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Has Human Influence On The Seasonal Cycle Been Found?

Dr David Whitehouse, GWPF Science Editor

Has a newly identified human-caused signal emerged from the background of climate noise?

This is the claim by Santer et al in a paper just published in Science. The signal is increased seasonal range of tropospheric temperatures observed in mid-latitudes.

Santer et al. say that most such AGW “fingerprint” studies rely on comparing climate model predictions with annual or decadal averages of specific climate parameters. They consider shorter timeframes; the seasonal cycle in the troposphere comparing the warmest and coldest months of the year with a “multimodal ensemble of simulations.”

As well as the paper itself Science contains two explanative articles on the research. One of them starts thus: ‘The identification of anthropogenically forced climate change from observational data is challenging. Climate-change effects over the time scale of decades are relatively small compared to natural variability but become progressively larger and influential as time proceeds.’

That’s an intro that ten years ago would have got the writer called a climate denier. That was a period when only references to a strong anthropogenic signal would have been regarded favourably.

Seasonal Amplification

Looking at 40 years of satellite data on tropospheric temperature the researchers see the amplification of the background seasonal pattern seen in model simulations. It is observed in mid-latitudes in both the northern and southern hemisphere, though to a much greater extent in the north. The strongest signals are seen in eastern Canada and eastern Siberia.

But, as one would expect when dealing with such complex systems, it is not all straightforward.

There seems to be some evidence that as far as surface temperature goes the largest warming occurs in the winter and decreases the background cycle, in contrast to the finding from the present study. Indeed, looking back over 40 years of surface temperature most studies show no seasonal cycle effect as seen by Santer et al in the lower troposphere.

The comparison of data with the computer models is also not straightforward, as a comparison between them shows how to cherry-pick desired feature and ignore others. (Click on image to enlarge). What’s more, despite it emerging from the model simulations the specific mechanism for the amplification is not known.

All things considered this is a very interesting paper, but given the inconsistencies and limitations I wouldn’t say it is as “powerful” new evidence as the authors of the paper contend.