It's a serious suggestion, aslthough it might not be good for the ozone layer. I recall an estimate that said something like 40 billion dollars a year spent on areosols could have a significant impact on global temperatures. Currently I think that money would be better spent on replaceing fossil fuels in transport and electrical generation, or possibly sequestering carbon through seeding the southern ocean with iron and/or changing agricultural practices since that's more of a long term solution with less possilbly nasty side effects such as increased acid rain.