In the latest Time magazine there is a list of 51 things you can do to cut global warming. Number 46 is “Plant a tree in the tropics”. In part it says “Recent studies have shown that trees in temperate latitudes—including most of the U.S.—actually have a net warming effect on the climate. The heat that dark leaves absorb outweighs the carbon they soak up.”
Okay, so if there is a tree there, the dark leaves absorb the heat. If there is no tree there, where does the heat go? If it hits the ground doesn’t the ground absorb it? The sun’s rays don’t pass harmlessly through the planet, they have to be absorbed by something. Certainly my experience tells me it’s hotter in Times Square than in Central Park.
The only effect I can think of that would cause this is if the trees somehow prevented the radiation of ground heat to space. Could this be reasonable?
I don’t know specifically about trees, but ground cover can have a huge effect on local air temperature.
I attended a symposium on science in the National Parks where one presenter described how changes in grassland from native perennials to exotic annuals over parks in Marin County, California accounts for a 1.2[sup]o[/sup]C change on a typical August mid-day, about 6[sup]o[/sup]C higher in unusually hot summers. One of the differences is in the albedo (reflection of light) of the grassland - the perennials have significantly higher albedo than annuals do.
(In case you’re interested, the presenter was Laurie Koteen, Ph.D. student at UC Berkeley. I don’t know if she’s published her work yet.)
The ground doesn’t just absorb energy, it also reflects it.
Nope. Both bare ground and dry grass and herbage are generally very reflective. You can see this yourself by doing a Google search for some false colour satellite images. Bare ground shows up much warmer in space because they are, well emitting more heat into space. In contrast trees or green herbage are a dark colour, and they also contain a lot of water that absorbs heat.
Mind you, this effect is far more pronounced in the tropics, so the idea that somehow planting a tree in the tropics is a panacea is just silly.
The evaporation of water from the leaves cools the trees and the area beneath the canapy. A hill with a unbroken heavy tree canapy on it’s sides ,and a field at the top, can demonstrate this. Stand in the feild and note the heat, walk to the tree line staying in the sun, and you can feel the cooler air flowing out of the woods. The air flows up the hill and out the top because of convection currents, after things heat up. You can go under the trees to feel the difference, but I wanted to show it wasn’t because you were out of the sun. I know a couple places that are like this, and are great when the weather is a scorcher. Tree are a carbon sink also so they take carbon out of the atmosphere and look it into the tree for a long time.
Nope. This thread pertains to the warming effect of trees as it applies to global warming From a global warming perspective evaporative cooling totally irrlevant. It simply transfers heat energy from the leaf into water vapour without ever removing it from the atmosphere. The net amount of heat in the atmosphere remains exactly the same, it has simply changed location. In actual fact because evaporative cooling transfers heat from the leaf surface (which would readily radiate it back into space) to water vapour (which can not do so as readily) it increases global warming.
This has nothing to do with evaporative cooling. The bare point on the top of the hill warms up producing an updraft. That updraft draws air out of the cooler shaded area at ground level, which is the cool breeze you feel. It is the shading effect of the trees on the ground that makes the ground cooler. Nothing whatsoever to do with evaporation, which is all taking place in the canopy well above where you could possiby feel it. You would get exactly the same effect if you shaded the ground with cloth or iron.
Growing trees are a carbon sink. Mature trees are carbon neutral while old moribund trees are a carbon source.
Blake I realise that there is more to the cooling than evaporation, bit it does provide some. I garantee you won’t feel as cool under a pole shed with open sides and a hot aluminum roof.