olelog

Around 14,800 years ago, a strengthening of the summer monsoons - moist tropical Atlantic monsoons from south-west - led to a dramatic climatic change in North Africa and created a “green Sahara”. How did this North African humid period come to an end and lead to the the world’s largest warm desert today. Was it abruptly or gradually?

The drying of the Sahara in the Holocene, that is approximately the last 11,550 years, is widely believed to have been an abrupt event, completed within a few hundred years, but new research published in Science of 9 may 2008 indicates that it happened gradually over the last 6000 years.

The authors of Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years studied a sediment record from Lake Yoa in northern Chad. Lake Yoa is one of the very few Saharan lakes in which sediments have accumulated without a break during the Holocene. Despite its extremely arid location, the lake is fed by ancient groundwater and therefore does not dry up.

The vegetation history of the surroundings is reconstructed from pollen. The reconstructed salinity values provide a record of changing precipitation. The input of atmospheric dust to the lake reflects wind regimes and the extent of vegetation cover in the surrounding landscape. The results show that vegetation and dust flux changed gradually over the past 6000 years, accompanied by the slowly weakening monsoon. The pollen source area implies that average north-easterly wind strength must have increased during this time, either because wintertime trade-wind circulation intensified or because a change in the mean position of the Libyan high-pressure cell now channeled low-level northeasterly flow more effectively through the Tibesti-Ennedi corridor.

Tibesti Mountains is a volcanic region to the west of Lake Yoa and the Ennedi Plateau, which is located to the east of the lake, is a sandstone plateau surrounded on all sides by sands, that encroach the deep valleys of the Ennedi.

However fast the drying occurred, it pushed people out of north-central Africa, and that climatically forced migrations might have led to the rise of the pharaohs and Egyptian civilization.

According to the lead author there are now signs of a tiny shift back towards greener conditions in parts of the Sahara, apparently because of global warming.

* http://www.sciencemag.org/cgi/content/short/320/5877/752
* http://sciencenow.sciencemag.org/cgi/content/full/2008/508/2?rss=1
* http://www.abc.net.au/science/articles/2008/05/09/2240138.htm
* http://www.nytimes.com/2008/05/09/science/09sahara.html?_r=1&partner=rssnyt&emc=rss&oref=slogin
* http://www.iht.com/articles/2008/05/09/africa/09saha.php
* http://www.redorbit.com/news/science/1378928/sands_of_sahara_moved_slowly/index.html?source=r_science

Over 30 people have been killed this month and hundreds more injured as ‘norwesters’, wreaked havoc across Bangladesh.

Nor'wester thunderstorms, locally known as Kal-baishakhi, often blow over Bangladesh in April-May from a northwesterly direction. Nor'wester thunderstorm coincides with the setting in of the summer season. From mid-March to April the temperature in Bangladesh rises sharply compared to the preceding months (i.e. winter months). In the middle of April the whole country, especially the northwestern part, records a sharp rise in day temperature. Presence of warm and moist air in the lower layer of the atmosphere is an essential precondition for the development of a nor'wester.

The main reasons behind the nor'wester is the warm and moist air coming from the southeast which rises up to 2 kilometres, mixes with the relatively cold and dry jet streams coming from the northwesterly and westerly directions. The mixing of these two dissimilar air masses causes storms. The warm and moist air rises due to the Chotanagpur Plateau, Himalayan ranges, and Assam Plateau. Thunder and lightning is common with a nor'wester. Nor'westers are more frequent in the late afternoon because of the influence of surface heating in producing convection currents in the atmosphere. In the western region of Bangladesh, nor'westers come in the late afternoon and before evening but in the eastern side it comes generally after evening, moving from a northwesterly to a easterly and southeasterly direction. In this season the morning remains calm. Temperature begins to rise from noon creating a convective current and the storm is formed. The average wind speed of a nor'wester is 40-60 km per hour. But in exceptional circumstances the wind speed may exceed 100 km. The duration of the storm is generally less than an hour but sometimes it may exceed an hour.

Much of the country’s rural population lives in huts made of corrugated iron or mud and straw which are ill-equipped to withstand winds powerful enough to uproot trees and knock down electricity pylons.

Norwesters may also strike later in November. Given their ferocity and destructive capacity `norwesters’ are also referred to as tornadoes. For nearly 60 days during the two storm seasons, locally generated storms have hit various parts of the country almost on a daily basis. So far there is no effective early warning system for these storms. They are so local in nature and take shape so suddenly that modern tracking devices can only locate them when they begin to move, at times with a whirling speed of 200 km.

In Dhaka, trees were uprooted and thatched roofs blown away after a powerful storm struck the capital on 2 May 2008. The storms are so frequent in number and so destructive in nature that the total damage done by them is perhaps only second to the damage caused by the annual floods. In terms of damage to life and property, they do more than the floods.

Norwesters often strike when the country’s ‘boro’ crop - the country’s main rice harvest - is ready for harvest, and jute, a major cash crop for the impoverished nation, is at a critical stage of growth. According to the department of meteorology, 30-50 percent of standing crops are damaged in areas where ‘norwesters’ hit.

The crop loss could be minimised or even avoided if the pattern of cultivation is changed, either by planting the crops two to three weeks earlier than now, or by shortening the harvesting season. Most of those who die, die indoors, crushed under mud walls or hit by flying tin roofs - the construction of disaster-resilient houses could save thousands of lives lost under falling roofs and walls.

http://www.irinnews.org/report.aspx?ReportId=78089
http://banglapedia.search.com.bd/HT/N_0208.htm

If you have been near a smoking volcano or solfatara you may have experienced the foul odour of rotten eggs, which comes from the hydrogen sulphide that occurs in volcanic gases.

Huaynaputina (Quechua: "New Volcano") is a stratovolcano located in a volcanic upland in southern Peru. The volcano has also been variously known as Omate, Quinistaquillas, Chiquimote, and Chequepuquina. The volcano does not have an identifiable mountain profile, but instead has the form of a large complex 2.5 km diameter explosion crater with a maximum elevation of 4,800 m above sea level and edifice height of no more than 500 m. On 19 February 1600 it exploded catastrophically, in the largest volcanic explosion in South America in historic times. The eruption caused substantial damage to the major cities of Arequipa and Moquengua. It blanketed nearby villages with glowing rock and ash, and killed some 1,500 people.

The eruption is known to have put a large amount of sulphur into the atmosphere, and tree ring studies show that 1601 was a cold year. Sulphur reacts with water in the air to form droplets of sulphuric acid, which cool the planet by reducing the amount of sunlight reaching the Earth's surface. But the droplets soon fall back to Earth, so the cooling effects last only a year or so. In the Northern Hemisphere 1601 was the coldest year in six centuries. In Greenland the sulphuric acid spike was larger than that from Krakatau (1883). Regional agricultural economies took 150 years to fully recover. According to a new study of contemporary records the eruption had a global impact on human society. In Russia 1601-1603 brought the worst famine in the country's history, leading to the overthrow of the reigning tsar. Records from Switzerland, Latvia and Estonia record exceptionally cold winters in 1600-1602. In France, the 1601 wine harvest was late, and wine production collapsed in Germany and colonial Peru. In China, peach trees bloomed late, and Lake Suwa in Japan had one of its earliest freezing dates in 500 years.

"The volcano that changed the world" - Ken Verosub and his coauthor, student Jake Lippman, explore the effects the 1600 Huaynaputina eruption had on the global agricultural economy. Their work appears in the April 11th issue of American Geophysical Union newsletter EOS.

* http://www.eurekalert.org/pub_releases/2008-04/uoc--1ec042308.php
* http://www.physorg.com/news128177951.html
* http://www.scientificblogging.com/news_releases/the_climate_disruption_from_the_huaynaputina_eruption
* http://en.wikipedia.org/wiki/Huaynaputina
* http://www.nature.com/news/2008/080411/full/news.2008.747.html
* http://www.nature.com/nature/journal/v393/n6684/abs/393455a0.html

In my latest post I mentioned that the Madden-Julian Oscillation, or in short MJO might serve as tool to extend the weather forecasts beyond the usual 5 days.

The MJO is a cyclical wave in Earth’s atmosphere - a cyclical pattern of slow, eastward-moving waves of clouds, rainfall and large-scale atmospheric circulation anomalies that can strongly influence long-term weather patterns around the world. It is a 30-to-90-day cycle, and it spans nearly half of Earth's equator, primarily over the Indian Ocean and western Pacific. The MJO affects precipitation over the tropical monsoon regions. It affects the winter jet stream and atmospheric circulation in the Pacific/North America region, causing anomalies that can lead to extreme rainfall events. It can also change summer rainfall patterns in Mexico and South America and may trigger El Niño events.

Fortunately it now seems possible to predict the MJO itself according to an article in Science of 14 December 2007, titled A Madden-Julian Oscillation Event Realistically Simulated by a Global Cloud-Resolving Model. The results of the study demonstrate the potential making of month-long MJO predictions when global cloudresolving models with realistic initial conditions are used. The predictions are based on infrared satellite images of clouds. It is indeed expected that weather forecasts beyond 10 days could be improved if the MJO representations in global weather prediction models were more realistic.

Another article in the same issue of Science titled Deep Ocean Impact of a Madden-Julian Oscillation Observed by Argo Floats shows that these atmospheric waves also affect the oceans - and to a greater depth than previously known. The authors used a data set of unprecedented size obtained from autonomous, free-drifting instruments, called Argo floats, to show that the surface wind stress associated with the MJO can force eastward-propagating oceanic Kelvin waves that extend to a depth of at least 1500 meters and that have amplitudes of as much as six times those of annual-cycle Kelvin waves. These amplitudes are significantly greater than those predicted by ocean models, so that the MJO could affect a much larger volume of the Pacific Ocean than just the ocean surface.

Argo floats are described here. and the use of these floats in oceanic studies mentioned in an editorial in Science of 15 December 2006. I wrote about oceanic Kelvin waves here.

Oceanic equatorial Kelvin waves in the central and eastern Pacific are forced by MJO wind stress anomalies. The MJO can also influence the deep ocean in high latitudes and has has a direct impact on the ocean biosphere, with implications for the fishing industry.

• http://www.sciencemag.org/cgi/content/short/318/5857/1763
• http://www.sciencemag.org/cgi/content/short/318/5857/1765
• http://www.sciencemag.org/cgi/content/summary/314/5806/1657
• http://www.argo.ucsd.edu/

Back in the sixties I bought a 24 volume (American) Collier’s Encyclopedia. There is not one word about Yedoma in it. There is a Danish saying that something is "a town in Russia" if something is unknown, incomprehensible, or unfamiliar. And actually Yedoma is a town in Russia, more precisely in Siberia, located at Latitude: 61° 45' 0 N, Longitude: 45° 10' 60 E.

Apart from being a town in Siberia, yedoma is a geological term, that has turned up in climate change papers the latest couple of years because of its greenhouse gas releasing potential. Before then it was also known to palaeontologists as finding place for mammoths. For example, the “mammoth cemetery”, containing remains of 156 mammoths, was in a yedoma mound.

A couple of maps may help me explain the term. First a map of the maximum ice sheets in the Northern hemisphere during the last ice ages.


In my post on loess about a week ago, I explained how loess was formed from an accumulation of dust carried by the wind during the ice ages. I mentioned that loess was found south of the ice sheets. It is remarkable that northern Siberia was practically ice free during most of the ice age. On the other hand it has been covered by permafrost ever since. You will see from the following map, that there is a remarkable coincidence between the ice free area and todays area of yedoma.


Loess was blown over this area, mixed with ice, and frozen ever since.

Yedoma is an organic-rich (about 2% carbon by mass) Ice Age loess permafrost with ice content of 50–90% by volume. Several Russian geologists have concluded that the ice was formed simultaneously with the soil. Yedoma currently occupies an area of more than one million km² in northeast Siberia (marked as orange on the map), and in many regions is tens of meters thick. It mostly lies under lakes, but also forms mounds. During the Last Glacial Maximum, when the global sea level was 120 m lower than that of today, similar deposits covered substantial areas of the exposed northeast Eurasian continental shelves (marked as light orange on the map).

The ice and loess were not deposited as hills. Instead, they were deposited as one thick layer. Later, as the ice began to melt in spots, water collected in the depressions, accelerating the melting near them.

Some Alaskan loess-related deposits resemble those of Siberian yedoma in several respects: their Ice Age origin, a depth of up to 40 m, similar organic carbon contents (2-4%), high ice volumes in the form of both massive syngenetic ice wedges and segregated ground ice, and formation of thermokarst lakes. Despite similarities, Alaskan deposits are not referred to as ‘yedoma’ in the scientific literature, the name given originally by Russian scientists to the ice-rich deposits in North Siberia.

Papers on thawing yedoma as a source of methane and carbon dioxide:
• http://www.sciencemag.org/cgi/content/abstract/318/5850/633
• http://www.sciencemag.org/cgi/content/summary/312/5780/1612