Only a very small fraction of the energy produced by nuclear fusion in the Sun (3.82 x 1023 kW) reaches the top of the Earth’s atmosphere (1.7 x 1014 kW), and then only about 50% of it reaches the Earth surface, where it is absorbed by the oceans and land. This energy is radiated back as longwave infrared radiation, which is partially absorbed and after reradiated downward to the surface by the greenhouse gases residing in the atmosphere.
The most abundant gases in the Earth’s atmosphere capable of absorbing infrared radiation are, in decreasing order of atmospheric mole fraction, water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and fluorinated gases. Circa 50% of the greenhouse effect is caused by water vapor. However, this vapor is not considered as a climate change driver, given the small human influence on its atmospheric concentration and its small residence time (» 9 days), that partially translates in a 100-year effective GWP (Global Warming Potential; GWP-100) of −0.001 to +0.0005 for the total anthropogenic emissions. Although the GWP-100 of CO2 is 1 to 4 orders of magnitude lower than that of other greenhouse gases (e.g., 28 times lower than methane), the exceedingly higher volumes of anthropogenic carbon dioxide emissions make it the most effective (> 75%) human-generated greenhouse gas contributing to radiative forcing and, thus, to global warming.
The present-day (May 2024) CO2 content of the atmosphere is 427 ppm, which, given an atmospheric mass of 5.15 x 1018 kg, corresponds to ca. 2.20 x 1015 kg of carbon dioxide nowadays residing in the atmosphere. CO2 concentration has been continuously rising since the beginning of industrial era, depicting 49.8% increase between 1850 (285 ppm) and the present day. This goes in hand with the increase of both anthropogenic CO2 emissions, which reached 3.68 X 1013 kg in 2023, and the increased global mean temperature, which in 2023 became 1.48ºC warmer than the mean 1850-1900 pre-industrial level, and 0.60ºC higher than the 1991-2020 average. This led the Intergovernmental Panel on Climate Change (IPCC) to assign the fundamental role of increasing anthropogenic CO2 to global warming.
The role of volcanism to the CO2 concentration in the atmosphere is addressed below.
VOLCANISM AND MASS EXTINTIONS
A CO2 flux of 6.9 X 1010 kg.a-1 was estimated for subaerial volcanism, reaching a global upper limit of 2.5 x 1011 kg.a-1 if including submarine volcanism. This combined value corresponds to a yearly CO2 volcanic addition to the atmosphere of a volume equalling »0.05 ppm. In the last 174 years (1850 – 2024) the CO2 concentration in the atmosphere increased by 142 ppm (see above), while the CO2 volcanic addition in the same period, assuming a constant volume of emissions, would correspond to only »8 ppm.
Plinian column from May 18, 1980 eruption of Mount St. Helens (USA). Photo: Robert Krimmel.
However, volcanic activity is not uniformly distributed over time. Indeed, Earth’s history has been punctuated by the formation of the Large Igneous Provinces (LIPs), characterized by huge volumes of magmatic rocks (frequently > 1 x 106 km3) generated during short periods of time (typically < 5 Ma).
In the last ca. 500 Ma of the Earth’s evolution 5 mass extinctions took place leading to the disappearance of more than 75% of species each time, in short (few Ma) geological periods. These events occurred at approximately 444 Ma (86% of species lost), 360 Ma (75%…), 250 Ma (96%…), 200 Ma (80%…) and 65 Ma (76%…), defining the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous geological periods, respectively. A temporal link between the Phanerozoic LIP events and these mass extinctions has been demonstrated, with the most robust correlations being associated with Siberian Traps (252 Ma), the Central Atlantic Magmatic Province (CAMP; 201 Ma) and the Deccan Traps (66 Ma). Whilst these correlations exist, they do not prove that mass extinctions are exclusively driven by such huge magmatic events. For example, it has been long suggested that the K/Pg extinction was a consequence of the 66 Ma Chicxulub impact, or at least to the combined effect of the LIP and the impact. The climate forcing caused by LIP events has been considered highly probable, with temperature increases, directly or indirectly caused by CO2 and CH4 emissions, up to 15ºC.
Among the pairs that pass the filter of Hg (a proxy used to determine magmatic contributions to the sedimentary record) is the CAMP – end of Triassic mass extinction, the causal link being considered indisputable. The CAMP is thus an ideal candidate to draw comparisons between LIP-derived and anthropogenic-derived CO2 greenhouse emissions and establish the potential impact of the latter.
Central Atlantic Magmatic Province at Middle Atlas (Morocco). Photo: Andrea Marzoli; CC license.
CO2 anthropogenic emissions surpass that of LIPs
Several attempts have been made to estimate the CO2 delivered by LIP events. A recent one took profit of the increasingly accurate knowledge of volcanic ages and it was based on a study of CO2 imprisoned in fluid inclusions of CAMP basalts (Capriolo et al., 2022).
Melt inclusions with exsolved bubbles of CO2 hosted in CAMP clinopyroxenes from a) Nova Scotia (Canada) and b) Morocco. Capriolo et al., (2020).
The CAMP was erupted and intruded in continental crust during a time lapse of 4 Ma, occupying an area of 107 km2 with a volume of 5-6 x 106 km3, plumbing system included. Taking into account the mass extinction dated at 201.654 ±0.015 Ma, this has been linked to the 1st and main CAMP volcanic phase (201.6 – 201.5 Ma). Indeed, fluid inclusions studies have provided strong evidence that, during this 1st phase, a total of 2.9 x 104 Gt of CO2 were emitted to the atmosphere, which would have had a key role on triggering the climate/environmental changes leading to the mass extinction event.
Anthropogenic CO2 emissions. CC license.
In 2023, 37.4 Gt of CO2 were emitted by human activities alone, which maintaining the same level of emissions, would correspond to 3.74 x 105 Gt of CO2 in 0.1 Ma, i.e. around 13 times the CO2 delivered by the CAMP during its main phase (which lasted 0.1 Ma). As a consequence of the anthropogenic CO2 emissions, we are presently faced with high extinction rates and population declines of many species, which has been considered as highlighting the start of a new mass extinction event.
