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Did global warming stop in 1998?

The skeptic argument...

For the years 1998-2005, temperature did not increase. And yes, this eight-year period of temperature stasis did coincide with society's continued power station and SUV-inspired pumping of yet more CO2 into the atmosphere. (Source: There IS a problem with global warming... it stopped in 1998 by Bob Carter)

What the science says...

While 1998 was an unusually hot year due to El Nino, the long term trend since 1998 is still that of warming.

Temperature data shows year-to-year variations independent of long term warming or cooling trends. Volcanoes cause temporary cooling over several years while the El Niño-Southern Oscillation cycle lasts around 4 to 5 years. Fawcett 2007 examines the temperature data of three different data-sets (see also an updated version of the paper) to determine the long term trend amidst the short term variations.

Using moving averages to discern the long term trend

The easiest way to remove short term variations, revealing any underlying trend, is to plot a moving average. Figure 1 displays the 11 year moving average - an average calculated over the year itself and five years either side. They've used three different data-sets - NCDC, NASA GISS and the British HadCRUT3. In all three data-sets, the moving average shows no sign that the warming trend has reversed.


Figure 1: Globally-averaged annual mean temperature anomalies in degrees Celsius, together with 11-year unweighted moving averages (solid lines). Blue circles from the Hadley Centre (British). Red diamonds from NASA GISS. Green squares from NOAA NCDC. NASA GISS and NOAA NCDC are offset in vertical direction by increments of 0.5°C for visual clarity.

The linear trend from 1998 to 2007

Next, Fawcett and Jones look for a cooling trend in the 10 years since 1998. They find the linear trend over 1998 to 2007 is a warming trend in all three data-sets. Note that HadCRUT3 displays less warming than NASA GISS and NCDC. This is most likely due to the fact that HadCRUT data doesn't cover parts of the Arctic where there has been strong warming in recent years.


Figure 2: Linear trends (solid lines) in the three global annual mean temperature anomaly time series over the decade 1998-2007.

Removing ENSO signal from the temperature record

The reason that 1998 was such an anomalously warm year was due to a strong El Niño that year. Fawcett and Jones remove the ENSO signal by calculating a linear regression of global temperatures against the Southern Oscillation Index. A detailed description of the process is found in Fawcett 2007. The result is shown in Figure 3.


Figure 3: Three time series of globally-averaged annual mean temperature anomalies (circles) in degrees Celsius, together with ENSO-adjusted versions (lines), for the period 1910-2007.

All 3 data sets demonstrate that the anomalously hot 1998 was due to the strong El Niño of 1997/98. When ENSO-adjusted, 1998 looks much less remarkable than it does in the original data. In all 3 ENSO-adjusted data-sets, 2006 is the hottest year on record and the trend from 1998 to 2007 is that of warming.

Further reading

Tamino further explores the warming trend since 1998 in Garbage is Forever and Wiggles.

I've kept my original treatment of the subject as other websites hotlink to the images. It uses similar arguments to Fawcett and Jones 2008 although their analysis is much more rigorous. 

Comments

  1. While I don't disagree with the statement that the overall warming trend continues, the line-of-best-fit-graph is a bit disingenuous. You should show the 2 sigma bounds on the fit line, which would likely show that the probability of no increase in the last 10 years is not unreasonable.

    Further I presume that you used least-squares to fit the line. This assumes that temperature anomalies are normally distributed. Glancing at the graph, this doesn't seem to be the case (several major excursions from the mean trend line), so the outliers will unduly influence the line of best fit. Least squares is extremely non-robust to non-normal data.

    The second graph doesn't help your argument much, and might actually hinder it some. If you could send me or point me to the raw data, I'd be happy to do a different analysis on it and see if there is a meaningful trend in the data.
    [ Response: Good question, the data comes from GISS Temp, using the Global-mean monthly land-ocean temperature index. ]
  2. These posts are useful. plfreeman, there's enough data from monthly, weekly, or daily summaries, that the regression and correlation are both statistically significant, even after correcting for autocorrelation and using various methods. Here's a good link to check it out, by a mathematician specializing in time series analysis:
    http://tamino.wordpress.com/2007/08/31/garbage-is-forever/

    The strong version of Carter's claim that warming stopped in 1998 requires the slope to be zero or negative since then. It isn't, therefore the strong version is proven false (and the 60 individuals who signed that have demonstrated their commitment to ideology over data). Even the weak version ('no significant warming') is shown false.
  3. Wondering Aloud at 06:10 AM on 23 January, 2008
    According to Carter the slope is indeed negative since then. However, I believe he is quite clear in saying the amount is not really significant.

    The data above does not match the data set he is using.
    [ Response: I have corresponded with Bob Carter about the data he uses - in articles where he states the temperature trend is negligible or even cooling, he's erroneously using upper troposphere data. See the footnote of Satellites show no warming. ]
  4. Has it warmed over the past decade?

    Based on the global surface record compiled by the Hadley Centre and the global UAH satellite record there has been warming over the past decade.
    http://www.cru.uea.ac.uk/cru/data/temperature/hadcrut3vgl.txt
    http://vortex.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.2

    Plotting the two temperature records for the last 10 years shows that:
    · The surface record showed a linear increase of 0.062 degrees C per decade
    · The satellite record showed a linear increase of 0.059 degrees C per decade

    The two warmest years during this period were 1998 (a strong ENSO year) and 2005 (a somewhat weaker ENSO year).

    Both of these rates of increase are considerably lower than the average rate of increase over the past 28 years, when satellite readings first became available:
    · The surface record showed a linear increase of 0.171 degrees C per decade
    · The satellite record showed a linear increase of 0.142 degrees C per decade
  5. Bob Carter’s article is almost 18 months old now, and we now have temperature records for both 2006 and 2007.

    Interestingly, both years were predicted to be “record warm” years, with the usual media hype about this being additional proof of alarming global warming caused by man.

    Both turned out to be rather normal years, but there was not much media hype, since (as we all know) “good news is no news”.

    But as I showed earlier, if we compare the past decade we see that there is still warming, but that the rate has slowed down considerably in comparison with the earlier record.

    Sure, there are two ENSO years in the record, 1998 and 2005 (which also turn out to be the two warmest years). But, then again, there will always be ENSO years and scientists have been unable to predict when these will occur or explain exactly why they occur when they do.

    The question that this site raised should not have been whether or not it has warmed since 1998 but rather whether or not the rate of warming has decreased since 1998 as compared to earlier decades, and if so, whether or not this indicates a trend of slowdown in temperature increase or just an anomaly caused by individual ENSO years.
  6. Good evening. Maybe this discussion could be enriched with comments from the IPCC chairman Rajenda Pachauri and the United Kingdom's Meteorological office (Royal, I presume).

    From a recent Reuters' piece: "Rajenda Pachauri, the head of the U.N. Panel that shared the 2007 Nobel Peace Prize with former U.S. Vice President Al Gore, said he would look into the apparent temperature plateau so far this century.

    "One would really have to see on the basis of some analysis what this really represents," he told Reuters, adding "are there natural factors compensating?" for increases in greenhouse gases from human activities."

    Source: http://tinyurl.com/3doxvc

    And then the Met office. They recently issued a forecast for 2008, stating this much: "The forecast value for 2008 mean temperature is considered indistinguishable from any of the years 2001-2007, given the uncertainties in the data." Source: http://tinyurl.com/2ezepk

    So, the IPCC is looking into "the plateau", and the Met Office says it persists into its' eight year, and who knows, after another eight years it may be very very cold again. Over at Tamino's Open mind there was some discussion about whether we should set 2015 as the year when the science could be declared settled;-). But the details seem to take some time to agree upon. The problem with outliers!
  7. Not that one month means much, but the RSS temperature reading for Jan. 2008 has arrived. It was the coldest month since 2000, enforcing the current decadal cooling trend.
    [ Response: The cool temperatures of Jan 2008 are due to an unusually strong La Nina effect (the strongest in a decade). ]
  8. Den siste mohikanen at 19:29 PM on 14 March, 2008
    With no major volcanic outbreak since 1991, current temperatures are the same as before that major outbreak (Pinatabu). (Actually, current temperature is lower than then but it is fair to allow for a lower average temperature the years before Pinatabu and we need to keep in mind that current temperatures are a bit lower than previous years.)

    Anyhow we can conclude than since prior to Pinatabu there has been no statistically significant warming trend (its all within error bounds).

    Now how much was the upper projection of the IPCC? +6°C to the year 2100. From now on that roughly means at least five times the warming rate we've seen so far since satellite measurements began and more than ten times the warming rate since we started to increase our CO2 emissions (around 1940-45). That anyone can believe this without the IPCC presenting any evidence at all is flabbergasting to be honest.

    I'll admit that there is a risk (albeit minor IMHO) for a two degree warming, but such a risk you handle differently and you certainly won't scare children nor keep people in poverty because of it.
    [ Response: You can read the IPCC projections versus observations here. ]
  9. OK, so now we have the data from the ARGO measuring system, and they show that the oceans have not warmed at all since 2003. So there is more than just one La Nina behind the flattening of global temperatures, wouldn't you agree?

    Sulphur aerosols from China?
    [ Response: The significance of cooling oceans is discussed here. ]
  10. Jürgen Hubert at 22:11 PM on 17 April, 2008
    I've analyzed the data used by Christopher Monckton in a recent essay where he claims a cooling trend, and the results may be surprising. Or possibly not, for the readers of this website...

    http://jhubert.livejournal.com/181274.html

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