BOX 3.2


Sustained measurements are needed to distinguish short-term variability in the Earth system from long-term trends. Sea level, for example, is monitored with a radar altimeter that measures the height of the ocean relative to a fixed reference level. Sea level must be measured with accuracy sufficient to distinguish 50-mm seasonal variations from the 3-mm climate signal (Figure 3.2.1, top). The exceptionally long data record of the TOPEX/Poseidon (T/P) mission gave an estimate of global sea-level rise of about 3 mm/year. If T/P had failed late in 1997, the increase in sea level in 1997 would have appeared to represent an acceleration of sea-level rise rather than an anomalous peak in a longer-term trend. A follow-on mission to T/P, Jason-1, a cooperative effort of the U.S. and French space and operational agencies, was launched before T/P failed. An overlap period of 4 years between the two missions allowed the science and engineering teams to detect and correct for a slow degradation of the T/P tracking system to give a continuous record of sea-level rise.

Such overlap is particularly important in climate observations. The design of systems for climate observing and monitoring from space must ensure the establishment of global, long-term climate records that are of high accuracy, tested for systematic errors on-orbit, and tied to irrefutable standards, such as those maintained in the United States by the National Institute of Standards and Technology. For societal objectives that require long-term climate records, the accuracy of core benchmark observations must be verified against absolute standards on-orbit by fundamentally independent methods so that the accuracy of the record archived today can be verified by future generations. Societal objectives also require a long-term record that is not susceptible to compromise by interruptions. Climate observations are different from weather observations; for example,the continuing debate over the reliability of surface-temperature records and the community’s inability to establish the upper-air temperature record over the last several decades stem from attempts to create climate records from what are essentially weather-focused observations.

The issue of sea-level rise also illustrates the importance of sustained in situ measurements. Observations of ocean temperatures from a network of drifting buoys, Argo, provided the foundation for estimating the contribution of a warming ocean (about 1.7 mm/year) to sea-level rise. The residual of 1.3 mm/year (Figure 3.2.1, bottom) is the result of other processes that presumably melt ice sheets—a contribution that may accelerate sea-level rise in the future. The residual sea-level rise can then be compared with estimates of changes in ice volume to verify that presumption. Trends in all the measurements are needed to calibrate climate models that predict changes in sea level and in other climate variables. This example illustrates the importance of avoiding gaps in the data record, of coordinating satellites with other measurement programs, and of supporting science and engineering teams in maintaining and interpreting the observations.

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