Apart from the potentially volatile topic, i'd say it seems very unlikely to see such dramatic drops in solar collection on a global scale, except in places like Hong Kong where the main culprit is air pollution.
Keep in mind at the same time in the last few years solar collection efficiency has been on the rise
. Gore and his buddies made global warming such a political topic that more knowledgeable experts like Dr. Richard Lindzen of MIT were sort of drowned out by Hollywood. But look up Lindzen's articles and you hear a calmer voice on the topic.