"Volcanic Vent Found Three Miles Undersea" says the title of a current newspaper story.  A colleague of mine a couple of years ago co-authored a scientific journal publication showing that about half of the claimed global warming can be attributed to variances in solar intensity alone.  It's occurred to many of us that there could be numerous other natural factors, for example volcanic activity levels.  The vent in the referenced article is pouring tremendous quantities of heat energy into the ocean, and inasmuch as we've just discovered it, we have no idea how that flow has varied over the years as the tectonic plates have shifted around.  Volcanoes above sea level could change temperatures in either direction.  For example, the explosion of Krakatoa in the Dutch East Indies in 1883 discharged so much sun-blocking ash and fine debris into the circulating atmosphere that Boston didn't have summer the following year.  Perhaps the good news is that if warming (no matter what the cause) were actually to become a worldwide climate problem, the United States could effect a cool-down through a few explosions in sandy regions of the globe.