There is no agreed “hiatus” period in the scientific literature

We catalogued a corpus of peer-reviewed articles published between 2009 and 2014 that specifically addressed the presumed “hiatus” in global warming. Table 1 shows that the term “hiatus” was used more than 550 times in this corpus and the word “pause” in excess of 70 times.

Table 1 Summary of literature on the “hiatus”. Full size table

Many articles assumed that the “hiatus” commenced around 1998, at which time temperature anomalies were considerably above the long-term trend. There is, however, considerable heterogeneity in published onset times, with the range spanning a decade (1993–2003). Similarly, there is considerable heterogeneity in the presumed duration of the “hiatus” across the same corpus of articles, with a range 10–20 (median 13 years, , ). For each article, we took the duration to be the number of years since the assumed onset of the “hiatus” to the end of the period being analyzed. This constitutes a lower bound on the presumed duration of the “hiatus” as some authors may have presumed that the “hiatus” was ongoing at the time they published an article. Figure 1 shows the modern global temperature data together with a histogram of the distribution of presumed onset times of the “hiatus” derived from the corpus.

Figure 1 Global mean surface temperature (GMST) anomalies estimated by NASA’s Goddard Institute for Space Studies (GISS) data set (40 http://data.giss.nasa.gov/gistemp/, all analyses based on dataset downloaded on 17 January 2015). The histogram at the bottom represents the distribution of presumed start years for the presumed “hiatus” in the corpus of articles ( ; see Table 1) considered for this analysis. The vertical lines represent the 5th (1993) and 95th (2001) percentile, respectively, of presumed starting years for the “hiatus”. The small inset shows the overall historical temperature anomalies recorded since 1880. Full size image

The heterogeneity in onset and duration raises the possibility that the use of the term “hiatus” departs from normal scientific practice, which strives to define phenomena on the basis of clear and generally accepted criteria. The heterogeneity may be explained by the supposition that authors defined the “hiatus” retrospectively, via an ad hoc analysis of the recent trend leading up to the time of writing, rather than on the basis of a priori criteria. This apparent lack of clear and a priori criteria must be of concern in the statistical environment in which the “hiatus” has unfolded, which is known to be sensitive to the particular choice of start and end points that define short-term trends and the comparison baseline12.

The “hiatus” is an unexceptional fluctuation

If the definitions of the presumed hiatus are highly variable, with many different time periods proposed in the literature, how can we determine whether or not there is one? In order to answer this question, we compared the distribution of decadal warming trends during the “hiatus”—as defined by the articles in the corpus—against the distribution of all possible trends that have been observed during the period of modern global warming. The results are shown in Fig. 2, using three different onset dates for global warming.

Figure 2 (A) distribution of observed decadal temperature trends (GISS) within the “hiatus” windows defined by the corpus of articles considered for this analysis (blue), compared to the distribution of all possible temperature trends from 1950 till 2012, the reference period used by the IPCC to establish the long-term warming trend (pink). (B) same distribution of temperature trends within the “hiatus” windows (blue) compared to the distribution of all possible temperature trends from 1964 till 2012 (pink). The year 1964 is the lower bound for the 95% confidence interval of a recent change-point analysis that sought to identify the onset of modern global warming. (C) same distribution of temperature trends within the “hiatus” windows (blue) compared to the distribution of all possible temperature trends from 1976 till 2012 (pink). The year 1976 is the upper bound for the 95% confidence interval of a recent change-point analysis that sought to identify the onset of modern global warming. In all panels, the distribution of all possible trends is obtained by computing all trends of a given duration from all possible years within the time period considered. The duration of trends is weighted by the propensity of presumed “hiatus” durations in the corpus. Thus, each 10-year trend is replicated 8 times (as 8 articles in the corpus presumed the “hiatus” to extend over 10 years), each 11-year trend 5 times and so on. See Table 1 for details of the distribution of presumed “hiatus” durations in the corpus. The vertical red lines in each panel represents the long-term trend (1951–2012) that was used by the IPCC in their Fifth Assessment Report as a benchmark for comparison with the “hiatus.” The solid line is for the GISS dataset40 analyzed here and the dashed line is the same long-term trend using the UK Met Office’s HadCRUT4 data set39 used by the IPCC. Full size image

The question of when, precisely, greenhouse-driven warming began to be observable against background natural variability is itself contested. An early review13 that examined the literature back to 1824 finds that scientific concern about global warming arose as early as 1938. Every decade since then has seen increased scientific attention and concern13, although no consensual onset date for global warming has been identified. Figure 2 therefore uses three different onset dates for the computation of all possible trends. Panel A uses the period 1951–2012, which was used by the IPCC in AR5 as the long term trend against which to define the “hiatus”6. Panel B uses 1964 as the onset of modern global warming, whereas Panel C uses 1976. Those two years are two standard deviations ( ) below and above, respectively, of the best estimate (1970) of the onset of modern global warming in the GISS data set reported in a recent change-point analysis14. Panels B and C therefore approximate the lower and upper bound, respectively, of the 95% confidence interval for the onset of modern global warming by the change-point measure. All panels include data through 2012 because many of the articles in the corpus were written when the latest available data were for 2012 (or even earlier). (See the Online Supplementary Material for an extension of our analysis to the entire instrumental record.)

To permit a commensurable comparison, in all panels the distribution of all possible trends has the same propensity of trend durations as the “hiatus” in the corpus. Thus, each possible 10-year trend is replicated 8 times (as 8 articles in the corpus presumed the “hiatus” to extend over 10 years), each 11-year trend 5 times and so on as determined by the propensity of trend durations in the corpus. The distribution of trend durations is therefore identical between the two histograms in each panel.

Figure 2 demonstrates that, although the distribution of trends during the “hiatus” is shifted downward compared to the overall distribution of trends of the same durations, the “hiatus” distribution falls within the overall envelope of historically observed trends. For the IPCC base period (1951–2012; Panel A) there is little discernable difference between the two distributions. For the two years that bracket the most likely change-point onset of the modern warming period (Panels B and C), the “hiatus” distribution is more clearly offset towards the lower end but it is by no means unusual or extreme.

Moreover, for nearly 15% of imputed “hiatus” trends (5 out of 40 articles in the corpus), the warming exceeded the long-term trend used by the IPCC (1951–2012; vertical red lines in Fig. 2). Similarly, nearly 20% of operationalizations (7/40) referred to a period during which temperatures increased significantly (i.e., in OLS regression), which is not consistent with a “hiatus.”

The results in Fig. 2 show that all operationalizations of the “hiatus” in the literature are unexceptional in the context of equivalent-length trends in the record of modern global warming. At most, the operationalizations in the literature support the conclusion that the rates of warming over some recent intervals have been toward the lower end of the historically-observed surface temperature record. However, they do not support the conclusion that there is a “pause” or “hiatus” in the warming.

The “hiatus” has always been there when sample size is small

We next analyzed the GMST data from all possible different vantage points (end years looking back in time) to examine whether a scientist in, say, 2014 or 2010 would have been justified in accepting the existence of a “hiatus” in warming relative to what would have been detectable at any other prior point in time.

Figure 3(A) shows the warming trends that were observable, given the available data at the time, for any vantage point between 1984 and 2014 (horizontal axis). For each vantage point, between 3 and 25 years were included in the trend calculation (vertical axis). The Online Supplementary Material extends this analysis to even longer time scales. Timescales of at least 17 years are known to be necessary for noise reduction and detection of a signal15.

Figure 3 (A) Observed magnitude of temperature trends (GISS, K/decade) as a function of vantage year and the number of years included in the computation of the trend. Trends are capped at ±1 K for plotting. For each vantage year, trends are computed for all possible windows between 3 and 25 years duration, all of which end with the particular vantage year. The dots indicate which trends are significant ( ) in an ordinary least squares analysis of annual means and the horizontal dashed line indicates the number of years that must be included ( ) for the trend to be significant from all vantage points. The open circles identify combinations of onset and duration that have been used to identify the “hiatus” by articles in the corpus. Multiple articles may contribute to a given circle. The Online Supplementary Material shows that the basic conclusions are unaffected by consideration of autocorrelations, although an additional 2 years are required to reach significance for all vantage points across the entire 30-year period. (B) Level of statistical significance for trends (GISS, K/decade) as a function of vantage year and the number of years included in the computation of the trend. Trends that are clearly non-significant ( ) are shown in beige, those that approach significance ( ) are shown in shades of gray and significant trends ( ) are shown in shades of terracotta. The diagonal lines identify calendar years that contribute to the analysis. Any observation in the grid that lies to the Southeast of a given line includes only observations from the stated year onward and any observation to the Northwest also includes earlier years. The observation in the top-left corner is 1960 (i.e., looking backward 25 years from 1984). Full size image

Figure 3(A) shows that at every year (vantage point) during the past 30 years, the immediately preceding warming trend was always significant when 17 years (or more) were included in the calculation (dots denote ). Figure 3(B) presents the same data using a ternary classification of p-values for the linear trend into non-informative (beige), partially informative but not conventionally significant (gray) and significant (terracotta). This panel also includes three diagonal lines that identify the earliest calendar year included in the analysis. Thus, any observation to the Southeast of the line labeled “1975” only includes observations later than that and so on for the other two lines. The observations to the Northwest of “1965” go back to 1960 (top-left corner; looking back 25 years from 1984 inclusive).

The large beige area in Panel B highlights the well-known fact that when sample size is small, statistical power is often insufficient to differentiate signal from noise. Conversely, the large terracotta area highlights the fact that when power is sufficient, the warming signal has been detectable at any point during the last 30 years, irrespective of vantage point. When one extends the period looking backwards in time, the warming trend is always significant and the most recent vantage point(s) do not differ systematically from earlier vantage points. It follows that the data do not permit identification of a “pause” or “hiatus” during the last 10–20 years. Significantly, this conclusion is unaffected by the choice of year taken to represent the onset of modern warming (i.e., areas to the Southeast of all 3 diagonal lines in Figure 3(B) permit the same conclusion). The conclusion is also unaffected by the choice of the year during which the “pause” was examined (i.e., the vantage point).

Conversely, if one uses shorter time periods of analysis, one can find many “pauses.” Using the operationalizations found in the corpus (mean duration 13.5 years) and a null hypothesis of no warming, we find that the climate “paused” strikingly often during the last 30 years. During that period, the 14-year trend escaped significance 10 times and the 13-year trend 13 times, suggesting that a “pause” occupied between 30% and 43% of a time period during which the climate warmed 0.6 K overall (Fig. 1). If the duration of the defined “hiatus” drops to below 12 years—which applies to 13 out of 40 articles (i.e., 32.5%) in the corpus—then almost everything is a “hiatus”, as signified by the preponderance of beige for trends of this duration in Fig. 3(B). Anyone making a “hiatus” claim of this duration will almost always find one, not because something new and different is happening, but because of the fundamental fact that small sample sizes provide insufficient statistical power for the detection of trends. Thus, a third of the articles in the corpus either presumed that the climate has nearly always “paused” during the last 30 years (rendering the term meaningless), or they inconsistently highlighted only one of many events that would qualify with their definition.

These results have been replicated using a variety of additional methods that incorporate autocorrelations in the time series (see the Online Supplementary Material). The results are not sensitive to the trend detection methods employed and they are also not sensitive to the choice of GMST data set (see the Online Supplementary Material).

We conclude that the evidence does not support the notion of a “pause” or “hiatus” as an identifiable phenomenon that is implied by standard dictionary definitions and common understandings of these terms.