Our understanding of the climate system and other important human and environmental systems has benefitted significantly through the use of satellite observations over the past 30 years (NRC, 2008c). For example, data from the Earth Observing System (EOS) series of satellites deployed in the late 1990s and early 2000s provide critical input into process and climate models that have provided key insights into Artic sea ice decline, sea level rise, changes in freshwater systems, ozone changes over Antarctica, changes in solar activity, ocean ecoystem change, and changes in land use, to name just a few. Box 4.5 provides an example of a key satellite-based measurement that has promoted enhanced understanding of the physical climate system and how it is changing over time.
TABLE 4.6 Examples of Science Needs Related to Observations and Observing Systems (see Part II for additional details)
• Extend and expand long-term observations of atmosphere and ocean temperatures; sea level; ice extent, mass, and volume; and other critical physical climate system variables.
• Extend and expand long-term observations of hydrologic changes and related changes relevant for water management decision making.
• Expand observing and monitoring systems for ecosystems, agriculture and fisheries, air and water quality, and other critical impact areas.
• Improve observations that allow analysis of multiple stressors, including changes in climate, land use changes, pollutant deposition, invasions of nonnative species, and other human-caused changes.
• Develop improved observations and monitoring capabilities to support vulnerability assessments of coupled human-environment systems at the scale of cities, states, nations, and regions, and for tracking and analyzing human health and well-being.
• Develop improved observations for vulnerability assessments related to military operations and infrastructure.
• Establish long-term monitoring systems that are capable of monitoring and assessing the effectiveness of actions taken to limit or adapt to climate change.
• Develop observations, protocols, and technologies for monitoring and verifying compliance with international emissions-reduction agreements.
BOX 4.5 Ocean Altimetry"
Ocean altimetry measurements provide an illustrative example of how satellites have advanced scientific understanding of climate and climate change. Sea level changes are a fundamental indicator of changes in global climate and have profound socioeconomic implications (see Chapter 7). Variations in sea level also provide insight into natural climate processes such as the El Nino-Southern Oscillation cycle (see Chapter 6) and have the potential to inform a broad array of other climate science disciplines including ocean science, cryospheric science, hydrology, and climate modeling applications (see, e.g., Rahmstorf et al., 2007).
Prior to the satellite era, tide gauge measurements were the primary means of monitoring sea level change. However, their limited spatial distribution and ambiguous nature (e.g., vertical land motion can cause erroneous signals that mimic the effects of climate change at some sites) limited their use for climate research. With the launch of TOPEX/Poseidon in 1992, satellite altimeter measurements with sufficient accuracy and orbital characteristics to monitor small (on the order of millimeters per year) sea level changes became available (Cazenave and Nerem, 2004). Jason-1, launched at the end of 2001, continued the TOPEX/Poseidon measurements in the same orbit, including a critical 6-month overlap that allowed intercalibration to ensure the continuity of records. It is important to note that tide gauges remain a critical component of the sea level observing system, providing an independent source of data in coastal areas that can be used to calibrate and interpret satellite data records. The integration of tide gauge and satellite data provides an excellent example of how satellite and surface-based observations are essential complements to one another within an integrated observing system.
Together, the TOPEX/Poseidon and Jason-1 missions have produced a continuous 15-year time series of precisely calibrated measurements of global sea level. These measurements show that sea level rose at an average rate of ~3.5 mm/year (0.14 inches/year) during the TOPEX/Jason-1 period, nearly double the rate inferred from tide gauges over the 20th century (Beckley et al., 2007; Leuliette et al., 2004). Since sea level rise is driven by a combination of ocean warming and shrinking glaciers and ice sheets (see Chapter 7), these altimetry results are also important for refining and constraining estimates of ocean heat content and ice loss. Another powerful aspect of satellite altimetry is that it provides maps of the spatial variability of the sea level-rise signal (see figure on facing page), which is valuable for the identification of sea level "fingerprints" associated with climate change (see also Mitrovica et al., 2001). Sea level measurements are also used extensively in ocean reanalysis efforts and short-term climate predictions.
Jason-2,fa which carries similar but improved instrumentation, was launched in June 2008. By design, Jason-2 overlaps with the Jason-1 mission, thus providing the requisite intercalibration period and securing the continuity of high-accuracy satellite altimetry observations. Funds have been requested
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