22
atmospheric CO 2 (blue bars) after the eruption of Mt. Agung (1963) and Mt. Pinatubo
(1991) is suppressed relative to prior years. The best estimate of contemporary release
of atmospheric CO 2 by volcanoes and deep sea vents reveals release of about 0.26 Gt
of atmospheric CO 2 per year (Marty and Tolstikhin 1998 ), less than 1 % of the human
burden. Interestingly and with touching irony for those who refuse to accept the human
influence on global warming, the volcanic release of CO 2 during the 9 h explosive
phase of Mt. Pinatubo on 15 June 1991 likely matched the total, global release of CO 2
by humans on that day (Gerlach 2011 ). More than 20,000 days have passed since the
start of modern measurements of atmospheric CO 2. On one day and one day only,
global human release of atmospheric CO 2 was likely matched by a volcano. Human
release of CO 2 has dwarfed volcanic release on the other 19,999 days.
Why have volcanoes been so dominant in the past, yet so unimportant in the
present? One factor is that modern human civilization has not yet experienced a
volcanic eruption of the magnitude known to have occurred in the past. The Volcanic
Explosivity Index (VEI) denotes the size of volcanic eruptions (Newhall and Self
1982 ), much like the Richter Scale for earthquakes. Mt. Agung, El Chichón, and
Mt. Pinatubo had VEIs of 5, 5, and 6, respectively. The most violent eruption Earth
has experienced over the past 36 million years was the VEI of 9.1–9.2 eruption of
La Garita^16 about 27.8 Mypb (Mason et al. 2004 ). Since the VEI scale is logarithmic
with respect to volume of ejecta, a VEI 9 eruption would eject about 1000 times
more mass than Mt. Pinatubo.^17 Had Mt. Pinatubo been VEI 9, it may have matched
human emission of CO 2 over the prior 1000 days. In this case, of course, the ejection
of CO 2 by such a monstrous event would have been the least of our concerns.
The other factor responsible for the minor role of volcanoes with respect to con-
temporary atmospheric CO 2 is that Earth is presently in a geologically dormant
period. The Deccan Traps of India is one of the largest, most well-studied ancient
volcanic features on Earth. Eruptions of this massive province, approximately
65 Mybp, may have been characterized by a decades-long explosive events (Self
et al. 2006 ). The perturbation to atmospheric CO 2 by the Deccan Traps is the subject
of active research, with some studies (Dessert et al. 2001 ) suggesting a considerably
larger influence than others (Self et al. 2006 ).
We conclude by providing a brief overview of the latest understanding of the
factors that control atmospheric CO 2. More detailed information, updated annually,
is maintained at http://www.globalcarbonproject.org/carbonbudget.
It is well established that a substantial portion of the CO 2 released to the atmosphere
by human activity is absorbed by trees and plants (i.e., the terrestrial biosphere) as well
as the world’s oceans (Le Quéré et al. 2015 ). Uptake of anthropogenic CO 2 by plants is
facilitated by three factors: higher levels of atmospheric CO 2 promote faster growth of
plants (Zhu et al. 2016 ), global warming has increased the length of the growing season
(Le Quéré et al. 2015 ), and human supply of fixed nitrogen to the biosphere promotes
a more fertile environment for plant growth (Galloway et al. 2014 ). The world’s oceans
(^16) The caldera of this ancient volcano is near the town of Creede, Colorado in the United States.
(^17) For those with a mathematical background, the calculation is straightforward. The ejected mass
is proportional to 10 raised to the power of VEI; therefore, the ratio of mass ejected by a VEI 9
eruption to Mount Pinatubo is 10^9 divided by 10^6 , which equals 10^3 or 1000.
1 Earth’s Climate System