Is 1 °C Halfway to Hell?

Pro-anthropogenic Global Warming web sites are concerned that we are about to exceed the 1 °C value for the temperature anomaly (see Met Office, New Scientist and Skeptical Science) but is this really “uncharted territory” (Met Office) or halfway to New Scientist’s global warming hell of 2 °C?

It is shown in the following discussion that “1 °C: Halfway to Hell” is an ill-chosen headline – a more appropriate headline would be, “1 °C: Halfway to the Optimum”.

HadCRUT4 Data

The HadCRUT temperature anomalies are shown in Figure 1.

Figure 1-HadCRUT4 Overlay on 1850-1900 Ave-CompressedFigure 1: HadCRUT 1850-1900 Temperature Anomaly with 1961-1990 Overlay (After: Met Office Chart)

The Met Office chart in Figure 1 is unusual in that the HadCRUT anomaly data are usually referenced to the 1961-1990 mean temperature. Using a pre-industrial mean of 1850-1900 is not strictly correct because the industrial revolution began from approximately 1760 to sometime between 1820 and 1840 (Wikipedia). Consequently, the use of an 1850-1900 mean for pre-industrial temperatures is an arbitrary choice.

Therefore, I have overlain the HadCRUT4 data (1961-1990 mean), plotted as the blue line and it is evident from Figure 1 that using a mean of 1850-1900 raises the anomaly by ≈ 0.3 °C, when compared with the 1961-1990 mean. This gives the pre-industrial anomaly greater visual impact than the usual HadCRUT4 values.

The use of the 1850-1900 mean as the basis for the 1 °C rise is unusual because the IPCC reports have always refer to the 1961-1990 mean from HadCRUT for climate projections, e.g., see IPCC AR4 FAQ 3.1, Figure 1. However, we can use other data to determine a real pre-industrial mean as discussed below.

A Different Pre-industrial Benchmark

A chart from Ljungqvist (2010) is presented in Figure 2 that shows temperature fluctuations in the northern hemisphere for the last two millennia.

LjungqvistFigure 2: Reconstructed Extra-tropical (30-90 °N) Decadal Temperature Anomaly to 1961-1990 mean (after Ljungqvist, 2010)

The purpose of Ljungqvist (2010) is to assess the amplitude of the pre-industrial temperature variability. It is evident from Figure 2 that there have been previous warm and cold periods over the last two millennia but that most of the temperatures have been cooler than the 1961-1990 mean. Furthermore, Ljungqvist notes that the Roman Warm Period (RWP) and the Medieval Warm Period (MWP)

seem to have equalled or exceeded the AD 1961-1990 mean temperature level in the extra-tropical Northern Hemisphere.”

The following points are worth noting from Figure 2:

  1. The instrumental date (shown in red at the right hand side of the chart) represents a comparatively small portion of the data available.
  2. The modern proxy peak temperature is 0.082 °C, which is 0.114 °C lower than the MWP peak of 0.196 °C.
  3. Using 1850-1900 as the base for pre-industrial temperature is a relatively cold benchmark for temperature measurements. For example, the 1850-1920 instrumental mean is -0.299 °C, which is 0.495 °C lower the MWP peak.
  4. It is apparent that we could use the 1961-1990 as a suitable base for pre-industrial temperatures. We could even use the mean of the Medieval Warm Period, which is 0.041 °C higher than the 1961-1990 mean.

It is also worth noting that one of Ljungqvist’s conclusions is that,

“Since AD 1990, though, average temperatures in the extra-tropical Northern Hemisphere exceed those of any other warm decades the last two millennia, even the peak of the Medieval Warm Period, if we look at the instrumental temperature data spliced to the proxy reconstruction. However, Ljungqvist stresses that, “However, this sharp rise in temperature compared to the magnitude of warmth in previous warm periods should be cautiously interpreted since it is not visible in the proxy reconstruction itself [my emphasis]

Indeed, Ljungqvist states that the proxy records result in “flattening out” the values,

“that makes us unable to capture the true magnitude of the cold and warm periods in the reconstruction… What we then actually get is an average of the temperature over one or two centuries.”

In other words when comparing earlier temperatures we should be careful when comparing temperature readings – we should only compare proxies with proxies and not proxies with thermometers.

Same Data: Different Perception

There are good scientific reasons for displaying temperatures as anomalies because it allows widely different temperatures from geographically disparate regions to be compared. Nevertheless, they do not need to be displayed in the format of Figure 1. It appears that one of the intentions of displaying temperatures in the Figure 1 format is to depict the temperature rise as being unusual and rising rapidly. However, this is not the case.

A temperature change of approximately 1 °C for 165 years from 1850 to 2015 is almost undetectable by human beings. To illustrate this I use actual HadCRUT4 global temperatures (not anomalies) in Figure 3.

Global Ave Temperature 1850-1900Figure 3: Global Average Temperature (1850-2015)

The chart presented in Figure 3 uses the HadCRUT 1961-1990 mean (14 °C) global temperature as its baseline (see FAQ here) and the actual temperatures are derived by adding or subtracting the anomalies data here from the 14 °C baseline. Figure 3 is based on a diagram by Anthony Watts (published in Climate Change the Facts, 2014) and it gives a less alarming view of global warming when compared with the anomaly diagram in Figure 1.

The following points are worth noting from Figure 3:

  1. The current high value for temperature (September 2015) is 14.70 °C and the lowest recorded value of 13.45 °C occurred in 1911. The starting value of the HadCRUT4 series is 13.63 °C in 1850.
  2. I have used temperatures from my own personal experience to determine a reasonable scale for the vertical axis, ranging from a high of 51 °C in Dubai to a low of -16 °C in Scotland.
  3. I also show temperatures from my new home in Sydney. These are more benign than those in item (2) above but they still show a large range from a high of 45.8 °C to a low of 2.1 °C.

Furthermore, by using temperature data from environs in which I have lived, it is evident that a temperature change of 1 or 2 °C is very small and is not unusual for most flora, fauna and humans. Nevertheless, let us examine if a 2 °C rise would cause serious climatic damage by discussing the Holocene Optimum.

The Holocene Optimum

The first IPCC report FAR presented the diagrams shown if Figure 4 for temperature variations over the last ten thousand years, 4(b), and the last one thousand years, 4(c).

The charts in Figure 4 are based on the work of Lamb, who was the founding director of the Climatic Research Centre (CRU) that produces the HadCRUT temperature data in conjunction with the Met Office. HadCRUT data is used extensively by the IPCC.

Schematic of Glbal Temp Variation-Lamb-IPCC FARFigure 4: Schematic Diagrams of Global Temperature Variations (Source: FAR Figure 7.1)

The similarity between Figure 4(c) and Ljungqvist’s chart in Figure 2 is remarkable, considering that Figure 4 was published in 1990. The dotted line in diagrams 4(b) and 4(c) is stated in FAR as nominally representing conditions near to the beginning of the twentieth century. Unfortunately, the diagrams in Figure 4 do not show values for the temperature scale. Therefore, I use Marcott et al (2013), which is referenced in AR5 WG1, to supply these values.

Marcott et al (2013) has been criticised for showing a spurious uptick in temperature in the 20th century. Indeed, Marcott stated in the paper and in RealClimate that is uptick is “probably not robust.” Consequently, I have used Roger Pielke, Jr’s version of Marcott’s diagram as Figure 5, in which the spurious data are deleted.

Marcott Fig-1B Pielke-AmendmentFigure 5: Holocene Global Temperature Variations (Source: Marcott Figure 1B amended by Pielke)

Approximately 80% of the Marcott et al (2013) proxies are of marine origin and consequently underestimate the variability in land temperatures. Nevertheless, several useful conclusions are obtained by Marcott et al (2013), namely:

  1. “Global temperature for the decade 2000-2009 has not exceeded the warmest temperatures in the early Holocene.”
  2. “The early Holocene warm interval is followed by a long-term global cooling of ≈ 0.7 °C from 5,500 BP to 1850.”
  3. “The Northern Hemisphere (30-90°) experienced a temperature decrease of ≈ 2 °C from 7,000 BP to 1850.”

Spatial Distribution of Temperature during Holocene Climatic Optimum

Renssen et al (2012) use a computer simulation to derive early Holocene temperature anomalies. They call the Holocene Climatic Optimum the Holocene Thermal Maximum (HTM) and, in referring to their simulation, they state that,

“The simulated timing and magnitude of the HTM are generally consistent with global proxy evidence, with some notable exceptions in the Mediterranean region, SW North America and eastern Eurasia.”

The Renssen et al (2012) computer simulation is cited in AR5 WG1 and it presents the spatial distribution of peak temperature anomalies during the Holocene Climatic Optimum relative to a  1000-200 BP pre-industrial mean (see Figure 6).

Global characterization of the Holocene Thermal Maximum-Renssen et al-2012-Fig 3A.-compressedFigure 6: Global Variation of Holocene Thermal Maximum Anomalies (Source: Renssen et al, 2012)

It is evident from Figure 6 that most of Europe and North America experienced an anomaly of 2-3 °C during the Holocene Thermal Maximum (HTM) and Renssen et al (2012) offer the following conclusions:

  1. “At high latitudes in both hemispheres, the HTM anomaly reached 5 °C.”
  2. “Over mid-to-high latitude continents the HTM anomaly was between 1 and 4 °C.”
  3. “The weakest HTM signal was simulated over low-latitude oceans (less than 0.5 C) and low latitude continents (0.5-1.5 °C).”

I reiterate that Renssen et al (2012) use a pre-industrial mean of (1,000 to 200) BP, which is ≈ 0.3 °C less than the HadCRUT4 (1961-1990) mean. Therefore, we should add ≈ 0.3 °C to their values when comparing them with modern-day temperatures. Not withstanding the aforementioned, it should be noted that the Renssen et al values are peak values and that global temperatures would be lower than their peak values.


Current temperatures are examined with regard to the approaching 1 °C anomaly and the following standpoints are evident from the discussion:

  1. Portraying current temperatures as an anomaly from the 1850-1900 mean gives the false impression that current temperatures are high because it is shown that temperatures during this period were very low when compared with other warm periods, either in the last two millennia (Ljungqvist, 2010) or in the early Holocene (Marcott et al, 2013 or Renssen et al, 2012).
  2. A reasonable mean for pre-industrial temperatures would be 1961-1900 because this mean compares well with actual mean temperatures that occurred during times that really were pre-industrial, e.g., the Roman Warm Period and the Medieval Warm Period.
  3. The change in temperature during the last 165 years is hardly visible in Figure 3 but such plots wouldn’t normally get people overly concerned. Conversely, when an anomaly plot is deployed, the vertical scale is highly magnified as shown in Figure 1. The magnified vertical scale gives a steep slope to the temperature rise in modern times, which conveys the impression that global warming is proceeding rapidly. To the contrary, and in reality, Figure 3 shows that temperatures have been very stable over the last century and a half.
  4. Less worrying anomaly plots than that shown in Figure 1 are presented in Figures 2 and 5. These show that current temperatures are not unusual when compared with earlier warm periods.
  5. Figure 6 (Renssen et al, 2012) shows that many parts of the world experienced temperatures during the early Holocene that were significantly greater than 2 °C above the pre-industrial


The following conclusions are evident from the above:

  1. Portraying current temperatures as an anomaly from an 1850-1900 pre-industrial mean gives the false impression that current temperatures are high because temperatures during 1850-1900 were amongst the lowest in the last 10,000 years.
  2. Global temperature for the decade 2000-2009 has not reached the warmest temperatures in the early Holocene.
  3. Northern Hemisphere temperatures would need to increase by at least 2 °C above the (1850-1900) pre-industrial mean to reach temperatures experienced during the Holocene Climatic Optimum.

I contend that “1 °C: Halfway to Hell” is an inappropriate headline – a more appropriate headline would be, “1 °C: Halfway to the Optimum”, especially, if you live in the Northern Hemisphere.

Climate Models 2011: Same Data – Different Conclusions

In his blog post 2011 Updates to model-data comparisons at Real Climate, Gavin Schmidt shows the diagram in Figure 1.

Figure 1: Real World Temperatures Compared with IPCC Model Ensemble (Schmidt, 2012)

Gavin states that, “Overall, given the latest set of data points, we can conclude (once again) that global warming continues.” My perception was that there had been some cooling over the last 15 years, therefore I have decided to check Gavin’s claims.

Gavin explains that the chart shows the annual mean anomalies from the IPCC AR4 models plotted against the surface temperature records from the HadCRUT3v, NCDC and GISTEMP products (it really doesn’t matter which). Everything has been baselined to 1980-1999 (as in the 2007 IPCC report) and the envelope in grey encloses 95% of the model runs.

At first glance the chart seems to show a good correspondence between real world temperature and the average of the IPCC models. However, the correspondence does not look quite so good when you compare the chart with the AR4 charts. I have updated AR4 Figures 1.1 and TS.26 to include the HadCRUT data up to May 2012 and discuss these as follows.

Figure 2 is derived from Figure 1.1 of IPCC AR4.

Figure 2: Global Average Temperature Compared with FAR, SAR & TAR (after AR4 Figure 1.1)

It should be noted in Figure 2 that I could not get the HadCRUT3 temperature to match exactly with the values in Figure 1.1 in AR4. Therefore, I had to adjust the HadCRUT3 data by adding 0.026 °C. I am not sure why I had to make the adjustment in the HadCRUT3 data, perhaps it is just a printing error in the AR4 diagram but this error also repeats elsewhere. It may be coincidence but the average temperature for 1961-1990 on which HadCRUT3 is based is 0.026 °C. Therefore, it may be that the AR4 chart is normalised to a zero temperature for the 1961-1990 period. However, I can find no information that confirms that this adjustment should be made.

Notwithstanding the above, it is evident from Figure 2 that the correlation between the adjusted HadCRUT3 data and the original AR4 Figure 1.1 data is very good. This applies to both the individual data points and the smoothed data. It is also evident that the temperature trend is significantly below the FAR estimate and is at the very low ends of the SAR and TAR estimates.

In order to compare Gavin’s diagram with actual global temperatures, I use Figure TS.26 from AR4 a shown in Figure 3.

Figure 3: Model Projections of Global Mean Warming Compared with Observed Warming (after AR4 Figure TS.26)

The following points should be noted regarding Figure 3 compared with AR4 Figure TS.26:

  1. I have deleted the FAR, SAR and TAR graphic from Figure TS.26 in Figure 3 because they make the diagram more difficult to understand and because they are already presented in Figure 2, in a form that is much easier to assimilate.
  2. The temperature data shown in AR4 Figure 1.1 does not correspond to that shown in Figure TS.26. The Figure 1.1 data appear to be approximately 0.02 °C higher than the corresponding data in Figure TS.26. I have assumed that this is a typographical error. Therefore, I have used the same 0.026 °C adjustment to the HadCRUT3 data in Figure 3 that was used for Figure 2.
  3. My adjusted HadCRUT3 data points are typically higher than those presented in Figure TS.26.
  4. Despite items (1), (2) and (3) above, there is very good agreement between the smoothed data in TS.26 and the adjusted HadCRUT3 data, particularly for the 1995-2005 period. It should be noted that AR4 uses a 13-point filter to smooth the data whereas HadCRUT uses a 21-point filter. Nevertheless, AR4 states that the 13-point filter gives similar results to the 21-point filter.

Comparing Gavin’s projections in the RC chart in Figure 1 with the official AR4 projections in Figure 3, the following points are evident:

  1. The emissions scenarios and their corresponding temperature outcomes are clearly shown in the AR4 chart. Scenarios A2, A1B and B1 are included in the AR4 chart – scenario A1B is the business-as-usual scenario. None of these scenarios are shown in the RC chart.
  2. Real world temperature (smoothed HadCRUT3) is tracking below the lower estimates for the Commitment emissions scenario., i.e., emissions-held-at-year-2000 level in the AR4 chart. There is no commitment scenario in the RC chart to allow this comparison.
  3. The smoothed curve is significantly below the estimates for the A2, A1B and B1 emissions scenarios. Furthermore, this curve is below the error bars for these scenarios, yet Gavin shows this data to be well within the error bands.
  4. The RC chart shows real world temperatures compared with predictions from models that are an “ensemble of opportunity”. Consequently, Gavin states, “Thus while they do span a large range of possible situations, the average of these simulations is not ‘truth’.” [My emphasis].

In summary, TS.26 from AR4 is useful for comparing real world temperature data with the relevant emissions scenarios. To the contrary, Gavin uses a chart which compares real world temperature data with average model data for which he states does not represent “truth.” I suggest that this is not much of a comparison and I conclude that the AR4 chart is a much more informative comparison.

I also conclude that it is evident from Figure 3 (AR4 Figure TS.26) that there has been a pause in global warming and that some cooling is occurring. It is certainly not as Gavin concluded that, “Overall, given the latest set of data points, we can conclude (once again) that global warming continues.” Whether or not this cooling pause is a longer-term phenomenon or temporary pause only time will tell.

2010 – The Hottest Year on Record: Is this a Cause for Concern?

GISS report that 2010 has tied with 2005 as being the hottest year on record. James Hansen, the director of GISS, said that, “If the warming trend continues, as is expected, if greenhouse gases continue to increase, the 2010 record will not stand for long.”

Is this a cause for concern?

GISS Data Compared with Hansen’s Scenarios (2006)

The GISS Land Ocean Temperature Index (LOTI) data up to January 2011 are shown in Figure 1. They are compared with the global warming models presented by Hansen (2006).

Figure 1: Scenarios A, B and C Compared with Measured GISS Land-Ocean Temperature Index (after Hansen, 2006)

The blue line in Figure 1 denotes the GISS LOTI data and Scenarios A, B and C describe various CO2 emission outcomes. Scenarios A and C are upper and lower bounds. Scenario A is “on the high side of reality” with an exponential increase in emissions. Scenario C has “a drastic curtailment of emissions”, with no increase in emissions after 2000. Scenario B is described as “most plausible” which is expected to be closest to reality. The original diagram can be found in Hansen (2006). It is interesting to note that, in his testimony to US Congress, Hansen (1988) describes Scenario A as “business as usual”, which somewhat contradicts his “on the high side of reality” statement in 2006.

It is evident from Figure 1 that the best fit for actual temperature measurements is currently the emissions-held-at-year-2000-level Scenario C. The current temperature anomaly is 0.61  °C. Therefore, even with temperatures at record highs, we are not experiencing the runaway temperatures predicted for the “business-as-usual” Scenario A. Indeed, for Scenario C with emissions curtailed at year-2000 levels, the rate of temperature increase is an insignificant 0.01 °C/decade.

It is also worth noting that we are currently at the lower end of the range of estimated temperatures for the Holocene optimum and the prior interglacial period. These occurred without human intervention or huge increases in carbon dioxide.

HadCRUT3 Compared with IPCC AR4

The above comparison based on Hansen (2006) uses relatively old climate models. Therefore, I have compared current HadCRUT3 temperature data with the latest IPPC AR4 (2007) models in Figure 2.

Figure 2: IPCC Scenarios A1B, A2 & B1 Compared with HadCRUT3 Temperature Data (after AR4, 2007)

Figure 2, which is based on IPCC AR4 Figure TS.26. I have added the HadCRUT3 data as blue dots. The black dots in the original TS.26 diagram appear to be HadCRUT3 data but are slightly misaligned. Therefore, I offset the HadCRUT3 data by adding 0.018°C to achieve a reasonable fit with the individual data points shown in AR4. The blue line with white dots the smoothed HadCRUT3 data. It is evident from Figure 2 that the smoothed curve gives an excellent fit with observed data presented as the solid black line in AR4. The current temperature anomaly is 0.52  °C.

The observed temperature trends in Figure 2 are significantly below the “likely” warming scenarios presented in AR4. Furthermore, as with the GISS data, the current HadCRUT3 trend is similar to the emissions-held-at-year-2000-level scenario.


Two comparisons are presented that compare GISS LOTI data and HadCRUT3 data with their respective temperature simulation models and the following conclusions are offered:

  1. Observed temperatures are significantly below the “most plausible” or “likely” high emissions scenarios. Instead, they are on a trajectory that is similar to the emissions-held-at-year-2000-level scenarios.
  2. Current temperatures are at the lower end of the range of estimated temperatures for the Holocene optimum and the prior interglacial period. These temperatures occurred without human intervention.

In summary, global temperatures may be reaching record highs but they are not following “runaway” trajectories suggested by computer models. Instead, they are following an insignificant warming trend of approximately 0.01 °C/decade.

Notwithstanding the above, it should be noted that time period for the comparison of actual temperature measurements with those predicted by computer models is still relatively short. Hansen (2006) suggests that we could expect reasonable results for distinction between the scenarios and useful comparison with the real world by 2015.