Can volcanos change the climate?
Yes, but it has to be a very big one, like once every 100,000 years, perhaps every million years. The volcanic eruption of 75,000 years ago may have kickstarted, or assisted the onset of the last ice age, though debates on that remain unresolved. It may have also threatened the survival of the human race and caused extensive ecological disasters.
See Toba Catastrophy Theory.
I have heard politicians claiming that volcanoes are the sole cause of
global warming and using so called "NASA data" to show that the Earth
is actually cooling instead of warming.
All of this is wrong on so many levels. Volcanoes are a very small contributor of CO2 and they make it colder on average, not warmer.
While the nature of this site is for specific questions and not for
political debate, we all know from the Year Without a Summer that
volcanic gases can impact the weather severely making it very cold or
Most volcanic eruptions have very little effect.
Mt. Pinatubo in 1991 is thought to have cooled the planet for a couple of years, by maybe 1/2 of 1 degree, if that. Mt. Tambora in 1815 was much larger and had a more drastic cooling effect, but only for a few years and only one year was the year without summer. The reason for this is the cooling volcanic gas and particulates, primarily SO2, has a very short half life in the atmosphere. By some estimates, it's half life is as short as a few days, though for larger amounts, because it primarily rains out of the atmosphere, the half life might be longer for big eruptions.
Source and Source. (best sources I could find with a quick google search).
It's an interesting question as to why Volcanoes are as good at cooling the earth as they are, given that SO2 has a half life of just days, but the answer to this is at least in part, that only very large volcanoes measurably cool the planet, and it might not take that much SO2 to reflect enough sunlight to cool the earth, at least for a month or two, which might be enough to cause a measured drop in temperature for the year, and in extreme cases, a year without summer.
But the short answer is that Volcanoes don't heat the Earth, they cool it, and they're not particularly good at cooling it either. Their effect is usually quite temporary. If a Volcano is strong enough to stop man made climate change in it's tracks, it would probably also cover half a continent with ash and significantly hurt our food supply for the year and perhaps, remove most of the ozone layer for a time. The temperature effects would be the least of our problems. (Hopefully, an eruption like that won't happen for several tens of thousands of years, perhaps a few hundred thousand years, but, a gonzo eruption will happen again at some point, probably not in our lifetime though.
My question is this:
Can a volcanic eruption, either a single or multiple explosions, be
the sole cause of global warming at the planetary scale? Can volcanoes
really change the climate so fast, relative to the geological
timescale, that they produce the abnormal levels that we see today?
Volcanoes are primarily agents of cooling not warming, so one might be able to trigger an ice age, though warm northern hemisphere summers and the current CO2 levels are unlikely to let an ice age actually take hold, so that's a long shot and it would need to be a huge eruption.
It's far more rare for volcanoes to cause warming. What can happen, if the volcano is large enough is that it could kill much of the plant life on earth and decomposition could lead to an increase in Greenhouse gas, but that would probably be temporary as plant life tends to grow fairly quickly and volcanic ash is actually fairly fertile. Another way is if the volcano sets fire to a large bed of coal, which is thought to have been the case with the Siberian traps, mentioned by David Hammen, thought to be the largest extinction in the history of the Earth.
The cooling effect of SO2 and particulates that reflect sunlight away from the earth is short lived. CO2, in large enough amounts is much longer lasting in the atmosphere, 100 years half life or longer, especially if the oceans start warming and releasing some of their stored CO2, so the burning of an entire bed of coal over half a continent may have added enough CO2 to the air, that once the SO2 and Particulate driven cooling was over, the Earth could have warmed measurably, so, it is possible for a Volcano to cause global warming, but it has to be positioned under a large coal deposit and enormously big. It's very rare, to say the least.
Generally speaking, anyone who says Volcanoes can cause global warming is either lying or ignorant, but if a volcano is an extinction level event, hundreds of times larger than the one that caused the year without summer, which happens every, Oh, I don't know, once every million to several million years, then such a volcano might trigger global warming, but if that happens, the warming will be years away and very far from our biggest concern.