Beitrag des Grundwassers zum Meeresspiegel-Anstieg überschätzt

Angesichts einzelner Horrorszenarien zum globalen Meeresspiegelanstieg darf nicht vergessen werden, dass es sich dabei um Meinungen von wenigen Einzelforschern handelt. Das Gros der Experten geht von Anstiegsraten aus, wie sie auch im aktuellen IPCC-Bericht angegeben sind. Der Joker der Alarmszenarien sind die großen Eismassen in Grönland und der Antarktis. Diese sind in Wirklichkeit wohl viel stabiler als angenommen, wie die  Stanford University am 3. September 2015 bekanntgab:

Ice sheets may be more resilient than thought, say Stanford scientists

Stanford study suggests that today’s ice sheets may be more resilient to increased carbon dioxide levels than previously thought.

Sea level rise poses one of the biggest threats to human systems in a globally warming world, potentially causing trillions of dollars’ worth of damages to flooded cities around the world. As surface temperatures rise, ice sheets are melting at record rates and sea levels are rising. But there may be some good news amid the worry. Sea levels may not rise as high as assumed. To predict sea level changes, scientists look to Earth’s distant past, when climate conditions were similar to today, and investigate how the planet’s ice sheets responded then to warmer temperatures brought on by increased carbon dioxide in the atmosphere.

In a recently published study in the journal Geology, PhD students Matthew Winnick and Jeremy Caves at Stanford School of Earth, Energy & Environmental Sciences explored these very old conditions and found that sea level might not have risen as much as previously thought – and thus may not rise as fast as predicted now. To better understand global sea level rise, Winnick and Caves analyzed the middle Pliocene warm period, the last time in Earth’s history, approximately 3 million years ago, when carbon dioxide levels in the atmosphere were close to their present values (350-450 parts per million).

“The Pliocene is an important analogue for today’s planet not only because of the related greenhouse gas concentrations, but because the continents were roughly where they are today, meaning ocean and climate circulation patterns are comparable,” said Winnick. These similarities are why the Intergovernmental Panel on Climate Change (IPCC), the group responsible for global sea level rise projections, focuses on the mid-Pliocene warm period to inform their computer models.

Previous studies of the mid-Pliocene warm period used oxygen isotope records to determine the volume of Earth’s ice sheets and, by proxy, sea level. Effectively, the oxygen isotope records act as a fingerprint of Earth’s ice sheets. By combining the fingerprint with models of ice sheet meltwater, many previous researchers thought that sea level was likely 82 to 98 feet (25 to 30 meters) higher during the Pliocene. Such high sea level would require a full deglaciation of the Greenland Ice Sheet and the West Antarctic Ice Sheet, and as much as 30 percent of the East Antarctic Ice Sheet – enough to cover New York City under 50 feet of water. But these estimates arose because the researchers assumed that the Antarctic ice of the Pliocene had the same isotopic composition, that is, the same fingerprint, as it does today – an assumption that Winnick and Caves challenge in their new report.

To understand the isotopic composition of Pliocene ice, Winnick and Caves began in the present day using well-established relationships between temperature and the geochemical fingerprint. By combining this modern relationship with estimates of ancient Pliocene surface temperatures, they were able to better refine the fingerprint of the Antarctic ice millions of years ago. In re-thinking this critical assumption, and by extending their analysis to incorporate ice sheet models, Winnick and Caves recalculated the global sea level of the Pliocene and found that it was 30 to 44 feet (9 to 13.5 meters) higher, significantly lower than the previous estimate.

“Our results are tentatively good news,” Winnick said. “They suggest that global sea level is less sensitive to high atmospheric carbon dioxide concentrations than previously thought. In particular, we argue that this is due to the stability of the East Antarctic Ice Sheet, which might be more resilient than previous studies have suggested.” However, a rise in global sea level by up to 44 feet (13.5 meters) is still enough to inundate Miami, New Orleans and New York City, and threaten large portions of San Francisco, Winnick cautioned. While the study helps refine our understanding of Pliocene sea level, both Winnick and Caves point out that it’s not straightforward to apply these results to today’s planet. “Ice sheets typically take centuries to millennia to respond to increased carbon dioxide, so it’s more difficult to say what will happen on shorter time scales, like the next few decades,” Winnick said.

“Add that to the fact that CO2 levels were relatively consistent in the Pliocene, and we’re increasing them much more rapidly today, and it really highlights the importance of understanding how sea level responds to rising temperatures. Estimates of Pliocene sea level might provide a powerful tool for testing the ability of our ice sheet models to predict future changes in sea level.”

Mehr als anderthalb Meter Meeresspiegelanstieg bis 2100, davor warnen einige Alarmisten. Das entspräche einem durchschnittlichen Anstieg von 18 mm pro Jahr. In Wahrheit messen die Küstenpegel jedoch nur ein Zehntel hiervon, wie auf der NOAA-Webseite nachzulesen ist. Dort werden Messwerte für einzelne Regionen der Erde zusammengefasst:

Global Regional Trends Comparison (4 Main Regions, various subregions)
The graphs compare the 95% confidence intervals of relative mean sea level trends for CO-OPS and global stations. Trends with the narrowest confidence intervals are based on the longest data sets. Trends with the widest confidence intervals are based on only 30-40 years of data. The graphs can provide an overarching indication of the differing rates of regional vertical land motion, given that the absolute global sea level rise is believed to be 1.7-1.8 millimeters/year. Note that they are relative sea level trends, and are not corrected for local land movement. The calculated trends for all CO-OPS stations are available as a table in millimeters/year and in feet/century. A complete table of non-CO-OPS station trends are available as a table in millimeters/year and in feet/century.

Diese Anstiegsrate passt gut zur durchschnittlichen globalen Anstiegsrate des 20. Jahrhunderts auf Basis von Küstenpegelmessungen, die von Thompson et al. (2016) mit 1,7 mm/Jahr ermittelt wurde.

Neue Forschungsresultate zeigen nun, dass die Grundwassergewinnung und der Abfluss in die Ozeane wohl einen deutlich geringeren Einfluss auf den Meeresspiegelanstieg haben als gedacht. Der Beitrag des Grundwassers wurde offenbar um den Faktor drei überschätzt wie das International Institute for Applied Systems Analysis am 2. Mai 2016 mitteilte:

How much does groundwater contribute to sea level rise?
Land water, including groundwater extraction, contributes far less to sea level rise than previously thought, according to a new study.

Groundwater extraction and other land water contribute about three times less to sea level rise than previous estimates, according to a new study published in the journal Nature Climate Change. The study does not change the overall picture of future sea level rise, but provides a much more accurate understanding of the interactions between water on land,  in the atmosphere, and the oceans, which could help to improve future models of sea level rise. “Projecting accurate sea level rise is important, because rising sea level is a threat to people who live near the ocean and in small islands,” explains IIASA researcher Yoshihide Wada, who led the study. “Some low-lying areas will have more frequent flooding, and very low-lying land could be submerged completely. This could also damage substantially coastal infrastructure.”

Sea level has risen 1.7 mm per year over the 20th and the early 21st century, a trend that is expected to continue as climate change further warms the planet.  Researchers have attributed the rising seas to a combination of factors including melting ice caps and glaciers, thermal expansion (water expands as it gets warmer), and the extraction of groundwater for human use. Land water contributions are small in comparison to the contribution of ice melt and thermal expansion, yet they have been increasing, leading to concerns that this could exacerbate the problem of sea level rise caused by climate change.

However, much uncertainty remains about how much different sources contribute to sea level rise. In fact, sea level has actually risen more than researchers could account for from the known sources, leading to a gap between observed and modeled global sea-level budget. Previous studies, including estimates used in the IPCC Fifth Assessment Report, had assumed that nearly 100% of extracted groundwater ended up in the ocean. The new study improves on previous estimates by accounting for feedbacks between the land, ocean, and atmosphere. It finds that number is closer to 80%. That means that the gap between modeled and observed sea level rise is even wider, suggesting that other processes are contributing more water than previously estimated. “During the 20th century and early 21st century, cumulative groundwater contribution to global sea level was overestimated by at least 10 mm,” says Wada. In fact, the new study shows that from 1971 to 2010, the contribution of land water to global sea level rise was actually slightly negative – meaning that more water was stored in groundwater and also due to reservoir impoundment behind dams. From 1993 to 2010, the study estimates terrestrial water as contributing positive 0.12 mm per year to sea level rise.

The study does not change the fact that future groundwater contribution to sea level will increase as groundwater extraction increases. And the increasing trend in groundwater depletion has impacts beyond sea level rise. Wada explains, “The water stored in the ground can be compared to money in the bank. If you withdraw money at a faster rate than you deposit it, you will eventually start having account-supply problems. If we use groundwater unsustainably, in the future there might not be enough groundwater to use for food production. Groundwater depletion can also cause severe environmental problems like reduction of water in streams and lakes, deterioration of water quality, increased pumping costs, and land subsidence.”

Reference
Wada Y, Lo MH, Yeh PJF, Reager JT, Famiglietti JS, Wu RJ, Tseng YH (2016). Fate of water pumped from underground and contributions to sea-level rise. Nature Climate Change. doi:10.1038/NCLIMATE3001

Der Standard berichtete am 4. Mai 2016:

Studie: Beitrag des Grundwassers zum Meeresspiegel-Anstieg überschätzt
IIASA-Forscher: Es landet weniger entnommenes Wasser im Meer als gedacht Wien – Der Meeresspiegel ist im 20. und frühen 21. Jahrhundert um 1,7 Millimeter pro Jahr gestiegen. Angesichts der Klimaerwärmung wird damit gerechnet, dass dieser Trend weiter anhält. Als wichtigste Gründe dafür nennen Wissenschafter das weltweite Abschmelzen der Eisschilde und Gletscher sowie die Ausdehnung des immer wärmer werdenden Wassers der Ozeane. Eine wichtige Rolle könnte auch die steigende Entnahme von Grundwasser durch den Menschen spielen, wurde bislang gedacht. Allerdings sei dieser Faktor überschätzt worden, bilanziert nun eine Studie, die im Fachjournal “Nature Climate Change” erschienen ist.

Weiterlesen im Standard.

Es kommt noch besser. Denn eine Studie der University of California, Irvine, gibt an, dass im vergangenen Jahrzehnt auf globaler Sicht die in Seen, Grundwasser und Bodenfeuchte gespeicherte Wassermenge insgesamt zu- und nicht etwa abgenommen hat. Hier die Pressemitteilung vom 11. Februar 2016:

Decade of rising seas slowed by land soaking up extra water, UCI and NASA find

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 [2016] 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.”