Stockholm — The world's politicians have a choice:
* They can do nothing to prevent the threat to the environment posed by acid rain, with consequences that could be disastrous. An example: If no action is taken, most of Sweden's 100,000 lakes could be ''dead'' within 50 years.
* They can take action with the existing techniques and technology, now said to be sufficient to deal with acid rain. But they lack the political will to do so.
''We have discussed possibilities and options,'' Prof. Arne Jernelov of the Stockholm Institute of Technology said. ''Now it is up to the politicians.''
This is the picture being presented to an international symposium of scientists and world environment ministers that opened in Stockholm June 28.
Acid rain is a phenomenon caused when sulfureous waste from heavy industry falls in precipitation. Scientists agree the maximum amount of sulfur that could be permitted to fall in rain or snow without causing acidification of lakes would be 0.5 grams per square meter.
The present amount falling in southern Sweden is 3.5 grams per square meter. Scandinavia and Canada are particularly vulnerable to acid rain because of lack of chalk in the soil. Chalk tends to neutralize the acidification.
Goran Persson, head of the research department of the National Swedish Environment Protection Board, says that if nothing is done about acid rain: ''Our lakes will be very clear and beautiful but they will be unable to support plant or fish life.'' Ground water would be undrinkable without treatment, and large sections of Scandinavia's forests would be destroyed.
The 100 scientists from 20 countries, including the United States, who took part in the ''Environment 82'' symposium found that existing techniques and technology were sufficient to deal with acid rain if the political will was there.