What was done
Using a variety of tree ring and ice core proxy records, the authors reconstructed temperatures in the Northern Hemisphere over the past one thousand years.

What was learned
Northern Hemisphere temperatures were shown to be "relatively warm ... earlier in the millennium," but gave way to a prolonged cooling trend following the 14th century that "could be viewed as the initial onset of the Little Ice Age."  Temperatures of the 20th century rose to values similar to those experienced during the 11th and 12th centuries, while temperatures of both the last decade and year (1998) were reportedly "the warmest for the Northern Hemisphere this millennium."  However, the authors note that the number of proxy indicators available for use in the study decline prior to AD 1400, suggesting that "a more widespread network of quality millennial proxy climate indicators will be required for more confident inferences."

What it means
The warming of the last century in conjunction with rising atmospheric CO2 concentrations is often offered as "evidence" that future increases in CO2 will result in catastrophic global warming.  However, as we pointed out in our Volume 2 Number 7 Editorial CO2 and Temperature: The Great Geophysical Waltz and in the Journal Review CO2 and Temperature: Ice Core Correlations, CO2-temperature trends over a much longer period of time (250,000 years) indicate that, contrary to current climate model implications, this simplistic notion has little real-world data to support it.  Following the penultimate deglaciation, for example, atmospheric CO2 concentrations exhibited no net change for approximately 15,000 years, during which period air temperatures dropped all the way back to values characteristic of glacial times.  Then, when CO2 finally began to decline, air temperatures remained constant for a few thousand years, after which they actually rose for about 6,000 years.  And even when the two parameters increased in unison, as they did during the three most recent glacial terminations, temperature always rose first, followed by CO2 concentrations some 400 to 1,000 years later.

Clearly, the concomitant increase in atmospheric CO2 and air temperature over the last century or so proves nothing of a cause-and-effect nature.  When all available CO2 and temperature records are analyzed, one can find much longer periods of absolutely no correlation and even opposing trends.  Thus, the balance of evidence would tend to suggest that the recent century-long rise in temperatures is not driven by the ongoing rise in the air's CO2 content.