AGW Observer

Observations of anthropogenic global warming

On quick feedback determinations

Posted by Ari Jokimäki on April 22, 2010

This article was originally published in Ilmastotieto-blog in Finnish.

In last few years there has been some studies where climate system feedback has been tried to determine from short-term measurements of surface temperature and outgoing radiation. They have studied what happens to the radiation leaving the Earth when surface temperature changes. Such studies have been Forster & Gregory (2006), Spencer et al. (2007), Spencer & Braswell (2008, 2009) and Lindzen & Choi (2009). Of these, teams of Spencer and Lindzen have found negative values for the feedback against common understanding (negative value of feedback would mean that the total warming effect from carbon dioxide would be small).

In a new study, Lin et al. (2010a) have looked into this and they focus especially to checking the results of Spencer & Braswell (2009, SB09). This new study hasn’t been officially published yet but it has been peer-reviewed and it is in press to be published in near future. Lin et al. use basically the same modelling and analysis methods as SB09, but Lin et al. include more realistic climate system which also includes climate “memory”. They refer to their earlier study (Lin et al., 2010b) where they studied the climate memory with climate feedback and found a positive feedback, and they also found that the feedback is more positive if the climate memory is longer.

First Lin et al. perform their modelling and analysis without climate memory and using the same feedback parameter value as SB09 to see if they can replicate the results of SB09. They do get the same result as SB09. Next Lin et al. perform the same analysis but now they include a one year memory to the climate system. In their earlier study they had found that climate memory is at least about 8 years. Now they use a one year memory because they wish to show that the total feedback of the climate system does not show up with quick surface temperature changes, even if there would be a short memory in the system. They already studied longer memories in their earlier study and found a positive feedback. Now they use one year memory and they then use different values for the feedback parameter from the extremely small value used in SB09 to bigger values. They find that the climate feedback with this short, one year climate memory, varies from very negative to slightly positive. They say:

During a short time period, there could be significant global surface temperature fluctuations. But the averaged temperature change during this period should be small. Thus, long-term feedbacks would be significantly weaker compared to short-term feedbacks during this period and cannot be detected by short-term observations.

They think that in order to determine the climate system feedback, one must use long-term observations and model runs. If an analysis uses only short-term observations and model runs, it may produce a feedback value that is significantly in error. In a short-term analysis it is likely to happen that the long-term response in the climate system determined by the climate memories cannot be seen from the noise of the fast climate processes. From Lin et al. new study, it seems that SB09 and other short-term studies have been measuring this noise.

Thanks to Tuomas for material supply and AJ for commenting the text and the subject.

Main reference: Lin, Bing, Qilong Minb, Wenbo Sunc, Yongxiang Hua and Tai-Fang Fan, 2010a, Can climate sensitivity be estimated from short-term relationships of top-of-atmosphere net radiation and surface temperature?, Journal of Quantitative Spectroscopy and Radiative Transfer, Article in Press, doi:10.1016/j.jqsrt.2010.03.012. [abstract]

Other referred articles:
Forster, P., Gregory, J., 2006, The climate sensitivity and its components diagnosed from earth radiation budget data, Journal of Climate 2006; 19: 39-52. [abstract, full text]

Lin, B., L. Chambers, P. Stackhouse Jr., B. Wielicki, Y. Hu, P. Minnis, N. Loeb, W. Sun, G. Potter, Q. Min, G. Schuster, and T.-F. Fan, 2010b, Estimations of climate sensitivity based on top-of-atmosphere radiation imbalance, Atmos. Chem. Phys., 10, 1923-1930, 2010. [abstract, full text]

Lindzen, Richard S, Yong-Sang Choi, 2009, On the determination of climate feedbacks from ERBE data, GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L16705, 6 PP., 2009 doi:10.1029/2009GL039628. [abstract, full text]

Spencer, Roy W., William D. Braswell, John R. Christy, Justin Hnilo, 2007, Cloud and radiation budget changes associated with tropical intraseasonal oscillations, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L15707, 5 PP., 2007
doi:10.1029/2007GL029698. [abstract, full text]

Spencer, Roy W., William D. Braswell, 2008, Potential Biases in Feedback Diagnosis from Observational Data: A Simple Model Demonstration, Journal of Climate 2008; 21: 5624-5628. [abstract, full text]

Spencer, Roy W., William D. Braswell, 2009, On the Diagnosis of Radiative Feedback in the Presence of Unknown Radiative Forcing, American Geophysical Union, Fall Meeting 2009, abstract #A32A-03. [abstract, presentation material]


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