How long is 'a climate'?

In Oklahoma we used to say,'If you don't like the weather, wait an hour, it will change.' So Oklahoma weather is measured in 'an hour.' But this begs the question: How long is a climate?

In the 1930s, Oklahoma and major parts of the midwest were called 'the dust bowl.' I remember at least one Oklahoma dust storm in the 1950s. But agriculture improved and now there are so many farm ponds it gets pretty humid in the summer.

So what defines the time boundaries when we declare some interval of accumulated weather history 'a climate'? A decade? Twenty years? Fifty years? A century?

Bob Wilson

 

I like 30 years for anything within current human life-span. The 10 year, moving window is OK since that also coincides with the census. But dealing with older records . . .

I'm partial instrumented versus non-instrumented as one divide. During the instrumented era, use 30 year averages using the middle year, which always ends in a "5" for a climate data point.

The next era would be recorded history but then we have to rely on proxies. Here it makes sense to use something generational, say a century. Again, use the "50" as the least significant year digits to charactrerize that climate data point.

Then comes the pre-history that is totally based upon proxy records. Since it spans such a huge time interval, a log scale makes sense. Inititally 1,000 year but soon going to 10k, 100k, and 1,000k as the upper limit. Again, I'm partial to using "5" as the least precision digit to mark the midpoint of the climate record.

Why is this important?

We have a 'framing' problem that allows taking one data point and blowing it out of proportion. But if we can constain the time-scale to reasonable, well defined intervals, citing specific "weather" events gets rolled into a climate scale that allows understanding of the phenomena. Without a ruler, we just bounce off the rails.

Bob Wilson

 

"Climate" has a definition: Trewartha climate classification - Wikipedia, the free encyclopedia

Group A: Tropical climates
Group B: Dry (arid and semi-arid) climates
Group C: Subtropical climates
Group D: Temperate and continental climates
Group E: Boreal climates
Group F: Polar climates
Group H: Highland climates
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For our species, the challenge is mapping the boundaries between the different climates. As climate changes, migrations are likely.

Then there is map:

Source: Atmosphere | Free Full-Text | Assessing the Transferability of the Regional Climate Model REMO to Different COordinated Regional Climate Downscaling EXperiment (CORDEX) Regions | HTML

A set of PowerPoint charts discussing how climates are classified: Chapter 10-Reg clim.ppt

Bob Wilson

 

Found another, associated metric:

Source: RealClimate » Blog Archive » Hiatus or Bye-atus?

. . .
The figure shows that at every year during the past 30 years of modern global warming, the immediately preceding warming trend was always significant when 17 years (or more) were included in the calculation. In a number of cases—including in 2014—fewer years were required to reach significance, but never more than 17. This result should not be surprising: Significance requires statistical power to be detected, and the more observations are considered the greater the power of the analysis. The fact that a trend fails to reach significance with, say, only 5 or 10 years of data is therefore non-informative: no matter how robust the warming trend, once natural variability is superimposed on the trend, it will escape statistical detection with a small sample. To conclude from that that global warming has “stopped” is unwarranted. Nevertheless, a number of papers in the peer-reviewed literature have done just that. That is, they concluded that there was a pause in warming using a time period that was too short to achieve conventional statistical significance.

To illustrate, we used the definitions of the pause found in our corpus of articles (mean duration 13.5 years), and asked how often the null hypothesis of no warming would fail to be rejected during the last 30 years. It turns out that during those 3 decades, the 14-year trend escaped significance 10 times and the 13-year trend 13 times, suggesting either that global surface warming “paused” between 30% and 43% of a time period during which the Earth warmed 0.6K overall, or that global surface warming never paused and what we have been observing are routine fluctuations superimposed on a warming trend.
. . .​

Now a fair criticism is that this analysis was done to address whether or not there was a pause in the global warming. In lay terms, a nail in search of a hammer. I found a simpler answer. By adding or subtraction one year from 1998, an exceptionally hot year, the pause disappears. Hanging a pause on one data point is not much of a trend.

So it was only a matter of time before global warming would fade the "pause" into just another rabbit in a noisy data record. Serendipity, 17 years between 1998 and 2015. Still, I wanted to make sure.

I used another approach, plotting minimums and maximums from the Berkeley Earth shown here with the mean sea level:

Both were smoothed using my favorite, a Gaussian filter, with a 12 month span. The data suggests we are due for a local maximum. Come August 2016, we'll be able to see what the summer of 2015 looks like.

If there is a trend, not just the straight-line averages but also the local minimums and maximum will have the same slope. This is not the case which means the straight-line average a false trend line . . . "cherry picked."

BTW, I don't toss out anomalies as sometimes they can lead to new insights. But to do the job right, you have to deal with a lot of data . . . more than the casual stuff we've been paying with.

Bob Wilson