Today's Guest Blogger, Lisa Moore, is a scientist in the Climate and Air Program.
The fiery centers of volcanoes burn carbon-containing rocks from deep within the earth, and thus emit the greenhouse gas carbon dioxide (CO2). There are a fair number of volcanoes in this world, all emitting CO2, so couldn't this be the cause of global warming?
In a word, no. Here's how scientists know that climate change is not from volcanic activity.
On average, volcanoes spew over 130 million tons of CO2 into the atmosphere each year. That sounds like a lot, but compare it to global fossil fuel emissions: in 2005, we emitted more than 27 billion tons of CO2. Emissions from human activity are more than 200 times the emissions from volcanic activity.
Those numbers are direct measurements from scientists reported on U.S. government sites, so I could stop here. But for those who still are skeptical, I'll demonstrate it another way as well.
If the increase in CO2 came from volcanoes, we would expect to see abrupt increases in CO2 after large eruptions, but we don't! The figure below shows monthly mean CO2 levels since 1958, measured at the Mauna Loa observatory in Hawaii.
As you can see, CO2 is steadily increasing. The arrows show the five largest volcanic eruptions during that time period. They didn't leave much of a mark, did they?
We can see evidence of volcanic eruptions in another dataset: temperatures. But the effect of eruptions is to cool the globe, rather than heat it!
Huge volcanic eruptions can shoot significant amounts of sulfur dioxide into the stratosphere. This gas is converted to sulfate aerosols, which reflect sunlight and have a cooling effect. Because of the way the atmosphere circulates, tropical volcanoes have a stronger cooling effect than mid- and high-latitude eruptions of the same magnitude.
The graph below is Figure 1 in Chapter 8 [PDF] of the latest IPCC report. The black line shows temperatures, the yellow lines show results from 14 different models, and the red line is the average of all the simulations. As you can see, the four biggest tropical eruptions over the past century had slight cooling effects.
The Mount Pinatubo eruption was especially interesting because it provided a great early test for climate models. After Pinatubo exploded, scientists entered emission measurements into their climate models and compared the results with actual observations. The next figure shows that comparison (a larger version is available here). The solid line shows temperature measurements and the dashed lines show model simulations.
Two things really jump out of this graph. First, the reflective aerosols from the eruption had a substantial cooling effect. And second, the climate model did a pretty darn good job of predicting the resulting climate change.
One more point worth noting: this particular model experiment was published 10 years ago. Scientists haven't stopped testing or improving their models since then, so today's models are even more accurate.
Speaking of models… that is a big topic! Maybe Bill should do a series about climate modeling – what do you think?