* Temperatures above Medieval Warming levels
* Warmest in Svalbard in 1,800 years; underscores Arctic melt
OSLO, Sept 27 (Reuters) - Temperatures high in the Norwegian Arctic are above those in a natural warm period in Viking times, underscoring a thaw opening the region to everything from oil exploration to shipping, scientists said on Thursday.
Last week, sea ice on the Arctic Ocean set a record low since satellite observations began in the 1970s. In recent years, mussels have been found off the Norwegian archipelago of Svalbard for the first time since the Viking era 1,000 years ago.
The study showed that summertime temperatures on Svalbard were higher now than at any time in the past 1,800 years, including in the Medieval Warm Period from 950 to 1200, scientists wrote in the journal Geology.
Summer temperatures were 2 to 2.5 degrees Celsius (3.6 to 4.5 F) higher since 1987 than during the Medieval Warm Period, lead author William D'Andrea, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, told Reuters.
People sceptical that mankind is the main cause of global warming sometimes point to the Medieval spike in temperatures as evidence that natural variations can bring large climate swings, Columbia wrote in a statement.
"The warming of the past 25 years or so is more than in this record for the Medieval period," D'Andrea said. The Medieval warming has been linked to shifts in solar output and volcanic eruptions.
"It has been pretty well established...that the modern warming is largely due to human contributions of greenhouse gases to the atmosphere," D'Andrea added.
The data also indicated that the Medieval warming was not uniform across the northern Hemisphere. Studies in Greenland and parts of North America show temperatures were warmer from 950 to 1250 than today.
In Svalbard, the scientists studied sediments of algae buried in Kongressvatnet Lake that left indications of temperatures in the types of fats they laid down.
The Arctic is warming at twice the rate of the rest of the globe due to emissions of greenhouse gases from burning fossil fuels, according to a U.N. panel of scientists.
When white ice and snow retreat, they uncover water or ground that are darker and so soak up more heat. The melt is threatening the hunting lifestyles of indigenous peoples and creatures such as polar bears, the Arctic Council says.
It is also making the region more accessible to oil exploration by companies such as Shell or Statoil and opening up areas for mining and for shipping across the Arctic Ocean.