[TED解説]酸性雨問題はどうなったのか？ジョセフ ゴフマン

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In 1963, scientists studying Hubbard Brook Experimental Forest in New Hampshire made a shocking discovery.

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Their most recent rainfall samples were nearly 100 times more acidic than usual.

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At these levels, additional downpours of acid rain would destroy the region’s marine and arboreal ecosystems in a matter of decades.

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Urgently sharing their findings with fellow researchers, they were determined to answer two questions:

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what was causing this deadly rainfall? And what could be done to stop it?

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Rain is never just composed of water.

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Chemicals and particulates in the atmosphere can be found in every drop, and some compounds— like carbon dioxide— make even regular rainfall slightly acidic.

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But this pales in comparison to the powerful acids produced when water interacts with oxides of nitrogen or sulfur dioxide.

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On the pH scale which measures acidity, each whole number is 10 times more acidic than the one above it.

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And where normal rain has a pH of roughly 5.4, rain that’s interacted with these gases can rank as low as 3.7.

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Oxides of nitrogen and sulfur dioxide can appear naturally as a short-lived byproduct of volcanic eruptions or lightning strikes.

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But power plants, refineries, and vehicles that use fossil fuels consistently pump large quantities into the air.

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These dangerous gases travel with the wind spreading hundreds of kilometers from the pollution’s source.

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Acting like roaming clouds of destruction, their presence dramatically increases the acidity of local precipitation, creating acid rain, acid snow, and acid fog.

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These all acidify lakes and streams, kill crops and forests, and damage soil to inhibit future growth.

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Over time, acid rain can even corrode human structures made of stone or metal.

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By the 1970s, scientists in North America and Europe classified acid rain as a major environmental threat.

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But despite clear evidence tying the problem to air pollution, companies denied responsibility and cast doubt on the research.

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In the United States, corporations lobbied against regulating pollution, and convinced politicians that such policies would raise energy costs and threaten jobs.

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These obstacles led the government to delay changes, and mandate further research into the issue.

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But after a decade of mounting concern, Congress finally took action.

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Since the bulk of sulfur dioxide emissions came from power plants, the government set a limit on the total amount of it the electric power sector could emit each year.