The largest consortium of scientists ever assembled to work on the climate of the past 2000 years combined detailed regional data sets and analyses to make an systematic analysis of climate change over the past two millennia. They determined that the period from 1971 to 2000 was the warmest since a warm spike over over 1300 years ago. The recent decade 1981 to 2010 has apparently reversed 2000 years of natural cooling resulting from declining summertime insolation in the northern hemisphere related to earth's natural orbital variations. This is the largest and most comprehensive validation yet of the recent rapid warming trend found by Dr. Michael Mann, derided by "skeptics" as "the hockey stick".
Our study is the most comprehensive evaluation to date of temperature change at the surface of Earth’s continents over the past one to two thousand years.
Previous studies have focused on hemispheric or global-scale temperature reconstructions, which are useful for understanding overall average conditions, but overlook important differences at the continental scale.
Temperature reconstructions from Antarctica, Australasia and South America clarify the temperature history for Southern Hemisphere continents.
The consortium of authors comprises 78 regional experts from 24 countries. The authors are familiar with the proxy temperature records from individual sites and how they relate to the climate of that region.
All of the data for the 511 individual proxy temperature records that were used for the continental-scale reconstructions, including some that extend back 2000 years, have been assembled for public release in a uniform format for future analyses.
(1) The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the 19th century.
- The regional rate of cooling varied between about 0.1 and 0.3°C per 1000 years.
- A preliminary analysis using a climate model indicates that the overall cooling was caused by a combination of decreased solar irradiance and increased volcanic activity, as well as changes in land cover and slow changes in the Earth’s orbit. The simulations show that the relative importance of each factor differs between regions.
(2) Temperatures did not fluctuate uniformly among all regions at multi-decadal to centennial scales. For example, there were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age.
- The period from around 830 to 1100 CE generally encompassed a sustained warm interval in all four Northern Hemisphere regions. In contrast, in South America and Australasia, a sustained warm period occurred later, from around 1160 to 1370 CE.
- The transition to colder regional climates between 1200 and 1500 CE is evident earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere.
- By around 1580 CE all regions except Antarctica entered a protracted, multi-centennial cold period, which prevailed until late in the 19th century.
- Cooler 30-year periods between the years 830 and 1910 CE were particularly pronounced during times of weak solar activity and strong tropical volcanic eruptions. Both phenomena often occurred simultaneously. This demonstrates how temperature changes over large regions are related to changes in climate-forcing mechanisms. Future climate can be expected to respond to such forcings in similar ways.
(3) The 20th century ranked as the warmest or nearly the warmest century in all regions except Antarctica. During the last 30-year period in the reconstructions (1971-2000 CE), the average reconstructed temperature among all of the regions was likely higher than anytime in nearly 1400 years. However, some regions experienced 30-year intervals that were warmer than 1971-2000. In Europe, for example, the average temperature between 21 and 80 CE was warmer than during 1971-2000.