Substantial glacier ice loss in Central Asia's largest mountain range

Glaciers in Central Asia experience substantial losses in glacier mass and area. Along the Tien Shan, Central Asia's largest mountain range, glaciers have lost 27% of their mass and 18% of their area during the last 50 years. An international research team led by the GFZ German Research Centre for Geosciences and including the institute of the French Centre National de la Recherche Scientifique (CNRS) at Rennes University in particular, estimated that almost 3000 square kilometres of glaciers and an average of 5.4 gigatons of ice per year have been lost since the 1960s. In the current online issue of Nature Geoscience, the authors estimate that about half of Tien Shan's glacier volume could be depleted by the 2050s.

Glaciological field measurements on a glacier in the Jetim-Bel range, Kyrgyzstan. 
Systematic in-situ surveys as pictured are essential for providing ground truth 
data against which satellite observation can be compared 
[Credit: D. Farinotti/GFZ/WSL]

Glaciers play an important role in the water cycle of Central Asia. Snow and glacier melt from the Tien Shan is essential for the water supply of Kazakhstan, Kyrgyzstan, Uzbekistan, and parts of China. "Despite this importance, only little was known about how glaciers in this region changed over the last century" the principal investigator Daniel Farinotti explains. Most of the direct monitoring programs, that were shut down with the collapse of the Soviet Union, are resumed only today, and modern observation techniques often cover a limited time span only.

A lake mirrors some fresh-snow covered glaciers in the Teskey Ala-Too, 
Kyrgyzstan. Yet the appearance is deceiving: Glaciers in the 
Tien Shan are losing mass at a rapid pace 
[Credit: D. Farinotti/GFZ/WSL]

GFZ-researcher Farinotti and colleagues now present a reconstruction of the glacier evolution in the Tien Shan. "We combined various methods based on satellite gravimetry, laser altimetry and glaciological modelling" Farinotti says. "This way, we were able to reconstruct the evolution of every single glacier. Currently, the Tien Shan is losing ice at a pace that is roughly twice the annual water consumption of entire Germany."

Glaciers in Central Asia

Glaciers can store water as glacier ice over decades, and transfer winter precipitation into the summer months by releasing it as melt water. This is particularly important in seasonally arid regions, i.e. regions that have months with virtually no precipitation, since local water supply is then closely linked to meltwater availability.

Two unnamed glaciers in the Terskey range, Kyrgyzstan. The topographic control
 on glacier occurrence is striking: North-facing slopes are glacierized, 
whilst south-facing slopes are completely free of ice
[Credit: D. Farinotti/GFZ/WSL]

Central Asia is the outstanding case for human dependence on water seasonally delayed by glaciers. Nowhere the question about the glacier state is linked so closely to questions of water availability and, thus, food security.

North facing slope of the Jetim-Bel range, Kyrgyzstan. Glacier melt is an essential 
water resource in an otherwise dry environment Two unnamed glaciers in the
 Terskey range, Kyrgyzstan. The topographic control on glacier occurrence
 is striking: North-facing slopes are glacierized, whilst 
south-facing slopes are completely free of ice 
[Credit: D. Farinotti/GFZ/WSL]

The pace of glacier retreat noticeably accelerated between the 1970s and the 1980s. Daniel Farinotti: "The long-term signal is clearly related to the overall rise in temperature." In fact, the study shows that the rise in temperature, and summer temperature in particular, is a primary control for glacier evolution in the region. "For Central Asia, this statement is less trivial than it might seem at first glance: Since the winter months in the region are very dry and the mountains are that high, glaciers receive most of their snowfalls during summer." Farinotti explains. "This means that an increased temperature contributes to both, increased melt and reduced glacier nourishment -- and obviously, both contributes to glacier wastage."

By using the latest climate projections, which anticipate an additional 2 °C warming of summer temperatures in the period 2021 -- 2050, the authors also provide a first outlook for the future evolution: Half of the total glacier ice volume present in the Tien Shan today could be lost by the 2050s.

Source: GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre [August 17, 2015]

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1,800 years of global ocean cooling halted by global warming

Prior to the advent of human-caused global warming in the 19th century, the surface layer of Earth's oceans had undergone 1,800 years of a steady cooling trend, according to a new study. During the latter half of this cooling period, the trend was most likely driven by large and frequent volcanic eruptions.

Compared to the atmosphere, the oceans can absorb much more heat and trap it for
 longer periods of time. Thus the ocean can buffer short-term changes in global
 temperature. But when events such as volcanic eruptions cluster together
 in a relatively short period of time, the temperature changes can 
become prolonged [Credit: Kathryn Hansen/NASA]

An international team of researchers reported these findings in the August 17, 2015 issue of the journal Nature Geoscience. The study also indicates that the coolest temperatures occurred during the Little Ice Age--a period that spanned the 16th through 18th centuries and was known for cooler average temperatures over land.

The concurrence of cooling events on both land and sea suggests that a global cooling phenomenon was erased by subsequent human-caused global warming.

"Today, the Earth is warming about 20 times faster than it cooled during the past 1,800 years," said Michael Evans, second author of the study and an associate professor in the University of Maryland's Department of Geology and Earth System Science Interdisciplinary Center (ESSIC). "This study truly highlights the profound effects we are having on our climate today."

Compared to the atmosphere, the oceans can absorb much more heat and trap it for longer periods of time. Thus the ocean can buffer short-term changes in global temperature. But when events such as volcanic eruptions cluster together in a relatively short period of time, the temperature changes can become prolonged.

"Volcanic eruptions have a short-term cooling effect on the atmosphere, but our results showed that when volcanic eruptions occurred more frequently, there was long-term ocean cooling," said lead author Helen McGregor, an Australian Research Council (ARC) Future Fellow at the University of Wollongong in Australia. "With this research, we now have new insight into the century-scale global sea-surface temperature variations that came before human-made greenhouse gas forcing."

Results of the global sea surface temperature compilation from Ocean2k: A cooling over the past two millenium was reversed only in the most recent two centuries. Fifty-seven previously published and publicly available marine sea surface temperature reconstructions were combined and compiled into 200-year brackets, represented by the boxes. The thin horizontal lines dividing each box are the median of the values in that box. The thick blue line is the median of these values weighted for differences in the region of the global ocean in which they were found. (More in Figure 2a in the paper and Supplementary Table S13). [Credit: Modified version of Fig 2a in McGregor et al. 
Nature Geoscience 2015]

The scientists are the first to combine 57 previously published marine surface temperature reconstructions that cover all of the world's oceans, from near-polar to tropical regions. The team compiled the data within 200-year brackets to observe long-term trends, and then compared the findings to land-based reconstructions, which revealed similar cooling trends.

"No matter how we divided the data set, the cooling trend stands out as a robust signal," McGregor said.

To investigate the cause of the cooling trend, the researchers turned to climate models. They examined how sea-surface temperatures reacted to various "forcing" factors, such as changes in solar output, Earth's orbit, land use, volcanic activity and greenhouse gases. Only volcanic events resulted in a cooling trend that matched the team's real-world observations.

Understanding how forcing factors changed ocean temperatures in the past can open a window into future climate change.

"Model simulations by others have shown us that the oceans can impart a substantial delay in the warming of the surface climate," said Evans, who is also the lead of the Ocean2k working group of the Past Global Changes (PAGES) program. "With much of the heat from global warming entering our oceans, recent ocean surface warming may foreshadow additional future warming, in the same way ocean cooling appeared as a long-term response to large and frequent volcanic events in recent centuries."

"We are still learning how the oceans mediate climate variations," Evans added. "Further work combining both observations and simulations of ocean climate will refine our understanding of the ocean's role in climate change."

Source: University of Maryland [August 17, 2015]

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Heat release from stagnant deep sea helped end last Ice Age

The build-up and subsequent release of warm, stagnant water from the deep Arctic Ocean and Nordic Seas played a role in ending the last Ice Age within the Arctic region, according to new research led by a UCL scientist.

Calving ice sheet in Spitzbergen 
[Credit: David Thornalley]

The study, published today in Science, examined how the circulation of the ocean north of Iceland -- the combined Arctic Ocean and Nordic Seas, called the Arctic Mediterranean -- changed since the end of the last Ice Age (~20,000-30,000 years ago).

Today, the ocean is cooled by the atmosphere during winter, producing large volumes of dense water that sink and flush through the deep Arctic Mediterranean. However, in contrast to the vigorous circulation of today, the research found that during the last Ice Age, the deep Arctic Mediterranean became like a giant stagnant pond, with deep waters not being replenished for up to 10,000 years.

This is thought to have been caused by the thick and extensive layer of sea ice and fresh water that covered much of the Arctic Mediterranean during the Ice Age, preventing the atmosphere from cooling and densifying the underlying ocean.

Dr David Thornalley (UCL Geography) said: "As well as being stagnant, these deep waters were also warm. Sitting around at the bottom of the ocean, they slowly accumulated geothermal heat from the seafloor, until a critical point was reached when the ocean became unstable.

"Suddenly, the heat previously stored in the deep Arctic Mediterranean was released to the upper ocean. The timing of this event coincides with the occurrence of evidence for a massive release of meltwater into the Nordic Seas. We hypothesize that this input of melt water was caused by the release of deep ocean heat, which melted icebergs, sea-ice and surrounding marine-terminating ice sheets."

A schematic of the changes in the Arctic Mediterranean at the
 end of the last Ice Age [Credit: UCL Geography]

This study highlights the important impact that changes in ocean circulation can have on climate, due to the ocean's capacity to redistribute vast quantities of heat around the globe. For example, scientists are currently concerned that ongoing changes in ocean circulation may result in warmer subsurface water that will cause enhanced melting and retreat of certain ice sheets in Greenland and Antarctica.

Dr Thornalley added: "To help predict the role of the ocean in future climate change, it is useful to investigate how ocean circulation changed in the past and what the associated climate effects were."

In this study, researchers from UCL, Woods Hole Oceanographic Institute and other partner institutions analysed the composition of calcite shells of small single-celled organisms (called foraminifera) that are found in ocean floor sediment. The shells of these organisms record the chemistry of the deep ocean at the time they were living, enabling the researchers to reconstruct past changes in ocean circulation.

By measuring the radiocarbon content of these shells, the research team was able to determine how rapidly deep water was being formed in the Arctic Mediterranean. A number of different techniques were then used to constrain past temperature changes, including measuring the ratio of magnesium and calcium, and the arrangement of isotopes of carbon and oxygen within the calcite shells of the foraminifera, both of which vary according to the temperature of the water in which the foraminifera grew.

A warmer, deep Arctic Mediterranean during glacial times has been suggested in previous studies, too. As summarised by co-author Dr Henning Bauch (GEOMAR/Germany) "It is good to see that new, independent proxy data would give strong support now to these former hypotheses."

Source: University College London [August 13, 2015]

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New insights into how continents break up

When the western part of the super-continent Gondwana broke up around 130 Million years ago, today's Africa and South-America started to separate and the South Atlantic was born. It is commonly assumed that enormous masses of magma ascended from the deep mantle up to higher levels, and that this hot mantle plume (the Tristan mantle plume) weakened the continental lithosphere, eventually causing the break-up of the continental plate of Gondwana.

Overnight camp in Northern Namibia 
[Credit: C. Haberland, GFZ]

A group of German scientists are now questioning this theory. On the basis of seismic measurements published in the current issue of the journal Geology, scientists from Potsdam (GFZ German Research Centre for Geosciences), Bremerhaven (Alfred-Wegener-Institute, AWI) and Kiel (GEOMAR) show that impacts of the mantle plume on the continental crust are actually surprisingly small. This is by no means in agreement with a large plume playing an important role in the break-up process. Accordingly, a dominant or controlling role of a mantle plume for the break-up of the continent is thus questionable.

Hot ascending mantle plumes in the Earth's mantle are an important driving force in plate tectonics. With an assumed diameter of the plume heads of up to several thousand kilometers, the amount of hot material ascending from the core mantle boundary at 2900 km depth is sufficient to migrate through the continental lithosphere.

This process leads to the eruption of large volcanic material (flood basalts) at the Earth's surface. This is also the case for Southern Africa and South America: the Parana/Etendeka/ flood basalt provinces are the direct consequences of the Gondwana break-up starting some 130 million years ago. Traces of the break-up process can be found on the newly formed ocean floor: the Walvis Ridge off the coast of Northern Namibia images the track of the mantle plume.

Drilling activities for seismic investigations
[Credit: C. Haberland, GFZ]

In order to study these processes, German scientists investigated structures which are related to the break-up process of Gondwana in the South Atlantic. The upwelling of large amounts of hot mantle material produces regions of crustal and mantle rocks with different seismic velocities (with respect to the surrounding, unaffected regions).

In cooperation with colleagues from AWI and GEOMAR, and with the support of the Geological Survey of Namibia, scientists from GFZ have carried out extensive seismic investigations on and offshore Northern Namibia. The crustal structure was investigated along several seismic lines.

"For the first time we could obtain images of the deeper crustal structure in the region where the Walvis Ridge joins the African continent, in order to study the impact of a mantle plume" explains Trond Ryberg from GFZ. "Our seismic investigations found a distinctive high-velocity anomaly in the lower crust between 20 and 40 km depth."

This region of anomalously high seismic velocities can be related to the intrusion of magmatic material in the lower crust of the Earth. This was expected according to the current perception. But surprisingly, the size of the anomaly was far too small to be created by a large plume head playing an active role in the break-up process.

Trond Ryberg: "The crustal structure in the investigated region reflects the general process of continental break-up rather than the immediate impact of the plume head on the lithosphere. In addition, we were able to reconstruct the direction of the mantle plume flow." It seems that the break-up of Gondwana in the South Atlantic and, in general, the role of mantle plumes during the continental break-up needs to be carefully re-evaluated.

Source: GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre [August 13, 2015]

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