01 April 2013

When was climate normal?

It's been a couple years since I took up the question of normal climate, so time for another go.  At that time, I used monthly data from Hadley, and arrived at the observation that if you're younger than 26, you've never seen a month where the global average as as cold as the 1850-2011 average, 317 consecutive months (at that point, now over 330) of warmer than 'normal' temperatures.  I'll cheat and give you some answers first, read on to see how they're established:
  • Climate was 'normal' only between 1936-1977
  • Every year 1987-present has been warmer than any year before that
  • 1976 was warmer than any year before 1926
  • 1978 (next coldest year of the recent run) was warmer than any year before 1940
Do read on to see what 'normal' winds up meaning; it's important!  One part of 'normal', as we intuitively think about it, is that you should some times above it, and sometimes below.  Having many consecutive years above 'normal' says that normal isn't really very good.  To help get quantitative about how to proceed, consider this plot of NCDC's data (warmer/colder than the 1880-2012 average -- the length of the entire record).

From this we see that every year 1977-present is above the zero line, much the same as we had in my earlier note.  But, which I paid little attention to at the time, every year in the early part of the record -- everything to 1936 -- is also below the 'normal' (0 anomaly) line.  A long run of negatives is as good a disproof of things being part of the 'normal' as a long run of positives.  Further, 1976 is much colder than all the years which follow and 1937 is much warmer than all the preceding years.  So this makes a good span, 1936-1977, to try to call 'normal'.  It does still satisfy our requirement that to call temperatures climate we want at least 20-30 years, though, at 42, we can't afford to lose many off either end of the record.

This satisfies one of the traits for 'normal' -- no long runs of always above or always below normal at the start or end of the record.  But another trait we often require is no trend.  That is a continuation of the idea that climate is stable in some meaningful sense.  The trend for 1936-1977 is indeed zero.  So for this span, it is indeed reasonable to say that there's a normal climate that weather is bouncing around.  Here's what the data themselves look like, using 1936-1977 for the reference:
This also looks a lot like what intuitions say should be the case for a 'normal' climate, not just the math coming out that way.  As it's 42 years, it's long enough to satisfy our usual requirements for a climate reference.  So now let's look at the whole record against this 'normal':
After seeing this, I have to say that our intuitive demands lead us to a highly unusual 'normal'.  The 1936-1977 span is the only long span with a trend of zero.  The first 30ish years show cooling, next 30ish are warming, the 42 year plateau 1936-1977, and then a 35 (and counting) year warming trend.

If by normal we mean what happens most often, then (for 30 year trends) what's most common is for climate to be changing, not to be some stable reference around which weather bounces. 


Robert Grumbine said...

I'll add that this is not an 'April Fool'. Also that blogger might be having a hard time with comments. I find it hard to believe that nobody has tried to comment, so this is also a test.

ms. susan grumbine said...

Okay bro, bottom line it for me. How long before I can enjoy a Chicago winter without having to shovel my drive?

L Hamilton said...

Probably you're going somewhere richer with this ... but I think about the choice of baseline as statistically arbitrary, in the sense that no matter what you use, it makes no difference for correlation/regression-type analysis (and that's a very broad type). It's just a linear transformation, like converting Celsius to Fahrenheit.

At the same time, the issue of what baseline to use causes confusion ranging from rhetorical flights to conspiracy theories among folks who don't think statistically. So in that other sense it does matter, whether it should or not.

Robert Grumbine said...

Well sis, you should already be there. My niece can see over the top of a shovel. Be good exercise for her to shovel your drive! You've already experienced a big drop in snowfall -- ever since I moved to points east. Not needing to shove drives at all ... a few decades maybe?

As you say L Hamilton, it's all just a linear transform away, so quite irrelevant to examining the data for trends. This note was partly prompted by my encountering elsewhere a scientist (good publication record, though, as usual, not in climate) who ascribed far more meaning to the reference figures than even the rhetorical flights of fancy you're thinking about.

Then again, I was surprised that the 'just varying around some more or less stable average' turned out to be such a poor descriptor of the data. In the recent decades, it's a linear trend of warming. In the first several, it's a pretty good parabola, cooling then warming. To the extent we think of climate as stable, and weather just bouncing around it, we're apparently thinking pretty incorrectly.

Evan said...

Is there a reason you're only using data from the last century? Why not use a much longer data set?

The data you're using are affected by C02, industrial particulates, and desertification. So we basically have three good reasons why we should expect that 'normal' for the last 100 or so years should be a changing climate.

It's not so obvious that's the case in the long term, especially not if you accept larger fluctuations on longer intervals.

Evan said...

Also (off topic), as an idea for a future post, I'd love to see your comments on this, since it's currently relevant and related to your specialty :)


Robert Grumbine said...

Thanks for the reminder about the Antarctic Evan. Definitely have to look in to it.

For the data sets... Global NCDC 1880-present, or GISS for the same period are about as long as you can go for global mean annual temperature anomalies. Hadley can give you another 30 years, to 1850, which doesn't, on climate scales, give us a lot more data.

We can get longer, but at the expense of area and time resolution. I've downloaded the Central England series and started looking at it. From that, everything 1883-present is quite different than the 200 years before then.

Or we can get a lot of years from Greenland cores, though I don't recall that they give us the annually-resolving data.

But, yes, definitely one should consider the fact that since the industrial revolution, we've started affecting the planet. If William Ruddiman is right, we started affecting planetary climate with the agricultural revolution 10,000 years ago.

Aaron said...

Why does the baseline have to be in the period 1880-2013? 1880 is well after the start of the industrial revolution. If you want "normal" look at the period before large scale industrialization. Just because you have more data for later period does not mean that the climate was normal in that later period.

Once we admit to AGW, we have admitted that our weather data after industrialization is contaminated.

I think the shape of the curve from 1937 - 1977 says more about aerosols than about CO2.