Volcanoes spewing sun-reflecting particles into the atmosphere have partly offset the effects of man's carbon emissions over a 15-year period that has become a global-warming battleground, researchers say.
A so-called hiatus in warming since 1998 has pitched climate sceptics against mainstream scientists.
While temperatures have risen relentlessly and 13 of the l4 warmest years on record occurred since the start of the century, they tracked far below the increase in man-made greenhouse gases.
This gap between the expected and actual temperatures has been cited by sceptics as proof that human-induced global warming is either a green scare or bad science.
But a study published in the journal Nature Geoscience said volcanic eruptions helped explain the apparent slowdown in warming.
Researchers using satellite data found a link between surface temperatures and the impact from nearly 20 volcanic eruptions since 2000.
Sulphuric droplets released by the volcanoes reflected sunlight and slightly cooled the lower atmosphere, they said.
The effect of those "aerosols" accounted for as much as 15 per cent of the gap between expected and measured temperatures between 1998 and 2012, according to the team's figures.
"The 'warming hiatus' since 1998 has a number of different causes," said study co-author Ben Santer of the Lawrence Livermore National Laboratory in California. "The cooling caused by early 21st century volcanic eruptions is one of the causes."
Other explanations for the "hiatus" have been a bigger than expected take-up of atmospheric heat by the ocean, or a decline in solar activity.
Giant eruptions, notably Mount Pinatubo in the Philippines in 1991, were known to discernible cooled Earth's surface.
But volcanoes have not featured in the "hiatus" debate mainly because there had been no major eruptions since the mooted pause began in 1998, only smaller ones whose impact was harder to measure.
This was a gap, as it left computer models of climate change incomplete, the new study suggested.
"Better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these ... in climate model simulations," it said.
Global warming sceptics have pointed to the "hiatus" as proof of flaws in models used to predict warming and thus play a key role in driving policies to tackle climate change.
They contend that these models exaggerate the heat-trapping effect from carbon dioxide emitted by fossil fuel burning.
"We've been lucky that a natural cooling influence [increased 21st century volcanic activity] has partly counteracted human-caused warming," Santer said.
"We do not know how volcanic activity will evolve over the coming decades, and therefore do not know how long our luck will continue."
Experts generally agree that Earth is on track for greatly exceeding the maximum two degrees Celsius of warming targeted in UN climate negotiations.
Last year the level of carbon dioxide in the atmosphere crossed a threshold of 400 parts per million (ppm) and is rising at two or three ppm per year, driven especially by the burning of coal in emerging economies.
Winters will still be killers
Global warming will fail to reduce high winter death rates as some officials have predicted because there will be more harmful weather extremes even as it gets less cold, a British study shows.
A draft UN report due for publication next month says that, overall, climate change will harm human health, but adds: “Positive effects will include modest improvements in coldrelated mortality and morbidity in some areas due to fewer cold extremes, shifts in food production and reduced capacity of diseasecarrying vectors.”
However a report in the journal Nature Climate Change on the situation in England and Wales said climate warming was not likely to cut winter mortality in those places. It suggested more volatile winters, with swings from cold to mild linked to rising greenhouse gas emissions, might even raise death rates.
Lead author Philip Staddon of the University of Exeter said that the findings were likely to apply to other developed countries in temperate regions that risked more extreme weather as temperatures rose.
Excess winter deaths, the number of people who die in winter compared to other times of the year, roughly halved to 31,000 in England and Wales in 2012-13 from 60,000 typical in the 1950s. Staddon’s report said the decline was due to better home insulation, heating, health care and other non-climate factors with no link to a decline in the number of cold days.
“Winter cold severity no longer predicts the numbers affected,” the authors wrote.
Staddon’s findings are at odds with a 2012 Climate Change Risk Assessment by the British Department for Environment, Food and Rural Affairs that said global warming would bring “some potential benefits, for example, a projected reduction in winter mortality”.