A commentary by a North Saanich resident.
The Aug. 6 headline “Should B.C. cities rethink their shade priorities?” asks the wrong question. “Should B.C. cities rethink their tree priorities?” would be a much better one.
When the sun’s rays strike an object, some of the energy (heat) is reflected and some is absorbed. If that object is above ground level, it casts a shadow over an area where this heat will not reach the ground.
These objects can be buildings, trees and many other things; in this case the writer is considering buildings and trees.
But trees, in addition to reflecting and absorbing heat from the sun, also use (as in lose) a very large amount of heat in the summer time, transpiring water vapour into the atmosphere.
Evapotranspiration moves heat from Earth’s surface to the atmosphere. Where we have decreased evapotranspiration, we have increased surface temperatures. Other vegetation does this, but not as well.
Neither concrete nor pavement does. And there has to be enough water stored in the ground to supply the vegetation and trees during the summer.
I have published results in peer-reviewed scientific journals showing that the ground surface temperature increased at the times of permanent deforestation.
The sites I used for detailed work are in B.C. and Yukon, from 50 to 63 degrees North, where forest fires occurred, road right-of-ways were cleared, or clearcuts were not replanted.
These results show that a permanent increase in the average ground surface temperature occurred at the time of deforestation, and that the average ground surface temperature increased by up to 2.6 degrees C.
Since the transpiration occurs mainly in the summertime, this means that the average summer time ground surface temperature increased by up to 10 C.
In contrast, warming caused by so-called greenhouse gases should be increasing gradually over decades, and not have large seasonal variations.
If such an increase is present in these data, it is much smaller than the main step increase detected at each of these sites.
When I spoke to Dr. Paul Hoffman, a member of the Royal Society of Canada, about this, he smiled at me and said, “You’re preaching to the converted.”
He then told me that when working in the summer in the eastern end of Great Slave Lake, he saw a map of the forested islands in the sky!
Much more water vapour was going up into the atmosphere above the forests than above the lake water, producing clouds above the islands. Forests produce more water vapour on a hot summer day than lakes do.
Why is this cause of climate change not widely acknowledged?
“Any factor which alters the radiation received from the sun or lost to space, or which alters the redistribution of energy within the atmosphere, and between the atmosphere, land and ocean, can affect climate,” according to IPCC Climate Change 1995. (I was an invited contributor to it.)
I have presented these data and conclusions at many scientific conferences and never received contrary comments.
Note that nothing said here contradicts the fact that greenhouse gases are a large contributor to climate change. Our development of Earth’s surface causing a decrease in evapotranspiration cannot produce all of the warming observed on Earth.
Constructing a building producing the same amount of shadow as the trees removed causes an increase in the temperature. Urban forests are wonderful.
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