Climate-change–driven accelerated sea-level rise detected in the altimeter era

• sea level
• acceleration
• climate change
• satellite altimetry

Satellite altimeter data collected since 1993 have measured a rise in global mean sea level (GMSL) of ∼3 ± 0.4 mm/y (1, 2), resulting in more than 7 cm of total sea-level rise over the last 25 y. This rate of sea-level rise is expected to accelerate as the melting of the ice sheets and ocean heat content increases as greenhouse gas concentrations rise. Acceleration of sea-level rise over the 20th century has already been inferred from tide-gauge data (3⇓–5), although sampling and data issues preclude a precise quantification. The satellite altimeter record of sea-level change from TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3 is now approaching 25 y in length, making it possible to begin probing the record for climate-change–driven acceleration of the rate of GMSL change (6). Unlike tide-gauge data, these retrievals sample the open ocean and allow for precise quantitative statements regarding global sea level. However, detecting acceleration is difficult because of (i) interannual variability in GMSL largely driven by changes in terrestrial water storage (TWS) (7⇓–9), (ii) decadal variability in TWS (10), thermosteric sea level, and ice sheet mass loss (11) that might masquerade as a long-term acceleration over a 25-y record, (iii) episodic variability driven by large volcanic eruptions (12), and (iv) errors in the altimeter data, in particular, potential drifts in the instruments over time (13). With careful attention to each of these issues, however, a preliminary satellite-based estimate of the climate-change–driven acceleration of sea-level rise can be obtained. This estimate is useful for understanding how the Earth is responding to warming, and thus better informs us of how it might change in the future.

The satellite altimeter record of GMSL is shown in Fig. 1 (14). These data reflect a recent update to the TOPEX measurements discussed in ref. 15 following earlier empirical work by refs. 16 and 17. The acceleration obtained by fitting a quadratic to this updated 25-y record is 0.097 mm/y². As described in ref. 12, the eruption of Mount Pinatubo in 1991 caused a decrease in GMSL just before the launch of TOPEX, followed by a slow recovery that resulted in an apparent deceleration of sea level of −0.02 ± 0.01 mm/y² over the 25-y record. To isolate the climate-change–driven acceleration, we remove this effect (Fig. 1), which increases the acceleration of the adjusted GMSL record from 0.097 to 0.117 mm/y².

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Fig. 1.

GMSL from the adjusted processing of ref. 15 (blue) and after removing an estimate for the impacts of the eruption of Mount Pinatubo (12) (red), and after also removing the influence of ENSO (green), fit with a quadratic (black). The acceleration (0.084 mm/y²) is twice the quadratic coefficient.

There is considerable interannual variability in the GMSL time series due to changes in TWS, mainly driven by El Niño Southern Oscillation (ENSO) effects (7⇓–9). We use a multivariable empirical orthogonal function (EOF) analysis to isolate the ENSO effects (Fig. 2) and remove them from the GMSL curve. This reduces the GMSL acceleration estimate by 0.033 mm/y², resulting in a final “climate-change–driven” acceleration of 0.084 mm/y². Climate-change–driven in this case means we have tried to adjust the GMSL measurements for as many natural interannual and decadal effects as we can to try to isolate the longer-term, potentially anthropogenic, acceleration––any remaining effects are considered in the error analysis.

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Fig. 2.

ENSO GMSL correction (blue) compared with detrended GMSL (red, Pinatubo effects removed).

We also must consider the impact of errors in the altimeter measurements, especially instrument drift. To assess instrument drift, we examine sea-level differences between altimetry and tide gauges (13) over time. This technique has been used for the last two decades to assess the instrument drift, but not to assess errors in GMSL acceleration estimates. Fig. 3 shows a time series of these differences. Using an AR1 noise model, we find that these differences imply a 1σ uncertainty in the acceleration of the instrument drift of 0.011 mm/y².

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Fig. 3.

Differences between altimeter and tide gauge observed sea level used to estimate the error in the acceleration estimate (13).

Finally, we must also consider the impact of decadal variability in GMSL on acceleration estimates derived from a 25-y record. While estimates of the impact of decadal variability on the 25-y-long time series are difficult to obtain (18), satellite altimetry is far less sensitive to decadal variability than tide-gauge measurements (19) because of its superior global coverage. Estimates of decadal variability from tide-gauge data (18) are uncertain due to their poor geographic sampling and other effects, so estimates must be assembled from measurements of the contributions to GMSL including TWS (10), the cryosphere (11), and thermosteric sea-level change (20). We have removed most of the impact of changes in TWS, but allow 0.01-mm/y² residual impact on the acceleration (based on ref. 10). Estimates of the decadal variability in ice sheet mass loss (11) suggest the impact on acceleration estimates is ∼0.014 mm/y² for a 25-y time series, in the absence of rapid dynamical changes in the ice sheets. The impact of decadal variability in thermosteric sea level was estimated at 0.01 mm/y² using a control run for the National Center for Atmospheric Research (NCAR) Large Ensemble (LE) (21). We also include an estimate from the NCAR LE of the impact of decadal variability in precipitable water in the atmosphere, which can impact GMSL. Therefore, a conservative estimate of the total impact of decadal variability on our acceleration estimate is the root sum square (RSS) of these contributions, which is 0.017 mm/y².

Table 1 shows a summary of the different error estimates. The final error estimate for the climate-change–driven GMSL acceleration is the RSS of the measurement errors (0.011 mm/y²), the error in the Pinatubo correction (0.01 mm/y²), the error in the ENSO correction (0.01 mm/y²), and the errors due to decadal variability (0.017 mm/y²). Their joint consideration yields a final acceleration estimate of 0.084 ± 0.025 mm/y².

We can perform an approximate validation of the altimeter-based GMSL acceleration estimate by examining other satellite and in situ measurements of the components that contribute to the GMSL acceleration. The Gravity Recovery and Climate Experiment (GRACE) mission provides estimates of the cryospheric contributions to the acceleration of sea-level rise, including Greenland, Antarctica, and small ice caps and mountain glaciers (22), although these measurements only start in 2002. As shown in Table 2, Greenland and Antarctica account for most of the observed GMSL acceleration (6). The acceleration of thermosteric sea level was determined from an update to ref. 23. The thermosteric acceleration is small compared with the ice sheets, but on par with the acceleration from mountain glaciers and small ice caps.

Table 2 summarizes estimates of the contribution of these components to the acceleration of GMSL. While the time periods are shorter than covered by the altimetry record, they provide a rough validation of the altimeter-based acceleration estimate. Shortening the altimeter record to match GRACE increases the acceleration, but also significantly increases the error bar. The main consequence of the shorter time periods is the potential influence of interannual and decadal variability. Nevertheless, the agreement between the climate-change–driven acceleration (adjusted for ENSO and Pinatubo effects) observed from 25 y of satellite altimetry and independent acceleration estimates from the components contributing to GMSL is quite good.

Our estimate of the 25-y GMSL acceleration is 0.084 ± 0.025 mm/y² (1σ) after removing the Pinatubo effect and accounting for the impact of ENSO variations. The probability that the acceleration is actually zero is less than 1%. The error includes both the altimeter drift error and the impact of decadal variability. When taken with a rate of sea-level rise of 2.9 ± 0.4 mm/y (epoch 2005.0), the extrapolation of the quadratic gives 654 ± 119 mm of sea-level rise by 2100 relative to 2005, which is similar to the processed-based model projections of sea level for representative concentration pathways 8.5 in the IPCC Fifth Assessment Report (24). Stated alternatively, the observed acceleration will more than double the amount of sea-level rise by 2100 compared with the current rate of sea-level rise continuing unchanged. This projection of future sea-level rise is based only on the satellite-observed changes over the last 25 y, assuming that sea level changes similarly in the future. If sea level begins changing more rapidly, for example due to rapid changes in ice sheet dynamics, then this simple extrapolation will likely represent a conservative lower bound on future sea-level change. In contrast, few potential processes exist to suggest that this estimate is too high. Projections over shorter time frames (25, 50 y, etc.) are therefore likely more reliable, but will also be more sensitive to internal climate variability and volcanic eruptions.

• Copyright © 2018 the Author(s). Published by PNAS.