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Earth Observations from Space: The First 50 Years of Scientific Achievements
ecosystems will require global-scale long-term observations from carefully calibrated satelliteborne sensors.
Early carbon cycle models that were used to investigate sources and sinks of anthropogenic CO2 ignored the effects of marine productivity, which was thought to be in equilibrium on annual timescales. Since marine productivity is not limited by carbon, it was reasoned that increases in CO2 would not affect oceanic productivity. More recently, modelers have investigated how marine productivity might be affected indirectly by climate change through its effect on oceanic and atmospheric circulation patterns.
Because phytoplankton life cycles are orders of magnitude shorter (days versus years or decades) than those of terrestrial plants, phytoplankton may respond to climate influences on ocean circulation, mixing, and the supply of nutrients and light much more quickly than plants in terrestrial ecosystems. Given that oceanic primary productivity is estimated to be roughly half of all global primary productivity, the oceanic component of the carbon cycle will respond more quickly to climate changes.
For example, there are vast areas of the Pacific and Southern Oceans, where phytoplankton productivity might be limited by iron (Martin et al. 1994). In contrast to the other limiting nutrients, which are supplied primarily by the deep ocean, atmospheric dust deposition is one of the main sources of iron to the open ocean. Paleorecords indicate that the Southern Ocean responded with increased productivity during colder periods when iron atmospheric deposition was enhanced due to the expansion of arid regions. This led to the notion that these areas in the Pacific and Southern Oceans could be stimulated to draw down large amounts of atmospheric CO2 if they were provided with iron. Several experiments conducted in the late 1990s and early 2000s proved conclusively that iron does limit production in these regions (Coale et al. 2004). Iron is supplied to the open ocean by atmospheric transport (dust deposition), by lateral advection of waters from the continental margins, and by upwelling of deep iron-rich waters. Long-term monitoring of the ocean phytoplankton will reveal whether climate change will affect these iron supplies potentially fertilizing the Southern Ocean or the Pacific.
With 10 years of continuous ocean color data (since 1997), we now have the ability to observe year-to-year variability in global oceanic primary production and begin to assess longer-term trends in ocean carbon uptake. Behrenfeld et al. (2006) describe a steady climate-driven decrease in oceanic NPP related to the warming of permanently stratified ocean waters at mid- to low latitudes over the past 8 years. This period of decreasing NPP followed the rise in NPP between the El Niño and La Niña phases. Satellite observations afford the only means of estimating and monitoring the role of the ocean biomass as a sink for carbon.
LONG-TERM ECOSYSTEM RECORD REVEALSATMOSPHERE-BIOSPHERE COUPLING
Although early studies established that red and near-infrared satellite bands could track changes in plant growth and development (Box 9.1), the large number of Landsat images (~5,000) required to assemble a global database, combined with computational requirements and frequent cloud cover, have prevented analysis of complete global or time series of Landsat data sets. Launched in 1978, the Coastal Zone Color Scanner showed that ocean productivity could be observed using visible and near-infrared bands; however, CZCS measurements were saturated over land and thus unusable.
The Advanced Very High Resolution Radiometer on the National Oceanic and Atmospheric Administration’s (NOAA) polar-orbiting weather satellites has obtained a continuous record of daily global observations since 1978, acquiring both red and near-infrared bands. Because AVHRR was not designed for observing the terrestrial biosphere and the 1- to 8-km scale of AVHRR pixels was significantly larger than theoretical understanding of ecosystem processes, scientists were initially skeptical about whether biospheric patterns and trends could be observed. However, scientists have managed to overcome technical problems such as maintaining calibrations, screening clouds, and adjusting for different observational angles. Thanks to the pioneering efforts of Compton Tucker, the daily AVHRR data set now spans more than 25 years and is the longest continuous global record available of terrestrial productivity, phenology, and ecosystem change for monitoring biospheric responses to climate change and variability. Although AVHRR was not designed for climate monitoring, continuing improvements in calibration and reanalysis have produced a consistent record for monitoring and assessing past and future biospheric responses resulting from climate change and variability and anthropogenic activities.
Initial studies using AVHRR followed seasonal and annual trends in ecosystem production and vegetation phenology at regional and continental scales (Tucker et al. 1985, Townshend et al. 1985) and at the global scale (Justice et al. 1985). In the early 1990s some key papers introduced the use of remote sensing data to ecology (Roughgarden et al. 1991, Ustin et al. 1991) and stressed the need for ecologists to focus on global ecological problems (Mooney 1991). These ideas led to the resurgence in ecosystem research and modeling of biogeochemical processes and significant advances in understanding the Earth as a system.
By the mid-1990s, global ecosystem and biogeochemical models used satellite data to establish variable vegetation composition and abundance (e.g., Biome BioGeochemical Cycles [BGC], Running and Hunt 1993; CASA, Potter et al. 1993). The concept of resource limitations as the controlling mechanism determining NPP was established in the late 1980s (Chapin et al. 1987). This placed a premium on direct