Changes

Change is the essential property of the universe.

— Spock

Global climate is changing, as is global temperature. But is the change itself changing? With 2015’s temperature so much higher than any previously recorded, talk of the “hiatus” or “pause” has, in some quarters, been replaced with talk of a “surge.” It was a mistake to talk about a slowdown of global warming when real evidence of it was lacking. But lately the question has changed to whether or not we’re seeing the beginning of an acceleration of global warming.

In yesterday’s joint NASA/NOAA press conference, Gavin Schmidt distinguished himself (not that Tom Karl was any slouch) on more than one occasion. His best statement, as I’ve mentioned, was to respond to a question starting “If this trend continues …” with an answer beginning “It’s not a question of ‘if’ …” But another outstanding response was when asked about possible acceleration of global temperature. He pointed out — quite rightly, I think — that there’s not yet any evidence of a change in the long-term trend.

Perhaps the plainest way to see this is to fit a straight line to temperature (I’ll use NASA GISS data) since 1970:

Then we can look at the residuals:

The smooth fit indicates no real departure of the residuals, i.e. no real change in the rate of increase of global temperature. Changepoint analysis (and other methods too) confirm that the evidence for a change in the rate of global warming, just isn’t there. We might be seeing the start of a “surge,” but so far it’s just speculation.

What about other variables? The much-discussed lower-troposphere data (TLT) from RSS give a similar result:

Again, the lack of evidence is supported by changepoint analysis. The balloon data (RATPAC) for lower-troposphere temperature agrees:

There are, however, changes which are changing. One we’ve already mentioned is the buildup of CO₂ in the atmosphere. But the climate forcing due to CO₂ is logarithmic, so let’s look not at CO₂ concentration, but its climate forcing (using the approximation ). Since I’m interested in recent climate change, I’ll restrict this (and all variables) to data from 1970 onward:

Yes, whether we use CO₂ concentration or its climate forcing, there has been acceleration.

One might wonder: if CO₂ forcing has accelerated, why hasn’t temperature increase — at least, noticeably? For one thing, CO₂ isn’t the only forcing, and some things (solar output, for instance) have recently decreased (at least, slightly). For another thing, surface temperature isn’t the only response to climate forcing. The dominant response is from ocean heat content. Here it is, again since 1970:

Ocean heat content has shown acceleration, rising faster since at least the early 1990s. One should also bear in mind, that the relationship between forcing and temperature (be it at the surface or in the oceans) isn’t instantaneous.

Some other variables are of interest. Arctic sea ice, for instance (I’ll use annual averages):

There’s visual evidence of acceleration of sea ice melt, but we need analysis to overcome the human tendency to see patterns when they’re not there. Changepoint analysis gives a p-value of 0.126, i.e. there’s not really evidence of an acceleration.

For the Antarctic sea ice we get this:

Again there’s visual evidence of a change from the linear pattern, and this time changepoint analysis agrees. Antarctic sea ice has recently grown, although the statistics don’t confirm that there’s a persistent upward trend, they just show that it hasn’t followed a straight line — it might be a temporary “blip” since 2013. But if it is a “blip” it’s not just a random one — something has changed recently.

I doubt that’s cause for celebration, or to doubt global warming — quite the opposite. One of the possible reasons for the recent increase in Antarctic sea ice is all the extra fresh water from melting of the Antarctic ice sheets (since fresher water freezes more easily than more salty water). If the increase in Antarctic sea ice is due to the speedup of ice sheet melt, that’s not a good sign, it’s a bad one.

Another interesting case is total snow cover. It has been declining during spring/summer/fall months, but shows signs of possible increase in winter. Using annual averages since 1970 gives this:

It shows visual signs of having slowed or even stopped its decline since around 1990, which is confirmed by changepoint analysis. The most likely explanation is that the increased water vapor content of the atmosphere (due to global warming) has increased the intensity of winter snowfalls, while summer snow cover continues to decline (confirmed by studying individual months’ data).

All in all, we see that some of the changes are indeed changing. Most notable is the increasing pace of CO₂ forcing, and the accelerated warming rate of ocean heat content. Surface temperature, however, shows not the least sign of accelerating — at least, not yet. So talk of a “surge” should be, in my opinion, discouraged. It’s well to be prepared for that possibility — a very real possibility — but let’s not speak of it as a “known” until the evidence is there.

---

As you may have guessed, a good bit of work went into this analysis. You can support this effort by donating at Peaseblossom’s Closet.