Climate skeptics are fond of pointing to volcanoes when disputing the facts of global warming, citing these natural events as major agents in climate change. They’re right – volcanoes do play a major role in the climate, but not the way they think: the net effect of volcanic eruptions is to cool the planet. Volcanoes shoot large amounts of sulfur gas into the stratosphere, where they form sulfur aerosols – minute droplets of liquid – that can stay suspended in the air for long periods of time. These aerosols act as tiny mirrors in the atmosphere, reflecting incoming sunlight, and decreasing the amount of the sun’s energy that reaches the surface.
A recent article examines the impact that past volcanic eruptions have had on the Earth’s climate. The summer of 1816 saw severe climate abnormalities, which resulted in crop failures in Northern Europe, Eastern Canada and the Northeastern United states. For many years, this was attributed to the 1815 eruption of Tambora, which killed 88,000 million people, and sent 100 million tons of sulfur gas into the atmosphere. However, climate records show that the cooling trend started five years earlier and the entire decade of 1810 – 1819 was the coldest in the past 500 years.
Now, Jihong Cole-Dai of South Dakota State University and his colleagues have discovered evidence of a 1809 volcanic eruption, which started the trend. Large amounts of volcanic sulfuric acid were found in snow layers dated to 1809 and 1810. The traces were found in ice cores both Greenland and Antarctica, attesting to the global reach of the eruption, which probably occurred in the tropics. Scientists estimate that the eruption was about half the size of the one in Tambora. It was the combined impact of the two that led to the disastrous summer of 1816.
More recent volcanoes have also altered the climate. The 1883 eruption of Krakatoa cooled the globe by 4.2 degrees Fahrenheit and produced spectacular sunsets over Northern Europe.
The eruption of Mt. Pinatubo in 1991 is still impacting the climate. Without its cooling aerosols, which are still in the atmosphere, current global temperatures would be even greater. This is also true of the sulfur aerosols that have been emitted by the burning of fossil fuels: as we reduce these emissions due to well-founded public health concerns, the temperature increase that they have been masking will be felt. (Some scientists have even proposed seeding the atmosphere with sulfur particles as a last-ditch way to halt global warming, a truly frightening example of geo-engneering).
What about the millions of tons of CO2 emitted by volcanoes, your skeptic might ask? Answer: The US Geological Survey estimates that current volcanic activity emits 130 million metric tons of CO2 a year – that’s a drop in the bucket compared to the 30 billion tonnes emitted by human activities such as the burning of fossil fuels.
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