Decade of rising seas slowed by land soaking up extra water

New measurements from a NASA satellite have allowed researchers to identify and quantify, for the first time, how climate-driven increases of liquid water storage on land have affected the rate of sea level rise.

A new study by scientists at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and the University of California, Irvine, shows that while ice sheets and glaciers continue to melt, changes in weather and climate over the past decade have caused Earth’s continents to soak up and store an extra 3.2 trillion tons of water in soils, lakes and underground aquifers, temporarily slowing the rate of sea level rise by about 20 percent.

The water gains over land were spread globally, but taken together they equal the volume of Lake Huron, the world’s seventh largest lake. The study is published in the Feb. 12 issue of the journal Science.

Each year, a huge amount of water evaporates from the ocean, then falls over land as rain or snow, and returns to the ocean through runoff and river flows. This is known as the global hydrological cycle. Scientists have long known that small changes in Earth’s water cycle could lead to large, although temporary, changes in the rate of sea level rise. They did not know how large this effect could be, however, because there were no instruments that could measure these changes on a global scale.

The 2002 launch of NASA’s Gravity Recovery and Climate Experiment (GRACE) twin satellites provided the first tool capable of quantifying these trends. By measuring the distance between the two satellites to within the width of a strand of human hair as they orbit the planet, researchers can record changes in Earth’s gravitational pull that result from water moving across its surface. Careful analysis of these data, allowed the scientists to measure the change in water storage over land.

“We always assumed that people’s increased reliance on groundwater for irrigation and consumption was resulting in a net transfer of water from the land to the ocean,” said lead author J.T. Reager of JPL, who began the research project as a UCI graduate student. “What we didn’t realize until now is that over the past decade, changes in the global water cycle more than offset the losses that occurred from groundwater pumping, causing the land to act like a sponge.

“These new data are vital for understanding variations in sea level change,” added Reager. “The information will be a critical complement to future long-term projections of sea level rise, which depend on melting ice and warming oceans.”

Jay Famiglietti, UCI Earth system science professor and also senior water scientist at JPL is senior author of the paper. “This is the first study to observe these changing water storage patterns on land and their impact on modulating current rates of sea level rise,” Famiglietti said. “Our work will certainly sound the alarm about the possible effects of climate change on shifting patterns of freshwater availability, as well as the potential for modulating future rates of sea level rise by managing the amount of freshwater stored on land.”

Famiglietti also noted that the study is the first to observe global patterns of wetting and drying on land, with wet areas getting wetter and dry areas getting drier. “These patterns are consistent with projections under a warming climate,” he said. “But we’ll need a much longer data record to fully understand the underlying cause of the patterns and whether they will persist.”

2 COMMENTS

  1. This article reports that over then last decade 3.2 trillion tons of water has been added to aquifers, lakes and soils.

    3.2 trillion tons of water
    = 3200 GT
    / 1.1023 Gt/GT
    = 2903 Gt
    / 362 Gt/mm-SLR
    = 8 mm of SLR over ten years

    That’s the equivalent of 0.8 mm/year of sea-level rise, a rate of just over 3” per century.

    But the article states that that represents a 20% reduction in the rate of sea-level rise, which is too low. It would imply that measured sea-level rise averages 3.2 mm/year, which is way too high.

    The actual, measured rate of sea-level rise, averaged over the best long-term coastal tide gauges, is just under 1.5 mm/yr.

    0.8 / (1.5 + 0.8)= 35%, not 20%.

    It appears that the authors are referencing their calculations to satellite altimetry measurements of sea-level, rather than coastal sea-level measured by tide gauges. That is a mistake.

    Most fundamentally, satellite altimeters measure the wrong thing. Their measurements are distorted by “sea-level rise” caused by thermal expansion when the upper layer of the ocean warms. But that is a strictly local effect, that doesn’t affect the quantity of water in the oceans (and is of little consequence to the coasts), and doesn’t affect sea-level elsewhere (e.g., at the coasts). So it shouldn’t be compared to the amount of water removed from the oceans and stored in aquifers and lakes.

    Also, that sea-level rise only matters at the coasts, but satellite altimeters are incapable of measuring sea-level at the coasts. Tide gauges measure sea-level at the coasts, where it matters.

    Also, tide gauge measurements of sea-level are much higher quality than satellite altimetry measurements, which are of questionable reliability, and vary considerably from one satellite to another. Also, some of the tide-gauge records of sea-level measurements are nearly ten times as long as the combined satellite measurement record, and twenty times as long as any single satellite measurement record.
    .

    Prof. Peltier estimates that meltwater load from the melting of the great ice sheets (~10k years ago) is causing the ocean floors to sink by enough to cause a 0.3 mm/yr fall in sea-level, absent other factors. For water mass budget calculations like this study’s, it is reasonable to add that number (0.3 mm/yr) to measured rates of sea-level, even though the resulting sum is not truly sea-level, and is not useful for projecting sea-level for coastal planning. (It’s an attempt to calculate what the rate of sea-level rise would be, were it not for the hypothesized sinking of the ocean floor.) But 1.5 mm/yr + 0.3 mm/yr is still just 1.8 mm/yr, and 0.8 mm/yr would represent a 31% reduction (from 1.8 + 0.8 = 2.6), not just 20%.
    .

    NOAA has done linear regression analysis on sea-level measurements (relative sea-level) from 225 long term tide gauges around the world, which have data spanning at least 50 years. (Note: the literature indicates that at least 50-60 years of data are required to determine a robust long term sea-level trend from a tide gauge record.)

    It is important to realize that there’s been no sign of any acceleration (increase in rate) in most of those tide-gauge records, in over three-quarters of a century.

    The rate of measured sea-level rise (SLR) varies from -17.59 mm/yr at Skagway, Alaska, to +9.39 mm/yr at Kushiro, Japan. 197 of 225 stations (87.6%) have recorded less than 3.3 mm/yr sea-level rise. At 47 of 225 stations (20.9%) sea level is falling, rather than rising. Just 28 of 225 stations (12.4%) have recorded more than 3.3 mm/yr sea-level rise. The average SLR at those 225 gauges is +0.90 mm/yr. The median is +1.41 mm/yr.

    That’s probably slightly less than the true global average, because a disproportionate number of those 225 stations are northern hemisphere stations affected by PRG (i.e., the land is rising). OTOH, quite a few long-term tide gauges are substantially affected by subsidence (i.e., the land is sinking), often due to extraction of water, oil, or natural gas, or due to the location having been elevated with fill dirt which is compacting (like Galveston).

    I downloaded the two sea-level measurement spreadsheet files (U.S. and global) from NOAA’s page, and combined them into a single Excel spreadsheet. For ease of sorting, I changed the U.S. station ID numbers by adding an “A-” prefix (“A” for “American”). I also added “average” and “median” lines at the end of the spreadsheet. The average of all 375 NOAA-analyzed stations is 1.28 mm/yr, and the median is 1.71 mm/yr:
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08.xls or
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08.htm
    .

    NOAA says that the average is 1.7-1.8 mm/yr. Some of the difference between the calculated average and NOAA’s figure for MSL rise may be due to the addition of model-derived GIA adjustments to the measured rates when calculating the average, to attempt to account for Post-Glacial Rebound (PGR). My guess is that they’re using Prof. Richard Peltier’s figures. (Unfortunately, those figures are only very loosely correlated with what is actually happening at the tide-gauge locations.)
    .

    Unfortunately, many of the tide station records in NOAA’s expanded list of 375 are too short to be appropriate for measuring sea-level trends. The literature indicates that at least 50-60 years of data are needed to establish a robust sea-level trend from a tide station record. But the shortest record in NOAA’s list is Apra Harbor, Guam, with just 21 years of data. (The text at the top of NOAA’s page says, “Trends with the widest confidence intervals are based on only 30-40 years of data,” but that is incorrect. I suspect they wrote it before they added the gauges with very short records.)

    So I also made a version of this spreadsheet in which stations with records shorter than 50 years are omitted.

    Considering only tide stations with records of at least 50 years, the average and median rates of MSL rise (of the 225 remaining stations) are 0.90 mm/yr and 1.41 mm/yr, respectively:
    http://sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr.xls or
    http://sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr.htm

    (I also tried limiting it to stations with records of at least 60 years, with very similar results: average 0.77 mm/yr, and median 1.37 mm/yr.)

    The average (0.90 mm/yr) is probably unrealistically low, due to the disproportionate number of stations in northern Europe which see low or negative rates of measured sea-level rise due to PGR. The fact that the average is less than the median also suggests that there are a disproportionate number of low-end outliers.
    .

    I also tried another approach, in which I excluded the most extreme latitudes. I started with just the “50+ year” stations, and included only stations within a latitude range of 45 (i.e., I excluded stations above 45 north or below 45 south). The resulting average and median for 137 stations were 2.22 mm/y and 2.02 mm/yr, respectively:
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_lowLat.xls or
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_lowLat.htm

    That approach largely solves the problem of low-side bias introduced by stations which are affected by PGR (which lowers the calculated average), but it doesn’t solve the problem of high-side bias introduced by stations affected by subsidence (which raises the calculated average). So the average (2.22 mm/yr) is probably unrealistically high. The fact that the average is greater than the median also suggests that there are a disproportionate number of high-end outliers.
    .

    So I tried another approach, this time explicitly eliminating “outliers.” I started with just the “50+ year” stations, but excluded the 40 stations with the lowest rate of sea-level rise (including most of those experiencing falling sea-level), and the 30 stations with the highest rate of sea-level rise (including most of those experiencing severe land subsidence, like Galveston, which is built on sinking fill dirt). The resulting average and median rates of sea-level rise (calculated from 155 stations) are both 1.48 mm/yr:
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_less_high30_and_low40.xls or
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_less_high30_and_low40.htm

    That figure, 1.48 mm/yr, is my current best estimate of globally averaged coastal sea-level rise. At first glance, excluding more low outliers than high outliers might seem to bias the result to the high end. But I think it is justifiable, because of the disproportionate number of northern European and North American stations at locations where the land is rising due to PGR. The fact that the median and average are equal suggests that there aren’t disproportionate numbers of either high or low outliers. (I also tried excluding the low and high 35 stations, and the result was an average MSL rise of 1.36 mm/yr, and median 1.41 mm/yr, which suggests that it includes more low outliers than high outliers.)

    Note that 1.48 mm/yr is less than six inches per century.

    Note, too, that if you add Peltier’s +0.3 mm/yr GIA to that calculated 1.48 mm/yr global average rate of MSL rise, the sum is within NOAA’s 1.7-1.8 mm/yr range.

    It is not possible to torture the tide-gauge data into yielding a globally averaged rate of sea-level rise anywhere near 3.2 mm/yr.

  2. This article reports that over then last decade 3.2 trillion tons of water has been added to aquifers, lakes and soils.

    3.2 trillion tons of water
    = 3200 GT
    / 1.1023 Gt/GT
    = 2903 Gt
    / 362 Gt/mm-SLR
    = 8 mm of SLR over ten years

    That’s the equivalent of 0.8 mm/year of sea-level rise, a rate of just over 3” per century.

    But the article states that that represents a 20% reduction in the rate of sea-level rise, which is too low. It would imply that measured sea-level rise averages 3.2 mm/year, which is way too high.

    The actual, measured rate of sea-level rise, averaged over the best long-term coastal tide gauges, is just under 1.5 mm/yr.

    0.8 / (1.5 + 0.8)= 35%, not 20%.

    It appears that the authors are referencing their calculations to satellite altimetry measurements of sea-level, rather than coastal sea-level measured by tide gauges. That is a mistake.

    Most fundamentally, satellite altimeters measure the wrong thing. Their measurements are distorted by “sea-level rise” caused by thermal expansion when the upper layer of the ocean warms. But that is a strictly local effect, that doesn’t affect the quantity of water in the oceans (and is of little consequence to the coasts), and doesn’t affect sea-level elsewhere (e.g., at the coasts). So it shouldn’t be compared to the amount of water removed from the oceans and stored in aquifers and lakes.

    Also, that sea-level rise only matters at the coasts, but satellite altimeters are incapable of measuring sea-level at the coasts. Tide gauges measure sea-level at the coasts, where it matters.

    Also, tide gauge measurements of sea-level are much higher quality than satellite altimetry measurements, which are of questionable reliability, and vary considerably from one satellite to another. Also, some of the tide-gauge records of sea-level measurements are nearly ten times as long as the combined satellite measurement record, and twenty times as long as any single satellite measurement record.

    Prof. Peltier estimates that meltwater load from the melting of the great ice sheets (~10k years ago) is causing the ocean floors to sink by enough to cause a 0.3 mm/yr fall in sea-level, absent other factors. For water mass budget calculations like this study’s, it is reasonable to add that number (0.3 mm/yr) to measured rates of sea-level, even though the resulting sum is not truly sea-level, and is not useful for projecting sea-level for coastal planning. (It’s an attempt to calculate what the rate of sea-level rise would be, were it not for the hypothesized sinking of the ocean floor.) But 1.5 mm/yr + 0.3 mm/yr is still just 1.8 mm/yr, and 0.8 mm/yr would represent a 31% reduction (from 1.8 + 0.8 = 2.6), not just 20%.

    NOAA has done linear regression analysis on sea-level measurements (relative sea-level) from 225 long term tide gauges around the world, which have data spanning at least 50 years. (Note: the literature indicates that at least 50-60 years of data are required to determine a robust long term sea-level trend from a tide gauge record.)

    It is important to realize that there’s been no sign of any acceleration (increase in rate) in most of those tide-gauge records, in over three-quarters of a century.

    The rate of measured sea-level rise (SLR) varies from -17.59 mm/yr at Skagway, Alaska, to +9.39 mm/yr at Kushiro, Japan. 197 of 225 stations (87.6%) have recorded less than 3.3 mm/yr sea-level rise. At 47 of 225 stations (20.9%) sea level is falling, rather than rising. Just 28 of 225 stations (12.4%) have recorded more than 3.3 mm/yr sea-level rise. The average SLR at those 225 gauges is +0.90 mm/yr. The median is +1.41 mm/yr.

    That’s probably slightly less than the true global average, because a disproportionate number of those 225 stations are northern hemisphere stations affected by PRG (i.e., the land is rising). OTOH, quite a few long-term tide gauges are substantially affected by subsidence (i.e., the land is sinking), often due to extraction of water, oil, or natural gas, or due to the location having been elevated with fill dirt which is compacting (like Galveston).

    I downloaded the two sea-level measurement spreadsheet files (U.S. and global) from NOAA’s page, and combined them into a single Excel spreadsheet. For ease of sorting, I changed the U.S. station ID numbers by adding an “A-” prefix (“A” for “American”). I also added “average” and “median” lines at the end of the spreadsheet. The average of all 375 NOAA-analyzed stations is 1.28 mm/yr, and the median is 1.71 mm/yr:
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08.xls or
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08.htm

    NOAA says that the average is 1.7-1.8 mm/yr. Some of the difference between the calculated average and NOAA’s figure for MSL rise may be due to the addition of model-derived GIA adjustments to the measured rates when calculating the average, to attempt to account for Post-Glacial Rebound (PGR). My guess is that they’re using Prof. Richard Peltier’s figures. (Unfortunately, those figures are only very loosely correlated with what is actually happening at the tide-gauge locations.)

    Unfortunately, many of the tide station records in NOAA’s expanded list of 375 are too short to be appropriate for measuring sea-level trends. The literature indicates that at least 50-60 years of data are needed to establish a robust sea-level trend from a tide station record. But the shortest record in NOAA’s list is Apra Harbor, Guam, with just 21 years of data. (The text at the top of NOAA’s page says, “Trends with the widest confidence intervals are based on only 30-40 years of data,” but that is incorrect. I suspect they wrote it before they added the gauges with very short records.)

    So I also made a version of this spreadsheet in which stations with records shorter than 50 years are omitted.

    Considering only tide stations with records of at least 50 years, the average and median rates of MSL rise (of the 225 remaining stations) are 0.90 mm/yr and 1.41 mm/yr, respectively:
    http://sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr.xls or
    http://sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr.htm

    (I also tried limiting it to stations with records of at least 60 years, with very similar results: average 0.77 mm/yr, and median 1.37 mm/yr.)

    The average (0.90 mm/yr) is probably unrealistically low, due to the disproportionate number of stations in northern Europe which see low or negative rates of measured sea-level rise due to PGR. The fact that the average is less than the median also suggests that there are a disproportionate number of low-end outliers.

    I also tried another approach, in which I excluded the most extreme latitudes. I started with just the “50+ year” stations, and included only stations within a latitude range of 45 (i.e., I excluded stations above 45 north or below 45 south). The resulting average and median for 137 stations were 2.22 mm/y and 2.02 mm/yr, respectively:
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_lowLat.xls or
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_lowLat.htm

    That approach largely solves the problem of low-side bias introduced by stations which are affected by PGR (which lowers the calculated average), but it doesn’t solve the problem of high-side bias introduced by stations affected by subsidence (which raises the calculated average). So the average (2.22 mm/yr) is probably unrealistically high. The fact that the average is greater than the median also suggests that there are a disproportionate number of high-end outliers.

    So I tried another approach, this time explicitly eliminating “outliers.” I started with just the “50+ year” stations, but excluded the 40 stations with the lowest rate of sea-level rise (including most of those experiencing falling sea-level), and the 30 stations with the highest rate of sea-level rise (including most of those experiencing severe land subsidence, like Galveston, which is built on sinking fill dirt). The resulting average and median rates of sea-level rise (calculated from 155 stations) are both 1.48 mm/yr:
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_less_high30_and_low40.xls or
    http://www.sealevel.info/NOAA_AllStationsLinearSeaLevelTrends_2015-08_50yr_less_high30_and_low40.htm

    That figure, 1.48 mm/yr, is my current best estimate of globally averaged coastal sea-level rise. At first glance, excluding more low outliers than high outliers might seem to bias the result to the high end. But I think it is justifiable, because of the disproportionate number of northern European and North American stations at locations where the land is rising due to PGR. The fact that the median and average are equal suggests that there aren’t disproportionate numbers of either high or low outliers. (I also tried excluding the low and high 35 stations, and the result was an average MSL rise of 1.36 mm/yr, and median 1.41 mm/yr, which suggests that it includes more low outliers than high outliers.)

    Note that 1.48 mm/yr is less than six inches per century.

    Note, too, that if you add Peltier’s +0.3 mm/yr GIA to that calculated 1.48 mm/yr global average rate of MSL rise, the sum is within NOAA’s 1.7-1.8 mm/yr range.

    It is not possible to torture the tide-gauge data into yielding a globally averaged rate of sea-level rise anywhere near 3.2 mm/yr.

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