Volcanic Versus Human Carbon Dioxide Emissions

A couple of weeks ago Iceland awaited with much anxiety as magma made its way to the surface. A volcanic eruption seemed imminent. That danger seems to have passed for now. Seismicity has abated and magma may not break through and erupt.

Misunderstandings regarding climate change though shows no signs of receding as this comment shows - 

 Source: X - https://twitter.com/dremtee/status/1723427183182446871

Ever so often it is worth putting up the numbers:

Anthropogenic CO2 emissions - About 40-50  billion tons per year.

Volcanic CO2 emissions - Approximately 500 million tons per year.

Terry Gerlach of the U.S. Geological Survey has compiled global data on volcanic emissions  -  Volcanic Versus Anthropogenic Carbon Dioxide, published in EOS Transactions American Geophysical Union. 

This article is from 2011, but there are good explanations on volcanic emission rates and the observed discrepancy (which has increased in the 12 years since publication) between anthropogenic and volcanic emissions. 

What conditions limit volcanic CO2 emissions on present day earth? 

On average, magma contains about 1.5 weight percent dissolved CO2. Estimated annual magma production on earth amounting to about 80 billion tons won't create near enough volcanic CO2 to match human emissions. About 850 cubic kilometers of magma would be needed to be generated annually to create volcanic CO2 on an anthropogenic scale. So much magma production either under land or sea would not have gone unnoticed. 

Short lived volcanic eruptions like past events in Iceland, or Mt. Pinatubo, or Mt. St Helen's, although violent and spectacular,  didn't emit more than a few million tons of CO2. These amounts are too small to have a discernible warming effect. Large explosive eruptions in fact might cool the earth by a degree or so for a short time because the sulphur particles they emit reflect sunlight back in to space.

Can volcanism cause global warming? Yes, but over much longer time scales. 

Weathering of surface silicate rocks consumes about 500 -700 million tons of CO2 per year, offsetting the amount emitted by volcanoes. There has to be sustained volcanism at high emission rates for decades to hundreds of years to create an imbalance between weathering and volcanism and change climate. 

Cin-Ty Lee and Slyvia Dee 's  commentary on this subject explores the role of volcanism on global climate.