Big Oops. Sea Level Rising Faster than Thought

I hate it when this happens.

Harvard University:

The acceleration of global sea level change from the end of the 20th century through the last two decades has been significantly swifter than scientists thought, according to a new Harvard study.

The study, co-authored by Carling Hay, a postdoctoral fellow in the Department of Earth and Planetary Sciences (EPS), and Eric Morrow, a recent Ph.D. graduate of EPS, shows that calculations of global sea-level rise from 1900 to 1990 had been overestimated by as much as 30 percent. The report, however, confirms estimates of sea-level change since 1990, suggesting that the rate of change is increasing more rapidly than previously understood. The research is described in a Jan. 14 paper in Nature.

“What this paper shows is that sea-level acceleration over the past century has been greater than had been estimated by others,” Morrow said. “It’s a larger problem than we initially thought.”

“Scientists now believe that most of the world’s ice sheets and mountain glaciers are melting in response to rising temperatures,” Hay added. “Melting ice sheets cause global mean sea level to rise. Understanding this contribution is critical in a warming world.”

Previous estimates had placed sea-level rise at between 1.5 and 1.8 millimeters annually in the 20th century. Hay and Morrow, however, suggest that from 1901 until 1990, the figure was closer to 1.2 millimeters per year. However, everyone agrees that global sea level has risen by about 3 millimeters annually since that time.

“Another concern with this is that many efforts to project sea-level change into the future use estimates of sea level over the time period from 1900 to 1990,” Morrow said. “If we’ve been overestimating the sea-level change during that period, it means that these models are not calibrated appropriately, and that calls into question the accuracy of projections out to the end of the 21st century.”

News story here, analysis from Stefan Rahmstorf below the fold:

AP via Washington Post:

The current sea level rise rate — which started in 1990 — is 2.5 times faster than it was from 1900 to 1990, according to a study published Wednesday in the journal Nature. Scientists say that faster pace of sea level rise is from melting ice sheets in Greenland and West Antarctica and shrinking glaciers, triggered by man-made global warming.

“We’re seeing a significant acceleration in the past few decades,” said study lead author Carling Hay, a geophysical researcher at Harvard University. “It’s concerning for cities along the U.S. East Coast” where water levels are rising even faster than the world average.

“It’s definitely something that can’t be ignored,” Hay said.

Previous research said that between 1900 and 1990, the seas rose about two-thirds of an inch a decade. The new study recalculates the 1900-1990 rate to less than half an inch a decade.

Old and new research both say that since 1990 seas are rising at about 1.2 inches a decade.

While hundreds of tide gauges around the world have been measuring sea levels since 1900, they have mostly been in Europe and North America with few in the polar regions or the middle of the oceans, Hay said. So past estimates of 20th century sea level rise gave an incomplete picture of the global effect, said study co-author Jerry Mitrovica, a geophysics professor at Harvard.

Stefan Rahmstorf in RealClimate:

All curves show the well-known modern sea level rise, but the exact extent and time evolution of the rise differ somewhat. Up to about 1970, the new (top of page) reconstruction of Hay et al. runs at the top of the existing uncertainty range. For the period from 1880 AD, however, it shows the same total increase as the current favorites by Church & White. Starting from 1900 AD it is about 25 mm less. This difference is at the margins of significance: the uncertainty ranges overlap.

It is also interesting to compare the rates of sea-level rise.

 

The graph shows that the rates vary over time and also differ between the curves. All reconstructions agree on one point: the rate of rise in the last two decades (about 3 cm per decade) is the highest on record. Hay et al. find that the acceleration of sea-level rise since 1900 AD is larger than in previous reconstructions, but it has been generally questioned whether the quadratic acceleration (derived from a parabolic fit) is a useful number in cases where a parabola doesn’t fit the data well (Rahmstorf and Vermeer 2011, Foster and Brown 2014). Taking a step back, in my view the “big picture” on acceleration is that we have moved from a stable preindustrial sea level to one now rising at 3 mm/year (see Fig. 1 here). The differences between the quadratic acceleration numbers come from differences in the decadal to multidecadal variability in the curves which I don’t consider very robust (we have shown in Rahmstorf et al. 2012 how strongly these can be affected by a small amount of “noise” in the sea-level data).

Why are there at all different reconstructions of the global sea level history? The reason lies in the challenge to calculate global sea level as accurately as possible from a suboptimal data base. Different research groups have developed different approaches for this.

The data problem looks like this:

• Tide gauge measurements are not available in sufficient number (especially in earlier times) and not distributed evenly over the oceans: the Northern Hemisphere, for example, is strongly over-represented and tide gauge stations are located along the coasts.
• Many of the time series have data gaps.
• Tide gauges (unlike satellites) measure sea level relative to the land, so these data are ‘contaminated’ by land uplift or subsidence.

A particular challenge is posed by the positioning of the gauges along the coasts, because coastal sea level can be affected by local effects such as the wind piling up water against the shore. Variability in the prevailing winds (which can extend over decades, England et al. 2014) will therefore lead to variability in the water level along the coasts – but of course we know that the wind cannot change global sea level at all as it merely redistributes the water. Nevertheless such variability induced by winds or currents may give a false impression of global sea level fluctuations in analyses of tide gauge data.

The new reconstruction of Hay et al. is an important addition to the body of sea-level work, coming from top experts. But is it better than previous ones? Which of the curves shown is “the best” is not easy to assess. No one knows the exact true sea-level evolution – so we have to consider what methodology is likely to be the most appropriate to cope with the challenges mentioned. I hope that the authors and other experts might stop by here at Realclimate for a discussion of the advantages and drawbacks of the different methods.

The until now widely favored method of reconstruction is that of Church & White (2006 and updated 2011). It uses the satellite data of sea level to determine the typical variability patterns of the sea surface and thus to establish the link between the locally measured tide gauge values and the global sea level. The big advantage is that one does not need questionable assumptions to extrapolate from the measurements on the coasts into the open ocean, but that empirical data on the actual relationship are used. The disadvantage is that unfortunately the satellite data exist only for about twenty years. This method thus relies on the relationship between sea level on the coast and in the rest of the ocean having remained essentially unchanged.

The new reconstruction of Hay et al. uses statistical methods for dealing with incomplete data which have already proven their worth in other applications. In addition, it also uses knowledge about the physics of sea level rise: it determines the components of the global sea-level rise (e.g. the contribution from ice melt in Greenland and Antarctica) taking into account the knowledge about the spatial pattern, the so-called ‘fingerprint’ associated with each of these components. On the other hand, it does not explicitly take into account the specific patterns of natural variability caused by winds or currents that can masquerade as a false global signal (as described above).

Hay et al perform a test in which they take their reconstruction as “truth” and see how well the method of Church & White performs in reproducing it. They find it to be biased high, although the obtained sea level trend in this case is lower than in the real Church & White reconstruction and fully encompasses the hypothetical “true” range. Although this is an important test, it is thus not entirely conclusive, also considering that it was performed just on one particular sea level pattern (that reconstructed by Hay et al.) so it would be premature to conclude that the method is biased high in general.

To sum up, in my view the strength of the method of Hay et al. is that it uses the expected “fingerprints” of the global warming signal, while the strength of Church & White is to take into account the empirical patterns of natural variability. Ideal would of course be a combination of both, and this could be the next step for further research. Ultimately, it is not clear down to what level of accuracy we will ever know the sea level evolution over the past hundred years or so. But for practical purposes, I don’t think it matters whether the rise from 1900 AD has been 3 centimetres more or less. I do not think this changes our outlook for future sea-level rise in any significant way.