Roughly 1,000 years ago, Europe enjoyed several centuries of balmier average temperatures. Dubbed the "Medieval Warm Period," it was the last time before the present that agriculture could flourish in Greenland. This era also shows yet again that changes to natural systems can drive local climate change—and provided fodder for countless misunderstandings about the nature of present day global warming. 

But new research shows that the MWP, as it is affectionately known in acronym-happy science circles, as well as the "Little Ice Age" that almost immediately followed it (and spelled doom for the Greenland Norse) were likely the result of fluctuations in the sun's strength and the frequency of volcanic eruptions, among other natural causes. 

Cores drilled from ancient ice sheets as well as of the coral reef and lake sediment varieties show that at the same time as Europe enjoyed average temperatures as warm as today, the tropical Pacific was unusually cold. This suggests that natural cycles—such as the succession of El Nino and La Nina conditions in the Pacific Ocean—forced these climate anomalies. 

Unfortunately, neither the sun nor other natural cycles can entirely explain the recent warming trend that has brought potatoes back to coastal Greenland. To date, the only explanation that matches those observations are a concurrent rise in greenhouse gas concentrations in the atmosphere.  

CO2 and its peers now are responsible for trapping an extra three watts per square meter of planet. And if that continues, the MWP will end up looking like an ice age. 

—David Biello