Tuesday, 22 May 2007

Global warming and heat island effects

Since writing my last blog on climate data available from the Bureau of Meteorology I’ve had some correspondence with Wilma Keppel of the US based www.managingwholes.com website. Wilma highlighted the impact of what are termed ‘heat islands’ on temperature and climate. In short, heat islands are built up areas such s towns, cities and even road-ways where the temperatures in and around them tend to be above that of the surrounding areas. It’s not hard to conclude that these must play a role in increased air temperatures more widely than just in those areas.

Here’s an extract from Wilma’s last email that I thought was worthy of sharing more widely.

“The heat island effect can be significant. The northernmost native persimmons that John ever found were in the heat island of central Omaha, Nebraska (almost exactly in the middle of the continental U.S.). This city of about half a million is 3-4 degrees C warmer than the surrounding areas.

As you can imagine, heat island effects greatly increase people's perceived need for air conditioning. Using air conditioning increases outdoor air temperatures -- both by pumping heat outdoors, and by running compressor motors. And when outside air temperature goes up, air conditioner efficiency goes down.

A big part of the problem is pavements and roofs that absorb heat. Installing high-reflectance roofs can have a HUGE effect. (On chicken coops, too -- see my chicken tractor article on Managing Wholes).

Another significant effect comes from big trees that shade roofs and pavements. This is also a significant effect on heat buildup in the countryside. I well remember driving through eastern Iowa, which used to be home to the western edge of the eastern hardwood forest that covered most of the east half of the temperate parts of North America. Sensible states like New Hampshire allow trees to grow right up to the edge of the roads, shading them and keeping them cool. Iowa has a policy of cutting trees within 10 meters or so of the road, which means no shade. And since most of the country has been plowed into farms with huge fields, cutting down the roadside trees has gotten rid of a large proportion of the remaining strip woodland windbreaks, so soil erosion has gone up. Cutting the trees also drastically decreases habitat for the remaining native wildlife. All this so drivers going too fast on icy road D/S won't spin off and hit a tree. (New Hampshire has narrower, steeper roads, and gets more snow, but expects people to drive sensibly.)”

Wilma’s comments also reminded me of a conversation I had not so long ago with a mate, Garry Coates, who used to (and probably still does) tour widely throughout Australia on a motor bike. When riding during the heat of the day out on the western plains areas throughout New South Wales he found that the temperature difference between heavily treed and open areas was significant. In the open areas he’d find himself suffering, and close to heat exhaustion. In the treed areas he was cool and not sweating at all. Garry’s view is that a lot of the Australian areas with large treed canopies (such as around Heallsville, Narbethong, Toolangi etc) are naturally drought resistant. Plants and animals survive there OK because the moisture gets locked in. Take away the large trees and the moisture goes as do the animals.

With regard to Garry’s observations Wilma mentioned that Peter Donovan (also of the US based www.managingwholes.com website) told her that in researching tropical rain forests, he discovered that they typically occur in at least somewhat brittle climates, and it's the tree canopy that keeps the forest underneath moist throughout the dry season.

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