.. the researchers drew up temperature curves for large regions at seven continents, using 511 local temperature records. These were based on the analysis of tree rings, pollen, corals, lake and marine sediments, ice cores and stalagmites as well as historical documents. In most cases the data used were highly resolved, attesting to short-term variations over decades or less, rather than smoothing over centuries. In Africa, there were too few records to accurately determine long-term temperature changes for that continent.
The researchers found that, while some areas and eras had been warmer than the 20th century, there has been an overall 2,000-year global cooling trend that was reversed around 1900. As the abstract reports:
Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.
The chart below from the study compares their data with temperature trend data from other researchers.
LiveScience adds:
The most consistent feature across the regions over the last 2,000 years was a long-term cooling trend, which was likely caused by a combination of factors such as an overall increase in volcanic activity, a decrease in solar irradiance, changes in land cover, and slow changes in earth's orbit. This cooling only came to an end toward the end of the 19th century.
The warming during the last century has reversed this long-term cooling, the study found. It remained cold only in Antarctica. An analysis of the average temperatures over 30-year periods indicates that interval from 1971-2000 was probably warmer than any other 30-year period in the last 1,400 years.
Cooler 30-year periods between the years 830 and 1910 AD were particularly pronounced during weak solar activity and strong tropical volcanic eruptions. Both phenomena often occurred simultaneously and led to a drop in the average temperature during five distinct 30- to 90-year intervals between 1251 and 1820.
Warming in the 20th century was on average twice as large in the northern continents as it was in the Southern Hemisphere. During the past 2,000 years, some regions experienced warmer 30-year intervals than during the late 20th century. For example, in Europe the years between 21 and 80 AD were possibly warmer than the period 1971-2000.
While the study does not attempt to attribute temperature changes to natural or man-made factors, one of the lead authors, Northern Arizona University earth scientist Darrell Kaufman added:
"The pre-industrial trend was likely caused by natural factors that continued to operate through the 20th century, making 20th century warming more difficult to explain if not for the likely impact of increased greenhouse gasses."
Back in March, another team put together a contested global temperture trend analysis encompassing the past 14,000 years based proxy temperature data. That report also found a long-term cooling trend until 1900. However, the proxy data in that study turned out to be not sufficiently robust to resolve 20th century temperature trends.
