in rainfall and river discharge (e.g., Oman et al., 2006; Trenberth and Dai, 2007) and the occurrence of tropical cyclones in the North Atlantic (Guevara-Murua et al., 2015). Documentation of the atmospheric impact of recent explosive eruptions provides important constraints for testing short-term climate model predictions and for exploring the effects of proposed geoengineering solutions to global warming (e.g., Robock et al., 2008, 2009).

Large effusive eruptions have a somewhat different effect on the atmosphere because of their long durations (e.g., Schmidt et al., 2016; Thordarson and Self, 2003). Basaltic eruptions, in particular, can be both voluminous and long lived, and can therefore affect local, regional, and possibly global climate. Historical examples from Iceland, such as the Laki eruption of 1783–1784 and the Bárðarbunga eruption of 2014–2015, provide an interesting contrast. The former had a regional (Northern Hemisphere) impact in the form of dry fogs of sulfuric acid (H2SO4), while the latter produced dangerously high local levels of SO2. The difference reflects not only the larger volume of the Laki eruption, but also the season (summer versus winter) because sunlight plays an important role in the oxidation of SO2 to H2SO4 (Gislason et al., 2015; Schmidt et al., 2010). In the extreme, the large volume and long duration of ancient flood basalts may have perturbed the atmosphere over time scales of decades to centuries to even millennia (Figure 4.1).

The effects of injecting large amounts of water by volcanic eruptions into the dry stratosphere could affect climate by accelerating the formation of sulfate aerosol by OH radicals or by decreasing the ozone formation potential of the system (Glaze et al., 1997; LeGrande et al., 2016). Studies of very large flood basalt eruptions suggest that both the formation of sulfate aerosols and the depletion of ozone played a significant role on climate over Earth’s history (Black et al., 2014). These examples emphasize the need to better characterize plume gas and aerosol chemistry as well as coupling of gas-phase chemistry with aerosol microphysics in climate models. Because satellite-based remote sensing observations of volcanic gases are heavily biased toward SO2 (e.g., Carn et al., 2016), obtaining a complete volatile inventory for explosive eruptions required for a full chemistry simulation of volcanic plumes is still a major challenge.

Large eruptions affect Earth’s oceans in a variety of ways. Volcanic ash may be a key source of nutrients such as iron and thus capable of stimulating biogeochemical responses (Duggen et al., 2010; Langmann et al., 2010). During the week following the 2003 VEI 4 eruption of Anatahan, Northern Mariana Islands, for example, satellite-based remote sensing detected a 2–5-fold increase in biological productivity in the ocean area affected by the volcanic ash plume (Lin et al., 2011). These impacts can be particularly pronounced in low-nutrient regions of the oceans. A more indirect and longer-term impact of very large volcanic eruptions is caused by the rapid addition of CO2 and SO2 to the atmosphere, which affects seawater pH and carbonate saturation. Carbon-cycle model calculations (Berner and Beerling, 2007) have shown that CO2 and SO2 degassed from the 201-million-year-old basalt eruptions of the Central Atlantic Magmatic Province could have affected the surface ocean for 20,000–40,000 years if total degassing took place in less than 50,000–100,000 years. Ocean acidification from the increased atmospheric CO2 may have caused near-total collapse of coral reefs (Rampino and Self, 2015). Rapid injection of large amounts of CO2 into the atmosphere by volcanic eruptions also provides the best analog for studying the long-term effects of 20th-century CO2 increases on ocean chemistry. Targeted investigations of these large eruptions have the potential to establish quantitative estimates of the volatile release and residence in the atmosphere as well as the effects on ocean acidification, carbon saturation, coral mortality, and biodiversity.

Over the long term, large eruptions can release thousands of gigatons of methane from organic-rich sediments. Light δ13C signatures interpreted to represent such a release (Svensen et al., 2009) have been recognized in carbon isotope stratigraphic records at the Permian–Triassic (252 Ma) and Triassic–Jurassic (201 Ma) boundaries, as well as in the Paleogene (56 Ma; Saltzman and Thomas, 2012). The latter represents a well-documented thermal maximum associated with extensive volcanism that accompanied the opening of the North Atlantic Ocean. Reconstructing the volcanic carbon emission record through geologic time and assessing the potential for large releases of reduced carbon from organic sediments is challenging and requires