In our last World Climate Report article, we described new findings that verified older findings that the patterns of sea surface temperature (SST) variations in the Atlantic Ocean (including in the tropical Atlantic region which is the birthplace of Atlantic tropical storms and hurricanes), are largely a reflection of natural variability, with some anthropogenic warming thrown in for good measure.

This time, we report on new research that finds that rather than a large dose of anthropogenic warming, a decline in the amount of dust coming off of the Saharan desert may have collaborated with multidecadal natural oscillations to produce the observed warming trend in Atlantic tropical SST over recent decades. An implication of this finding is to further lessen any impact than human emissions of greenhouse gases may have had on the observed behavior of Atlantic hurricanes, including the recent upturn in activity.

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There is a degree of disagreement among climate scientists as to whether or not a phenomenon known as the Atlantic Multidecadal Oscillation (AMO) is a true physical mechanism operating in the Atlantic Ocean (e.g., Delworth and Mann, 2000; Knight et al., 2005; Zhang, 2007), or whether it is largely a manifestation of the pattern of the anthropogenic influence on the earth’s climate (Mann and Emanuel, 2006). The subject is of considerable interest in that many researchers have identified other climate phenomenon that seem to be related to the patterns of the AMO—primary among which are the patterns of Atlantic hurricane activity (e.g. Goldenberg et al., 2001). Thus, the source of the AMO likely sheds light on the source of Atlantic hurricane frequency and intensity fluctuations—are they primarily natural in origin, or are they primarily caused by human emissions of greenhouse gases and aerosols?

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Guest Commentary

Craig Loehle, PhD
National Council for Air and Stream Improvement, Inc. (NCASI)

Tree rings are widely used for reconstructing climate and past climates are critical for putting the current climate (including global temperatures) into the proper perspective. Is current warming unusual? Only a comparison to the past can tell.

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