In the climate-change debate, skeptics have made much play of the so-called “pause” in the rise in global surface temperatures over the last 15 years or so. But rather than some fabled mechanism of Gaia regulating Earth’s climate, a new study led by the Lawrence Livermore National Laboratory points to a series of volcanic eruptions in the early years of the 21st century having contributed to cooling the planet’s surface and lower atmosphere.
The research indicates that cooling caused by such eruptions has partly offset the warming that greenhouse gases would otherwise have produced.
As the team from LLNL outlines, despite continuing increases in atmospheric levels of greenhouse gases, an example being the historical figures from the US National Oceanic & Atmospheric Administration, and despite rises in the total heat content of the ocean, global-mean temperatures at the surface of the planet and in the troposphere (the lowest portion of the Earth's atmosphere) have shown relatively little warming since 1998.
Much has been made of this apparent slowdown by climate change skeptics, but as Skeptical Science highlighted in October last year, that “speed bump” which some popular media are so fond of pointing to, casting doubt on whether climate change is actually happening, relates only to a slowing of global surface temperatures. What skeptics generally fail to grasp, according to Skeptical Science, is that overall warming of the entire climate system has continued rapidly over the past 15 years. Not only that, but even faster than the 15 preceding years.
The LLNL researchers found that volcanic eruptions have moderated temperatures, cooling the Earth’s surface and the lower atmosphere. But a graph from the 2013 report of the Intergovernmental Panel on Climate Change helps put these moderating effects in perspective.
The graph breaks down the total energy accumulation of the different components of Earth’s climate system between 1971 and 2010. It’s readily apparent that energized oceans are far and away the major factor. Not only that, the deep ocean appears to be heating up at a constant rate. In contrast, way down at the bottom, and barely registering, are land and atmospheric temperatures.
Land temperatures virtually flat-lined in the early part of the 21st century. It’s that hiatus which the LLNL researchers homed in on. Their findings are published this week in the journal Nature Geoscience.
When volcanoes erupt, the gas sulfur dioxide is ejected into Earth’s atmosphere. Some eruptions are sufficiently violent for sulfur dioxide to be ejected beyond the Earth-hugging troposphere and into the next layer up, the stratosphere. Once in the atmosphere, the gas combines with water to form tiny droplets of sulphuric acid, sometimes known as "volcanic aerosols."
The reflective qualities of these droplets mean that a proportion of the sunlight that would otherwise reach Earth, thereby heating the surface and lower atmosphere, is reflected back into space.
Lead author of the study and LLNL climate scientist Benjamin Santer explained what has happened so far this century, "In the last decade, the amount of volcanic aerosol in the stratosphere has increased, so more sunlight is being reflected back into space," continuing, "This has created a natural cooling of the planet and has partly offset the increase in surface and atmospheric temperatures due to human influence."
Levels of greenhouse gases in the atmosphere increased over the study period 2000 to 2012, just as they did for decades before that as the second graph measuring atmospheric levels of carbon dioxide at the Mauna Loa Observatory in Hawaii illustrates.
The insulating properties of greenhouse gases are well-known, trapping the sun’s rays and causing the Earth to heat up. Typically, the effects of greenhouse gases warm the lower atmosphere, the troposphere and cool the stratosphere. But on the other hand, volcanic eruptions do the reverse, cooling the troposphere and warming the stratosphere.
In the 21st century this “regulator,” in the shape of volcanic eruptions, has contributed to the recent warming hiatus, say the researchers. They believe most climate models have not accurately accounted for this effect.
"The recent slowdown in observed surface and tropospheric warming is a fascinating detective story," Santer said. "There is not a single culprit, as some scientists have claimed. Multiple factors are implicated. One is the temporary cooling effect of internal climate noise. Other factors are the external cooling influences of 21st century volcanic activity, an unusually low and long minimum in the last solar cycle, and an uptick in Chinese emissions of sulfur dioxide.
"The real scientific challenge is to obtain hard quantitative estimates of the contributions of each of these factors to the slowdown."
The LLNL scientists used two distinct statistical tests to determine whether recent volcanic eruptions have cooling effects that can be distinguished from the intrinsic variability of the climate. They found evidence for significant correlations between observed volcanic aerosols and satellite-based estimates of lower tropospheric temperatures as well as the sunlight reflected back into space by the aerosol particles.
Co-author Susan Solomon, professor of atmospheric chemistry and climate science at MIT commented, "This is the most comprehensive observational evaluation of the role of volcanic activity on climate in the early part of the 21st century."
So where does that leave climate science?
Firstly, it provides scientific rebuttal for those arguing that global warming has somehow stalled. Surface warming has been on pause, but surfacing warming is not by any stretch the largest component of global warming.
Secondly, if volcanic eruptions can be identified as the cause for a hiatus in rising surface temperatures, then it’d be foolish to rely on such variable events as a reason for inaction. Just as during the first decade of this century volcanoes appear to have been particularly active, what happens when they simmer down and we can’t rely on their kindly aerosol effects?
Unfortunately, the answer to that is particularly grim. If volcanoes go on vacation, their moderating effect on Earth’s surface and lower atmospheric temperatures will also take a holiday.
As a consequence, expect accelerated glacial melt with a consequent rise in sea levels. Worse, currently locked up or frozen gaseous hydrocarbons in the form of methane, a greenhouse gas far more potent than carbon dioxide, will be unleashed from permafrost and the ocean depths.
When that happens, no amount of Krakatoas will reverse global warming.
Lawrence Livermore National Laboratory
IPCC Report 2013