The first scientific paper to speculate that volcanoes could alter the world's climate was written by Benjamin Franklin. After observing a strange "dry fog" that covered Europe and parts of North America in 1783, Franklin wrote that the source of the mysterious fog-and the extremely cold weather it caused-might have been "the vast quantity of smoke" issued by a volcano near Iceland, and which "spread by various winds over the northern part of the world." The idea that volcanoes can alter the global climate remained mostly speculation. In recent years, however, satellite technology and computer models have allowed scientists to precisely measure such effects.
Each year, tons of ash and gas are spewed into the air by volcanoes around the world. Most of it falls to the ground within weeks, or washes out in rain. But a truly powerful eruption can shoot millions of tons of volcanic gases into the stratosphere, many kilometers high, where it may circle the globe for years. (The stratosphere is the top layer of the atmosphere; the lowest layer, the troposphere, is where all surface weather takes place.) Major eruptions can create a haze in the stratosphere that shades and cools Earth enough to cause climate changes.
In 1991 a large eruption from Mount Pinatubo in the Philippines shot 15 million tons of sulfur dioxide into the stratosphere. Although sulfur dioxide is an invisible gas, it reacts with water in the stratosphere and creates tiny droplets of sulfuric acid. These particles form a yellow haze similar to the smog caused by urban pollution.
Small drops or particles that float in the air are called aerosols. Mount Pinatubo put more aerosols into the stratosphere than any other volcano in the 20th century. The aerosol haze from Pinatubo blocked enough sunlight to cool the entire Earth by 0.6º C (1º F)-quite a feat for a single volcano.
Why Don't Aerosols Fall Out of the Stratosphere?
Unlike air in the troposphere, which is continuously churning, air in the stratosphere stays in separate layers. ("Strata" is the scientific word for layers.) When aerosol particles become trapped in one of these layers, they can float in the stratosphere for years. In the troposphere, most such particles would fall to the ground within weeks.
The stratosphere and troposphere behave differently because they are heated differently. The troposphere is heated from below; warm air near the ground rises to the top of the troposphere, cools, and falls again. This cycle repeats over and over, like a Lava Lamp. In the process, airborne particles in the troposphere are swept down to Earth's surface. Rain and snow also wash particles out of the troposphere.
The stratosphere, in contrast, is heated from above, by sunlight. Since the warmest air is already at the top, there is no vertical circulation in the stratosphere. The lack of clouds and rain in the stratosphere also keep particles from washing to the ground. Because the winds in the stratosphere are very strong, stratospheric aerosols rapidly spread around the globe.
Why Don't All Volcanoes Affect Climate?
The 1980 eruption of Mount St. Helens was nearly as powerful as Pinatubo, yet it had almost no effect on the global climate. Why not? Many factors determine whether and how a volcano will affect the climate:
SIZE. To send gas and ash into the stratosphere, an eruption must be big and explosive: strong enough to hurl volcanic matter at least 15 km high, above the troposphere.
SHAPE AND DIRECTION. Even a very large eruption will not reach the stratosphere if it explodes sideways. Mount St. Helens erupted so explosively that it blew off an entire mountaintop-yet much of its ash and gas shot out the side, so the material never reached the stratosphere.
LOCATION. To shroud the entire Earth in volcanic aerosols, the eruption must occur near the equator, so the ash and gases can spread both north and south.
SULFUR CONTENT. A volcano that doesn't emit much sulfur won't produce many aerosols. The magma that feeds some explosive volcanoes doesn't contain much sulfur, so gigantic explosions like the 1956 eruption of the Russian volcano Bezymianny, which was nearly as big as Pinatubo, have almost no effect on the global climate.
WATER CONTENT. The more water in the magma that feeds a volcano, the more explosively that volcano will erupt. When still below ground, magma is under great pressure; the pressure serves to keep gases, including water vapor, dissolved within the magma. When water-charged magma reaches the surface, the dissolved water blasts out of the magma like steam from a kettle. As the water expands to 100 times its original volume, it can cause an explosive eruption.
PINATUBO: ALL OF THE ABOVE. Mount Pinatubo met every one of these criteria. It was a very explosive eruption; its magma contained large amounts of both sulfur and water; and the eruption occurred very near the equator, causing its aerosols to spiral over every part of the Earth.
Other Climate Effects
Volcanic aerosols do more than just cool the planet. Their effects range from beautiful sunsets, to erosion of the ozone layer in the Earth's stratosphere, to severe disruptions of regional weather patterns.
In 1992, record floods struck the midwestern United States. The nightly television news regularly featured images of farms and entire cities deluged by water. Mount Pinatubo may have partially caused this flooding. Here's how:
When volcanic aerosols cool the planet, the resulting changes in temperature are more extreme in mid-continent than near the coasts, because oceans moderate temperature swings. (Minneapolis has colder winters and hotter summers than Seattle, even though they are at the same latitude, because the Pacific Ocean moderates Seattle's temperatures.)
Rain and snow in the Midwest are created when clouds carrying warm, moist air from the Pacific Ocean and the Gulf of Mexico hit a cold front and drop their load. The cooler temperatures caused by Pinatubo in 1992 may have generated more cold fronts and thereby contributed to the extensive floods.
Of course, the timing of the floods could have been a coincidence. Lots of events other than a volcano can cause rain and flooding. Why blame Pinatubo? Scientists point to a strong piece of evidence. If the extra rain were simply caused by more rain clouds passing over the Midwest, more rain should fall on the East Coast too. Yet in 1992, while the Midwest suffered record flooding, the Northeast experienced a major drought. The discrepancy may indicate that the extra rain in the Midwest was caused not just by more rain clouds, but by more cold fronts, which wrung the clouds dry before they reached the East Coast. Since Pinatubo's cooling affects the interiors of continents more than the coasts, it may account for an increased number of cold fronts.
Is there any way to know for sure whether the Midwest floods were influenced by Mount Pinatubo's aerosol haze? Is it possible to "turn back the clock" and repeat the 1990's to see what the weather would have been without the volcano? In fact, scientists at NASA's Goddard Institute for Space Studies have done so, many times-not in the real world, but in a computer model of the global climate. Read the other sections of this story to find out how the model works, and how Mount Pinatubo helped them test and improve it.