## Has Global Warming Stopped?

#### Posted on 2 August 2010 by Alden Griffith

**Guest post by Alden Griffith, creator of Fool Me Once, a new blog featuring video presentations explaining climate science. This blog post is a written version of his first video addressing the argument 'Global warming has stopped'.**

Has global warming stopped? This claim has been around for several years, but received new attention this winter after a BBC interview with Phil Jones, the former director of the Climate Research Unit at the University of East Anglia (which maintains the HadCRU global temperature record).

BBC:Do you agree that from 1995 to the present there has been no statistically-significant global warming?

Phil Jones:Yes, but only just. I also calculated the trend for the period 1995 to 2009. This trend (0.12C per decade) is positive, but not significant at the 95% significance level. The positive trend is quite close to the significance level. Achieving statistical significance in scientific terms is much more likely for longer periods, and much less likely for shorter periods.

Those pushing the “global warming has stopped” argument immediately jumped on this as validation, and various media outlets ran with the story, e.g. “Climategate U-turn as scientist at centre of row admits: There has been no global warming since 1995” (Daily Mail).

Well, what can we take away from Dr. Jones’ answer? He says that the positive temperature trend is “quite close to the significance level” and that achieving statistical significance is “much less likely for shorter periods.” What does all of this mean? What can we learn about global temperature trends from the past 15 years of data?

*Figure 1:* Global temperature anomalies for the 15-year period from 1995 to 2009 according to the HadCRUT3v analysis. The black line shows the linear trend.

First though, it’s worth briefly discussing what “statistically significant” means. This is referring to the linear regression test that informs our decision to conclude whether the slope of the trend line is truly different from zero. In other words, is the positive temperature trend that we observed really any different from what we would expect to see from just random temperature variation? By convention, statistical significance is usually set at 5% (Dr. Jones has simply inverted it to 95%). This 5% refers to the probability that we would have observed such a positive trend if in reality there is no trend. The lower this probability, the more we are compelled to conclude that the trend is indeed real.

Using the dataset available at the time, the statistical significance of the 15-year period from 1995 to 2009 is 7.6%, slightly above 5% (the most recent HadCRU dataset gives 7.1% for this period).

What can we conclude from the statistical test alone? If one was to make any real conclusion, it should probably lean toward there being a positive temperature trend (as the slope is quite close to being statistically significant). We certainly cannot strongly conclude that there’s no trend. Really though, we cannot conclude much at all from such a short time period. Although a 15-year period may seem like a long time, it is relatively short when thinking about changes in climate. So what to do? How can we tell if global warming has stopped or not?

First we need to **identify the important questions**:

- Do 15 years tell us anything about the long-term temperature trend?
- What temperatures should we expect to see if global warming is continuing?

The first question is essentially putting the skeptics’ logic to the test. The logic is that a 15-year period without a statistically significant trend means that global warming has stopped, or at the very least that it contradicts a warming world. So let’s look further back and see if there are any other 15-year periods without a statistically significant trend:

*Figure 2:* Global temperature anomalies since 1900 according to the HadCRUT3v analysis. The trend lines represent recent 15-year periods without statistically significant warming.

Lo and behold! If we just focus on the most recent period of rapid warming, we see several 15-year periods with trends that are "not significant at the 95% significance level" (actually, since 1965 there are 8 nonsignificant 15-year periods, several of which overlap, and 39 nonsignificant 15-year periods since 1900). So according to the logic, global warming keeps on stopping even though temperatures keep on rising. Clearly this makes no sense! That’s because 15 years of temperature data do not tell us much about temperature trends. Concluding that global warming has stopped from looking at the last 15 years is wishful thinking at best.

The second question is really what we should be asking: What temperatures should we expect to see if global warming is continuing? This is very easy to do. Let’s take the most recent warming trend beginning in 1960 and stop at 1994, just before the last 15-year period. Warming over this period is highly statistically significant (<0.0001%). We can then calculate what’s known as the 95% prediction interval. This gives us the range in which we would expect to see future temperature values if the trend is indeed continuing (i.e. if global warming is still happening at the same rate).

*Figure 3:* 95% prediction interval (dashed lines) if the linear trend from 1960-1994 is continuing. Temperatures from 1995 to 2009 are plotted in blue.

Lo and behold! The last 15 years are not only within this range, but temperatures are at the upper end of it. In fact, 1998, 2002, and 2003 were even warmer than the predicted range. If you do this analysis for the entire HadCRU time span (1850-2009) you can see that the last 15 years are almost entirely above the predicted range.

*Figure 4:* 95% prediction interval (dashed lines) if the linear trend from 1850-1994 is continuing. Temperatures from 1995 to 2009 are plotted in blue.

So here are two requirements for those wishing to conclude that global warming has stopped based on the interview with Phil Jones:

- Accept the backwards logic that allows global warming to keep on stopping while temperatures keep on rising.
- Ignore the real question of whether the last 15 years is consistent with a continued warming trend (which it is).

So no, global warming has not stopped. It takes some serious wishful thinking to say that it has.

[Lastly, I want to make the prediction that global warming will once again “stop” in 2013. Even if temperatures continue to rise over the next 3 years, the 15-year period from 1998 to 2012 will begin with the record setting 1998 El Niño year, which will make statistical significance unlikely. Beware, the return of the “global warming has stopped” argument!]

**NOTE:** be sure to check out a video presentation of this material at Fool Me Once.

**NOTE:** This post was updated on 11 Aug 2010.

KRat 13:32 PM on 4 August, 2010anyinformation on the underlying physical processes, use the simplest fit that goes through the center of variance. Otherwise (as you clearly show with your last figure) you're overfitting into la-la land.apeescapeat 13:39 PM on 4 August, 2010Of coursethe R^2 is going to be lower as you increase the degree. You can use poly fits if you think there is a physical mechanism that shows a curvilinear relationship, you can go ahead and use it (and any other transformation). But because you're using it for such a short series (with decently noisy data), the poly fit gets great fits for the short term fluctuations instead of the trend. The poly fit that you have here is more reminiscent of the 1998 burst, instead of the trend. As a simple sensitivity analysis, you can change the value of 1998, and see the change in R^2 for the linear fits and the poly fits. I would bet the latter will be affected more strongly.apeescapeat 13:40 PM on 4 August, 2010higheras you increase the degree."Mikeat 22:01 PM on 4 August, 2010Alden Griffithat 02:05 AM on 5 August, 2010Peter Hogarthat 02:57 AM on 5 August, 2010Dikran Marsupialat 05:13 AM on 5 August, 2010kdkdat 07:24 AM on 5 August, 2010GeorgeSPat 07:40 AM on 7 August, 2010GeorgeSPat 20:32 PM on 7 August, 2010muoncounterat 09:32 AM on 8 August, 2010greenhousegaseousat 02:52 AM on 10 August, 2010fydijkstraat 20:43 PM on 11 August, 2010‘There is no physical base for that!’could be the shortest and most friendly summary of these comments. Well, that’s true! There is no physical basis for using a polynomial function to describe climatic processes, regardless of which order the function is, first (linear), second (quadratic) of higher. Such functions cannot be used for predictions, as Aldin also states: we are only speaking about the trend‘to the present’. Aldin did not use any physical argument in his trend analysis, and neither did I, apart from the suggestion about ‘saturation.’ A polynomial function of low order can be very convenient to reduce the noise and show a smoothed development. Nothing more than that. It has nothing to do with‘manipulating [as a] substitute of knowing what one is doing’(GeorgeSP, #61). A polynomial function should not be extrapolated. So far about the statistical arguments. Is there really no physical argument why global warming could slow down or stop? Yes there are such arguments. As Akasofu has shown, the development of the global temperature after 1800 can be explained as a combination of the multi-decadal oscillation and a recovery from the Little Ice Age. See the following figure. The MDO has been discussed in several peer-reviewed papers, and they tend to the conclusion, that we could expect a cooling phase of this oscillation for the coming decades. So, the phrase ‘global warming has stopped’ could be true for the time being. The facts do not contradict this. What causes this recovery from the Little Ice Age, and how long will this recovery proceed? That could be a multi century oscillation. When we look at Roy Spencers ‘2000 years of global temperatures’ we see an oscillation with a wavelength of about 1400 years: minima in 200 and 1600, maximum in 800. The next maximum could be in 2200.Dikran Marsupialat 22:28 PM on 11 August, 2010Pete Ridleyat 02:18 AM on 13 August, 2010Moderator Response:The name and entire premise of the website you refer to is itself a violation of the comments policy here. Find a better source to cite for the satellite instrumentation issue, preferably one concerned primarily with science as opposed to conspiracy theories.Pete Ridleyat 19:08 PM on 13 August, 2010Pete Ridleyat 19:09 PM on 13 August, 2010SNRatioat 01:19 AM on 14 August, 2010SNRatioat 01:30 AM on 14 August, 2010Moderator Response:You're right that discussion of the technical details of satellite measurement of temperatures is off-topic for this thread. Fortunately, there is a new thread dedicated to clearing up the confusion over this subject:Of satellites and temperaturesmuoncounterat 11:25 AM on 14 August, 2010fydijkstraat 05:48 AM on 15 August, 2010“but it is super-imposed on a linear function of time, so it too goes on to infinity.”Yes, the oscillation in Akasofu’s model is super-imposed on a linear trend. We don’t know how long this trend will continue. Not to infinity of course, because nothing in the climate goes on to infinity. We can look at Roy Spencers reconstruction of 2000 years of global temperatures. I gave the link in my previous posting, but here is the graph. “it would be interesting to see the error bars on your flattening model. I suspect there are not enough observations to greatly constrain the behaviour of the model beyond the calibration period, in which case the model [is] not giving useful predictions.” It is not possible to calculate error bars with only 10 points on a flattening curve. I used this flattening function only to show, that the data fit better to a flattening curve than to a straight line. This is only about the data onto the present, it is not a prediction. When it is said that‘global warming has stopped’this is only about the data onto the present. Nobody denies that it is possible that global warming will resume. SNRatio (#69):“The simple Akasofu formula "anomaly = LIA recovery + MDO" predicts falling temperatures now - and therefore I wonder if it is not already partly falsified.”No, the Akasofu model does not exactly predict the year when the falling temperatures should continue. Moreover, just as with the model of ever rising temperatures, there is noise in the data. Akasofu’s model perfectly fits with the data so far.“The trend also seems rather speculative: What is the physical basis for this continuing "LIA recovery" in the 21st century?”See my above given reply to Dikran.Moderator Response:See the Skeptical Science posts are "We’re coming out of the Little Ice Age" and "Climate’s changed before."doug_bostromat 06:15 AM on 15 August, 2010chrisat 09:13 AM on 15 August, 2010ad hoc-ery to be realistic fydijkstra. (i) Braun et al (2005) explicitly rule out the 1470 year cycle for Holocene events. Their tentative conclusion for the driving of Dansgaard Oescher phenomena relates to the possibility of threshold events resulting from meltwater pulses involving massive N. Hemisphere ice sheets that result in large temporarily perturbation of the thermohaline circulation with dramatic and rapid effects on temperature in the N. Atlantic. We know these processes have nothing to do with current global warming. In any case the current warming is out of phase with the supposed "cycle" (if the peak of the last cycle was around 800-900 AD then we shouldn't be getting a new peak until 2300-2400). Or are you suggesting that we've got another 300-400 years of relentless warming due to some uncharacterised putative cycle? (ii) This seems a little unlikely in the context of the Spencer/Loehle and Akasofu's notions. Firstly, if one were to take the Spencer/Loehle sketches at face value, then we should take on board that their sketches only go to 1935. If we add on the real world warming since then, current temperatures are already well above the supposed maximum of the Loehle/Spencer sketch you reproduced. We're surely much warmer than we should be if our temperatures were dominated by your 1500 year cycle which projects a substantial warming from natural causes still to come... (iii) Akasofu proposes a linear "recovery" from the LIA that continues to this day and through the next ~ 100 years. That seems astonishing to me. It implies that the Earth has amuch highersensitivity to changes in forcings than current understanding would support, and that the climate system has such an extraordinary inertia that "recoveries" (from temperature perturbations) aredominatedby processes with time constants on the century timescale or longer (how can this possibly be true?). Let's hope that Akasofu isn't correct else we're probably in a lot more trouble than we think we are! (iii) Of course we probably don't believe Akasofu'sad hoc-ery if we think about it for a bit. Looking at the temperature record (reconstructions and direct measurements from the mid 19th century) indicates that "recovery" from the LIA was largely complete by the early 19th century. (iv) I suppose the other problem inherent inad hoc-ery is that thead hocdecision to project the "Braun et al" cycle into the Holocene (where Braun et al state it doesn't apply) seems entirely incompatible with Akasofu'sad hocconstruction. Akasofu's sketch doesn't show any of these supposed 1500 year cycles? And while according to Akasofu, we should be already heading into a cooling phase which will continue for another 20-odd years, according to the 1500 year cycle idea we should still be on a rather relentless warming "curve" that should continue for another 300 or more years... (v) Is there a good reason for rejecting everything we know about the climate system, and basing our ideas on mutually incompatiblead hocnotions? I can't think of one!Dikran Marsupialat 09:47 AM on 15 August, 2010Peter Hogarthat 07:06 AM on 16 August, 2010John Russellat 00:08 AM on 28 May, 2011"...there has (only just) been no statistically significant warming since 1995... etc."?Response:[DB] Tamino examined this issue back in January here.

Based on his analysis, the warming since 2000 is statistically significant (the error bars do not include zero):

Considering the Aughts (the decade 2001-2010) were the warmest in the instrumental record, with 2010 being tied for the warmest year on record, you can safely say that global warming is still happening today.

John Russellat 01:46 AM on 28 May, 2011Eric (skeptic)at 06:58 AM on 11 February, 2012Dikran Marsupialat 07:13 AM on 11 February, 2012