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: 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.

Figure 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.

Figure 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.

Figure 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 20^th 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.

Figure 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).

Figure 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.

Discussion

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

Conclusions

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.