Decade-to-Century-Scale Climate Variability and Change A Science Strategy (1998) / Chapter Skim
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6 Crosscutting Issues
6 Crosscutting Issues
Pages 111-123
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From page 111... ...
, and the relative scarcity of global measurements throughout the century is only the first. Changes in the methods of measuring land and marine surface-air temperatures from ships, buoys, and land-surface stations; changes in instrumentation, instrument exposures, and sampling times; urbanization effects-these are but a few of the time-varying biases that have plagued the interpretation of the surface air-temperature records for the twentieth century.
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From page 112... ...
DECADE-TO-CENTURY-SCALE CLIMATE VARIABILITY AND CHANGE: A SCIENCE STRATEGY Recent global-scale measurements of layer-averaged atmospheric temperatures and sea surface temperatures from instruments aboard satellites have greatly aided our ability to monitor global temperature change (Spencer and Christy, 1992a,b; Reynolds, 1988) , but the situation is far from satisfactory (Hurrell and Trenberth, 1996~.
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From page 113... ...
(It is now known that the data from these balloons high in the atmosphere have an inadvertent temporal bias, because of improvements in shielding from direct and reflected solar radiation (tours and Eskridge, 1995~.) Even if the instrumental records of ground, atmosphere, and sea surface temperatures were accurate enough, the length of the records would still be an issue.
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From page 114... ...
Since CO2 is a long-lived atmospheric constituent and it is well mixed through the atmosphere, a moderate number of well-placed stations can provide a very robust estimate of global changes in carbon dioxide as long as they operate for the primary purpose of monitoring seasonal-to-decadal changes. To understand the causes of the increase in atmospheric carbon dioxide, however, we must understand how the carbon cycle operates and how the anthropogenic carbon budget is balanced.
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From page 115... ...
Much wiser implementation and monitoring practices must be adhered to for both space-based and surface-based observing systems if we are to adequately understand global changes. A number of steps can be taken to improve our ability to monitor climate and global change: · When changes are made to existing environmental monitoring systems, or new observing systems are introduced, standard practices should include an assessment of the impact of these changes on our ability to monitor environmental variations and alteration.
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From page 116... ...
The large-scale spatial and seasonal variations of properties distributed by the climate system provide critical tests of the transport processes simulated by climate models. For example, the distribution and rates of accumulation of anthropogenic or chemically reactive gases and aerosols in the atmosphere, or the distribution of inorganic carbon and freshwater in the ocean, are fields whose simulation critically tests both the source/sink and transport processes in models.
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From page 117... ...
Paleoclimate reconstructions also allow us to observe the response of climate to changes in forcings or boundary conditions for example, the relationship between drought and solar variability, or the behavior of ENSO during periods of warmer background SST or lower sea level. By improving our understanding of the natural variability and sensitivity of climate over dec-cen time scales, paleoclimatic reconstructions enable us to evaluate model behavior over these time scales (see, e.g., Knutson et al., 19971.
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From page 118... ...
A combination of reconstructions from ice cores and findings at archaeological sites in Greenland support the theory that Norse settlements were abandoned because of colder summertime temperatures associated with the early stages of the transition into the Little Ice Age which gripped the North Atlantic region for several centuries (Pringle, 19971. Calibration of Proxy Records The extraction of climatological quantities from proxy data requires an understanding of how the signal is incorporated into the proxy, and of any competing influences on the record.
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From page 119... ...
A global "ridded annual-mean time series of, for example, surface temperature and precipitation for the past 1,000-2,000 years from reconstructed paleodata would allow the application of standard statistical techniques for comparison with modern data. Summary Proxy data provide a unique contribution to the objectives of a research program aimed at understanding climate change on dec-cen time scales.
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From page 120... ...
Analysis Products and Model Output A special hybrid climate-information product involves the interpolation of climatic datasets into spatially consistent fields, through the use of climate modeling and assimilation techniques. In essence, the inaccurate, irregularly spaced, and often sparse observations are blended with a model simulation to produce a globally consistent sequence of climatological fields.
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From page 121... ...
The GCMs' spatially detailed, local description of phenomena and their short-term variability limits these models' ability to provide such a description for sufficiently long time intervals and sufficiently broad ranges of the many unknown parameters that enter the governing equations. Simple models are flexible enough to explore broad swaths of parameter space and study variability on the long time scales of interest for dec-cen climate variability, but they can be evaluated against observations only through the use of intermediate models with greater spatial and physical detail.
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From page 122... ...
Obtaining reliable, long-term observations of factors that change thermal forcing, which drives the longterm variations of a climate model. These factors include not only the increase in greenhouse gases, but also changes in solar irradiance and in aerosol loading in the atmosphere.
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From page 123... ...
Records from past ob servations of both ocean and the atmosphere should be compiled and analyzed for variables such as concentration of greenhouse gases in ice cores, sea-level pressure, surface and subsurface temperature and salinity in oceans, air tem perature and humidity at the surfaces, and temperature and geopotential height at selected pressure levels in the atmo sphere. It is also essential to improve model parameteriza tions of various feedback processes, in particular those in volving cloud, snow, and sea-ice cover, all of which substantially!
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