Climate Benchmarks

Benchmark Measurements for Model Testing

The most important single requirement on climate data is that we leave for future investigators completely reliable and reproducible measurements of the climate. We also require measurements that are of very high accuracy if we are to detect the fingerprint of anthropogenic climate change before it becomes a major problem. Space measurements, in general, have not been designed to this level of accuracy, nor have they had the means for in-flight verification demonstrating that the required accuracy has been achieved. However, if accuracy is given the highest priority it is possible to realize key space Benchmarks with existing technology. Two examples of realizable space Benchmarks are radio refractivities from occultations of GPS satellite transmissions; and absolute, spectrally resolved thermal radiances. In order to study long-term climate change, data on spectral radiances must be comparable from one satellite to another, regardless of when or by whom the measurement is made. This requires a Benchmark strategy under which observations are independent of the local environment and are tied to absolute standards. History has chronicled the development of precision metrology in the laboratory, and the principles that have been developed there are critical to scientific strategies addressing long-term climate change.

Resolved radiances measured by downward-directed spectrometers in Earth orbit are the most important and versatile of climate variables that can be observed from space for the following reasons. Both the radiative forcing of the atmosphere resulting from greenhouse gas emissions and aerosols and the response of the atmospheric variables are clearly observable in the spectral signal of outgoing radiance. The increases in greenhouse gas concentrations and aerosol loading imply changes in the spectral distribution of outgoing infrared radiation. Similarly, large differences among model projections of temperature, water vapor and cloud distributions imply, for each model, different predicted changes in outgoing radiation. The spectrum of IR radiance, if observed accurately and over the full terrestrial band, carries decisive diagnostic signatures in frequency, spatial distribution and time. At satellite altitudes, the boundary conditions on radiative processes can be measured without interference, and spectral resolution allows us to study individual forcings and their responses, both thermal and in the water cycle, including cloud formation. There is no other single measurement that can yield such a wide range of relevant climate information.

IRIS SpectrumIn order to study long-term climate change, data on spectral radiances must be comparable from one satellite to another, regardless of when or by whom the measurement is made. This requires a Benchmark strategy under which observations are independent of the local environment and are tied to absolute standards. History has chronicled the development of precision metrology in the laboratory, and the principles that have been developed there are critical to scientific strategies addressing long-term climate change.