Perlwitz et al. begin their narrative by noting that there has been "a decade-long decline (1998-2007) in globally averaged temperatures from the record heat of 1998," citing Easterling and Wehner (2009). And in further describing this phenomenon, they say that U.S. temperatures in 2008 "not only declined from near-record warmth of prior years, but were in fact colder than the official 30-year reference climatology (-0.2°C versus the 1971-2000 mean) and further were the coldest since at least 1996."

With respect to the geographical origin of this "natural cooling," as they describe it, the five researchers point to "a widespread coolness of the tropical-wide oceans and the northeastern Pacific," focusing on the Niņo 4 region, where they report that "anomalies of about -1.1°C suggest a condition colder than any in the instrumental record since 1871."

So, pushing the cause of the global and U.S. coolings that sparked their original interest back another link in the chain which -- in their estimation -- connects them with other more primary phenomena, they ask themselves what caused these latter anomalous and significant oceanic coolings?

Perlwitz et al. first discount volcanic eruptions, because they say "there were no significant volcanic events in the last few years." Secondly, they write that solar forcing "is also unlikely," because its radiative magnitude is considered to be too weak to elicit such a response. And these two castaway causes thus leave them with "coupled ocean-atmosphere-land variability" as what they consider to be the "most likely" cause of the anomalous coolings.

In regard to these three points, we agree with the first. With respect to Perlwitz et al.'s dismissal of solar forcing, however, we note that the jury is still out with respect to the interaction of the solar wind with the influx of cosmic rays to earth's atmosphere and their subsequent impact on cloud formation, which may yet prove to be substantial. And with respect to their final point, we note that the suite of real-world ocean-atmosphere-land interactions is highly complex and also not fully understood. Indeed, there may even be important phenomena operating within this realm of which the entire scientific community is ignorant. And some of those phenomena may well be strong enough to totally compensate for anthropogenic-induced increases in greenhouse gas emissions, so that other natural phenomena end up dictating the ever-changing state of earth's climate, as could well be what has been happening over the last decade or more.

In light of these considerations, therefore, as well as the substantial strength and longevity of the planet's current cooling phase, the path of wisdom would seem to us to be to wait and see what happens next, in the unfolding biogeophysical drama of earth's ever-changing climatic path to the future, before we undertake to attempt to change what we clearly do not fully comprehend.

Sherwood, Keith and Craig Idso

References
Easterling, D.R. and Wehner, M.F. 2009. Is the climate warming or cooling? Geophysical Research Letters 36: 10.1029/2009GL037810.

Perlwitz, J., Hoerling, M., Eischeid, J., Xu, T. and Kumar, A. 2009. A strong bout of natural cooling in 2008. Geophysical Research Letters 36: 10.1029/2009GL041188.