Obviously the administrative cost of taxing some other emissions source is going to depend on the nature of that emissions source, but if hypothetically evaluating emissions from certain sources turned out to be prohibitively burdensome, that would not be any excuse to avoid doing it for fuels at a minimum, which constitute the majority of emissions anyway.
And a more efficient way to handle imports would be to impose general tariffs on imports from countries that don't themselves impose a carbon tax, rather than trying to calculate the effects of something happening in a foreign country. Which would have a more significant effect anyway, because causing the entire country to reduce its own emissions would be a much bigger win than causing only their exports to do so.
And a more efficient way to handle imports would be to impose general tariffs on imports from countries that don't themselves impose a carbon tax, rather than trying to calculate the effects of something happening in a foreign country. Which would have a more significant effect anyway, because causing the entire country to reduce its own emissions would be a much bigger win than causing only their exports to do so.