Climate change and volcanoes

There was a bit of chatter around the (online) Whaleoil water-cooler about the effect of volcanoes on climate so here’s an answer: Quote.

Schematic overview of the climate effects after a very large volcanic eruption. Source: Timmreck, 2012.

The relationship between volcanoes and climate is a very complex one. From reading the media one gets the impression that they are some sort of climatic wild card. They are used to explain the cooling after the Pinatubo eruption, or the Little Ice Age cooling as a detriment to the solar hypothesis. But they are also used to explain the warming leading to mass extinctions in the distant past.

To be able to fulfill such a dual role, scientists take advantage of the different gas emissions from volcanoes. About 50-90 % of the gas emitted by volcanoes is water vapor. The rest is highly variable from one volcano to another, but CO2 can be 1-40 %, SO2 1-25 %, H2S 1-10 %, and HCl 1-10 %, plus a lot of other minor gases. H2S gets quickly oxidized to SO2.

If we want to concentrate on the cooling produced by volcanoes we look at SO2 (+H2S). Due to silent degassing, nobody knows how much SO2 volcanoes are producing, and the estimates vary wildly, but their contribution is very significant. SO2 placed in the troposphere gets converted to sulfate and coalesces into aerosols that affect cloud condensation and get precipitated as acid rain in a relatively short time. SO2 also reaches the stratosphere mainly during explosive eruptions that take place about every two years, but also from rising tropospheric SO2. Most of the stratospheric SO2 is of volcanic origin. In a period of weeks to months stratospheric SO2 gets converted to sulfate, dehydrating the stratosphere and building a sulfate peak in about 3 months after the eruption, that produces a stratospheric aerosol layer enhancement that lasts about 4 years.

The increase in sulfate aerosols in the stratosphere has three main climatic effects. The better known one is to scatter incoming solar radiation producing surface cooling. But they also absorb near IR (Infrared radiation) and LWR (longwave infrared radiation, incoming and outgoing) producing stratospheric heating, affecting atmospheric circulation and leading to warm winters in the Northern Hemisphere (NH) in the 1-2 years after a stratospheric eruption. The third effect is a very effective destruction of ozone due to altered chemistry and perturbed heating rates. The drop in the ozone level affects stratospheric climate leading to widespread atmospheric changes and to an increase in solar UV (ultraviolet) radiation reaching the surface.

To complicate things, the climatic effects of volcanoes depend a lot not only on their stratospheric injection, but also on the latitude of the volcano and the time of the year. These factors have a significant effect on the dispersal of the cloud. Thus, some volcanoes have a global effect while others only a hemispheric one, or no effect at all, like the Mt. St. Helen’s eruption of 1980, a strong volcanic eruption with no climatic effect.

When scientists want to concentrate on the warming produced by volcanoes, they look at CO2. Currently CO2 emissions from volcanoes are too small, about 1% of anthropogenic emissions. But, in the distant past the amount of CO2 produced by a much higher level of volcanic activity, particularly by large igneous provinces, might have been very considerable. Since mass extinctions, from the past in some cases, correlate well with large igneous provinces, there is a lively debate on whether SO2 and cooling, or CO2 and warming, caused the extinction, with the latter one being the most popular hypothesis today. End of quote.

The article goes on at some length and detail but concludes:  Quote.

Volcanic eruptions affect weather (a few years), not climate, and their only possible climatic effect is a hypothetical (but reasonable) feedback effect at glacial terminations through their significant contribution to COincrease.
However, climate change has a strong effect on volcanism. The effect of obliquity-linked ice unloading is very clear. Due to the asymmetric nature of ice changes, as melting is very rapid, but ice build-up is very slow, the effect of ice unloading has been well established, but that of ice loading has not. Anecdotal evidence since the 1970’s suggests that ice loading might also increase volcanic activity, as eruptions have been observed after decades-long cooling periods, however, the evidence for this is not significant. End of quote.

So, as you can see, the science is settled. /Tui

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