GeoLog

GeoLog

Imaggeo on Mondays: The shrinking of Earth’s saltiest lake

One of the consequences of the rapid fall of the water level (>1 m per year), is that vast areas of salt-rich ground of the shrunken Dead Sea are prone to strong dissolution and mechanical erosion of the subsurface processes.

The Dead Sea is one of the saltiest lakes on Earth, located at the lowest point of the globe.  For centuries it has been known for the restorative powers of its muds and waters. Their hypersalinity means it is possible to easily float on the lake’s surface.

Bordering Israel, the West Bank and Jordan, it is a unique environment in an otherwise arid region.  Changing climate, which is seeing temperatures rise in the Middle East, and the increased demand for water in the region (for irrigation) mean the areas on the banks of the lake are suffering a major water shortage. As a result, the lake is shrinking at an alarming rate.

The changing geomorphology of the Dead Sea region is now the focus of a large international project (DESERVE) to address the resulting geohazards at the Dead Sea.

One of the consequences of the rapid fall of the water level (>1 m per year), is that vast areas of salt-rich ground of the shrunken Dead Sea are prone to strong dissolution and mechanical erosion of the subsurface processes. This leads to the widespread land subsidence and the development of sinkholes, which pose a major geological hazard to infrastructure, local population, agriculture and industry in the Dead Sea area, writes Djamil Al-Halbouni in an abstract presented at the EGU 2016 General Assembly.

Today’s Imaggeo on Monday’s image was taken in the purpose of investigating the sinkhole phenomenon along the coastline.

“Near-surface aerial photography offer valuable hints on possible processes that lead to the formation of huge depression zones, e.g. the ground and surface water flow, the existence of vegetation and water sources or simply the morphology,” explains Djamil.

Sets of images are then combined into digital terrain models to quantitatively estimate hazard potentials and development of sinkholes via repeated measurements.

Specifically, this image was taken by a camera on a helikite balloon from 150m altitude. It shows a canyon penetrating the whitish pure salt shoreline at the Jordanian coast. It also reveals, in its’ magnitude surprising for the scientists involved, round structures under the shallow water, which are interpreted as submarine springs and possible submarine sinkholes close to the shore.

 By Laura Roberts and Djamil Al-Halbouni of the German Research Center for Geosciences, Physics of the Earth, Potsdam, German

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Geosciences Column: The complex links between shrinking sea ice and cloud cover

Sea ice breaking on the Chukchi Sea, Barrow, July 2014

The global climate system is complex. It is composed of, and governed by, a plethora of interconnect factors. Solar radiation, land surface, ice cover, the atmosphere and living things, as well as wind and ocean currents, play a crucial role in the climate system. These factors are intricately connected; changes to some can have significant effects on others, leading to overall consequences for the global climate.

Since the 1980s, sea ice has been decreasing gradually as a result of global warming. But the impact of retreating sea ice on the global climate system aren’ t yet fully understood. A new study published in the EGU’s open access journal, Atmospheric Chemistry and Physics, attempts to unravel the complex feedback systems between Arctic sea ice extent and cloud cover in the region.

The researchers, lead by Manabu Abe, of the Institute of Arctic Climate and Environmental Research in Japan, argue that shrinking sea ice extent in the Arctic is the cause for increased cloud cover in the region. This, in turn, further enhances the feedback processes of Arctic warming because it cause sea ice to retreat further.

Sea ice, is the ice that ‘grows’ as water in the poles is exposed to very low temperatures over long periods of time. Although some waters are covered by ice year round, most sea ice forms during the cold winter months and melts in the summer.

Global climate influences the annual growth of sea ice. This year, ocean waters in the Arctic are failing to freeze and sea ice isn’t forming as quickly as it normally would. Alarmingly, October 2016 registered the lowest sea ice extent since records began.

Scientists think that the unusually low amount of sea ice formed in the Arctic this year is the result of extraordinarily hot sea surface and air temperatures, which are essentially stopping the formation of ice on ocean waters.

But sea ice extent also influences global climate. Solar radiation is absorbed and reflected by the Earth’s atmosphere (including clouds) and surface. Ice is more reflective than water and land. So as ice cover across the globe decreases, so does the planet’s ability to reflect solar radiation, causing the Earth’s surface to warm further, which, in turn, causes more melting of ice. This is know as the ice-albedo feedback loop.

The effects of shrinking sea ice are not limited to surface warming. Ocean heat uptake and storage can be affected, as can be the formation of low-level cloud cover over the Arctic. While the surface of clouds reflect solar radiation, they also prevent heat from being lost from the Earth’s surface. That’s why, often, on overcast nights temperatures are higher than on clear nights.

A study back in 2012, proposed that increased cloud cover in the Arctic enhanced the radiation emitted by the atmosphere and clouds – known as longwave radiation (DLR) -, causing higher surface air temperatures in autumn. This would extend the sea ice melting season. But there is little data which measures radiation at the surface, making the claim controversial.

Other studies have used computer simulations of the global climate, to mimic the effects of reduced sea ice conditions on cloud cover. They show that the areas of open ocean created by the reduction in sea ice mean more moisture is transported from the ocean to the atmosphere, resulting in the formation of more clouds. But the simulations are not very good at representing polar clouds and so the results aren’t entirely reliable.

Now, Abe and his coworkers, used a new state-of-the-art climate simulation to try and shed light on the problem. They included data from as far back as 1850 in their study, as well as making it more robust by taking into account other factors, such as changing sea surface temperatures, greenhouse gases, aerosols and land use (from the 1980s to 2005), which might affect the formation of clouds.

Geographical map of the simulated linear trend in the total cloud cover (shaded) and sea ice concentration (contours) in (a) September, (b) October, and (c) November during the period 1976–2005. The units are decade. From M.Abe at al., 2016

Geographical map of the simulated linear trend in the total cloud cover (shaded) and sea ice concentration (contours) in (a) September, (b) October, and (c) November during the period 1976–2005. The units are decade. From M.Abe et al., 2016

The new simulation found that between 1976 and 2005, Arctic sea ice decreased through the summer and autumn months (which is corroborated by satellite observations). Meanwhile, cloud cover increased throughout autumn, winter and spring, reaching its peak in October.

The researchers argue that the link between the two trends is not coincidental. Reduced sea ice extent in the autumn months,coupled with a decrease in atmospheric temperatures, means more heat is exchanged from the oceans to the atmosphere, which fuels the formation of clouds. More clouds mean downwards longwave radiation (DLR) in October is increased by as much as 40 to 60% (compared with clear autumn skies). With less heat being reflected off the surface of the Earth, sea ice extent decreases further due to melting and so a feedback loop (not dissimilar to the ice-albedo loop) is established.

The results reinforce the findings of previous studies, but some questions remain unanswered. The scientists point out that, it is not only important to understand how much cloud cover increases by as a result of shrinking sea ice extent. In a warming climate, how increases in air temperature and humidity affect the vertical structure of clouds will play an important role in the sea ice-cloud feedback loop. The vertical profile of a cloud also strongly influences how and how much DLR is reflected back on the Earth’s surface, so there is a need for a better understanding of the feedback processes related to clouds too.

By Laura Roberts Artal, EGU Communications Officer

References

Abe, M., Nozawa, T., Ogura, T., and Takata, K.: Effect of retreating sea ice on Arctic cloud cover in simulated recent global warming, Atmos. Chem. Phys., 16, 14343-14356, doi:10.5194/acp-16-14343-2016, 2016.

Wu, D.L., and Lee, J.N.:Arctic low cloud changes as observed by MISR and CALIOP: Implication for the enhanced autumnal warming and sea ice loss, J. Geophys. Res.-Atmos., 117, D07107, doi:10.1029/2011JD017050, 2012

Imaggeo on Mondays: White rainbow in the Arctic

“Above the foggy strip, this white arch was shining, covering one third of the visible sky in the direction of the ship's bow,” he explains. “It was a so-called white, or fog rainbow, which appears on the fog droplets, which are much smaller then rain droplets and cause different optic effects, which is a reason of its white colour.”

Despite heading into the long polar night – the time when the sun doesn’t shine in the globe’s most northerly latitudes and when temperatures drop and thick sheets of sea ice form -the Arctic is reported to be 20° C warmer than average for this time of year.  Never has it been more important to understand the effects of climate change on Polar Regions.

Mikhail Varentsov, a climate and meteorology expert from the University of Moscow, was on board the Akademik Tryoshnikov research vessel during its scientific cruise, NABOS (Nansen and Amundsen Basin Observation System), in 2015.

“During the cruise we went through five Arctic seas from Arkhangelsk city in Russia to the latitude of 180 degrees East,” explains Mikhail.

The aim of the cruise was to collect oceanographic and meteorological data of the icy waters of the Arctic. The data would be used to gain an increased understanding of the challenges faced by the region due to rising temperatures. There was a particular focus on understanding what role warm Atlantic water plays in climate change in the Arctic.

“I was in the group responsible for measurements of parameters of sea and atmosphere interaction – heat and momentum fluxes, compounds of heat balance, cloudiness, etc., and during my watch I had to go to the upper deck to make observations about cloudiness and to check the state of our measurement equipment,” explains Mikhail.

Grey days, with total cloud cover were a common theme throughout the expedition – typical of the Arctic sea summer. But on an unusually clear day, with just a few fog patches in the horizon and remarkably cold water temperatures, Mikhail headed out to take his cloudiness measurements and was rewarded with an extraordinary view.

“Above the foggy strip, this white arch was shining, covering one third of the visible sky in the direction of the ship’s bow,” he explains. “It was a so-called white, or fog rainbow, which appears on the fog droplets, which are much smaller then rain droplets and cause different optic effects, which is a reason of its white colour.”

Despite the gruelling conditions and challenges of working in such a remote region, Mikhail feels privileged, “this trip made me love the simple and fragile beauty of the Arctic, however, this particular scene, with white rainbow above the ice was one of my strongest impressions from the trip.”

By Laura Roberts, EGU Communications Officer

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

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