Challenges for the Future

It is crucial to avoid treating cloud, radiation, and aerosol observations in isolation. Instead, we must study them in their meteorological context (i.e., temperature, humidity, and winds) because clouds are formed by dynamic and thermodynamic processes, and aerosols provide a perturbation to the formation and dissipation processes as well as to the physical and radiative properties of clouds once they have formed. Numerical weather prediction reanalyses provide the most consistent source of such data, and now they include output for the ERB and cloud fields as well. Reanalyses are thus an important resource for climate studies; however, although the representation of the clear-sky ERB can be good, there are still signifi cant errors in the simulation of clouds and of the all-sky ERB (Allan et al. 2004). A climate-quality reanalysis using a coupled ocean-atmosphere model, paying attention to the radiative and hydrological cycles, would be a major step forward and should be a high priority (Bengtsson et al. 2007).

Several independent cloud and ERB datasets suggest that there might have been a decrease in high-level cloud cover during the past several decades that has reduced LWCRF over time. ERBS, ISCCP flux dataset, and ocean heat content measurements indicate that this slight cooling effect has recently been opposed by a stronger warming effect caused by a weakening of SWCRF. Considering all the corrections that have been applied to the data, can we confidently conclude that these trends are real? If they truly exist, are they related to anthropogenic climate change or are they merely produced by some natural decadal cycle? What implications do the cloud and ERB trends have on our understanding of climate sensitivity?

Answering these questions with confidence will require a thorough reexam-ination and reprocessing of the different types of available data to reconstruct our best understanding of past variations in cloudiness and ERB and how they are associated with other parameters of the climate system. We should especially use knowledge gained from more recent detailed and comprehensive satellite measurements to improve older records. Better known quantities should be employed to constrain less known quantities when empirical calibration is unavoidable, and improved physical models should be developed to address issues like the perceived variation of cloud optical thickness with satellite view angle. Since the magnitude of systematic artifacts relative to real cloud and radiation variability is typically greatest at the largest scales, patterns of regional changes may be more robust than global changes. In particular, investigations should search for regional trends in cloudiness and ERB that are likely to be associated with anthropogenic global warming or aerosol.

Considering the many inadequacies in the historical data record, it may seem preferable to forget about it and instead to devote our efforts solely to making more precise, comprehensive, and stable measurements in the future. This would be a mistake, we believe, since many years of observation will be required to distinguish a climate change signal from natural variability. We do not have the luxury of waiting for those years to come, but rather must make better use of the record that we do have.

Since there have been damaging gaps in the past record of ERB measurements, which have made it difficult to quantify decadal variability or to identify climate trends, it is essential that we maintain continuity in the future. Any trends in ERB are expected to be small and thus require careful cross-calibration of the various instruments flown at different periods, and without overlaps. Such cross-calibration is very difficult, if not impossible, to achieve. Independent parameters, such as ocean heat content, should also be monitored to provide additional constraints on ERB. As global warming signals begin to emerge clearly above the level of natural variability, one of the greatest challenges facing attempts to ensure continuity of ERB and other measurements is the difficulty of obtaining adequate funding for climate monitoring missions, despite their obvious importance. It is relatively straightforward to justify the continuity of missions to support numerical weather prediction, because of the requirements of operational agencies. However, no satellite agency has yet been able to commit to continuous funding of climate monitoring. Thus, there is a real danger of losing the continuous record of ERB measurements because of future funding constraints. The numbers presented above suggest that the minimum requirement for monitoring changes in the ERB associated with global warming would be continuation of the coverage from the CERES or similar instruments at the current level. Other complementary measurements that might be useful include radiation budget measurements of the sunlit portion of the globe from a satellite at the Lagrange point L1. A particularly valuable mission that has been proposed is a climate calibration observatory, which would be used to cross-calibrate instruments on other satellites to make better use of their data by bringing them together to a common reference scale that is compatible with the CERES data.

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