PARIS – Volcanoes spewing sun-reflecting particles into the atmosphere have partly offset the effects of man’s carbon emissions over the last 15 years, a period that has become a battleground in the ongoing debate over global warming, researchers said Sunday.
A so-called hiatus in warming since 1998 has pitched climate skeptics against mainstream scientists.
While temperatures have risen relentlessly — 13 of the 14 warmest years on record have occurred in the 21st century — the increase in man-made greenhouse gases has been bigger.
This gap between the expected and actual temperatures has been cited by skeptics as proof that human-induced global warming is either a green scare or bad science.
But a study in the journal Nature Geoscience said volcanic eruptions helped explain the apparent warming slowdown.
Researchers using satellite data found a link between surface temperatures and the impact from nearly 20 volcanic eruptions since 2000.
Sulfuric droplets disgorged by the volcanoes reflected sunlight, leading to a slight cooling of the lower atmosphere, they said.
The effect of these “aerosols” accounted for as much as 15 percent 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,” study co-author Ben Santer of the Lawrence Livermore National Laboratory in California said in an email. “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 absorption of atmospheric heat by the ocean, or a decline in solar activity.
Blockbuster eruptions, notably that of Mount Pinatubo in the Philippines in 1991, were known to have discernible cooling effects on the Earth’s surface.
But volcanoes have not featured in the hiatus debate, mainly because there had been no major eruptions since the pause began, only small ones, whose impact is harder to measure.
This is 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 skeptics have pointed to the hiatus as proof of flaws in models used to predict warming and justify policies to tackle climate change.
They contend that these models exaggerate the heat-trapping effect from carbon dioxide emitted by fossil fuel burning.
Santer said the new findings “do not support” such an argument. “We’ve been lucky that a natural cooling influence has partly counteracted human-caused warming,” he said.
“We do not know how volcanic activity will evolve over the coming decades, and thus we do not know how long our luck will continue.”
Experts generally agree that Earth is on track for greatly exceeding the maximum 2 degrees Celsius (3.6 degrees Fahrenheit) of warming targeted in U.N. climate negotiations.
Last year the level of carbon dioxide in the atmosphere crossed a threshold of 400 parts per million — a level never experienced by humans. Carbon dioxide concentrations are rising at 2 or 3 ppm per year, driven especially by the burning of coal in emerging economies.
Commenting on the study, Piers Forster, a professor of climate change at the University of Leeds, said, “Volcanoes give us only a temporary respite from the relentless warming pressure of continued increases in CO2.”