

Research indicates that the Arctic's albedo may be declining much faster than was estimated only a few years ago. Earlier this year a paper in Proceedings of the National Academy of Sciences reported that declining Arctic albedo between 1979 and 2011 constituted 25% of the heating effect from carbon dioxide over the same time.

According to Danish glaciologist Jason Box, who heads the Dark Snow project to measure the effect of dust and other darkening agents on Greenland's ice sheet, Arctic ice sheet reflectivity has been at a near record low for much of 2014. Even a minor decrease in the brightness of the ice sheet can double the average yearly rate of ice loss, seen from 1992 to 2010....

Snow scene, shot by , Wikimedia Commons via Flickr, under the Creative Commons Attribution 2.0 Generic license

Straw albedo mitigates extreme heat

Peter Ruegg in a press release from ETH Zurich: Fields that are not tilled after crop harvesting reflect a greater amount of solar radiation than tilled fields. This phenomenon can reduce temperatures in heat waves by as much as 2 °C, as researchers from ETH have demonstrated in a recent study.

Wheat fields are often tilled immediately after the crop is harvested, removing the light-coloured stubble and crop residues from the soil surface and bringing dark bare earth to the top. Post-harvest tilling is a widely practised and common management technique in Europe. However, ploughed fields can have a negative effect on the local climate during a heat wave. This effect was addressed in a recent study conducted by researchers at ETH Zurich led by Edouard Davin, senior lecturer at the Institute for Atmospheric and Climate Science, and Sonia Seneviratne, professor of land-climate dynamics, which has now been published in the scientific journal PNAS.

Unploughed stubble is lighter in colour and reflects more solar radiation than tilled surfaces. Measurements taken show that approximately 30% of sunlight is reflected back due to the albedo effect – the albedo is a measure of the reflectance capacity of reflective surfaces. Ploughed fields reflect only 20% of incoming solar radiation. Model simulations have shown that this difference results in a 50% higher level of reflection in unploughed fields and that this in turn has a significant effect in extreme heat. In the event of a heat wave, such as the one in Europe in 2003, unploughed farm fields could reduce the local temperature by as much as 2 °C.

The hotter it becomes, the greater the albedo effect and the resulting temperature reduction. "Cropland albedo management has more effect during heat waves because there is almost no clouds during these events and more radiation can be reflected back into space", says first author Edouard Davin. However, this effect is only short term and local — perhaps at the most regional, but never trans-regional.

...The scientists also report that the cooling effect of an unploughed field cannot be attributed solely to changes in albedo values. Crop residue acts as an insulating layer that holds back moisture from deeper soil strata and releases it only slowly — this long process of evaporation also helps reduce the air temperature during a heat wave. In a ploughed field, on the other hand, moisture evaporates more rapidly and almost completely in extreme heat. Thus, there is an additional cooling effect of no-till farming through slow evaporation....

Harvested wheat field with round straw bales. Shot by Arne Hückelheim, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license

NASA satellites see Arctic surface darkening faster

NASA: The retreat of sea ice in the Arctic Ocean is diminishing Earth's albedo, or reflectivity, by an amount considerably larger than previously estimated, according to a new study that uses data from instruments that fly aboard several NASA satellites.

The study, conducted by researchers at Scripps Institution of Oceanography, at the University of California, San Diego, uses data from the Clouds and Earth's Radiant Energy System, or CERES, instrument. There are CERES instruments aboard NASA's Tropical Rainfall Measurement Mission, or TRMM, satellite, Terra, Aqua and NASA-NOAA's Suomi National Polar-orbiting Partnership (Suomi NPP) satellites. The first CERES instrument was launched in December of 1997 aboard TRMM.

As the sea ice melts, its white reflective surface is replaced by a relatively dark ocean surface. This diminishes the amount of sunlight being reflected back to space, causing Earth to abso
rb an increasing amount of solar energy.

The Arctic has warmed by 3.6 F (2 C) since the 1970s. The summer minimum Arctic sea ice extent has decreased by 40 percent during the same time period. These factors have decreased the region's albedo, or the fraction of incoming light that Earth reflects back into space – a change that the CERES instruments are able to measure.

..."Scientists have talked about Arctic melting and albedo decrease for nearly 50 years," said Ramanathan, a distinguished professor of climate and atmospheric sciences at Scripps who has previously conducted similar research on the global dimming effects of aerosols. "This is the first time this darkening effect has been documented on the scale of the entire Arctic."

Eisenman, an assistant professor of climate dynamics, said that the results of the study show that the heating resulting from albedo changes caused by Arctic sea ice retreat is "quite large." Averaged over the entire globe, it's one-fourth as large as the heating caused by increasing atmospheric CO2 concentrations during the same period....

This image shows a visualization of Arctic sea ice cover on Sept. 12, 2013, with a yellow line showing the 30-year average minimum extent. A new study shows that the magnitude of surface darkening in the Arctic (due to the retreat of sea ice) is twice as large as that found in previous studies. Image Credit: NASA Goddard's Scientific Visualization Studio/Cindy Starr