Volcanoes Cool the Earth… at Least Temporarily

Could the eruption of Mount Agung in Indonesia cool the earth?

Very big volcanoes can disrupt the climate around the earth. In 1783, the Laki Craters eruption in Iceland produced more than 6 times as much sulfur dioxide as Mount Pinatubo. It also got more into the stratosphere, because the bottom of the stratosphere is lower near the poles – only eight miles up. In the next year, a great freeze settled over Europe, leading to a famine that contributed to the French Revolution. The continents did not heat up as much the next summer, so the monsoons didn’t develop. This contributed to droughts that killed one-sixth of the population of Egypt, 11 million people in India and over 1 million in Japan. Could the same thing happen because of the current eruption of Mount Agung in Indonesia?

The last time Mount Agung erupted in 1963, the average global temperature dropped for a few years.  This graph shows the average global temperature for the last 140 years, marking the times of five large volcanic eruptions. You can see how the world’s temperature drops for a few years after those eruptions.  Some, but not all, big volcanic eruptions have caused a drop in the global temperature. What makes the difference?

The answer is in the gases that come out of the lava. Magma often has a large amount of trapped gases that get released when the it reaches the surface. The most common are water, carbon dioxide, and sulfur dioxide. Water and carbon dioxide are already so common in the atmosphere that the volcanoes do not change the global concentrations. Mount Pinatubo in 1991 released 50 million metric tons of carbon dioxide. Human activity in the United States release 100 times that – 5 billion metric tons every year.  Besides, as we all know, carbon dioxide traps the infrared 

radiation of heat rising form the earth and tends to make the earth hotter.

The cooling effect of volcanoes comes from pushing larger particles into the stratosphere where they block sunlight coming in. Although volcanic ash blocks sunlight, it is heavy (it is rock, even though in very small pieces) and it falls back to the earth in a few days or weeks. The biggest culprit is the sulfur dioxide. Sulfur dioxide oxidizes into sulfuric acid and condenses into sulfate aerosols. In the lower atmosphere, sulfates are washed out of the atmosphere relatively quickly by rain. But above the main climate systems, in the much drier stratosphere, particles could be transported around the world, staying aloft for years. These sulfate particles are just the right size to scatter incoming sunlight, sending some of it back into space and, consequently, cooling the ground below. Volcanic eruptions that send a lot of sulfur into the stratosphere can have a substantial impact on the global temperature. Mount Pinatubo, erupting in 1991, cooled the world by 1.5°F, with an impact that could still be felt three years later.

Mount Agung in 1963 had a similar impact to Mount Pinatubo, and it could again. The next eruption would need to also have a high concentration of sulfur dioxide, and it would need to be powerful enough to force that sulfur dioxide up into the stratosphere, 12 miles above the earth’s surface. And as we see in the graph, it doesn’t stop the global warming we are creating with greenhouse gases; it just gives us a short break in the otherwise upward trend.

You can read more about the Laki eruption and other catastrophic natural disasters in my upcoming book, The Big Ones, available for pre-order here.