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:

nasa

Then we can look at the residuals:

nasa_res

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:

rss

rss_res

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

ratpac

ratpac_res

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 F = 5.35 \ln(CO_2 / 280)). Since I’m interested in recent climate change, I’ll restrict this (and all variables) to data from 1970 onward:

co2force

co2force_res

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:

ohc

ohc_res

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):

arctic_seaice

arctic_seaice_res

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:

antarctic_seaice

antarctic_seaice_res

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:

snow

snow_res

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.

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14 responses to “Changes

  1. I can’t think of a way to put this that doesn’t sound completely inane, but it comes down to the Earth being a complex system with interactions between subsystems that we only partially understand, and the oceans being a very large heat sink with a long time constant.

    I noticed this last year when doing piecewise linear regressions of 30-year surface temperature trends (1965-1995, 1975-2005, 1985-2015 est.) and finding that the warming rates were statistically indistinguishable.

  2. “Surge” – ©2015 Eli Rabett?

  3. Thanks Tamino. Yet another reason why your posts are very readable. An issue addressed early, covered well and in depth, and dismissed despite notionally being valuable for your side of the argument. Those of us on the side of truth and reason (with your valued assistance) have no need to clutch at straws for support…

  4. David B. Benson

    My understanding of the growth in Antarctic sea ice is that it is due to the increased circumpolar winds. The coriolis effect pushes the sea ice further north and it is cold enough that more sea ice forms in the gaps.

    Freshwater melt is a minor contributor.

  5. But but but…. there’s a clear temperature surge since 2013. Just remove the rest of the data and it becomes blindingly obvious.

  6. The measurements that do show acceleration do not vary much from year to year. That makes it easier to see the acceleration.

    That the measurements with more short-term variability do not show a statistically significant acceleration does not mean that there is none, just that we have no statistical evidence for it.

  7. Why don’t we use Monckton’s method of “calculating, not cherry picking” start points for periods of change? By that method, RSS temperatures have actually been “surging” since Feb 2007 – nearly 9 years. That is, the linear trend in RSS since Feb 2007 has been higher than it was at any time prior to the start of the Monckton pause (May 1997). So, using Monckton’s preferred method, the latter half of his pause has been been a surge…

  8. About the snow thing…

    We expect precip to increase by 1-3 % per degree C of global warming. I don’t think it makes much sense to say that’s causing the greater expanse of snow.

    I remember you looked at seasonal hiatus and the Kosaka & Xie modelling. Doesn’t the data show a short-term cooling over NH land in winter? That seems a more plausible contributor to me.

    • We expect precip to increase by 1-3 % per degree C of global warming. I don’t think it makes much sense to say that’s causing the greater expanse of snow.

      Why not? The experience of my young years in Sault (“Snow”) Ste. Marie, ON, led me to think that temperatures near freezing tended to be much more productive of snow than colder ones. I don’t know if the science available today supports that perception or not, but Clausius-Clapeyron would seem to provide a plausible link.

      • I haven’t done the full calculations but I’d be surprised. We’re looking at snow changes over a couple of decades or so where global warming is about 0.4 C, meaning global precip change from warming should be very small. Meanwhile, warming should mean more areas getting rained on rather than snowed on – snow area is largely determined by temperature as well.

        The cooling over NH land in winter means the area of potential snow survival has expanded and I think that’s a good suspect to explain the snow area changes. From Kosak and Xie, it’s related to changes in the Pacific rather than the forced response which suggests a retreat of winter snow in future if/when the Pacific switches back.

        In models we see about a 7 % per degree increase in water vapour because of Clausius-Clapeyron, but total precip increases less. This is because the atmosphere can’t cool efficiently enough to dump all the latent heat, which limits the rate of latent heat release and therefore precip. In CMIP5 you can see greater warming high up in the tropics, which reduces the surface-tropopause temperature gradient so rising air is not driven as strongly and the circulation slows down very slightly.

        One recent paper suggests that models closer to 1% precip increase per degree are better against obs:
        http://www.nature.com/nature/journal/v528/n7581/fig_tab/nature15770_SF7.html

    • There is definitely a relationship between temperature and snowfall. And there is definitely a temperature gradient from the poles to mid-latitudes.

      From this we can deduce that, with global warming, areas with ‘normal’ temperatures lower than the peak snow temperatures will warm and see more snow. Conversely, areas with ‘normal’ temperatures equivalent to that of peak snow will see reduced snow or a change in precipitation to rain.

      At *some point* the globe will warm enough to make the change in precipitation to rain be the main result. The real question then is have we reached that point yet?

      For instance, high latitudes are essentially deserts. They receive very little precipitation. We would expect this to change as the weather warms. Yet, there are obviously areas in the 40N latitudes where snow is rare and will become rarer.

      As Judah Cohen wrote:“In my mind there is no doubt that the globe is getting warmer and this will favour warmer temperatures in all seasons and in all locations; however, I do think that the increasing trend in snow cover has led to regional cooling as discussed in the paper and I see no reason why this won’t continue into the near future. Also if it continues to get much warmer in the fall, precipitation that currently falls as snow will fall as rain instead, eliminating the winter cooling.”

      A confounding factor would be changes in synoptic weather.