That Sinking Feeling: Megacities in Decline

Well, not really in decline in the way you’re thinking. Megacities, defined by the United Nations as areas with urban populations in excess of 5 million persons (think Tokyo, New York City, Mumbai), are slowly sinking into the earth – according to research from the Netherlands (and reported by the BBC). Coupled with raising sea levels due to climate change, humanity’s most densely packed population centers at risk of longer and deeper floods.

Sea Level Rise, City Subsidence (via BBC)

Land or the ground sinks into the Earth’s crust naturally, as any geographer will tell you. One method is tectonic plate movement. Depending on the plate, one may be subsumed under another with one being pushed upwards and the other being pushed under. These geologic processes also cause earthquakes and volcanic activity (see: the Pacific Ring of Fire). As the BBC article points out, this geologic activity may be responsible for about 1 mm of subsidence a year. According to the research, a longitudinal study using radar imagery (measuring elevation), concludes that human activity – particularly groundwater extraction – is the primary culprit for city subsidence.

In most cases, a city’s drinking water supply is sourced from local groundwater. As this water is extracted from underground aquifers, one would assume that heavier buildings and infrastructure would press down upon, and compact, the underlying soils. Of course, the relative amount of compaction would be dependent on the local soil (sand, clay, silt, and other factors). While some cities have reduced, if not almost wholly eliminated, municipal subsidence (the article mentions Tokyo and Venice) by halting groundwater extraction – this option isn’t a realistic solution for coastal megacities in less economically advanced countries (Dhaka, Lagos, Jakarta). In these and other “smaller” cities (between 1 and 5 million persons) municipal budgets are already strained coping with a vast informal housing sector (read: slums and shantytowns), a stagnant infrastructure, and poor administration. Adding a requirement for an entirely new source of drinking water for an entire city would be prohibitively expensive.

However, given rising sea levels, municipal subsidence, some 75% of humanity lives on the coast (but not necessarily in a city), and about half of humanity lives in cities (not necessarily on a coast) – we can easily see the scale of the problem. Fortunately, the problem is somewhat long-term, city subsidence and sea level rise occurs at rates of millimeters a year. However, though the number is small the results are disproportionately large. A National Geographic article, published Sep 2013, cited a OSCE report stating that a 20-inch sea level rise would leave 150 million people and $35 trillion dollars (about 9% of global GDP) at risk of coastal flooding. A city sinking 20-inches, an easy analogy, would take 40 years at 5 mm a year.

Of course, this timeline would compress markedly if cities are to contend with both rising sea levels and their own sinking. The timeline is likely to further compress if urban population growth rates remain high as new residents also demand access to water.

Even more long-term, Z Geography wonders if the growth of megacities will lead to their own decline. Could this natural hazard (coastal flooding) combine with other human-made hazards in cities (violent conflict, poverty) and lead to a period of deurbanization in the next 50, or 100, years? One could argue “yes”, in the true spirit of Malthus, but we shall watch for technological and economic innovation – perhaps a cheaper way to reduce our dependence on groundwater?

Super Moon (2013)

This overly short post is dedicated to our constant extraterrestrial companion, the Moon (or Luna). The video below is a NASA science video on the 2011 perigee (super) moon. One key takeaway – if you decide to take a picture try to capture it as it comes over the horizon as it passes common reference points on the ground (such as trees or buildings).

And Happy Belated Summer Solstice! What do these things have to do with Geography? If we’re discussing the entirety of Earth, the perigee (super) full moon of June 2013 represents the closest the moon comes to the Earth in 2013. As the video notes, while natural disasters are unlikely, the effect on tides will be the strongest during the perigee (super) full moon.

The summer solstice occurs when a planet’s semi-minor axis is at its maximum tilt towards its star. In the northern hemisphere of Earth, this translates to the longest (in terms of daylight) day of the year. By the same token the winter solstice, when Earth is tilted at its maximum away from the Sun, is the shortest day of the year.

From Earth Snapshot: Haze over Bangladesh

For those of you with readers, if you haven’t subscribed to Earth Snapshot yet, you should really consider it. Multiple daily satellite images of Earth, aside from their beauty they’re a valuable learning tool in their own right.

A few days ago, Earth Snapshot released this view (and reproduced below) of the eastern Bay of Bengal showing Bangladesh, Burma (Myanmar), and parts of northeastern India. The caption for the image discusses mountains natural ability to block smoke and haze from traveling over them. As we all know, heat rises, but when it is forced upwards by a mountain chain the air cools, forcing it back down if the mountain ridges are too high. This is an important aspect of the precipitation cycle, warm moist air low-lying areas and oceans are eventually forced upwards when they are pushed into mountain chains, as the air cools the moisture is released as precipitation, watering the mountainside and traveling downhill towards rivers and, eventually, larger bodies of water. In this image, we see haze from India and Bangladesh (attributed to agricultural fires and pollutants) locked out of Burma by the eastern Himalaya.

What is most striking to me is the clear delineation of the border between Bangladesh and the northeast Indian state of Meghalaya. Meghalaya’s landscape is dominated by the Khasi Hills, which are lush and heavily forested. Further north beyond the forested hills are the agricultural lands along the Assam Plain (on either side of the Brahmaputra river). The eastern boundary of Bangladesh with India and Burma is less discernible thanks to the forested Chittagong Hills, foothills of the eastern Himalaya. Finally while Bangladesh is overwhelmingly agricultural land (Dhaka is just obscured by a cloud), we can still make out the Sundarbans in the country’s south, at the western mouth of the combined Ganga-Brahmaputra (Padma-Jamuna) river system. This extensive mangrove forest is home to a wide variety of flora and fauna (including the endangered Royal Bengal tiger). Beyond the wildlife, the Sundarbans also absorb some of the devastation brought by cyclonic activity which routinely ravage the country.

In contrast with the haze over Bangladesh, the skies over Burma are clear allowing us to clearly see the mouth of the Irrawaddy river and its floodplain.


haze over India and Bangladesh (via eosnap)