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Urban climate
Workbook #11 | HUMAN FACTORS: SUSTAINABILITY AND HABITABILITY
Whereas a nomadic Australian Aborigine uses energy at a rate equivalent to the constant burning of a single 100-watt light bulb, the energy used per person in Australia today is equivalent to the continuous burning of about 6 1000-watt radiators.
Charles Birch, from ‘Confronting the Future’
Carry your cup in your hand and look around,
Leaves are brown
And the sky is a hazy shade of winter.
Lyrics from ‘A Hazy Shade of Winter’ by Simon & Garfunkel
One of my flat mates made it a habit (a bad one at that) to leave the geyser on the entire day (we’ve put a stop to that since), being the last one to leave the house in the morning. Once, he even left the electric stove on all night. He’s Indian and a nice guy, but he was brought up in America. According to the Sydney Morning Herald, the average American uses 40 squares of toilet paper a day. The connection isn’t difficult to make.
Perhaps I’m being unfair to Americans – we all know Brutus is (was) an honorable man; just ask Julius. Excessive consumption is pretty much a given with the more “developed” nations, Australia included. And with “developing” nations such as India, inferior technology and sheer weight of population numbers mean that even a low per capita consumption of energy translates into high emission of greenhouse gases and waste heat. The result, as we all know, is rising global temperatures, changing climate patterns, and doomsday movies like The Day After Tomorrow, which graphically demonstrates the possibility of extreme weather events triggered by changing salinity and temperature of ocean currents. Ice storms, waves of hurricanes, cities flooding… all images of destruction lovingly captured on celluloid. The worrying thing is the process is already well under way in reality.
Take global warming. The IPCC Third Assessment Report on Climate Change (2001) estimates a 0.6°C increase in global surface temperature since the late 19th century, with a 95% confidence interval of 0.4 to 0.8°C. The graph on the left (www.ipcc.ch) confirms this, with a marked
shift from negative temperature anomalies to positive (i.e. hotter) anomalies. The result is retreating ice shelves and glaciers, the melting ice dumping fresh water into the oceans with two major consequences: increasing sea levels, and decreasing salinity. In the movie, these disruptions actually cause the warm Gulf Stream to “shut down”, triggering off an Ice Age. Here’s what Peter N. Spotts had to say about the movie in the Christian Science Monitor:
In the movie, global warming melts the Arctic ice cap, fresh water shuts down critical currents in the North Atlantic that transport heat north, and the big chill begins. The impetus for that scenario comes from Dr. Broecker's early work on a 1,000-year chill-down known as the Younger Dryas, which occurred at the end of the last glacial period.
Today, the climate is far too warm to support such extensive amounts of ice. To shut the circulation down under today's conditions, he says, simulations indicate that global average temperatures would have to rise by 4 to 6 degrees. This would generate enough rainfall and river runoff to freshen the North Atlantic sufficiently to halt the "conveyor." But given current trends, he adds, it could take 70 to 100 years to get to that point - not 10 days, as the movie suggests.
10 days, 100 years, it’s all the same. There are always going to be people taking issue with what they see as factual inaccuracies or self-serving exaggeration, but there have been too many instances of “extreme weather events” in the recent past for any sane person to wish the problem away. It’s happening right here and now:
In 2003 (from www.heatisonline.org)
One of the strongest Pacific cyclones on record, with winds up to 220 miles-per-hour, destroyed more than 15 villages on the island of Tikopia in the south Pacific…
In early January, hurricane force winds and torrential rains battered Europe killing at least six people, flooding tens of thousands of homes and hampering rail, road and waterway traffic from Germany to Portugal
In South Asia nearly 400 people died in a severe cold snap. In Bangladesh alone, more than 260 people died from the cold as temperatures dipped toward the freezing mark
In early January, Agriculture Officials announced that last year's drought had cut U.S. wheat yields to their lowest level in 30 years
In Lesotho in Southern Africa, altered rainfall patterns, untimely frost and severe storms destroyed crops and contributed to unusual famine conditions. Said one local farmer: "Frost in the summertime! We never used to see weather like this. We don't know what to expect anymore from the skies. I think God is angry at us, but I don't know why."
In mid-January, a study indicated that Australia's worst drought in memory, which decimated the country's winter wheat crop and reduced its sheep population back to 1920 levels, was directly aggravated by climate change
In South Asia, a highly unusual four-week cold snap killed more than 1,300 people by mid-January. The 37-degree F. temperatures killed numerous homeless people in India, Nepal, and Bangladesh
In Canberra the worst firestorm in history destroyed at least 388 homes as one of the worst droughts to hit Australia in a century continued. Officials estimated the brushfires alone had already cost the country $100 million Australian (about $60 million US)
Meanwhile, in Jerusalem more than a foot of snow paralysed Israel and the West Bank and Gaza, bringing a rare respite to hostilities in the region
Meanwhile, in Alaska, a lack of snow cover and an unusual number of thawed rivers and lakes forced race organizers to reroute the traditional Iditarod dog sled race
Greenhouse gas emissions are largely blamed for the rise in global temperatures that seems to be responsible for the increasing frequency of extreme weather events. The Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC), an international treaty on global warming. When ratified, signatory countries will commit themselves to reducing their emissions of carbon dioxide and other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases. Emission trading (in this world anything can be bought or sold) means that countries with limits that are set above their current production can sell of this surplus to other countries in the open market. Countries also receive credits through various shared clean energy programs and carbon dioxide sinks in the form of forests and other systems that remove carbon dioxide from the atmosphere. Australia and the United States are not ratifying the protocol. No big surprise there. In the words of the great and enlightened leader George W. Bush:
America's unwillingness to embrace a flawed treaty should not be read by our friends and allies as any abdication of responsibility. To the contrary, my administration is committed to a leadership role on the issue of climate change. Our approach must be consistent with the long-term goal of stabilizing greenhouse gas concentrations in the atmosphere.
Riiight.
Image source: www.bushisms.com
Urban climate & urban form
In an earlier journal entry we saw how cities are heat islands, with temperatures increasing as one moves from the greener outskirts to the asphalt centre. Cities are parts of a larger ecosystem – they modify climate in and around them by virtue of the sheer physicality of their buildings, the heat generated from millions of people (anthropogenic energy) living cheek to jowl within a compressed volume of space, and the heat and GHG produced by the transport these millions of people require to move around. The embodied energy in the millions of tons of material that make up buildings, their thermal mass, the form of the buildings, and their spatial configuration tangibly affects local climate patterns and, perhaps, global patterns as well. A study carried out by Dr. J. Marshall Shepherd at the Goddard Space Flight Centre came up with the following observations:
Cities tend to be one to 10 degrees Fahrenheit (0.56 to 5.6 degrees Celsius) warmer than surrounding suburbs and rural areas.
Computer simulations carried out for the study sow how cities affect climate during different seasons, Shepherd taking Houston as a case study. During summer, heat absorbed by the city and pollution’s interference with raindrop formation interacts to cause clouds to build higher and larger before they shed their water. So pollution increases summer precipitation. During winter, moist air from over the ocean normally rises over the hills downwind of coastal cities, shedding rain and snow. But pollutants interfere with droplet formation, making them smaller so that instead of precipitating, most of the water evaporates. Urban pollution can reduce winter precipitation by as much as 15 to 25% (Shepherd, 2003).
Urbanization induces changes in the surface and atmospheric properties of a region, transforms the radiative, thermal, moisture and aerodynamic characteristics, and dislocates the natural solar and hydrologic balances (Cheung, 2001). To recap, these changes are induced by an increase of air pollutants, changes in the albedo of surface materials (i.e. their radiation “trapping” property), replacement of vegetation with impervious surfaces, block like geometry and anthropogenic heat exhaust from humans and their machines.
Air pollution affects precipitation. Albedo of individual reflecting surfaces and their geometrical arrangements in cities are generally lower than natural materials. Reduction of soft surfaces increases run-off causing decreasing groundwater levels in cities. The built morphology produces local effects like wind tunneling, “urban canyons” with reduced sky-view factor (SVF), downdrafts off the face of sheer glass cliffs, and “hot spots” on flat concrete rooftops that have been recorded at 71 degrees C in Salt Lake City (Urban Heat Island Pilot Project).
Urban canyons are surface units comprised of buildings in close proximity. These units comprise the urban canopy layer, the layer of air from ground to roof-level in an urban area. The geometric relationships within urban canyons ‘can influence the absorption and emission of incoming solar and outgoing long wave radiation within the urban area and can have a significant impact on the energy balance and temperature of an urban area.’ (Zutter, 1999) A typical canyon is basically the walls and ground (hard / soft) between two adjacent buildings defining the canyon, and the air volume bound by the surfaces (Oke, 1987). Knowing the relationship between canyon climate and geometry can help in designing urban form that reduces the heat island effect, by considering factors such as the SVF for radiation of heat back to the sky.
Studies to gauge this relationship have shown that geometry and materials influences air and surface temperatures within the canyons. Venting heat to the night sky, better served by better sky view, and using vegetation as well can alter these higher temperature pockets.
To summarize, the hypothesised causes of urban heat islands include:
References
Evaluation and application of automated methods to measure sky view factors for urban areas, Zutter, H., 1999
Inadvertent climate modification (Urban Areas), Cheung, I., 2001
IPCC Third Assessment Report on Climate Change, 2001
Paper published in European Wind Energy Conference & Exhibition, Copenhagen, 2-6 July 2001: Wind Energy for the Built Environment
Street design and urban canopy layer climate, Oke, T.R., 1987
www.gsfc.nasa.gov
www.heatisonline.org