Mixed news for rivers (Image: Steve Turner/Oxford Scientific/Getty)
Make your mind up. Big volcanic eruptions have a dramatic effect on rivers, sometimes boosting their flow, sometimes reducing it to a trickle.
We knew such eruptions throw aerosols into the stratosphere, reflecting sunlight back into and changing rainfall patterns, but it seems the effect is more potent than we thought.
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To investigate, Carly Iles and Gabriele Hegerlat the University of Edinburgh looked at the historical flow volume for 50 large rivers around the world.
They found that, in the two years after major eruptions, flow decreased in rivers in wet tropical regions, including the Amazon, the Nile and the Congo. Iles estimates that the Amazon river dried by around 10 per cent.
However, in some drier subtropical regions, river flow increased after eruptions, sometimes by as much as 25 per cent – although these are “back of the envelope” calculations, says Iles.
So why the two opposite effects? Volcanoes block out sunlight, resulting in less heat to drive atmospheric circulation patterns, which affect the distribution of rainfall.
Fake volcanoes
One example is the Hadley cell, in which air rises over the equator and falls over the subtropics. The rising air is moist, but most of the moisture is lost as condensation or rain as it rises, so the descending air is dry. Consequently, when this is disrupted, wet tropical areas get drier and dry subtropical areas get wetter.
In the areas where river flow is decreased by volcanic eruptions, the impact could be similar to during dry years that arise from other causes, such as El Niño, says Mark New, from the University of East Anglia, UK. This would mean a reduced water supply and possible crop failure in areas where the agriculture depends on floodplains.
Knowing in more detail how rivers are affected by volcanic eruptions presents an opportunity to improve climate models used for long-range forecasting. “Incorporation of volcanic aerosols into the seasonal forecast models might result in improved forecasts in years after the next big eruption,” says New.
It also has implications for geoengineering projects that propose to cool the climate by putting particles into the atmosphere, mimicking the effects of eruptions.
“Deliberately placing sulphate aerosol particles into the stratosphere is very likely to have detrimental effects on agriculture and food security in some of the most vulnerable regions in the world,” says Anja Schmidt, a researcher in volcanic impacts and hazards at the University of Leeds.
Journal reference: Nature Geoscience, DOI: 10.1038/ngeo2545
