By Javier

A most remarkable climate phenomenon is taking place under our very noses without anybody paying attention to it.

As nearly everybody knows, the planet is warming. Since its short-term rate of temperature change hasn’t changed much, the warming is essentially accomplished because the planet spends less time cooling than warming. Therefore, periods of cooling have become shorter and result in less cooling.

In the figure below, we can see the top 10 biggest periods of cooling in terms of temperature decrease since 1950. The data used is a 13-month centered average of the monthly HadCRUT 4.6 global dataset found here:

https://www.metoffice.gov.uk/hadobs/hadcrut4/data/current/time_series/HadCRUT.4.6.0.0.monthly_ns_avg.txt

The date given is that of the month of maximum temperature when the cooling started, and the length in months is to the month of minimum temperature when the cooling ended.

Notice how the top four took place before 1975, during the ENSO period dominated by La Niña conditions.

The data for the cooling period that started in 02/2016 (red in the figure) is provisional, as the cooling has not ended as of June 2018. If it continues for a few more months, it could easily make it to the top three.



Figure 1

The most remarkable thing is that every single top cooling period in the list has a reasonable explanation except the current one. Two of them are associated with the major volcanic eruptions of El Chichon and Pinatubo, and seven of them are associated with major La Niña events when the Multivariate ENSO Index (MEI) reached a standard deviation of -1 (see bottom panel in the figure).

Only the current cooling period remains there unexplained, without a big volcanic eruption and without a major La Niña event.

Climate scientists should be looking for a reason for this most interesting cooling instead of ignoring it.

Why is the planet cooling now?

Everybody can have their own favorite explanation. Personally, I see no other reasonable cause for the cooling but low solar activity.

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