Most Changes in Earth's Shape Are Due to Changes in Climate
Scientists using NASA satellite data found the shape of the Earth appears to be influenced by big climate events that cause changes in the mass of water stored in oceans, continents and atmosphere.
Image to right: Satellites in the SLR: This graphic shows the constellation of satellites supported by the Satellite Laser Ranging (SLR) network. Data spanning 28 years from 8 satellites are used for measuring the large-scale mass movements on the Earth and global solid Earth dynamics. Click on image to enlarge. Credit: NASA
The study's principal researchers are Minkang Cheng and Byron D. Tapley, of the Center for Space Research, University of Texas at Austin. They reviewed climate events like El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) that affect the amount of water moving in the oceans, atmosphere and continents.
The study shows significant variations in the shape of the Earth, defined by the Earth's gravity field, or geoid, during the past 28 years might be partially linked to climate events. The study examined Earth's oblateness, how much its rounded shape flattens at the poles and widens at the equator. Scientists measured the distance from ground stations to satellites by using Satellite Laser Ranging (SLR) data that are accurate within one millimeter.
Image to left: SLR Supports Sensing of Surface Elevations: Satellite Laser Ranging (SLR) provides direct, clear measurements of surface heights of sea level and land surfaces, using satellites and SLR stations on the ground that cross-check each other. Accurate SLR measurements provide changes in the global mean sea level to a few millimeters per year. Credit: NASA
The data reflected mass changes as water redistributed in oceans, atmosphere, and in soil. The redistribution resulted in slight changes of the Earth's gravity field, detectable with geodetic satellites, those that study of the size and shape of the Earth.
The researchers found over the past 28 years, two large variations in the Earth's oblateness were connected to strong ENSO events. Variations in mass distribution, which caused the change in the gravity field, were predominantly over the continents, with a smaller contribution due to changes over the ocean. The cause of a variation in the Earth's mass over the 21-year period between 1978 and 2001, however, still remains a mystery.
Cheng and Tapley's research relied on NASA's SLR data to measure changes in the longest wavelengths of the Earth's gravity field in order to see how the global-scale mass was redistributed around the world.
The Earth's gravity is an invisible force of attraction that pulls masses together. The relative motion of a small lighter object, such as a spacecraft, to a large heavy object such as the Earth, depends on how much mass each object has and how that mass is distributed. Scientists can measure the changes in Earth's gravitational pull using instruments on the ground to track satellites in space. So, water mass shifts on Earth and the changes in shape of the Earth can be detected.
Image to right: Variations in Earth's Oblateness: Variations in Earth's oblateness (flattened top, budging middle or "J2") are indicated on this graph over several areas. Over tropical areas (blue circles) and areas outside of the tropics (black circles) caused by soil moisture changes. Changes in oblateness (J2) from the global soil-moisture-induced changes from year to year in J2 (red line), compared with the unusual mean sea level pressure readings (gray line) warm phase (<0) and cold phase (>0). These signals are dominant in the observed interannual variations of the Earth’s oblateness during the strong El Nino/La Nina events. Click on image to enlarge. Credit: NASA
The scientists also found that another change in mass distribution may have started in late 2002, which coincides with the moderate El Nino that developed at that time.
"The main idea, however, is that the Earth's large scale transport of mass is related to the long-term global climate changes," said Cheng.
Image to left: El Nino Warm Waters: This image from NASA's Jason oceanography satellite, taken during a 10-day collection cycle ending December 2, 2002, shows the Pacific dominated by two significant areas of higher-than-normal sea surface temperatures. In the central equatorial Pacific, the large area of warmer than normal sea surface temperatures (large area outlined in red with a white center) associated with growing El Nino conditions has recently migrated eastward toward the coast of South America. Click on image to enlarge. Credit: NASA JPL
The long-term history of the SLR measurements make it possible for scientists to see the changes over time in melting glaciers and polar ice sheets and the associated sea level change. The SLR data have also been used to detect the motion of global tectonic plates on which landmasses rest, the deformation of the Earth's crusts near plate boundaries, and the orientation and rate of spin of the Earth.
Images above and below: What is an El Nino?: El Nino is a weak warm ocean current that runs southward along the coast of Peru around Christmas time. El Ninos disrupt fisheries and bring severe weather events worldwide. In a normal year, the trade winds blow westward and push warm surface water near Australia and New Guinea. During El Ninos, trade winds weaken and warm, nutrient-poor water occupies the entire tropical Pacific Ocean. Heavy rains that are tied to the warm water move into the central Pacific Ocean and cause drought in Indonesia and Australia, while causing floods on the east over Peru and Chile. Credit: NASA
In March 2002, NASA and the German Aerospace Center launched the Gravity Recovery and Climate Experiment (GRACE) to sense small-scale variations in Earth's gravitational pull from local changes in Earth's mass. GRACE data will assist with future studies similar to Cheng and Tapley's research. The GRACE satellite, together with 18 other NASA research satellites, have opened new windows to exploring Earth and to understanding the intricate processes that support life.
The study was published in a recent issue of the Journal of Geophysical Research-Solid Earth.
For more information contact:
NASA Goddard Space Flight Center, Greenbelt, Md.
University of Texas at Austin, Texas Space Grant Consortium, Austin, Texas
For more information about NASA's Satellite Laser Ranging Technologies, please visit: http://ranier.oact.hq.nasa.gov/Sensors_page/Laser/SLR.html
For information about the PDO, please visit on the Internet: http://sealevel.jpl.nasa.gov/science/pdo.html
University of Texas at Austin