From Watts Up With That?

By Renee Hannon

This post examines how present global surface temperatures compare to the past 12,000 years during the Holocene interglacial. The AR6 IPCC climate assessment report, Climate Change 2021: The Physical Science Basis, by Working Group 1 states in their Summary for Policymakers section A.2.2:

“Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence). Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago [0.2°C to 1°C relative to 1850–1900] (medium confidence). Prior to that, the next most recent warm period was about 125,000 years ago, when the multi-century temperature [0.5°C to 1.5°C relative to 1850–1900] overlaps the observations of the most recent decade (medium confidence).”AR6

Paleoclimate proxy data records have low temporal resolution.

Comparing present instrumental data to the past is no small task. Temperature data during the Holocene and older are indirect measurements based on proxies. Scientists have compiled and extensively analyzed these proxy data covering the past 10,000 years. The datasets contain 100’s of records and include terrestrial, marine, lake, and glacial ice proxy data, to name a few.

Unfortunately, lake and marine proxy data are smoothed due to sediment mixing and uncertain age control. Smoothing of paleoclimate proxy data also occurs due to averaging of multiple data types together which destroys higher frequency decadal data (Kaufman and McKay, 2022). Hence, proxy data during the Holocene is multi-century at best, representing an average temperature smoothed over a couple hundred years.

The IPCC statement above is correct but can be misleading. They compare decadal average temperatures to multi-century average proxy data. To better understand how modern temperatures compare to the past, one can either deconvolve past proxy data or smooth present instrumental temperatures to produce a similar temporal resolution comparison.

Kaufman and McKay, 2022, wrote a technical note comparing multi-century present and future temperatures to the past. They used the average of instrumental data plus AR6 model projections to show global mean temperatures of about 1 deg C during a 200-year period from 1900-2100 shown in Figure 1. These averages include 120 years of present instrumental data and 80 years of future modeled projections. The pre-industrial baseline is defined by the IPCC as the average global temperature during 1850-1900.

Figure 1. Global surface temperature from instrumental data used by the IPCC consists of 4 datasets (Hadcrut, NOAA, Berkeley Earth, Kadow) shown in black from Trewin, 2022. Global temperature projections using three emissions scenarios (low, intermediate, and high) from AR6 also shown. The length of the dashed line indicates the period over which data were averaged. All datasets have been calibrated to a 1850-1900 pre-industrial baseline. After Kaufman and McKay, 2022.

Instrumental temperature data have been around since 1850, about 170 years. These data are closely approaching a bicentennial timescale. To note, pre-1950 HadCRUT instrumental data is considered lower quality due to sparser data coverage and increased noise (McLean, 2018). Since IPCC scientists use simple averages for comparison to the past, then averaging instrumental data should also be considered as a present base case. Using the IPCC’s instrumental dataset, a simple average for the last 170 years shows a global temperature anomaly of a whopping 0.3 deg C, uncertainty range of 0.1, above the pre-industrial baseline shown in Figure 1.

Smoothing instrumental temperature during the last century and a half allows for a truer comparison to smoothed multi-century proxy data. This smoothed instrumental data is 70% cooler than the 1 deg C represented by present plus future temperature means over a 200-year period. Annual global instrumental temperatures have only been slightly at or above 1 deg C for about one decade. That’s not even close to being a multi-century comparison to the past.

A More Valid Comparison of the Present to the Past

Using instrumental data without adding in uncertain future modeled projections seems to be a better way to compare present temperatures to the past. Nobody knows how accurate model projections are especially considering the debates about their track record of not matching observed temperatures and past proxy data. A smoothed instrumental average for comparison to the past is absent in the AR6 report and never established, mentioned, or recognized by the IPCC. Figure 2 shows the 170-year instrumental temperature average (small black square) compared to past proxy data during the Holocene.

Figure 2: Millennial global surface temperature ranges from proxy data over the Holocene. Temperature 12k data from Kaufman, 2020. Data are calibrated to a 1850-1900 pre-industrial baseline. Instrumental data represents less than 1.5% of the past 12,000 years.

The Holocene climatic optimum (HCO) occurred 6000-7000 years ago with the warmest 200-year long interval estimated at 0.7 deg C with an uncertainty range of 0.3 to 1.8 deg C according to extensive proxy data compiled by Kaufman, 2020. An earlier proxy study by Marcott, 2013, shows an HCO temperature mean of 0.8 deg C with a two-standard deviation of 0.3 above the pre-industrial period. Marcott also confirms that proxy records completely remove centennial variability, and no variability is preserved at periods shorter than 300 years in his reconstruction. Andy May also performed a Holocene global reconstruction using proxy data here. His reconstruction shows an HCO of 0.85 deg C above the pre-industrial baseline and over 1 deg C warmer than the coldest time of Little Ice Age. Figure 3 shows the 170-year instrumental temperature average compared to the HCO temperature of these reconstructions.

Chemical, biological, and physical data supports a warmer Holocene past. A mid-Holocene climatic optimum is supported by pollen records which show expanded grass and shrub vegetation in the African Sahara, increased temperate forest cover in Northern Hemisphere mid-latitudes and boreal forest instead of tundra in the Arctic (Thompson, 2022). Glacier and ice cap fluctuations from lake studies in the Arctic were smaller than present or absent during the early and mid-Holocene (Larocca, 2022). Both Javier Vinos, 2022, and Kaufman, 2023, have a thorough discussion of empirical evidence at different latitudes supporting a warmer past mid-Holocene.

Figure 3: Histogram of proxy temperature reconstructions in gray showing the warmest temperature of the Holocene compared to the multi-century average of instrumental data in red. Error bars shown by black line. All temperature deltas are from the 1850-1900 pre-industrial period. No climate model data is included.

Even the IPCC states that around 6500 years ago temperatures ranged from 0.2°C to 1°C warmer relative to 1850–1900 pre-industrial period. Therefore, the present global temperature 170-yr average is mostly cooler than the past Holocene climatic optimum 6500 years ago. As a matter of fact, the present average temperature barely hits the 5% minimum error bar on one of the reconstructions and is just over the IPCC minimum range.

In the IPCC technical justification note, Kaufman and McKay 2022, conclude that global recent plus the modeled upcoming warming reaches a level unprecedented in more than 100,000 years. My emphasis on the word plus. Without including future modeled temperatures, present instrumental temperature, averaged over 170 years, does not exceed the warmest multi-century period of the Holocene based on proxy data. And it’s not even close to the last interglacial period when multi-century temperatures were almost 1.5 deg C warmer than the pre-industrial period. If, big IF, the climate models are considered reliable, then perhaps in the future 80 to 100 years, present global temperatures might be as warm as the past Holocene Climatic Optimum.

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