Two British scientists have used tree rings, ice cores, fossil shells and other data to confirm that roughly a millennium ago, between A.D. 890 and 1170, the world was considerably warmer than at any time until the past century.

Their research also shows that modern warming is much more extensive than it was then.

Around that time, known as the Medieval Warm Period, the Vikings occupied Greenland. In the Pacific, Polynesians were new in Hawai'i. New Zealand, or Aotearoa, and Easter Island, or Rapa Nui, were being settled.

Climate scientists Timothy Osborn and Keith Briffa of the University of East Anglia compared modern and ancient "climate proxies" such as tree rings from all over the Northern Hemisphere to arrive at their temperature estimates.

Many scientists believe that modern warming is partly caused by human activities such as burning fossil fuels.

But what was making it warm 1,000 years ago? By e-mail, Osborn said that it likely had to do with volcanic activity — or rather, the lack of it.

Osborn said that if methane and carbon dioxide are pumped into the atmosphere, temperatures rise. But if volcanic emissions are pumped into the atmosphere in the form of sulphate droplets, temperatures fall.

"It is the 'explosive' type of volcanoes that can have an influence on the climate, because those are the ones that can shoot material up into the stratosphere, where it can stay for a number of years and, therefore, affect the climate for a number of years," he wrote.

Osborn said the Medieval Warm Period was probably an era with very little volcanic activity, which allowed the planet to warm up more than normal. The sun also may have been putting out a little more energy than normal near the end of the period, adding to the warmth, he said.

While those things could be having a small effect today, Osborn feels humans are mainly responsible for modern warming.

"The combination of natural changes in sun energy and volcanic activity cannot explain the warming found in the last 100 years. They might contribute a small amount, but that is all. The bulk of the recent warming is likely to be caused by man-made emissions of greenhouse gases" such as carbon dioxide, Osborn said.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.