Volcanic eruptions have complex effects on the climate and the environment. Apart from the obvious destruction of the surroundings that occurs when an eruption plume or marauding lava flow trundle over everything in their paths, volcanic blasts also negatively influence the environment by releasing a combination of aerosols and gases into the atmosphere.
Despite what conspiracy-prone news outlets may tell you, the net effect of most volcanic eruptions is a cooling effect on the planet, not warming. Yes, carbon dioxide is released during volcanic eruptions, but this is almost always negated by the aerosols that are released at the same time. A new study in Nature Geoscience suggests that these aerosols and their associated cooling effect may be having a subtle, complex effect on the flow of rivers across the world.
Volcanic aerosols – suspended particles of matter that drift up into the atmosphere – are mostly sulphur dioxide compounds. Volcanic eruption columns from powerful eruptions breach the stratospheric layer of our atmosphere. Here, the compounds turn into drops of sulphuric acid by reacting with water vapor, which are then scattered and distributed around the globe. They are effective at reflecting incoming sunlight back into space, and if enough of them are present in the stratosphere the Earth can experience a noticeable cooling effect.
Benjamin Franklin, one of the United States’ founding fathers, was the first to link volcanic eruptions with climate. He suggested that the extremely harsh winter of 1783-84 in Europe was a direct consequence of Mount Laki’s paroxysmal volcanic eruption earlier that summer.
A recent example of this cooling effect was the 1991 cataclysmic eruption of Mount Pinatubo in the Philippines. The 17 megatons of sulphur dioxide released into the atmosphere cooled the planet by roughly half a degree Celsius (almost 1 degree Fahrenheit) by 1993.
Whenever a major volcanic eruption releases vast quantities of sulphur dioxide into the stratosphere, the climate cools. A cooling climate means that less evaporation of water from ground-based water sources will occur; consequently, less rain, snow and other forms of precipitation will fall. In addition to this, a cooler ground stabilizes the atmosphere, reducing its ability to hold water.
This alteration of the planet’s evaporation-precipitation cycles leads to regional changes in rainfall. The team from the University of Edinburgh looked at the effect that a wide range of major eruption events, including Krakatoa in 1883 and Pinatubo in 1991, had on the flows of over 50 major rivers across a long time span. They discovered a direct link: a year or two after a major, sulphur dioxide-releasing eruption, the flow of rivers in tropical regions generally decreased. Northern Asia, the Amazon, the Congo and the Nile all experienced dramatically decreased river flows.
In contrast to this, in some sub-tropical regions, including parts of South America and the southwestern United States, the flow of rivers increased. It appears that the disruption of the evaporation-precipitation cycles in the tropical regions provokes increased rainfall in other areas of the globe.
This will be a setback for scientists who are looking to a geoengineering solution to global warming. Cooling the climate by emitting masses of sulphur dioxide into the atmosphere could indeed work, but it appears that it will have unintended effects on the world's rivers. Major cities across the planet rely on rivers to deliver water; along rivers like the Nile, where the human population density is particularly high, a severe volcanic eruption – or a controlled emission of sulphur dioxide into the atmosphere – will reduce the river’s flow, which is likely to result in dire consequences.