Uncertainty Identified in GCMs With Respect to Predicting Global Albedos

There is a new paper which assesses global model skill at accurately representing albedo (thanks to Willie Soon for alerting me to it) . The article in the May 2006 issue of the journal Tellus (subscription required) is entitled

“22 views of the global albedo—comparison between 20 GCMs and two satellites”

By F. Bender, H. Rodhe, R. J. Charlson, A. M. L. Ekman and N. Loeb.

The abstract reads,

” comprehensive comparison of characteristics of the planetary albedo (α) in data from two satellite measurement campaigns (ERBE and CERES) and output from 20 GCMs, simulating the 20th-century climate, is performed. Discrepancies between different data sets and models exist; thus, it is clear that conclusions about absolute magnitude and accuracy of albedo should be drawn with caution. Yet, given the present calibrations, a bias is found between different estimates of α, with modelled global albedos being systematically higher than the observed. The difference between models and observations is larger for the more recent CERES measurements than the older ERBE measurements. Through the study of seasonal anomalies and space and time distribution of correaltions between models and observations, specific regions with large discrepancies can be identified. It is hereby found that models appear to over-estimate the albedo during boreal summer and under-estimate it during austral summer. Furthermore, the seasonal variations of albedo in subtropical areas dominated by low stratiform clouds, as well as in dry desert regions in the subtropics, seem to be poorly simulated by the models.”

In their discussion, they write,

“From our comparison between satellite observations and GCM simulations of the planetary albedo, we find that models and measurements differ in many respects. GCM-derived albedos are almost consistently higher than the values observed by satellites. For the period with global ERBE data (February 1985–May 1989), the modelled global mean albedo is on average 0.009 above the measured global mean. This corresponds to a difference in radiative flux of almost 3 Wm-2. The mean level of global mean albedo according to CERES (March 2000 December 2003) is an additional ca. 0.012 below the ERBE mean, corresponding to an additional flux difference of ca. 4 Wm-2.

The difference between the two measurement sets may indicate a real albedo difference betweeen the two measurement periods, but part of it is undoubtedly due to calibration and algorithm differences. The fact that the models are closer to the ERBE data and farther from the CERES data raises the question of how they are being and should be tuned to satellite measurements.

The models have difficulties capturing the difference between the higher NH winter solstice peak and the lower SH winter solstice peak in the seasonal cycle of albedo, seen in the ERBE and CERES observations. This is likely due to the representation of snow and ice in the models. Large and consistent differences in absolute values between ERBE and model albedos occur at high southern latitudes during boreal summer, where almost all models under-estimate the albedo by as much as 0.1…..

In this study, we have seen that there are substantial differences between models and measurements when it comes to regional and global albedo. By pointing out specific problems and apparent shortcomings, we hope to facilitate the improvement of GCMs in this respect. We have also seen that large uncertainties and poor/insufficient coverage in observations make it hard to draw conclusions with confidence, and we therefore take this opportunity to urge experimentalists to refine their products and quantify the uncertainties therein. Of course development of the two go hand in hand, and by improving both models and measurements we will decrease differences and increase understanding, which is a basic condition for our future attempts to understand climate stability. ”

This study shows that there is a fundamental issue with a critical aspect of radiative forcing (the albedo). The uncertainties are on a order of a few watts per meter squared. This uncertainty, regardless of the fraction that is due to the global models and the observations, respectively, is on the same order as the magnitudes of the the other radiative forcings! The assessment of albedo changes over time, as they influence the net radiative forcing at the level of tenths of watts per meter squared, is not yet achievable based on the results of this study.

This is yet another reason why we need to move away from using multi-decadal global climate models as the tool to communicate to policymakers. They are NOT skillful multi-decadal projection tools even in terms of global average radiative forcing.

Leave a comment

Filed under Climate Change Forcings & Feedbacks, Climate Models

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.