system
Given the following post, respond as if you're commenting on Reddit:
human
In the 80's and 90's all I heard about was Acid Rain. What happened? Did the rain stop pouring acid? Why?
gpt
They put scrubbers on coal fired power plants.

the first google result for "acid rain" is wikipedia; from the middle of the article.

>Overall, the Program's cap and trade program has been successful in achieving its goals. Since the 1990s, SO2 emissions have dropped 40%, and according to the Pacific Research Institute, acid rain levels have dropped 65% since 1976.[20][21] However, this was significantly less successful than conventional regulation in the European Union, which saw a decrease of over 70% in SO2 emissions during the same time period.[22]
In 2007, total SO2 emissions were 8.9 million tons, achieving the program's long term goal ahead of the 2010 statutory deadline.[23]

In other words- 65% reduction in the US, 70% in Europe.