The LIA was caused by a lengthy grand solar minimum, actually several minimums in a row, coupled with the normal increased volcanic activity that occurs during solar minimums and the changes that occur to the solar & terrestrial magnetic fields.
The last named minimum period was the Centennial Minimum, approx 1890 to 1930. Then decades of warmer climate until now.
The earth is now in the beginning decade of the modern minimum, named the Eddy Minimum, with decades longer to run.
Thank you for the correct info but seriously this really needs to be called the Al Gore Minimum ;-)