Read here. A new analysis of Greenland ice cores conclusively identifies the extremes of both the Medieval Warming and the Little Ice Age, represented by a 50-year average temperature measurement. This same measurement identifies the 1950's as a modern peak for the 50-year average, with a subsequent cooling through the mid-1990's.
This peer-reviewed analysis, by Kobashi et al., clearly shows that temperatures and climate exhibit a wide variation, regardless of atmospheric CO2 levels. In fact, these scientists state categorically that there are natural, decadal and century-long oscillations that are driving Arctic regional temperature changes over the past, the present and foreseeable future. These decadal/century periodicity events significantly pre-date human CO2 emission influence. (click on image to enlarge)
"Here, we reconstruct the last 1,000 years of central Greenland surface temperature from isotopes of N2 and Ar in air bubbles in an ice core. This technique provides constraints on decadal to centennial temperature fluctuations. We found that northern hemisphere temperature and Greenland temperature changed synchronously at periods of ∼20 years and 40–100 years. This quasi-periodic multi-decadal temperature fluctuation persisted throughout the last millennium, and is likely to continue into the future.....The data show clear evidence of the Medieval Warm Period and Little Ice Age in agreement with documentary evidence." [Takuro Kobashi · Jeffrey P. Severinghaus · Jean-Marc Barnola · Kenji Kawamura · Tara Carter · Tosiyuki Nakaegawa 2010: Climatic Change]