A critical question regarding climate change (CC) is how much of observed changes in temperature and climate can be attributed to anthropogenic (man-made) global warming (AGW). One way we can address this question is to compare the current temperature and temperature change since the start of the industrial revolution (when we began to inadvertently modify the atmosphere significantly) with natural variability that has occurred in Earth’s history. If either the temperature or rate of change of temperature is unprecedented that would provide key evidence towards the conclusion that mankind is responsible for the majority of recent warming as claimed by the Intergovernmental Panel on Climate Change (IPCC), and raise the level of concern and need for CC action.
In the first part of this two-part post (Global Warming - It's All Relative Part I ) we saw that the average global surface temperature (GST) has increased around 0.56 C over the last 40 years, as measured most accurately by satellites, and by approximately another 0.4 C in the hundred years prior to 1980, as measured by ground stations.
For comparison, to determine the temperature of the Earth for periods hundreds or thousands of years ago we must rely on temperature proxies. These proxies typically consist of some material that is deposited in layers (with the layer order providing age information) and the material having some physical property that varies with temperature at the deposition time. The National Climate Data Center is an excellent source for information and data for a wide variety of temperature (and other climate parameters) proxies at this link:
https://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets
One of the most widely utilized temperature proxies are ice cores drilled into thick ice sheets at the earth’s poles and on glaciers. Seasonal variability causes the ice to be deposited in identifiable annual layers that can be counted. The age of the layers are also cross-calibrated with other known chronological sources such as dust from volcanic sources or celestial impacts to determine the age of all of the various layers with reasonable accuracy. The temperature of the water that was the source of the snow used to create the ice pack in each layer is determined by looking at isotropic ratios, typically for hydrogen or oxygen.
One of these sites where a long term temperature record is available is the Dome C drilling site in Antarctica. Fig. 1 below shows the relative temperature change at Dome C over the last 740,000 years. (Data source: Jouzel, J., et al. 2004. EPICA Dome C Ice Cores Deuterium Data. IGBP PAGES/World Data Center for Paleo- climatology)
Figure 1. Dome C Temperature Anomaly
The most recent data point in this chart is on the right side of the graph and corresponds to 1880. The oldest record, corresponding to 740,000 years ago, is on the left in the graph. This particular temperature measurement has a time resolution of around a century for the most recent data with decreased resolution at earlier times.
This temperature record shows that Antarctica over this record has been much colder on average than the current temperature. We are not talking about small variations in temperature. The average temperature at Dome C would reach as low as 9 C (16.2 F) below the current average temperature! Ice cores and other temperature proxies all over the world prove that over the last four hundred fifty thousand years there have been four other warm interglacial periods with temperatures comparable to the present warm Holocene period, which is approx. 12 kyr ago to present. The interglacial warm periods are separated by very cold glacial periods of longer duration where a much higher percentage of the Earth is covered with ice sheets.
Fig. 2 shows similar data from Vostok station which is 560km distant from Dome C. (Petit et al, Nature 1999). (NOTE - time axis is reversed, current time to left). Fig. 2 also shows other data obtained from the ice cores including CO2 concentration in parts per million (pbm). The historical temperatures from the two Antarctic locations are extremely similar supporting a conclusion these two temperature profiles are representative of all of Antarctica. (Plot B with scale on RHS shows temperature anomaly.)
Fig. 2 (from Petit et al, Nature 1999, Plot B shows Temperature aberration)
What about data from the other polar region, the Arctic? Fig. 3 shows a temperature history (only 49,000 yrs old) for the central Greenland location of the GISP2 ice core drilling location. This data has time resolution on the order of a decade.
Fig. 3 (Source: Alley, R.B.. 2004. GISP2 Ice Core Temperature and Accumulation Data. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2004-013.)
The GISP2 data set shows periods during the most recent glacial period that are up to 25 C below the peak temperatures found during the Holocene! There is considerable uncertainty in the scale factors used to convert changes in isotopic ratios to temperature. Subsequent papers and refinements in isotropic analysis suggest that the absolute magnitude of the temperature variations seen in Fig 3 may be a factor of 2 too high. Still, the time history of the temperature changes seen here is considered accurate.
So now let's get back to the central question of this post. How do recent temperature changes and absolute temperature compare to historical temperatures? Clearly, Figures 1 and 2 show that the previous 4 interglacial periods had significantly warmer temperature peaks in Antarctica than the peak temperature of the Holocene (and the current temperature) - up to 4.4° C (7.9 F) warmer!! Additionally, Figure 3 clearly shows periods in the past where the rate of temperature change of the Arctic has been much greater than what we have observed recently. For example, 11,700 years ago the temperature at that location changed >10 degrees C in <140 years!
This analysis appears to show that recent temperature changes and absolute temperature are insignificant compared to historical, “natural” variations. In fact, numerous AGW sceptics have used similar comparisons to question the validity of the IPCC conclusion that is “extremely likely” that human activities are the dominant cause of the warming of the planet since the mid-twentieth century. However, this analysis is too simplistic.
Firstly, the ice core temperature records shown in the previous figures display regional temperature variations and NOT global temperature variation. Temperature variability in any one location or region is more variable and has a wider range than the global aggregate. The large temperature variation mentioned a few paragraphs earlier, 11,700 years ago in the Arctic, greatly diminishes in magnitude when the data there is combined with the temperature proxies throughout the world to obtain a global average.
Several multiproxy global temperature reconstructions, including other temperature proxies such as coral rings, alkenones, and Mg/Ca ratios in plankton and seashells, have shown that the global temperature variation between glacial maximums and interglacial temperature peaks is less than the temperature range at the poles, both in magnitude and rate of change. Figure 4 shows one recent average global surface temperature reconstruction.
Figure 4 - GST reconstruction (T. Friedrich et al, Science Advances 2016)
A more significant factor when comparing recent temperature history to the paleoclimate temperature record as shown in Fig. 4 is that the climate on earth is a very complex, nonlinear system highly dependent on previous conditions. It is simply not valid to compare temperature variations as observed over the last 150 years with temperature variations seen at or near the height of each glacial maximum when a significant portion of the northern hemisphere is covered with ice.
A more valid, albeit difficult, analysis is to compare the Antarctic temperature variations during the current Holocene period with the previous 4 interglacial periods (see Figure 5)
Figure 5 - Antarctica temperature anomalies of current interglacial (T1) and previous three interglacials (T2,T3, T4)
Figure 5 shows that in Antarctica all four interglacial periods have a relatively rapid increase in temperature in less than 8,000 years to a peak temperature, shown in Fig. 5 as time 0, followed by a decrease in temperature at a much slower rate. T1, the current interglacial period, and T2, the previous interglacial which started around 125,000 years ago, have similar profiles, with the most notable difference that the T2 temperature peak was around 4 C higher! Additionally, Fig 5 shows pre-industrial temperature fluctuations of at least 1.5 degree C, so any modern temperature fluctuation of similar or smaller magnitude can not be conclusively linked primarily to AGW.
Since we cannot compare the Antarctica temperature history with the modern global instrument record, how does the long-term temperature record compare to the modern instrument temperature record for Antarctica? Figure 6 shows the average monthly temperature history since 1955 at Antarctica manned stations (From Berkeley Earth)
Figure 6 Antarctica monthly temperatures 1955 to 2020
There are wide fluctuations seen over short time scales (<1 year) as you would expect from any one region of the world, but the 10-year average trend shows an increase of less than 0.5 degrees C over the 65 year record. This variation is less than the natural variations that have previously occurred over the current and previous interglacials. Thus, the temperature record in Antarctica DOES NOT support the climate alarmist or IPCC position.
But before jumping to the conclusion that we have our final answer, we must consider that this is just one data point. The evidence from the Arctic is more concerning. Recall that Figure 3 shows a high-resolution Holocene temperature history at a location in Greenland. For comparison, we have multi-century thermometer records from Iceland, which is approximately 400 miles from the location of the GRIP measurements.
Figure 7 shows the thermometer based temperature record for a composite of Iceland stations from Berkeley Earth.
Figure 7. Thermometer based annual average temperatures for Iceland (from Berkeley Earth)
Since 1855, there has been an approximately 1.5 degree C rise in the annual temperature in Iceland. Combining this record with the GRIP Greenland temperature record for the last 8,000 years we obtain Figure 8.
Figure 8 - GRIP ice core & Iceland thermometer temperature anomalies (relative to 1855)
The red line in Figure 8 is the thermometer record since 1855. Both the magnitude and rate of rise of the temperature in the last 150 years are unusual (for a randomly selected 150 year period over the last 8000 years) but not exceptual.
The temperature records at both of the poles do not (by themselves) support the IPCC claim that it is “extremely likely” that the earth’s warming over the last 50 or 150 years are mainly caused by mankind.
In the interest of fairness and completeness it should be noted that a highly referenced paper by Marcott et al (SA Marcott et al. Nature 514, 616-619 (2014) doi:10.1038/nature13799) compares the GST modern instrument temperature record with a multi-proxy temperature reconstruction and concludes that global temperature anomalies over the last 10,000 years are much smaller than that reported from ice cores (see Figure 9)
Figure 9 - Global Surface Temperature reconstruction from Marcott 2013
Marcott’s reconstruction shows natural global temperature variation of <1 C over the last 10,000 years with a flat or decreasing trend. This reconstruction has been controversial and has numerous potential issues including some inherent with any multi-proxy reconstruction (I may address in another post possibly). But if this reconstruction is representative of the GST for the last 10,000 it would constitute the one strongest pieces of evidence that the relatively rapid 1 degree C rise over the last 150 years has a large anthropogenic component.
