What is the definition of global mean sea level (GMSL) and its rate?

The term "sea level" has many meanings depending upon the context. In satellite altimetry, the measurements are made in a geocentric reference frame (relative to the center of the Earth). Tide gauges, on the other hand, measure sea level relative to the local land surface (see the tide gauge discussion and FAQ). The satellite altimeter estimate of interest is the distance between the sea surface illuminated by the radar altimeter and the center of the Earth (geocentric sea surface height or SSH). This distance is estimated by subtracting the measured distance between the satellite and sea surface (after correcting for many effects on the radar signal) from the very precise orbit of the satellite. At any location, the SSH changes over time due to many well understood factors (ocean tides, atmospheric pressure, glacial isostatic adjustment, etc.). By subtracting from the measured SSH an a priori mean sea surface (MSS), such as the CLS01 mean sea surface, and these known time-varying effects, we compute the sea surface height anomalies (SSHA). Each point in the global mean sea level (GMSL) time series plots is the area-weighted mean of all of the sea surface height anomalies measured by the altimeter in a single, 10-day satellite track repeat cycle (time for the satellite to begin repeating the same ground track). Our goal is to observe the changes of the GMSL due to less understood factors, such as ocean mass changes from melting land ice and groundwater depletion, thermal expansion and contraction of the oceans, and the interannual variability caused by phenomena such as the ENSO. Gaining more understanding of these different factors using other sources of information such as GRACE gravity measurements allows us to try to close the sea level budget and estimate the causes of GMSL changes (e.g., Leuliette & Miller, 2009 and Willis et al., 2008).

The term "global mean sea level" in the context of our research is defined as the area-weighted mean of all of the sea surface height anomalies measured by the altimeter in a single, 10-day satellite track repeat cycle.  It can also be thought of as the "eustatic sea level." The eustatic sea level is not a physical sea level (since the sea levels relative to local land surfaces vary depending on land motion and other factors), but it represents the level if all of the water in the oceans were contained in a single basin. Changes to this eustatic level are caused by changes in total ocean water mass (e.g., ice sheet runoff), changes in the size of the ocean basin (e.g., GIA), or density changes of the water (e.g., thermal expansion). The time series of the GMSL estimates over the TOPEX and Jason missions beginning in 1992 to the present indicates a mostly linear trend after correction for inter-mission biases between instruments. The GMSL rate corrected for GIA represents changes in water mass and density in the oceans. These changes are thought to be predominantly driven by thermal expansion of the oceans and land ice melt (Greenland and Antarctic ice sheets and glaciers).

Why is the GMSL different than local tide gauge measurements?

The global mean sea level (GMSL) we estimate is an average over the oceans (limited by the satellite inclination to ± 66 degrees latitude), and it cannot be used to predict relative sea level changes along the coasts. As an average, it indicates the general state of the sea level across the oceans and not any specific location. Local tide gauges measure the sea level at a single location relative to the local land surface, a measurement referred to as "relative sea level" (RSL). Because the land surfaces are dynamic, with some locations rising (e.g., Hudson Bay due to GIA) or sinking (e.g., New Orleans due to subsidence), relative sea level changes are different across world 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.

We do compare the altimeter sea level measurements against a network tide gauges to discover and monitor drift in the satellite (and sometimes tide gauge) measurements. This is discussed further in the tide gauge discussion.

GMSL is a good indicator of changes in the volume of water in the oceans due to mass influx (e.g., land ice melt) and density changes (e.g., thermal expansion), and is therefore of interest in detecting climate change.

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:

  1. How is the volume of the ocean changing?
  2. How much of this is due to thermal expansion?
  3. 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.

How often are the global mean sea level estimates updated?

We update the sea level data approximately bimonthly (every two months). The altimeter data are released by NASA/CNES as a 10-day group of files corresponding to the satellite track repeat cycle (10 days). There is also a two-month delay between the time the data are collected on the satellite to their final product generation (known as a final geophysical data record (GDR)). We use these final GDR products in the global mean sea level estimates. We are planning to shorten the time between our global mean sea level updates.

What is the format of the dates in the GMSL time series?

The time tag is simply YYYY.XXX, where YYYY is the year and XXX is the fractional part of a 365-day year (to be added to January 1, YYYY). This format allows the time series to be plotted easily without date formatting.

Do you account for plate tectonics in the global mean sea level trend?

The principal tectonic processes (ridge building, subduction, etc.) responsible for changing the ocean basins are measured in millions of years and are so slow that short-term global satellite records do not consider them. Glacial isostatic adjustment is comparatively a much shorter-term process (although still measured in thousands of years), and it does have a minor effect on ocean basin size. The current effect has been estimated to be -0.3 mm/yr of equivalent sea level rise due to increasing ocean basin size. This effect is corrected in the satellite altimeter global mean sea level time series and contributes 0.3 mm/yr to the estimated global mean sea level. This is considered a small effect since it is less than our estimated error of 0.4 mm/yr.

For further information see the reference "Sea Level Variations over Geologic Time."

Is sedimentation in the oceans accounted for in the GMSL estimate?

We do not account for sedimentation in the GMSL estimate because it is estimated to be a very small factor.  Unlike the glacial isostatic adjustment (GIA) correction, which is 0.3 mm/yr, the estimated effect of sedimentation on global mean sea level is an order of magnitude smaller at 0.02 mm/yr (Gornitz and Lebedeff, 1987; Milliman and Meade, 1983; Holeman, 1968).

What determines the x-intercept (i.e., "zero-crossing" or "base year") of the GMSL plots?

To understand what determines the x-intercept (i.e., "base year" or "zero crossing") of the global mean sea level (GMSL) time series, we need to review how the estimation is computed. Each point in the time series plots is the area-weighted mean of all of the sea surface height anomalies (SSHA) measured by the altimeter in a single, 10-day satellite track repeat cycle.  Sea surface height anomalies are the differences of the individual, altimeter-measured sea surface heights (SSH) from a modeled mean sea surface (MSS). Since we are interested in the time series of the global mean of the sea surface anomalies (i.e., how the GMSL is changing over time), the modeled mean sea surface must be referenced to a specific point in time.  The modeled mean sea surface we use is the CLS01 mean sea surface (we expect to soon update this to the improved CLS11 MSS). Other MSS models exist, but their use does not greatly alter the time series of the global mean sea level or its computed rate.  The CLS01 MSS we use is principally derived from the TOPEX/Poseidon altimeter data from 1993-1999 and nominally represents the mean sea surface at the center of this range in 1996.  Therefore, we would expect our time series of global mean sea level (i.e, global mean sea surface height anomalies) to have a zero-crossing at around 1996.  This is indeed the case for our time series, but using an improved MSS referenced to a more recent year, such as the CLS11 MSS, will shift the zero-crossing to the right in our plots. Therefore, the zero-crossing is sort of arbitrary if the desire is to analyze the time series and rate of the global mean sea level, and use of different mean sea suface models partially explains why some other research groups estimate differing time series and corresponding zero-crossings.