Challenged: 3.2 mm/year sea level rise since 1993
By guest author Kenneth Richard
Source data studies indicate recent only 0.8 – 1.6 mm/year sea level rise.
According to the IPCC (2013), sea levels rose at a rate of 1.7 mm/year (6.7 inches per century) for the 1901 to 2010 period, but 3.2 mm/year (12.6 inches per century) during the 18-year period between 1993 to 2010, a rate similar to that obtained from 1920 to 1950.
If sea levels truly were rising at rates of 3.2 mm/year between 1993 and 2010 as the IPCC (2013) claims with high confidence, then the thermal and ablation sources for that sea level rise should at least somewhat closely add up to 3.2 mm/year for the same period. But if the sources for the sea level rise as determined by published studies only reach between 0.8 and 1.6 mm/year when added together – a quarter to half of the 3.2 mm/year claimed – then that suggests that there is something seriously questionable about the 3.2 mm/year sea level rise estimates for 1993 to 2010 obtained by satellite altimetry.
What follows is an abridged list of recent papers showing that sea level rise estimations claimed by the IPCC are likely highly exaggerated.
“It is very likely that the global mean rate was 1.7 mm yr–1 between 1901 and 2010 for a total sea level rise of 0.19 m. Between 1993 and 2010, the rate was very likely higher at 3.2 mm yr–1; similarly high rates likely occurred between 1920 and 1950.”
Problematically, in recent decades the source contribution estimates for recent decades from published studies add up to half (or less) of this claimed rate.
According to recent estimates, the contribution to sea level rise from thermal expansion was 0.64 mm/yr (2.5 inches per century) for the period between 2005-2013 (Llovel et al., 2014, see below). The contribution from the Greenland (GIS) and Antarctic (AIS) ice sheets was 0.59 mm/yr (2.3 inches per century) for the 1992-2011 period (Shepherd et al., 2012). The contribution from global glaciers and ice caps that are not the GIS or AIS was 0.41 mm/yr (1.6 inches per century) for 2003-2010 (Jacob et al., 2012).
Llovel et al., 2014
Over the entire water column, independent estimates of ocean warming yield a contribution of 0.77 ± 0.28 mm yr−1 in sea-level rise … the deep ocean (below 2,000 m) contributes −0.13 ± 0.72 mm yr−1 to global sea-level rise [0.64 mm/yr total].
Shepherd et al., 2012
Since 1992, the polar ice sheets [Antarctica and Greenland] have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.
Jacob et al., 2012
Here we show that GICs [glaciers and ice caps], excluding the Greenland and Antarctic peripheral GICs, lost mass at a rate of 148 ± 30 Gt yr−1 from January 2003 to December 2010, contributing 0.41 ± 0.08 mm yr−1 to sea level rise.
These sources add up to about 1.6 mm/year, which is the equivalent of a little over 6 inches per century. Interestingly, this is the same or slightly less than the rate for the entire 1901 to 2010 period (1.7 mm/year, IPCC), meaning that sea level rise has not accelerated in recent decades according to the summation of source estimates.
Another recent estimate of the sea level contribution from the Antarctic ice sheet (Zwally et al., 2015, see below) for 1992-2008 has East Antarctica ice mass gains exceeding the losses from West Antarctica, indicating a net gain in ice mass for the continent as a whole and a negative sea level contribution of -0.23 mm/year (-0.9 of an inch per century) for 2003 to 2008.
Mass changes of the Antarctic ice sheet impact sea-level rise as climate changes, but recent rates have been uncertain. Ice, Cloud and land Elevation Satellite (ICESat) data (2003–08) show mass gains from snow accumulation exceeded discharge losses by 82 ± 25 Gt a–1, reducing global sea-level rise by 0.23 mm a–1. European Remote-sensing Satellite (ERS) data (1992–2001) give a similar gain of 112 ± 61 Gt a–1.
“The good news is that Antarctica is not currently contributing to sea level rise, but is taking 0.23 millimeters per year away,” Zwally said.
Along with the sea level decline equivalent of -0.23 mm/year for 2003-2008 due to ice gains of +82 Gt per year between 2003 and 2008, we can compute the sea level loss equivalent for 1992-2001 as -0.31 mm/yr, as 1992-2001 had a higher rate of mass gain (+112 Gt/year) for the Antarctic ice sheet as a whole. Averaged together, the Zwally et al. (2015) study indicates that Antarctica took away -0.28 mm/yr (-1.1 inches) from sea levels over 1992-2008.
Using this -0.28 mm/year estimate for Antarctica’s ice melt contribution instead of the Shepherd et al. (2012) estimate of 0.19 mm/year (0.7 of an inch per century), but yet continuing to use the Shepherd et al. (2012) estimate for Greenland’s ice sheet contribution (0.4 mm/year) reduces the polar ice sheet contribution from 0.59 mm/year to 0.12 mm/year, which is a sea level contribution of half an inch (0.5) per century from both polar ice sheets combined.
This change of -0.47 mm/year effectively reduces the 1.6 mm/year source estimate from above to between 1.1 and 1.2 mm/year, a little over 4 inches per century.
The template for the above source analysis of sea level rise contributions was provided in the journal Science by Cabanes et al., 2001. Here is their summary for the last few decades of the 20th century.
The third assessment report of the Intergovernmental Panel on Climate Change (IPCC) estimates the various factors that have contributed to the 20th century sea level rise. The largest contribution (0.7 mm/year sea level rise) arises from thermal expansion due to warming of the oceans that mainly occurred since the 1950s. Melting of continental glaciers produces 0.2 to 0.4 mm/year sea level rise. Estimated Greenland and Antarctica mass imbalance (accounting for a long-term readjustment since Last Glacial Maximum plus a climate-related response) contributes –0.2 to 0.6 mm/year. The least certain contribution is the change in terrestrial water storage that results partly from human activities, which is in the range of –1.1 to + 0.4 mm/year with a median value of −0.35 mm/year (i.e., corresponding to sea level drop). The sum of these contributions ranges from −0.8 to 2.2 mm/year, with a median value of 0.7 mm/year.
Notice that in this analysis, “terrestrial water storage” is included in the contribution budget, and this amounted to -0.35 mm/year (with a range of “–1.1 to + 0.4 mm/year”) for the late 20th century. A more recent estimate is apparently not available, but it is likely a fair assumption that these parameters for terrestrial water storage haven’t changed significantly during the first decade of the 21 century compared to the last decades of the 20th. Therefore, it would be reasonable to include this value among the source contributions to recent sea level rise. This would effectively reduce the 1.1 to 1.2 mm/year rate described above (using Zwally et al., 2015) to about 0.8 mm/year, which is very similar to the 0.7 mm/year calculated for the last decades of the 20th century by Cabanes et al., 2001. A rate of 0.8 mm/year is equivalent to about 3 inches per century.