2007-10-02, Greenhouse Gases 温室效应

（单词翻译）

On a restricted site in northern Wisconsin, the hissing¹ you hear is helping² scientists see the future. Inside towering rings of pipes, aspen, birch, and sugar maples³ feed on measured diets of greenhouse gases.

Our studies were looking at what forest will be faced with in, in about 50 years from now.

Emissions⁴ from automobiles⁵ and industry are increasing carbon dioxide and ozone⁶ levels in the atmosphere, so government agencies and research institutions have become partners in the ASPEN FACE project (Free-Air, Carbon Dioxide Enrichment).

Their goal - to see how future forest will grow in higher levels of greenhouse gases. This is big science for us; there are very few places in the world where forestry⁷ can garner⁸ these sorts of resources for a huge experiment. Inside the gas-enriched rings, researchers constantly monitor the forest environment. (Alright, why don't we go back into the canopy⁹ a little bit?) For example, they measure the rate at which trees convert carbon dioxide to food. As long as the leaf is active, it's taking in carbon dioxide out of that air stream, and it's giving off water vapour.

Their research has led to some striking early discoveries. In rings where the trees are fed just elevated levels of carbon dioxide. It's a little like an all-you-can-eat buffet¹⁰. Well, as far as the plants are concerned, it's entirely¹¹ a good thing. It's just like having more food on the dinner table.

Trees need carbon dioxide to survive and here they get plenty. One of the things that we have noticed with elevated CO2, at least early on in the canopy development is that we have a thicker canopy, the leaves are a little larger. If trees can absorb much more CO2, that might reduce its impact on global warming.

What's more troubling to researchers, in the short term, is the effect they can see from another major greenhouse gas - ozone. With ozone, the responses are absolutely the opposite. They are mainly negative responses in terms of, of growth. We are seeing increased levels of mortality; we are seeing increased levels of insect and disease attack on these, with the injury both on the lower leave surface and upper leave surface. The first effect is the air pollution. It's all the biotic stuff that moves in after that. It's the sharks that smell blood in the water, so to speak, that, that causes real problems.

This ladybug may eat 5000 aphis in a life time, ladybugs don't thrive in ozone, plant-eating aphis do. Scientists don't know whether there is such a thing as too much carbon dioxide or safe level of ozone for trees. What they have now.