olelog

No doubt large-scale destruction of protective mangroves along the coasts of Myanmar aggravated the devastation wreaked by the tropical cyclone Nargis. Mangrove provide significant flood protection in low coastal areas, but never the less Mangrove forests are one of the world’s most threatened tropical ecosystems. In January 2008, the UN Food and Agriculture Organization warned that Asia is fast losing its coastal mangroves, with more than 1.9 million hectares being destroyed each year. Mangroves are only suitable for planting on coastal mud-flats and lagoons, but such mud-flats and lagoons cover about 25% of the continental coastline of the Bay of Bengal.

The protective benefits were clearly demonstrated when the Boxing Day Tsunami on 26 December 2004 flooded the coasts of the Indian Ocean. In regions with lesser tsunami intensity areas with coastal tree vegetation were markedly less damaged than areas without. A study published in Science of 28 October 2005 titled The Asian Tsunami: A Protective Role for Coastal Vegetation describe how Cuddalore District in Tamil Nadu, India, provided a unique experimental setting to test the benefits of coastal tree vegetation in reducing coastal destruction by tsunamis (and other flooding events). Cuddalore has a relatively straight shoreline, a fairly uniform beach profile, and a homogenous continental slope. Moreover, the shoreline comprises vegetated as well as non-vegetated areas and was documented by cloud-free pre- and post-tsunami satellite images.

Salt-marshes, mangroves and other forested wetlands act as the front-line defence against incoming storms. They help minimise the impact of storms by reducing wind action, wave action and currents, while the roots of the plants help to hold the sediment in place. The Government of Bangladesh has invested considerable sums of money in re-planting mangroves in previously storm flooded areas to assist in storm protection.

Mangroves are also important for fisheries. Furthermore I must confess that I have had some of my best bird watching experiences in mangrove areas, which may though be of less economic importance.

Predicted effects of climate change over the next 50-100 years will place both coastal and inland wetlands in some parts of the world under a great deal of pressure through increased prevalence of tropical storms, changing patterns of precipitation, and sea level rise.

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

The Sea contains a much higher percentage of oxygen (34 %) than the atmosphere (ca. 21 %).
Most of the oxygen is of course bound to hydrogen in water molecules. A smaller part is bound in molecules of other substances (like calcium carbonate), and only part of the oxygen is freely available for respiration as oxygen gas dissolved in the water. Oxygen occurs as a by-product of photosynthesis in plants (plankton in the open sea). Oxygen is also dissolved at the interface between the sea surface and the atmosphere.
Most of the oxygen-rich ocean water and the animal life which uses oxygen for respiration is therefore found near the surface. As one descends into the depths, the amount of dissolved oxygen in the water drops rapidly, and so does animal life. In a zone occurring at depths of about 200 to 1,000 metres, depending on local circumstances, oxygen saturation in seawater in the ocean is at its lowest. This zone is called the Oxygen Minimum Zone (sometime referred to as the shadow zone). From the oxygen minimum layer downward the amount of dissolved oxygen increases initially, and another decrease occurs near the bottom.

Please note that oxygen profiles like the one shown here vary from location to location. Just now I would like to stress that the occurrence as such of a minimum oxygen zone is a natural phenomenon due to destruction of dissolved oxygen by respiration (something more or less like this CH₂O + O₂ = CO₂ + H₂O). I’ll come back to the degree of oxygen depletion later.

Surface ocean waters generally have oxygen concentrations close to equilibrium with the Earth's atmosphere. In general, colder waters hold more oxygen than warmer waters. This is obvious in the following image of annual mean sea surface dissolved oxygen for the World Ocean. Data from the World Ocean Atlas 2001.

Dissolved oxygen is measured in units as millilitres O₂ per litre (ml/l), millimoles O₂ per litre (mmol/l), milligrams O₂ per litre (mg/l) and moles O₂ m^-3. For example, in freshwater under atmospheric pressure at 20°C, O₂ saturation is 9.1 mg/l. (more about the unit mole at this Wikipedia page). 1 mol of oxygen (O₂) molecules weighs approximately 32 grams. For example an oxygen concentration in surface water of 0.27 mol O₂m^-3 approximates 6 millilitre per litre.

In the oxygen minimum layers the concentration of dissolved oxygen can approach zero, a condition called suboxic. Important mobile macroorganisms are stressed or die under hypoxic conditions; that is, when oxygen concentrations drop below ~60 to 120 mmol kg^-1 (3). Hypoxia occurs at different oxygen concentrations among various species of macroorganisms, so the threshold is not precise. Water lacking dissolved oxygen (0% saturation) is termed anoxic.

As I said above colder waters hold more oxygen than warmer waters. (Salinity is another important factor with differences between fresh water and sea water). The global ocean has warmed substantially over the past 50 years. So what happens during global warming. You would expect reduced oxygen levels and the oxygen minimum zones to expand. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies.

According to the study Expanding Oxygen-Minimum Zones in the Tropical Oceans by Stramma et al. published in Science of 2 may 2008 the oxygen minimum zones of tropical oceans are expanding, restricting habitats for fish and other marine life, the researchers found that oxygen levels here have declined significantly over the past fifty years. The study was concentrated on the eastern tropical Atlantic and the equatorial Pacific simply because these areas have the best historical data, the situation may in fact be worse (or better?) in other parts of the world’s oceans. The expected impacts on subtropical and subpolar regions are larger than in the Tropics. Long-term oxygen changes have been observed and reported in the subpolar and subtropical regions.

And now back to the eastern tropical Atlantic and the equatorial Pacific during the past 50 years. The data reveal a vertical expansion of the intermediate-depth low-oxygen zones. The oxygen decrease in the 300- to 700-m layer is 0.09 to 0.34 micromoles per kilogram per year. In the tropical North Atlantic the vertical extent of the layer with oxygen concentrations of <90 mmol kg^-1 increased 85%, from a thickness of 370 m in 1960 to 690 m in 2006. The tropical ocean oxygen minimum zones in the central and eastern tropical Atlantic and equatorial Pacific Oceans appear to have expanded and intensified during the past 50 years.

Oceanic dissolved oxygen concentrations have varied widely in the geologic past. The anoxic ocean at the end of the Permian (251 million years ago) is associated with elevated atmospheric CO₂ and massive terrestrial and oceanic extinctions.

The trends have fundamental implications for marine ecosystems and thereby fisheries.


• http://www.sciencemag.org/cgi/content/abstract/320/5876/655
• http://news.nationalgeographic.com/news/2008/05/080501-dead-zones.html
• http://www.terradaily.com/reports/Oxygen_depletion_threatens_ocean_habitats_study_999.html
• http://www.scientificblogging.com/news_releases/the_decline_in_ocean_oxygen

The tropical cyclone Nargis in the Bay of Bengal is on course towards the west coast of Myanmar. It is expected to hit the lowest central part of the country, the Ayeyarwady (or Irrawaddy) delta near the great city of Yangon (also known as Rangoon) and the low coast to the north of it. In the northern hemisphere a cyclone rotates counter-clockwise, because of the coriolis force, which means that water masses from the Bay of Bengal will be forced far over land on the right side of the cyclone, where the delta is. Further north the mountain range Arakan Yoma (also known as the Chin Hills) forms a wall towards the bay, and here Nargis is expected to release torrential rains, when the humid air is forced up over the up to 3000 m high mountain tops.

The majority of Burma's Myanmar's population lives in the Ayeyarwady valley. The upper and central portions of the Ayeyarwady delta are almost entirely under cultivation, principally for rice. The climate is monsoonal climate, with an average annual rainfall of about 1,500-2,000 mm in the north increasing to 2,500 mm in the southeast and 3,500 mm in the southwest. Over 90% of the rain falls between mid May and mid November.

Sorry at my age we sometimes have difficulties with "new" geographical names, but I do my utmost to keep to the official, politically correct, spelling.

• http://www.gdacs.org/reports.asp?eventType=TC&ID=NARGIS-08&system=asgard&alertlevel=Green&glide_no=&location=&country=&new=true
• http://www.arcbc.org.ph/wetlands/myanmar/mmr_irrdel.htm



PS of 30 April 2008
Bangladesh raises storm alert and urges swift rice harvest: http://www.alertnet.org/thenews/newsdesk/DHA224084.htm
The cyclone can also be tracked here, with up to 72 hours lead: http://tsr.mssl.ucl.ac.uk/

PS of 3 May 2008: See follow up here.