THE VALUE OF PRECISE MEASUREMENTS

The power of modern geodesy is matched by the challenges it faces, namely, detecting minute changes in Earth’s system over time. Over the past 50 years, space-based geodetic technologies have revolutionized the way we look at our planet, allowing us to measure and monitor changes in the Earth’s system with unprecedented levels of accuracy and detail. Modern geodesy delivers precision to one part per billion, and precision of one part per trillion can be envisioned in the foreseeable future (see Box 1.1). The current level of precision enables us to track the shrinking distance between Los Angeles and San Francisco to an accuracy of 1 millimeter as we keep watch for the next California earthquake; or a millimeter shift of the pole associated with a large earthquake in Chile; or a microsecond change in the length of day associated with an Asian monsoon; or the slight change in the gravity field due to a drought-induced drop in the water table across the Mississippi drainage basin.

Such exquisitely precise measurements provide critical information for many areas of science with a tangible societal impact. One particularly complex example is sea level change. Over the past decade, the global sea level has increased by an average of 3.3 millimeters per year and has been predicted to rise by as much as one meter by the end of the 21st century. The shallow slope of some shorelines will exacerbate the impact of this vertical change in sea level, so that one meter of sea level rise would flood 2.2 million square kilometers of coastline, displace 145 million people worldwide, and result in the loss of $944 billion in combined global gross domestic product (Anthoff, 2006) (see Figure 1.1).

Geodetic technologies have been critical to measuring past sea level change over time, but predicting future changes can be exceedingly complex, underlining the need for precision infrastructure for global sea level monitoring. Global sea level change is largely caused by two factors: (1) changes in the thermal and salinity conditions of the ocean (which result from the expansion of the ocean’s volume due to heating), and (2) flow of water into the ocean from melting ice sheets and glaciers. The combined measurements obtained from three geodetic observing systems—altimetry and gravity satellite missions, and tide gauge networks—allow us to estimate the contributions of these sources. Recent measurements indicate that melting of glaciers and polar ice sheets contribute approximately 2 millimeters per year to sea level rise. In the year 2007 alone, the Greenland ice sheet lost nearly 300 billion tons of its mass, enough to bury the District of Columbia under a mountain of ice nearly 2,000 meters high (about 6,500 feet), or blanket the entire state of California with three-quarters of a meter (2.5 feet) of ice. The west Antarctic ice sheet is melting at a comparable rate, and the Antarctic continent as a whole is undergoing changes that we will not fully grasp until the comple-

BOX 1.1

Precision Notation

Geodesists often use a “parts-per” shorthand notation to denote precision; this is the error in measurement of the distance between two points divided by the distance. For example, if the distance between two points separated by 1,000 km (1 billion millimeters) can be measured with a precision of 1 millimeter, then in “parts-per” notation the precision is 1 part-per-billion (e.g., Davis et al., 1988).

Distance

 

1 km

100 km

1,000 km

Precision

1 part per million

1 mm error

0.1 m error

1 m error

 

1 part per billion

0.001 mm error

0.1 mm error

1 mm error



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