Could trigger catastrophic global cooling for several years.
Although Eyjafjallajökull volcano, which caused havoc across Europe two years ago, may be quiet now, activity has been increasing at several nearby volcanoes, says this article on icelandreview.com.
This includes Katla, which has erupted about every 60 years (the last time in 1918), Hekla, which has erupted about every ten years(the last time in 2000), and Grímsvötn, which had a short eruption last year.
In the 17th and 19th centuries, Eyjafjallajökull eruptions were followed within months by eruptions of Katla, and now again, Katla is growing increasingly restless.
According to GPS measurements, there has been surface movement on Katla as well as bursts of high earthquake activity beneath its caldera, which indicatoes rising magma.
Depending on wind direction and other factors, a major eruption of Katla could cause flooding, poison crops, destroy property, affect air traffic across Europe, and even trigger catastrophic global cooling for several years.
(The Laki eruption of 1783-84 reduced global temperatures by up to 3 C. Such an eruption today “could lead to between 52,000 and 228,000 fatalities throughout Europe.” See https://iceagenow.info/2012/04/global-warming-exacerbating-iceland-volcanic-activity/
Increased activity has also been detected in the volcanoes under the largest ice cap, Vatnajökull, where Grímsvötn lies, and just last week we learned that Askja volcano might also be preparing to erupt.
Fun map of Iceland volcanoes:
Thanks to Thomas McHart for these links