Satellite observations of global sea levels began in 1993 following the launch of the international TOPEX/Poseidon mission. Since then, climate scientists have observed an average sea level increase of 3 millimeters per year, which is not quite what's expected. On a warming planet, this increase should be accelerating—every year, the sea level rise should be more than what was observed in the previous year.
According to an open-access study published Wednesday in Scientific Reports, the missing acceleration is likely the result of some bad timing. In 1991, a year before the launch of the TOPEX/Poseidon satellite (which was launched in late 1992 and went online in 1993), the second-largest volcanic eruption of the century occurred in the Philippines at Mount Pinatubo, which released 17 megatons of sulfur dioxide in the atmosphere. Sulfur dioxide reacted with water droplets to produce a thin haze of stratospheric sunlight-blocking sulfuric acid all across the globe.
The effect of this volcanic haze was an average temperature decrease of about .5 degrees Celsius, which persisted for at least two years. Global cooling.
Pinatubo-related cooling effects were observed almost immediately and have since been well-documented. Moreover, a 1992 paper by scientists at NASA's Goddard Space Flight Center suggested that the effects of the eruption would likely overwhelm those of global warming, at least temporarily.
The current National Center for Atmospheric Research-led study uses global climate models tweaked to remove the effects of the Pinatubo eruption. As it turns out, if we simulate geo-history such that the eruption never occurred, global sea levels rise at expected rates. That is, they accelerate. More rise every year.
Key to the simulations was evaluating as many models/scenarios as possible in an attempt to account for sea level conditions prior to the start of satellite-based observations. This is a tricky thing as earlier measurements mainly relied on tidal gauges, imperfect tools subject to myriad variations due to location and measurement technique. With a reasonable picture of this pre-satellite period, it became possible to more correctly evaluate the post-eruption conditions when the satellite went online.
"This study shows that large volcanic eruptions can significantly impact the satellite record of global average sea level change," notes study co-author Steve Nerem in a statement. "So we must be careful to consider these effects when we look for the effects of climate change in the satellite-based sea level record."
In the decade following the eruption, two abrupt oceanic changes took place. In the two years of pronounced global cooling following the eruption, ocean temperatures cooled accordingly. It was during this period that the TOPEX/Poseidon satellite began taking altimeter readings of ocean levels. Following the period of cooling, in the early months of altimeter observations, ocean temperatures very quickly recovered, raising ocean levels by up to 7 mm. Through the following decade of observations, this quick rise would have had the effect of masking the normal contributions of climate change. It'd be sort of like trying to take a hearing test at a gun range.
"Given however that no such major eruption has occurred since 1991, a reasonable expectation is also that an accelerated rate of rise may emerge in the near future, particularly as the influences of climate variability and instrument drift reported in previous studies abate," the current study concludes. In other words, we can expect the missing sea level rise acceleration to appear any day now.