Large volcanic eruptions cool the world ocean. In doing so, they temporarily reduce the increase in ocean heat content and the rise in sea level attributed to warming caused by greenhouse-gas emissions.
Global warming is producing a rise in sea level. Observations from tide gauges and satellite altimetry indicate that sea level has been rising by 1.8 millimetres per year since 1950 (ref. 1) and about 3 millimetres per year during the 1990s (ref. 2). The two causes are a thermal expansion of sea water in response to ocean warming, and the input of extra water from the melting of glaciers and ice sheets on land3. But against the background of this overall increase, global mean sea level displays interannual to decadal oscillations of the order of several millimetres. These oscillations have received scant attention to date. Some of them result from changes in ocean heat content associated with internal perturbations of the ocean–atmosphere system, such as the El Niño–Southern Oscillation and Pacific Decadal Oscillation4. But other processes, probably related to the ‘forcing’ effects of natural climate variation, also play a role.
On page 74 of this issue, Church et al.5 use climate simulations to reveal the effects of volcanic eruptions on sea level between 1890 and 2000. The simulations take account of anthropogenic forcing that stemmed from greenhouse gases, aerosols and ozone, and the natural climate forcing resulting from changes in volcanic activity and the input of solar radiation. These forcings affect the oceans through warming (or cooling) the upper ocean, leading to an increase (decrease) of ocean heat content and hence an increase (decrease) in sea level through ocean thermal expansion (contraction). Church et al. find that, in the first few months after large volcanic eruptions, there is a fall of several millimetres in global mean sea level. This is followed by a slow increase, lasting for a decade or more, towards the pre-eruption state.
The link here is that large volcanic eruptions inject particles and gases into the atmosphere, in particular sulphur gases that are converted into sulphate aerosols in the stratosphere, the layer of Earth's atmosphere immediately above the lowest layer, the troposphere. Their dominant effect is to increase the fraction of incident radiation reflected by the planet, and hence reduce the amount of solar energy reaching Earth's surface. The corresponding net surface-air cooling and its consequences on weather have been much studied6, unlike the impact on ocean heat content and sea level.
Church et al.5 show that, because of the reduction of the net solar flux at the ocean surface, volcanic eruptions induce an immediate cooling of the surface layers, and so a decrease in heat content and sea level. The predicted abrupt fall in ocean heat content is well correlated with observations7. Although surface-air temperature recovers within a few years, the cooling effects on the ocean persist for at least a decade. This is because of the large heat capacity of the oceans compared with that of the atmosphere and the slow redistribution of heat by the ocean circulation5,8.
Observational analyses of historical ocean temperatures show a net warming of the oceans since 1950, contributing about 85% of the total increase in heat content of the whole Earth system7 and consistent with the current imbalance between the energy absorbed and emitted by the planet9. Model studies suggest that most of this ocean warming results from human activities and the associated increase in levels of greenhouse gases10. However, during the past several decades, large volcanic eruptions — Mt Agung, Indonesia (1963), El Chichon, Mexico (1982) and Mt Pinatubo, Philippines (1991) — have temporarily reduced the anthropogenic ocean warming. Because the effects of subsurface ocean cooling can persist for one to several decades, they have masked at least part of the accelerating increase in sea level.
The simulations5 indicate that the eruption of Mt Pinatubo in 1991 (Fig. 1) produced a drop of some 6 mm in sea level within about a year, which was followed by a slow rise of about 0.5 mm yr−1 over the next decade or more. Thus, about 0.5 mm yr−1 of the steeper sea-level rise resulting from thermal expansion, estimated from ocean temperature data over the past decade (about 1.5 mm yr−1, compared with the 0.4 mm yr−1 mean rate of the past 50 years)11, may reflect a recovery from the Mt Pinatubo eruption. This in turn may explain part of the higher rate of sea-level rise observed by satellite altimetry since early 1993 (3 mm yr−1, compared with 1.8 mm yr−1 recorded by historical tide gauges since 1950; ref. 1). Although it remains unclear whether sea-level rise over the past decade indicates an accelerating trend, the study by Church et al.5 suggests that part of it (although not all) can be explained by natural variability on interannual to decadal timescales.
Climate-model predictions indicate that sea level will continue to rise in the coming decades, and even centuries, owing to thermal expansion of the ocean in response to anthropogenic warming3. But Church et al.5 clearly demonstrate that large volcanic eruptions can (partially and temporarily) mask that effect. Their study is one step towards a better understanding of the sea-level record — which is essential if we are to improve projections of sea-level rise, and prepare for the impact of that rise on vulnerable coastal regions and island nations.
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Quaternary International (2012)
Energy & Environment (2008)