By Larry Kummer. From the Fabius Maximus website.

Summary: There has been a daily drumbeat of dark climate news to accompany the IPCC’s new report, “Global Warming of 1.5 °C.” Millions of people are terrified that climate change will wreck or destroy the world. Here is some information they probably do not know, because journalists do not mention it.

By Sebastian Lüning and Fritz Vahrenholt.

Frontiers of Earth Science, 17 December 2017.

Abstract

This paper is ungated, and well worth reading in full for anyone interested in climate change – one of the key policy questions of our time.

“The Paris Agreement adopted in December 2015 during the COP21 conference stipulates that the increase in the global average temperature is to be kept well below 2°C above “pre-industrial levels” and that efforts are pursued to limit the temperature increase to 1.5°C above “pre-industrial levels.” In order to further increase public acceptance of these limits it is important to transparently place the target levels and their baselines in a paleoclimatic context of the past 150,000 years (Last Interglacial, LIG) and in particular of the last 10,000 years (Holocene; Present Interglacial, PIG).

“Intense paleoclimatological research of the past decade has firmed up that pre-industrial temperatures have been highly variable which needs to be reflected in the pre-industrial climate baseline definitions. The currently used reference level 1850–1900 represents the end of the Little Ice Age (LIA). The LIA represents the coldest phase of the last 10,000 years when mean temperatures deviated strongly negatively from the Holocene average and which therefore are hard to justify as a representative pre-industrial baseline. The temperature level reached during the interval 1940–1970 may serve as a better reference level as it appears to roughly correspond to the average pre-industrial temperature of the past two millennia.

“Placing the climate limits in an enlarged paleoclimatic context will help to demonstrate that the chosen climate targets are valid and represent dangerous extremes of the known natural range of Holocene temperature variability.”

Origin of two-degree climate change target.

The two-degree climate change target was first proposed by economist William Nordhaus in 1975 (here and refined here) with almost no basis in the physical sciences and no peer-review by physical scientists. It was pushed by papers from political and advocacy groups. Since then scientists have learned much about the temperature range of the Holocene.

“The Pleistocene comprises of the last 2.6 million years and is characterized by an alternation of cold glacial and warm interglacial phases. A typical glacial/interglacial cycle lasts 100,000 years, whereby the cold phase usually takes 90% of the time while the interglacial phases make up only 10% of the cycle. The cyclicity is controlled by Milankovitch Earth’s orbital cycles. The Last interglacial (LIG) is the Eemian …which occurred 126,000–115,000 years ago. …LIG temperatures have likely been 1.2°C above the most recent temperatures averaged over 1998–2016. …LIG already exceeded the 1.5°C climate limit by 0.5°C while it just about reached the upper limit of the 2°C temperature target. LIG temperatures correspond roughly with the upper end of the tolerable temperature window cited by the WBGU (German Advisory Council for Global Change, 1995). …

“After the end of the last glacial period, temperatures during the early Holocene started to increase again and reached a maximum during the mid-Holocene Thermal Maximum (HTM), an exceptionally warm period 8,000 to 5,000 years before present (BP). Alternative names for this phase are Holocene Climate Optimum (HCO) and Atlantic Period. The warm period is considered a result of Milankovitch Earth’s orbital effects, in particular changes in earth axial tilt and distance to the sun (e.g., Otto-Bliesner et al., 2016). Global average temperature during the HTM was generally warmer than today. An exact quantification of the HTM warming and its comparison to modern temperatures has unfortunately not yet been achieved and requires additional data and detailed correlation effort. …

“Following the HTM, long-term cooling commenced around 4,000 years BP, representing the onset of the Neoglaciation which culminated in the Little Ice Age 1400–1850 AD. Superimposed on the long-term cooling trend are natural temperature fluctuations, which differ somewhat in the various global reconstructions …). In several of these reconstructions pre-industrial phases occur during which temperatures have reached a similar intensity as today. Likewise, the pre-industrial average temperature for the period 0–1850 AD varies in the different reconstructions (Figure 2). …”

Pre-Industrial Temperature Variability.

The existing “pre-industrial” baseline used are the average temperature estimates for 1850–1900. This is the end of the Little Ice Age, the coldest period during the last 10,000 years. It is an inappropriate basis for setting a public policy target, and a number that misrepresents the public about the history and dynamics of climate change.

“Pre-industrial temperatures have undergone marked natural variations at every possible time-scale. Definition of an 1850–1900 ‘pre-industrial’ reference level is therefore simplistic and does not do justice to the significant natural dynamics of the pre-industrial temperature development. Unfortunately, key reports on the climate limits refer to the pre-industrial temperature as if it had been static during past millennia. …”

Choice and Paleoclimatic Context of Chosen Baseline Value.

“The baseline of the climate limit was defined by World Bank (2014) and UNFCCC (2015) {AKA the Paris Climate Agreement} as the temperature average 1850–1900. Notably, this period marks the end of the Little Ice Age, the latter representing the coldest phase of the entire last 10,000 years (Figures 1, 2). More recently, Hawkins et al. (2017) suggested 1720–1800 as pre-industrial baseline period, which however lies even closer to the coldest phase of the Little Ice Age (Figure 2). The choice of a baseline near the lower extreme of a variable parameter is uncommon in science. An average over a longer pre-industrial interval capturing several natural temperature fluctuations appears more adequate. Suitable averaging time windows may be e.g., the last 1000 years, 2000 years or 10,000 years, always excluding the last 150 years due to anthropogenic influence. …”

The authors’ conclusions.

“Pre-industrial temperatures have been more variable than previously thought which needs to be reflected in the baseline definitions of climate targets as part of the Paris Agreement. The currently used reference level represents the end of the Little Ice Age, the coldest phase of the entire last 10,000 years. The temperature value of this reference level deviates strongly negatively from the Holocene average temperature and therefore is hard to justify as a pre-industrial baseline. A better reference level may be equivalent to 1940–1970, when temperatures were reached that correspond to the pre-industrial global mean temperature of the past two millennia. …”

My thoughts about this important topic.

(1) About the temperature record.

The authors rely on the major published temperature reconstructions. From up in the peanut gallery, it looks to me that Steve McInyre’s criticism of them are devastating and disqualify them for use in setting public policy goals. See his posts on the latest reconstruction here and here.

Reconstruction of the temperature record is foundational information for public policy debate. The operation of these projects is paradigmatic of the state of climate science today: adequate academic science, but grossly inadequate for the public policy need. These reconstructions need more funding, involvement of a wider pool of experts, and vastly improved external review and supervision. It would be some of the best money the US government spent next year. The results might revitalized the policy machinery.

(2) The policy measures necessary to meet the 1.5C or 2C targets.

The targets are unrealistic. They require either devastating measures (e.g., carbon taxes at high levels that would disrupt the global economy) or technology that does not now exist (e.g., bio-energy with carbon capture and storage). Several experts have pointed this out, such as Oliver Geden (presentation, gated paper), Glen Peters (gated paper), and Roger Pielke Jr.

For More Information

See the new IPCC report: “Global Warming of 1.5 °C.” SR15 differs from AR15 on one major way: it assume +1.5°C over pre-industrial creates Armageddon. That’s odd, since we are already at 1°C over (much of that is natural warming). To understand the origin of these “red lines” see “The Invention of the Two-Degree Target” in Der Spiegel.

For an excellent introduction to carbon budgets and temperature targets, see this article at Carbon Brief.

The excerpts from the paper are cited in accordance with its stated copyright provisions.

For more information about this vital issue see the keys to understanding climate change and these posts about the climate wars…

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