Long-term, high-quality observations are essential for defining the current state of the Earth's system, discovering the magnitude and sources of past trends, and providing the scientific basis for future predictions and variability.  Observational capability should support determination of trends in means and changes in distributions of extremes, and cover spatial scales from global to local. The broader U.S. governmental context for Earth observations, including those for climate, has been documented by the OSTP in its September 2010 report, Achieving and Sustaining Earth Observations: A Preliminary Plan Based on a Strategic Assessment by the U.S. Group on Earth Observations.

The observational system must be an integrated mix of satellite and non-satellite systems.  These systems complement each other in a variety of ways--satellites provide the global coverage frequently inaccessible to surface-based measurements, while surface measurements can achieve greater spatial and/or temporal resolution, as well as measure parameters inaccessible to satellites and provide information on earlier periods in Earth system history.  The USGCRP agencies are leading contributors to the national and global observing system.  The determination of long-term trends and the requirement for measurement consistency place enormous burdens on the measurement systems in terms of calibration and validation, as well as the need for transparency in algorithm and instrument knowledge.  In implementing its measurement programs, the United States follows wherever possible the Monitoring Principles established by the Global Climate Observing System.

The collection of observational data and their importance to global change research makers the long-term stewardship of data an essential part of the observational program. Not only is the volume of these data sets ever-increasing, but they also come from a multiplicity of sources.  All of these data sets must be archived and disseminated in ways to conveniently facilitate their long-term use by the full range of user communities, especially by those involved in truly multidisciplinary scientific, adaptation, and assessment work.