There has been a lot in the news recently about current volcanic activity – Merapi in Indonesia and Bezymianny in the Kamchatka peninsula in Russia, but while most reports have focussed on the very real dangers to the local populace and air traffic, volcanoes can have important impacts on climate as well. However, there are a number of conditions that need to be fulfilled before an eruption will show up in the climate record.

The most important recent volcanic impact on climate was that of Mt. Pinatubo in the Phillipines which erupted in June 1991. Prior to that El Chichon (Mexico, in 1982), Mt. Agung (Indonesia, 1963), Santa Maria (Guatemala, 1902) and Krakatoa (Indonesia, 1883) all had noticeable cooling effects. The observant among you will notice that all of these volcanoes are in the tropics, and this turns out to be a crucial determinant of how much climate impact there is. Additionally, each of the these eruptions had a high VEI (Volcanic Explosivity Index) and high sulphur dioxide amounts in the ejecta. In each case, the eruption was so strong that significant amounts of sulphur dioxide (which gets converted to sulphate aerosols) were carried up into the stratosphere (12 to 15 km above the ground). There, because this is well above the clouds and rain of the troposphere, the sulphates can hang around for a long time (a few years) while sulphates in the lower atmosphere get quickly washed out and don’t generally have a long term impact (but there can be exceptions – see below). This can happen equally well in the mid and high latitudes, but the key factor in tropical eruptions is that the circulation of the stratosphere (rising in the tropics, subsidence in the high latitudes) favors the worldwide dispersion of tropical sulphates, but pushes high-latitude sulphates right back down again. So for tropical eruptions, the effects both last longer and are more widespread than for equally explosive high-latitude eruptions. The principle impact is a couple of years of cooling (since sulphates are reflective), but related changes in dynamics can cause ‘winter warming’ in Europe, and there are hints in the paleo-record of an impact on El Nino events – the probability of one may double after a big tropical eruption (Adams et al, 2003).

There can be some exceptions to the tropics-only rule, and at least one high latitude volcano appears to have had significant climate effects; Laki (Iceland, 1783-1784). The crucial factor was that the eruption was almost continuous for over 8 months which lead to significantly elevated sulphate concentrations for that whole time over much of the Atlantic and European regions, even though stratospheric concentrations were likely not particularly exceptional.

One point that is also worth making is that although volcanoes release some CO 2 into the atmosphere, this is completely negligable compared to anthropogenic emissions (about 0.15 Gt/year of carbon, compared to about 7 Gt/year of human related sources) . However, over very long times scales (millions of years), variations in vulcanism are important for the eventual balance of the carbon cycle, and may have helped kick the planet out of a ‘Snowball Earth’ state in the Neo-proterozoic 750 million years ago.

Volcanoes can provide great tests for climate models, and indeed, predictions of the impacts of Pinatubo (before they happened) proved very accurate (about a 0.5 C cooling after about 18 months) – for instance in Hansen et al (1992). Other studies have examined the dynamic impacts of volcanic eruptions (the ‘winter warming’ – Robock and Mao, 1992; Shindell et al 2004), the water vapour feedback as the planet cools (Soden et al, 2001), the impact of high-latitude eruptions (Oman et al, 2005), impacts on ENSO (Mann et al, 2005) and have shown that the models work pretty well.

So what of the current eruptions? Well Bezymianny appears to be explosive enough, but its latitude (55 N) will tend to preclude it having any big climate impact. Merapi is in the right location but doesn’t appear (so far) to be explosive enough to put anything in the stratosphere, and so this too seems unlikely to impact climate. At some point, there will be another climatically important eruption, but it hasn’t happened yet…