What is Driving Rising Sea Levels?
Heat Storage, Ice Melt, or Both – and How Do We Know?
We are so surrounded by technological wonders that sometimes we forget just how amazing our capacity is to view, monitor and understand the rapidly changing global environment. A recent paper with Norman Loeb as first author spurred a previous essay summarizing the suite of measurements, combing long-term observations with the newest technologies, that yielded a validated net energy balance for the whole Earth.
Measuring and understanding the Earth as a single, integrated system, the focus of the field called Earth System Science, is an international, interdisciplinary, and largely collaborative effort. Scientific papers and reports presenting new findings or global summaries tend to have several to dozens of authors. Occasionally the first “author” will be the name of a research consortium or project.
Often these research groups will be defined around new technologies yielding new kinds of data that address important issues. Looking all the way back to the 1950s, what has become known as “The Keeling Curve” of accumulating carbon dioxide in the atmosphere was initiated, in part, as a result of the invention of a better way to measure concentrations of that gas.
The atmosphere is a relatively well-mixed and accessible part of the Earth system. What about the oceans? What technologies and long-term data sets have been developed to understand the role of oceans in global climate change? While we live in the dynamic and chaotic atmosphere, it is the big, slow-moving cogs in the global climate system that drive our changing fate in the long run. The biggest of these is the global ocean and its interaction with the huge reservoir of fresh water stored in glaciers and ice caps.
The global ocean absorbs much of the carbon dioxide that we have added to the atmosphere (around 50%, depending on your time frame), and as much as 90% of the increased heat retained in the Earth system by this and other greenhouse gases. In return, marine systems hold the key to the long-term changes in the global environment, both through direct impacts of rising sea levels, and changes in ocean and atmospheric dynamics that control climate. Another recent essay explored the dynamics of one part of the global ocean circulation system, the Gulf Stream, as a potential tipping point, with a special focus on Northern Europe.
Sea level rise threatens every coastal community, and even the continued existence of some low-lying island nations. Average sea level can be measured directly relative to adjacent coastlines, and has been for many years, but understanding what controls that increase, and so predicting future increases, requires in-depth understanding and measurement of many different parts of the global water/ice system. Technological innovation has played a big role in recent improvements in this understanding.
Another characteristic of Earth System Science is how often an incredible number of measurements made with different techniques are combined into a single outcome, figure, or graph. For this essay, we will use the first figure here as a vehicle to address the measurements and technologies involved. The concept captured in this figure is relatively simple, two processes contribute to total measured sea level rise, but the underlying measurements are amazingly complex, and provide an internal check on the accuracy of each set of measures.
In this figure, the top black line, total sea level rise, is not the arithmetic sum of the other two. Rather, each line represents a separate set of measurements. The purple line IS the sum of the other two, and expresses the degree of agreement between this sum and the direct measurements.
The lower line in the figure (steric) is thermal expansion. All fluids increase in volume with increasing temperature. As 90% of the heat energy resulting from increased greenhouse gas concentrations in the atmosphere has been transferred to the oceans, water temperatures have increased, leading to thermal expansion.
The middle line (ocean mass) estimates the contribution of water to the oceans by the melting of land-based ice trapped in the major glaciers and ice caps of the Earth as well as storage on land by dams and the draw down of ground water. Warming temperatures have caused the well-documented retreat of continental glaciers globally, and especially in North America and Europe. Antarctica and Greenland are by far the largest reservoirs of frozen water, and various studies have shown that both are losing water faster through melt than they are gaining by snowfall (more on that in a minute).
The distinction between these two processes is important. Thermal expansion is a straightforward physical process. If we can predict changes in ocean temperature, thermal expansion will follow. On the other hand, the dynamics of glacial ice flow over solid ground, melting in the presence of sea water, variation in snowfall, and many other factors affect the net rate of ice gain or loss. The size of these water reservoirs and the complexity of their dynamics makes predictions more difficult.
OK- so this is now a standard story about sea level rise, but it was this figure that actually triggered this essay in that I had thought that thermal expansion was the primary cause of sea level rise, with large contributions from ice melt being more of a future concern. This summation suggests that ice melt is the larger influence.
My questions, then, was, “How do we know this?”
Digging back through the websites and reports from the different programs responsible for the measurements that underlie that first figure, one thing I find most intriguing is the combination of long-term observations accumulated in the simplest ways with data acquired by some truly amazing satellite and instrument technologies capable of precise and sensitive measurements of changes in the ocean/ice system.
For example, the level and timing of ocean tides has been critical for navigation since the age of sail and is routinely measured directly in harbors and other locations around the world. In some places, the record goes back more than 200 years. These data sets are curated and archived now at the University of Hawaii Sea Level Center, which is also part of the UN-initiated Global Sea Level Observing System (GLOSS) that alone supports about 290 reporting stations.
Since 1992, the TOPEX/Poseiden/Jason set of satellites, a joint program of U.S. and European agencies, has mapped ocean elevation at the scale of millimeters (yes, millimeters) using radar ranging and extremely precise technologies for determining where the satellites are relative to the Earth (see figure below). The fact that this is possible given the turbulence of waves and presence of storms is one of those technological marvels suggested at the top of this essay.
Direct measurements and satellite estimates of sea level rise generally agree. In some locations they disagree slightly because direct measures in the harbors can be affected by geological shifts in locations of those harbors. Continents and islands “bounce” up or down in response to several geological forces. An image of the differences between these two data sets notes that places of disagreement tend to be in seismically active areas, such as the coasts of Alaska and Peru. Correcting the direct measurements for geologic movement of the locations in which they were made brings them into better agreement with the satellite record. Again two very different measurements of the same process allow analysis of both agreement and potential corrections.
Thermal expansion estimates require an understanding of changes in ocean temperature. Imagine trying to detect subtle changes in a system as vast and complex as the oceans. Satellite systems are not enough here, as they cannot penetrate much beyond the surface. A major program called ARGO was initiated in 2000 to deploy a set of programmable buoys, traveling passively with ocean currents, but capable of sinking and rising in response to pre-programmed commands, and instrumented to measure temperature, pressure and salinity, among other parameters. As of 8 September 2021, 3864 of these buoys are active in all the oceans of the world, moving up and down through columns of ocean water, collecting data.
The ARGO data set maps changes in temperature in ocean waters up to a depth of 2000 meters. Results have shown the largest increases nearer to the surface (the top 700 meters) but with deeper waters also beginning to increase as the global circulation system moves heat through the global ocean.
As a comparison among technologies, it is interesting that TOPEX maps the largest increases in ocean elevation in the southern ocean (north of Antarctica – see figure above) at the same time that ARGO has projected the greatest increases in the heat content of oceans since 2006 between 20 to 60 degrees south latitude. These differences in where within the global ocean the additional heat is stored relate to and can actually help chart the global scale redistribution of ocean waters, especially by deep ocean currents.
In another cross-study check, ARGO estimates a net change in energy flux to the oceans since 1991 at .50 to .68 watts/square meter. In the Loeb article cited above, total global energy gain from 2005-2020 averaged about 0.7 watts per square meter). That these estimates are so similar helps to corroborate each one, and the relative numbers are also consistent with the idea that most of the additional heat in the global system is retained in the oceans.
Perhaps the most amazing newer satellite technology available for charting the global distribution of water has been applied to the mapping of glaciers, ice sheets and even groundwater on land. Again, there is a long-term observational data set on the retreat of glaciers globally, especially in North America and Europe. Images from a network of sites, as well as field trips that can be arranged to many retreating glaciers, yield dramatic evidence for the rapid retreat of ice on land.
But the big actors here are Greenland and Antarctica, where complex dynamics of snowfall, ice melt and glacial flow present a challenge to land-based techniques.
The recent technology I find so amazing has been deployed by NASA and operates under the title of GRACE (for Gravity Recovery and Climate Experiment). This consists of two satellites flying in tandem, each VERY precisely located and following the same orbital path. As they travel, minute changes in the force of Earth’s gravity field cause the satellites to change relative position as they fly over different parts of the Earth. Differences in relative position can be measured in microns (millionths of a meter – there are 25,400 microns in an inch). Changes in gravity fields causing these exceedingly minor orbital variations have been linked to the amount of water beneath the satellites, both in surface glaciers and in groundwater.
GRACE has been used since first launched in 2002 to map loss of ice across Greenland (as in the figure here) and even to chart draw down of water in aquifers, for example under agricultural regions in California.
So can all of these low-tech and cutting edge methods be combined into a single integrated view of sea level rise and its causes? In an effort analogous to the paper cited at the top by Loeb and colleagues, Thomas Frederikse and colleagues in a paper from 2020 (citation below) have done just that, effectively closing the global ocean water budget and documenting the role of different processes in determining changing sea levels since 1900.
First among the results in this paper is that estimates for sea level rise over time by direct measures (harbors, etc.), by satellite altimetry, and by the sum of all of the processes of ice melt, thermal expansion and changes in land storage of water, though measured independently, can be brought into agreement. This is what it means to “close” the water budget, and also demonstrates the value of using a wide diversity of methods to cross-check results from each. That these measurements can be brought together in an internally consistent view of causes of sea level rise is a major result.
Graphed trends in causes of sea level rise in this same paper show that ice melt has generally (since 1900) been a larger contributor than thermal expansion. The analysis was so sensitive that the paper presents a time period from the 1940s through the 1970s when the construction of large dam projects reduced water flow to the oceans sufficiently to temporarily slow the rate of sea level rise, and to cause thermal expansion to be the larger contributor for part of this time period.
In terms of sources of water by melting of ice, land-based glaciers have been the larger source for most of the period since 1900, with Greenland matching this contribution by 2020. Throughout, Antarctic ice is projected as a much smaller source.
Finally, this paper reinforces the pattern in the figure at the top of this essay that, in 2020, ice melt caused about two-thirds of the measured rate of sea level rise, and thermal expansion the remaining third. My misconception that launched this essay has been corrected!
From tidal gauges going back 200 years, to cutting edge technologies that measure water content by micron-level changes in satellite altitude, a wide variety of approaches has been reconciled by a large number of teams to produce that one simple figure of changes in sea level due to thermal expansion and loss of ice (and other land-based water components) at the top of this essay.
Kudos to those dedicated teams of researchers and technicians responsible for this incredibly detailed and well-validated set of measurements of the complex and dynamic ocean/ice system that will largely control our climate future.
Sources
The Loeb, et al, paper cited in that previous essay is:
Loeb, N. G., Johnson, G. C., Thorsen, T. J., Lyman, J. M., Rose, F. G., & Kato, S. (2021). Satellite and ocean data reveal marked increase in Earth’s heating rate. Geophysical Research Letters, 48, https://doi.org/10.1029/2021GL093047
The Keeling Curve initiated by, and curated for so many years by Charles Keeling can be found here: https://keelingcurve.ucsd.edu/
One source for the estimate that 90% of heat retained by greenhouse gases has been transferred to the oceans is here: https://www.ncei.noaa.gov/news/ocean-heat-content-rises
One source that ~50% of emitted carbon dioxide has been dissolved in the oceans is here: https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
NASA has developed a tool that projects sea level rise for a number of specific locations through 2150: https://sealevel.nasa.gov/data_tools/17
The first figure on total sea level rise and contributions by ice and thermal expansion is from:
A similar diagram as well as ARGO data on the distribution of increases in ocean heat content showing greater warming in the Southern Ocean can be found here:
https://argo.ucsd.edu/science/global-phenomenon/#heat
And a short piece that includes the estimate of heat transfer to the oceans is here:
http://goodtimesweb.org/industrial-policy/2019/science-oceans-warming-jan-11-2019.pdf
The University of Hawaii Sea Level Center is here: https://uhslc.soest.hawaii.edu/
This also has information about the UN-initiated GLOSS program
Information on TOPEX/Poseidon/Jason can be found here:
https://sealevel.jpl.nasa.gov/missions/topex-poseidon/summary/
https://svs.gsfc.nasa.gov/3206
Data on water loss from glaciers worldwide can be found at the World Glacier Monitoring Service: https://wgms.ch/
Information about and data from the GRACE mission and follow-on missions are here:
The image of ice loss from Greenland is here:
https://gracefo.jpl.nasa.gov/resources/33/greenland-ice-loss-2002-2016/
The paper by Frederikse, et al. is here: https://www.nature.com/articles/s41586-020-2591-3
And the preferred citation is: Frederikse, T., Landerer, F., Caron, L. et al. The causes of sea-level rise since 1900. Nature 584, 393–397 (2020). https://doi.org/10.1038/s41586-020-2591-3