What is glacial isostatic adjustment (GIA), and why do you correct for it?
The correction for glacial isostatic adjustment (GIA) accounts for the fact that the ocean basins are getting slightly larger since the end of the last glacial cycle. GIA is not caused by current glacier melt, but by the rebound of the Earth from the several kilometer thick ice sheets that covered much of North America and Europe around 20,000 years ago. Mantle material is still moving from under the oceans into previously glaciated regions on land. The effect is that currently some land surfaces are rising and some ocean bottoms are falling relative to the center of the Earth (the center of the reference frame of the satellite altimeter). Averaged over the global ocean surface, the mean rate of sea level change due to GIA is independently estimated from models at -0.3 mm/yr (Peltier, 2001, 2002, 2009; Peltier & Luthcke, 2009). The magnitude of this correction is small (smaller than the ±0.4 mm/yr uncertainty of the estimated GMSL rate), but the GIA uncertainty is at least 50 percent. However, since the ocean basins are getting larger due to GIA, this will reduce by a very small amount the relative sea level rise that is seen along the coasts. To understand the relative sea level effects of global oceanic volume changes (as estimated by the GMSL) at a specific location, issues such as GIA, tectonic uplift, and self attraction and loading (SAL, e.g., Tamisiea et al., 2010), must also be considered. For more discussion on the GMSL and how it relates to tide gauges, see the GMSL and tide gauge FAQs.
There are many different scientific questions that are being asked where GMSL measurements can contribute. We are focused on just a few of these:
- How is the volume of the ocean changing?
- How much of this is due to thermal expansion?
- How much of this is due to addition of water that was previously stored as ice on land?
In order to answer these questions, we have to account for the fact that the ocean is actually getting bigger due to GIA at the same time as the water volume is expanding. This means that if we measure a change in GMSL of 3 mm/yr, the volume change is actually closer to 3.3 mm/yr because of GIA. Removing known components of sea level change, such as GIA or the solid earth and ocean tides, reveals the remaining signals contained in the altimetry measurement. These can include water volume changes, steric effects, and the interannual variability caused by events such as the ENSO. We apply a correction for GIA because we want our sea level time series to reflect purely oceanographic phenomena. In essence, we would like our GMSL time series to be a proxy for ocean water volume changes. This is what is needed for comparisons to global climate models, for example, and other oceanographic datasets.
There are other science questions that researchers are investigating, such as the effect of ocean volume change on local sea level rates, but this is not the focus of our research. When studying local sea level rates, which is important for policy planning, one definitely needs to account for the fact that in areas where GIA is causing an uplift, this somewhat mitigates the ocean volume change. This is being taken into account in these investigations. Also note that GIA can cause subsidence far away from the source of the old ice sheet, and that there are even larger cases of uplift and subsidence unconnected to GIA that are 10-20 times larger. For example, large parts of New Orleans are subsiding more than 10 mm/year—three times the current rate of GMSL—and so they see a much higher rate of sea level rise that has nothing to do with climate change.
Prior to release 2011_rel1, we did not account for GIA in estimates of the global mean sea level rate, but this correction is now scientifically well-understood and is applied to GMSL estimates by nearly all research groups around the world. Including the GIA correction has the effect of increasing previous estimates of the global mean sea level rate by 0.3 mm/yr.