Monthly Archives: August 2016

Trend and Fluctuation (One More Time)

A recent comment says:


The Gistemp temperature anomaly for June 2015 was 0.79 degrees Celsius.
The Gistemp temperature anomaly for February 2016 was 1.32 degrees Celsius.
In 8 months the temperature anomaly increased by 0.53 degrees Celsius.

The question is, how much of the 0.53 degrees Celsius increase was caused by man-made global warming?

Let me propose an analogous question:

The temperature on March 1st reached 2°C. The temperature on March 2nd reached 18°C. In 1 day the temperature increased by 16°C. How much of the 16°C increase was due to the progress of the seasons from winter toward spring and on to summer?

Maybe it’s obvious, even to the author of the aforementioned comment. But just in case, the answer is: the day-to-day fluctuation in temperature swamps the amount of trend change in a single day.

Only a fool would claim that most, or even a sizeable fraction of, the 16°C rise from one day to the next was due to the passing of the seasons. Only a bigger fool would ask the question with a straight face. Only the biggest fool would attempt to use the day-to-day fluctuations in temperature as evidence that there’s no seasonal trend and no reliable evidence that summer will be hotter than winter.

Global average temperature data from NASA GISS looks like this:

giss

The black line follows the monthly average temperature anomaly. The thicker red line shows the trend estimated by linear regression. The blue circles mark the values in June 2015 and February 2016.

Fluctuations come and go, but over the long haul they average out to nothing. Trends persist, so over the long haul they can accumulate to large changes in spite of the fact that in the short term they amount to very little. That’s why, just since 1970, the net trend change in the NASA GISS data is 0.84°C, considerably bigger than the short-term change from June 2015 to February 2016 which is due to fluctuation.

Global warming is about the trend, not about the fluctuations. The progress of the seasons is about the trend, not the fluctuations.

If the trend continues at the present rate, then by January 2100 the net warming due to trend will be 2.34°C above the 1970 value. And of course, it’s possible the average rate over the remainder of this century will be higher than it has been recently, especially if warming triggers feedback in the carbon cycle (releasing natural stores of CO2 or CH4).

Let’s avoid that.

Let’s also stop treating claims or questions which conflate trend and fluctuation in the most ludicrous way, as being serious claims or questions, let alone worthy of “debate.”


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