As the world cools because of decreasing solar activity the dome of cold polar air expands. (drtimball.com)
For example, reconstructions suggest decreased solar activity and increased volcanic activity in the 17th century as compared to current conditions. (ipcc.ch)
While decreased solar activity will decidedly play a role as stated earlier, our good old friend global warming will be the main culprit for this trend. (hubpages.com)