Some researchers say the low volcanic dust levels in the atmosphere over the last dozen years could be contributing to global warming, but others dispute the claim.
During a lunar eclipse, Earth blocks sunlight from reaching the Moon directly. But some sunlight still gets through, refracted through Earth's atmosphere. The amount varies, depending mainly on how much dust from volcanic eruptions is floating around at high altitudes.
Because dust can block sunlight from passing through the atmosphere, more dust makes for a darker Moon during lunar eclipses. "All the big dimmings of the Moon during eclipses can be attributed to specific volcanoes," says Richard Keen of the University of Colorado in Boulder, US.
Keen and his collaborators have charted the brightness of eclipses back to 1960 and for a few years around the time of the 1883 eruption of Indonesia's Krakatoa volcano.
They are using the eclipse data to track changes in the opacity of Earth's atmosphere. While most of the light deflected by particles in the atmosphere is just temporarily diverted and eventually reaches the Earth's surface, the effects of atmospheric dust can have a significant, if temporary, impact on the climate, Keen says.
Earth-orbiting satellites can measure atmospheric opacity, but only for a small part of the atmosphere at any given time. A lunar eclipse, on the other hand, conveniently gives an average over all latitudes, Keen says. Eclipse measurements are also easily compared with old eclipse records, which extend back much further in time than the satellite measurements, he says.
The most recent lunar eclipse, on 20-21 February, was a bright one, measuring a 3 – the second-brightest level – on an eclipse-rating scale that ranges from 0 to 4.
That is in line with eclipse data taken since 1995. In that time, the stratosphere has been especially clear, with very little haze-producing volcanic activity compared to the previous three decades, from 1965 to 1995, Keen says.
Because more sunlight is reaching the surface, Earth should be 0.1 to 0.2° Celsius warmer in recent years than it was back in the late 1960s, Keen and his colleagues calculate. Over the same period, the average surface temperature of the Earth has risen by about 0.6° C.
According to the scientists that make up the Intergovernmental Panel on Climate Change (IPCC), which reports to the United Nations, most of the warming since the mid-20th century is due to the greenhouse gases released by human activity. Other factors, including fluctuating patterns in ocean circulation and slight changes in the Sun's brightness, also influence the climate.
"All of these have been contributing to a warming, adding on top of each other," Keen told New Scientist. "The difficulty is, of course, what are the relative magnitudes [of these effects]," he says.
Susan Solomon of the US National Oceanic and Atmospheric Administration in Boulder, Colorado, a member of the Nobel-prize-winning team that put together the 2007 IPCC report, says atmospheric haze, including haze from volcanoes, was included in computer models used for the report.
But she disputes Keen's conclusions. "There's no evidence for a significant warming trend over the last several decades [due to a decline in volcanic haze]," she told New Scientist. "In fact, it's exactly the opposite."
The amount of haze in the stratosphere has been higher – blocking more sunlight – in the past 40 years compared to the 20 years before that, she says. So over the past 60 years, there would have been a slight cooling trend if volcanic haze were the only influence on climate, she says.
Keen acknowledges that depending on the period chosen, volcanic haze can give a cooling rather than a warming trend. But he argues that the relatively long period with a clear atmosphere since 1995 could be having a big impact on climate, especially if the extra sunshine reaching the Earth's surface could create subtle, longer-term warming effects through the heating of ocean water, as some scientists propose.
He is now compiling more precise estimates of the brightness of the most recent eclipse by comparing the Moon's brightness to that of reference stars during the eclipse. This will allow the amount of haze in the stratosphere during the eclipse to be calculated more precisely.