The last part of the 20th century is considered by many scientists to be the warmest period since modern record-keeping began around the 1850s, but new research indicates the era is even more remarkable.

The warmth in which the Northern Hemisphere has basked since the middle of the 20th century has been the most widespread and longest period of unusual climate experienced at any time during at least the past 1,200 years, according to a research paper in the journal Science.

The finding, by a pair of climate researchers from the University of East Anglia in Norwich, U.K., was based on comparisons of the current warm period to other hot and cold intervals since the year 800.

Among these long periods of alternating temperatures were the "Little Ice Age" that sent Northern Europe into a deep freeze, and the Medieval Warm Period around 1000, when an interval of more benign climate coincided with the rise of the sea-faring Vikings.

The research was undertaken to help determine whether recent warming is a natural phenomenon, part of the normal long-term fluctuations of temperatures that have been observed around the world, or something whose intensity makes it without precedent.

The researchers think their work bolsters the case that global warming due to human activity has created a change in climate unlike anything seen in more than a millennium.

The findings "provide additional support for the case that recent warmth is unusual in the context of natural changes in the last 1,200 years," said Timothy Osborn, one of the researchers, who added that the results are "probably related" to greenhouse gas emissions from human activity.

Although direct temperature measurements extend back only about 150 years, the researchers were able to infer earlier readings throughout the Northern Hemisphere by looking at the signs that temperatures left in tree rings, ice cores and seashells from 14 sites across North America, Europe and Asia.

Two of the temperature estimates were from Canada -- one from tree-ring measurements taken in the Rockies near the Columbia Ice Field, the other from tree rings in a Northern Quebec boreal forest.

Other records were based on tree samples from the United States, Austria, Sweden, Russia and Mongolia, among others, along with Greenland ice cores and seashells from Chesapeake Bay in the United States.

Tree rings are an excellent gauge of past climate conditions, particularly if the trees are from cold-weather locations near the tree line, where growth is highly dependent on temperature conditions.

Dr. Osborn said that in such places, the amount of growth is limited by how warm the summer is. If it is warmer, the tree growth is greater and the tree ring is wider. Colder years have the opposite effect. Each year provides only one tree ring. "So counting back the rings can give the precise date to when each ring actually grew," he said.

Through recent measurements, scientists have been able to determine how much a tree will grow at any given temperature, allowing accurate estimates of previous temperatures.

There are usually periods in which some regions of the globe are warming and others are cooling.

But Dr. Osborn said the 20th century was remarkable for the geographical extent of the warming because "all records indicate warm conditions at the same time."

Reliable records from trees and other sources go back only about 1,200 years, but this allowed the researchers to measure the magnitude of the current bout of warming against two of the best known long-term weather conditions, the Little Ice Age from about 1580 to 1850, and the Medieval Warm Period from 890 to 1170.

These eras were not continuously warm or cool, but were punctuated by hot and cold spells.

However, their temperature extremes were not as pronounced as the current warming, according to the research.

Not all scientists agree that the 20th century is the warmest period in recent history.

In 2003, a team led by researchers from the Harvard-Smithsonian Center for Astrophysics announced that it believed the 20th century wasn't the warmest, nor the one with the most extreme weather of the past 1,000 years.

But this research has been criticized for its selection of the indicators used to estimate historic temperatures, among other problems.

The new paper tried to overcome some of these shortcomings by carefully selecting items, such as tree rings, that are directly connected to temperature changes.

Although the new paper looked at data up to only 1995, recent years have continued with even more pronounced warmth.

The World Meteorological Organization said late last year that the decade from 1996-2005 contained nine of the 10 warmest years on record.

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