Gases and ash injected by volcanic eruptions high into the atmosphere can act as a sunscreen, reflecting incoming sunlight and cooling the planet. But in a warmer, human-caused climate, this effect may become much weaker depending on the size of the eruption.
The researchers, from the University of Cambridge and the UK Meteorological Office, say that large-magnitude eruptions will have greater effects as the climate continues to warm. However, the cooling effects of small- and medium-sized eruptions could shrink by as much as 75%.
As the atmosphere warms due to climate change, the plumes of ash and gas emitted by large volcanic eruptions and driven by temperature and density differences will rise ever higher. Climate change will also accelerate the transport of volcanic material, like gases, volcanic ash and volcanic sulfate aerosols, from the tropics to higher latitudes.
For large eruptions, the combined effect of these phenomena will cause the haze created by volcanic aerosols to block more sunlight from reaching Earth's surface, ultimately amplifying the temporary cooling caused by volcanic eruptions.
When Mount Pinatubo in the Philippines erupted in 1991, the effects were felt worldwide. The plume from the eruption - the second largest of the 20th century - reached more than 30 kilometers (or 98,500 feet) into the sky, forming a layer of global haze. In 1992, this haze caused global temperatures to drop by as much as 0.5 degrees Celsius. In comparison, human activities have warmed global temperatures by over 1 degree Celsius since 1850. However, the effect of volcanic aerosols only persists for one or two years, while anthropogenic greenhouse gases will affect the climate for centuries.
"Beyond the data we have from recent eruptions like Pinatubo, we can also see the cooling effect of volcanoes going back two thousand years from the information contained in tree rings," said Dr. Thomas Aubry from Cambridge's Department of Geography, the paper's first author. "However, we wanted to look at the question from the opposite angle: how could a warming climate affect the cooling from volcanic eruptions?"
Volcanic plumes rise like hot air balloons: they keep rising to a height where they're naturally buoyant. The Cambridge study looked at how high in the atmosphere these plumes can rise and be transported globally under different warming scenarios.
The researchers used global climate models combined with volcanic plume models to simulate how the aerosols emitted by volcanic eruptions might be affected by climate change.
They found that for large eruptions like Mount Pinatubo, which typically occur once or twice per century, climate change will cause the plumes to rise higher and the aerosols to spread faster over the globe, resulting in a cooling effect amplified by 15%. Changes in ocean temperatures are expected to further amplify the cooling, and the melting of ice sheets and changing sea levels is also projected to increase volcanic eruptions frequency and size.
However, for moderate-sized eruptions such as the 2011 Nabro eruption in Eritrea, which typically occur on a yearly basis, the effect will be reduced by about 75% under a projected global warming scenario of 4 to 5 degrees. This is because the height of the tropopause - the boundary between the troposphere and the stratosphere above it - is predicted to increase, making it harder for volcanic plumes to reach the stratosphere. Aerosols from volcanic plumes confined to the troposphere are washed out by precipitation in a matter of weeks, making their climatic impacts relatively minor and much more localized.
Since these smaller eruptions are far more frequent, further research is needed to determine whether the net effect will be additional warming or cooling.
"Climate change isn't something that's coming, it's already here, as clearly demonstrated by this week's IPCC report," said co-author Dr. Anja Schmidt, from the Department of Geography and Yusuf Hamied Department of Chemistry. "The effects of climate change and some of the feedback loops it can cause are becoming more obvious now. But the climate system is complex: getting a grasp of all these feedback loops is critical to understanding our planet and making accurate climate projections."
"The new feedback loops between climate and volcanic eruptions that we highlight in this work are currently unaccounted for by IPCC," said Aubry. "It could shed new light on the evolution of future volcanic influences on climate. Even if volcanoes have a limited influence on climate compared to human greenhouse gas emissions, they are an important part of the system."
"Due to more frequent and more intense wildfires, as well as other extreme events, the composition of the upper atmosphere is changing in front of our eyes, and so is our understanding of the consequences of these changes," said Schmidt. "As we continue to emit greenhouse gases, the way that volcanic emissions interact with the atmosphere will continue to change and it is important to quantify these interactions in order to fully understand climate variability."
The authors hope to bring together more volcanologists and climate scientists to understand how climate affects volcanic eruptions.
The study is published as open access paper in the journal Nature Communications.
