Solar activity and natural variation from year to year (that doesn't form a long-term trend) could certainly open it up for short periods of time. The problem today is that solar activity is at a minimum yet these things are occurring, and starting to occur consistently.So this is the fourth time its opened in recorded history? If that is true then how come it is CO2 that is causing the warming when clearly there wasn't enough CO2 in the atmosphere the last three times to open it?
What was the mechanism of warming for the passage to be opened before?
Ahh but didn't we just learn (well most of us have known it for a long time) that events today were set in motion hundreds of years ago? We know from loads of empirical data that CO2 rises hundreds of years after the warming trend has begun. We, right now at this moment, are at almost the exact 800 year mark from the MWP which all evidence shows is the leading cause of the CO2 increase.