Thread by @valmasdel

🌋 What do we know of the climate effects of large explosive volcanic eruptions?

This thread is based on the 2021 @IPCC_CH, climate report, on the physical science basis of climate change
ipcc.ch/report/AR6/WG1

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Major volcanic eruptions can inject sulphur dioxide gas into the stratosphere, leading to the formation of reflective sulphate aerosols that can cool the planet during up to a few years by reflecting some incoming solar radiation.
Figure from volmip.org
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Major eruptions drive a range of climate system responses for several years, depending upon whether the eruption occurs in the tropics (with stratospheric aerosol dispersion in both hemispheres), or the extra-tropics (dispersion in the hemisphere of eruption).
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The history and climatic effects of volcanic activity have been traced through historical records, geological traces, and observations of major eruptions by aircraft, satellites, and other instruments.
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The climatic response also depends on the effective injection height, sulphur mass injected, and the time of year of the eruption.
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These factors determine the total mass, lifetime, and optical properties of volcanic aerosols in the stratosphere, and influence the stratospheric aerosol optical depth (sAOD).
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The radiative forcing (which measures the effect on the Earth's radiative budget) from these aerosols is assessed to be -20 ± 5 W/m2 per unit sAOD.
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Investigating the climate response to volcanic eruptions builds on the study of multiple events from the past centuries, in order to obtain a sufficient signal to noise ratio against the background of internal variability.
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Since the 1980s, aerosols have increasingly been integrated into comprehensive modelling studies of transient climate evolution, including through treatment of volcanic forcing.
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The radiative forcing of major volcanic eruptions has been considered in the first @IPCC_CH Assessment Report in 1990. In subsequent assessments, the effect of smaller eruptions has also been considered.
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So, what do we know about volcanic eruptions of the past 2500 years?
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Advances in analyses of sulphate records from Greenland and Antarctic ice cores have resulted in improved dating and completeness of stratospheric aerosol optical depth over the past 2500 years, a more uncertain extension back to 10 000 years...
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The period between successive large volcanic eruptions (with a radiative forcing larger than -1 W/m2) ranges from 3-130 years, with an average of 43±7.5 years between such eruptions over the past 2500 years. The most recent such eruption was that of Mt Pinatubo in 1991.
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During the last 2500 years, 8 larger eruptions (with a radiative forcing stronger than -5 W/m2) have occurred, notably Tambora (in 1815) and Samalas (in 1257).
Century-long periods that lack such eruptions occurred once every 400 years on average.
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The stratospheric aerosol optical depth averaged over 950-1250 CE was lower than for the period 1450-1850, and similar to the period 1850-1900. The 13th century was the most volcanically active, with 4 eruptions larger than the 1991 Pinatubo one.
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Volcanic forcing is now regarded as the dominant natural driver of forced variability in preindustrial global surface temperature.
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Before the global warming that began around 1850, a slow cooling in the northern hemisphere from roughtly 1450 to 1850 is recorded in paleoclimate archives.
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While this cooling primarily driven by an increased number of volcanic eruptions shows regional differences, the subsequent warming over the past 150 years (due to human influence) exhibit a global coherence that is unprecedented in the past 2000 years.
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Natural forcings such as past changes in solar irradiance and historical volcanic eruptions are represented in model simulations covering the past 2000 years
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The last generation climate models (contributing to CMIP6, the 6th phase of the Climate Model Intercomparison Project) reproduce surface temperature variations over the past millennium, including the cooling that follows periods of intense volcanism.
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Over the past millennium, and especially since around 1300 CE, simulated global surface temperature anomalies are well within the uncertainty of reconstructions,
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except for some short periods immediately following large volcanic eruptions for which different forcing datasets disagree.
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Before the year 1300, larger disagreements between models and temperature reconstructions can be expected because forcing and temperature reconstructions are increasingly uncertain further back in time (specific causes have not yet been identified conclusively).
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From around 860 CE to 1840 CE, global and north hemisphere temperature variations have been attributed mostly to volcanic forcing, with solar forcing playing a minor role. In contrast, the effect of forcings was not detectable in the southern hemisphere.
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Volcanic forcing is now regarded as the dominant natural driver of forced variability in preindustrial global surface temperature.
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Repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a multi-decadal to centennial global scale cooling trend, via a decline in mixed-layer oceanic heat content.
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Let's now move to the period since 1750.
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Since 1750, changes in the drivers of the climate system are dominated by the warming influence of ↗️ in atmospheric greenhouse gas concentrations, and a cooling influence from aerosols, both resulting from human activities; with a negligible long-term influence of volcanoes
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There were several large volcanic eruptions between 1750 and 1850.
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The net radiative forcing from changes in solar activity and volcanic activity is estimated to be small (less than 0.1 W/m2) in 1850-1900, compared to around 1750, while there was already a net anthropogenic forcing of 0 to 0.3 W/m2.
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The average magnitude and variability of volcanic aerosols since 1900 has not been unusual compared to at least the past 2500 years.
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Strong volcanic eruptions with periods of strong negative radiative forcing lasting 2-5 year in duration occurred in the late 19th and early 20th century,
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followed by a relatively quiescent period between around 1920 and 1960, and three strong eruptions occurred in 1963, 1982 and 1991, with a series of small-to-moderate eruptions since 2000.
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Strong volcanic eruptions can lead to temporary drops in global surface temperature lasting 2–5 years, with peak cooling between 0.1 and 0.5°C during the first year after the eruption.
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For the historical period, results for CMIP6 suggest that the qualitative history of surface temperature increase is well reproduced, including the increase in warming rates beginning in the 1960s and the temporary cooling that follows large volcanic eruptions.
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The radiative feedbacks governing the global surface temperature response to volcanic eruptions can differ from those governing long-term global warming
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Estimates of equilibrium climate sensitivity based on the response to volcanic eruptions agree with other lines of evidence, but do not provide direct estimates.
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Many studies have focused on the temporary slowdown in the rate of global surface temperature increase observed over 1988-2012, compared to 1951-2012.
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It was induced by internal variability (particularly Pacific Decadal Variability) and variations in solar irradiance and volcanic forcing that partly offset the anthropogenic warming over this period
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Global ocean heat content continued to increase throughout this period, indicating continuous warming of the entire climate system. Hot extremes also continued to increase during this period over land.
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Even in a continually warming climate, periods of reduced and increased trends in global surface temperature at decadal time scales will continue to occur in the 21st century
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The occurrence of volcanic eruptions can alter the water cycle for several years.
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Volcanic eruptions affect regional climate through their spatially heterogeneous effect on the radiative budget as well as through triggering dynamical responses
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Volcanic eruptions generally result in decreased global precipitation for up to a few years following the eruption, with climatologically wet regions drying, and climatologically dry regions wetting, which is opposite to the response under global warming.
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Changes in monsoon circulations occur, with a general weakening of tropical precipitation, and a decrease of extreme precipitation over global monsoon regions.
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Monsoon precipitation in one hemisphere tends to be enhanced by eruptions occurring in the other hemisphere, or reduced if they occur in the same hemisphere
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Volcanic eruptions of the past century caused detectable streamflow decreases (in north South America, central Africa, high latitude Asia, wet tropical – subtropical regions), and increases (in SW North America, and southern South America).
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Volcanic forcing has been shown to contribute in part to the cold phases of the Atlantic Multi-decadal Variability (AMV), can influence part of the NAO/NAM variability, and the annual SAM index.
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Volcanic eruptions have been linked to the onset of El Niño followed by La Nina, althought this connection remains contentious.
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There have been detectable increases in Arctic sea ice extent in response to past volcanic eruptions, with sea-ice / ocean feedbacks prolonging cooling after volcanic aerosols are removed. Recent studies indicate that explosive volcanism can drive glacier advances.
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The ocean buffers the atmospheric response to volcanic eruptions through temporary (<10 years) surface and subsurface cooling during large volcanic eruptions in the upper ocean, which reduces the magnitude of the peak atmospheric temperature anomaly.
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However, while explosive volcanic eruptions only disturb the Earth’s radiative budget and surface fluxes for a few years, the ocean preserves a heat content anomaly in the upper 500 m, with implications for thermosteric sea level, many years after the eruption.
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This in turn affects the atmosphere through air-sea fluxes, with surface conditions returning to normal after decades, or at the centennial scale in the case of repeated eruptions.
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The temporary climate consequences of major volcanic eruptions can also temporarily affect carbon sinks (eg cooler ocean, larger sink), and thus the airborne fraction of anthropogenic CO2 emissions, or reduce methane wetland emissions due to drier tropical conditions.
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There is evidence for large impacts upon contemporary society from eruptions such as 1257 Samalas and 1850 Tambora, the latter resulting in the “year without a summer”, with multiple harvest failures across the northern hemisphere.
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Let us now look into the future.
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Future scenario-based climate simulations account for projected changes in solar irradiance and the long-term mean background forcing from volcanoes (in addition to contrasted human influence), but not for (unpredictable) individual volcanic eruptions.
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Based on paleoclimate and historical evidence, it is likely that at least one large explosive volcanic eruption would occur during the 21st century.
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A sequence of large explosive volcanic eruptions within decades has occurred in the past, causing substantial global and regional climate perturbations over several decades. Such events cannot be ruled out in the future.
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By exploring possible volcanic futures under an intermediate emission scenario, it has been demonstrated that inclusion of time-varying volcanic forcing may enhance climate variability on annual to decadal timescales.
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Clustered eruptions would have substantial impact upon the global surface temperature throughout this century.
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For the near-term, the central estimate of crossing 1.5°C of global warming (for a 20-year period) occurs in the early 2030s, assuming no major volcanic eruption.
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A factor that could substantially alter projections in the near-term would be the occurrence of a large explosive volcanic eruption, or even a decadal to multi-decadal sequence of small-to-moderate volcanic eruptions as witnessed since 2000 CE.
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An eruption similar to Mount Pinatubo (1991) is expected to cause substantial northern hemisphere cooling, peaking between 0.1 and 0.4°C and lasting for 3 to 5 years.
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Although temporary, close to pre-industrial temperatures could be experienced globally for a few years after a 1257 Samalas-sized eruption.
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The central estimate of crossing 1.5°C of global warming (for a 20-year period) occurs in the early 2030s, assuming no major volcanic eruption.
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A factor that could substantially alter projections in the near-term would be the occurrence of a large explosive volcanic eruption, or even a decadal to multi-decadal sequence of small-to-moderate volcanic eruptions as witnessed since 2000 CE.
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In addition to effects on the water cycle described above, regional drought events may be enhanced by the occurrence of volcanic eruptions in addition to the effect of human-caused global warming.
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A major eruption would reduce global precipitation, especially over land, for one to three years, alter the global monsoon circulation, modify extreme precipitation and change many climatic impact-drivers.
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Considerable climate prediction skills can be attributed to volcanic eruptions.
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Monitoring volcanic eruptions is of course critical for early warning systems, related to local and regional hazards, associated tsunamis, atmospheric transport of ashes and harmful gases...
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Monitoring stratospheric effects of major eruptions is also crucial to assess stratospheric volcanic aerosols, and update near-term climate predictions, to inform risk management
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This illustrates the role of natural drivers in modulating human-caused climate changes in the near-term and at regional scales, and why it is important to take into account the eventuality of major volcanic eruptions in planning for the full range of possible changes.
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This thread has been prepared a few days after a violent eruption of Hunga Tonga, leading to widespread damage and urgent need for immediate aid, with intense ongoing scientific activities to assess from satellite data the potential implications of this eruption.

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