Rising sea levels due to melting glaciers and ice sheets is one of the dire consequences of climate change. But despite the onslaught of global warming, sea levels are not really rising quite as quickly as scientists have predicted. The reason: Land water storage slows down sea level rise.
A backgrounder on water cycle
Around 6 trillion tonnes of water cycles in the land surface of Earth every year. These waters end up as snows, surface water, soil moisture, and groundwater—these are collectively referred to as land water storage. The storage gradually flows into larger bodies of water such as seas and oceans. This entire process illustrates the global water cycle.
The amount of water cycled can vary from year to year due to natural variations in weather and climate. Humans however speed up this cycle by extracting groundwater and draining surface water. They also slow this down at times by storing water in manmade reservoirs. But extraction and draining activities exceeds the amount of water stored. These waters end up in oceans. In fact, extraction and draining of water have contributed to an annual 0.38 mm rise to global sea levels over the past two decades.
A study by J. T. Reager however revealed that natural occurrences such as variations in rainfall and evaporation also have an impact on how much water ends up in land water storage. Take note that an increase in rainfall also increases the amount of water stored in groundwater, surface water, and open manmade reservoirs. According to the study, over the past decade, variations in rainfall and evaporation have increased the amount of water stored on land.
Reager et al specifically revealed that continents are absorbing more of the water before it flows into the sea. They figured this out using gravitational measurement made between 2002 and 2014. To be specific, they used the Gravity Recovery and Climate Experiment or GRACE satellites of NASA to quantify variations in land water storage.
Gravitational measurement using GRACE
The GRACE satellites are a pair of satellites nicknamed Tom and Jerry. The two displays an orbital game of cat-and-mouse as they chase each other around the globe. When Jerry passes over a massive object like mountains, gravity would pull it down toward the larger mass. This gravitational pull increases the distance between Jerry and Tom. But Jerry would end up pulled back by the massive objective after passing over it. This decreases its distance with Tom.
Scientists monitors and measures the changes in distance between the two GRACE satellites. The changes in distance tell scientists about the changes in the strength of gravity at different areas in the surface of the Earth.
In an interview with the Los Angeles Times, Reager explained that the force of gravity depends on mass. This means that the greater the mass, the greater the gravitational acceleration. In the case of the Earth, the only thing heavy and mobile enough to affect gravity is water. The movement of the water affects the gravity field of Earth.
A gravitational measurement using data obtained by the GRACE satellites revealed differences in gravitational strength in relation to presence of water in land. For example, in Southern California, drought results in a decrease in the pull of gravity. A big flood event would also register an increase in gravitational pull.
Land water storage slows down sea level rise
Reager et al used the data obtained by the GRACE satellites not to determine variations in gravity pull but to quantify variations in land water storage. Subsequently, by combining the obtained data with estimates of mass loss by glaciers, the researchers found that net land water storage has been increasing.
The years that spanned from 2012 to 2014 brought more rain and snow and thereby, more water on land. The amount of water stored on land increased by 120 billion tonne per year from 2012 to 2014. The researchers noted that had this water ended up in seas and oceans, it would raise sea levels by an additional 0.33 mm per year.
In other words, based on the same data, the researchers found that the net land water storage has been increasing from 2012 and 2014. Furthermore, the greatest regional changes are associated with climate-driven variability in precipitation although a small portion of the increase in land water storage is associated with human activities—particularly by filling reservoirs. This means that land water storage slows down sea level rise by roughly 15 percent.
Reager et al concluded that between 2012 and 2014, variability in climate resulted in an “additional 3200 ± 900 gigatons of water being stored on land. This gain partially offset water losses from ice sheets, glaciers, and groundwater pumping, slowing the rate of sea level rise by 0.71 ± 0.20 millimetres per year.”
Further details of the study of Reager et al are in the article “A decade of sea level rise slowed down by climate-driven hydrology” published in February 2016 in the journal Science. Further details of the interview with Reager and his co-authors are in the article “Thirsty continents are slowing down expected sea level rise, scientists say” authored by Sean Green and published in February 2016 by the Los Angeles Times.