by Judith Curry
Three new papers to discuss on the topic of natural internal variability and sea level rise.
Contribution of the pacific decadal oscillation to global mean sea level trends
B.D. Hamlington, R.R. Leben, M.W. Strassburg, R.S. Nerem, K.-Y. Kim
Understanding and explaining the trend in GMSL has important implications for future projections of sea level rise. While measurements from satellite altimetry have provided accurate estimates of GMSL, the modern altimetry record has only now reached twenty years in length, making it difficult to assess the contribution of decadal to multi-decadal climate signals to the global trend. Here, we use a sea level reconstruction to study the twenty-year trends in sea level since 1950. In particular, we show that the Pacific Decadal Oscillation (PDO) contributes significantly to the twenty-year trends in GMSL. We estimate the PDO contribution to the GMSL trend over the past twenty years to be approximately 0.49 ± 0.25 mm/year, and find that removing the PDO contribution reduces the acceleration in GMSL estimated over the past sixty years.
[link] to abstract.
Uncertainty in future regional sea level rise due to internal climate variability
Aixue Hu and Clara Deser
Sea level rise (SLR) is an inescapable consequence of increasing greenhouse gas concentrations, with potentially harmful effects on human populations in coastal and island regions. Observational evidence indicates that global sea level has risen in the 20th century, and climate models project an acceleration of this trend in the coming decades. Here we analyze rates of future SLR on regional scales in a 40-member ensemble of climate change projections with the Community Climate System Model Version 3. This unique ensemble allows us to assess uncertainty in the magnitude of 21st century SLR due to internal climate variability alone. We find that simulated regional SLR at mid-century can vary by a factor of 2 depending on location, with the North Atlantic and Pacific showing the greatest range. This uncertainty in regional SLR results primarily from internal variations in the wind-driven and buoyancy-driven ocean circulations.
Citation: Hu, A. and C. Deser, 2013: Uncertainty in future regional sea level rise due to internal climate variability. Geophys. Res. Lett., 40, 2768-2772, , doi:10.1002/grl.50531. [link] to complete paper
Australia’s unique influence on global sea level in 2010–2011
John T. Fasullo, Carmen Boening, Felix W. Landerer, R. Steven Nerem
In 2011, a significant drop in global sea level occurred that was unprecedented in the altimeter era and concurrent with an exceptionally strong La Niña. This analysis examines multiple data sets in exploring the physical basis for the drop’s exceptional intensity and persistence. Australia’s hydrologic surface mass anomaly is shown to have been a dominant contributor to the 2011 global total, and associated precipitation anomalies were among the highest on record. The persistence of Australia’s mass anomaly is attributed to the continent’s unique surface hydrology, which includes expansive arheic and endorheic basins that impede runoff to ocean. Based on Australia’s key role, attribution of sea level variability is addressed. The modulating influences of the Indian Ocean Dipole and Southern Annular Mode on La Niña teleconnections are found to be key drivers of anomalous precipitation in the continent’s interior and the associated surface mass and sea level responses.
Citation: Fasullo, J.T., C. Boening, Landerer, F. W., Nerem, R. S., (2013). Australia’s unique influence on global sea level in 2010–2011. Geo. Res. Lett., DOI: 10.1002/grl.50834. [abstract]
Fasullo’s paper has received substantial media attention, see especially the interview with Fasullo in EENews.
For context, I provide excerpts from the ‘leaked’ final draft IPCC AR5 Summary for Policy Makers:
Global mean sea level has risen by 0.19 [0.17 to 0.21] m over the period 1901−2010 estimated from a linear trend , based on tide gauge records and additionally on satellite data since 1993. Based on proxy and instrumental data, it is virtually certain that the rate of global mean sea level rise has accelerated during the last two centuries. The current centennial rate of global mean sea level rise is unusually high in the context of centennial-scale variations over the last two millennia (medium confidence).
It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm yr–1 between 1901 and 2010 and 3.2 [2.8 to 3.6] mm yr–1 between 1993 and 2010. Tide-gauge and satellite altimeter data are consistent regarding the higher rate of the latter period. It is likely that similarly high rates 3 occurred between 1920 and 1950.
Using the IPCC’s numbers, the acceleration of sea-level rise that is being attributed to AGW is 3.2-1.7 = 1.5 mm/yr, with an inferred range for the amplification of 0.9 to 2.1 mm/yr.
The significance of the Hamblingdon et al. paper is that the attribution of 0.49 mm/yr to the PDO accounts for about 25-50% of the sea level rise amplification.
Fasullo comments on the 2011 drop of 7 mm, which is not included in the period considered by th AR5. Regarding sea level subsequent to 2011, Fasullo is quoted in EENews: In the last two years, he added, researchers have noticed a sharper-than-normal increase in sea-level rise, from the 3 mm yearly to 10 mm. “It’s never gone up that quickly in our observed record,” he said.
The significance of the Fasullo et al. paper is that there is high-amplitude interannual variability in sea level rise, particularly since 2010 that is not adequately understood. I spotted this also on the Wikipedia page re the 1998 El Nino: The strong 1997–1998 El Niño caused regional and global sea-level variations, including a temporary global increase of perhaps 20 mm.
Our confidence in all of this rests on our confidence in the satellite derived estimates of global mean sea level.
So what does the future hold? In the near term, with the cool phase of the PDO, we would expect more La Nina’s (sea level drop) and fewer El Nino’s.