FIGURE 1: CLIMATE SENSITIVITY IN A MOVING 60-YEAR WINDOW 1880-2018

A RESPONSE TO A TWEET 5/30/2019

The tweet says that humans and volcanoes caused nearly all of global heating in past 140 years and cites an article in The Guardian. The article cited may be found here:  [LINK]. The Guardian article in turn cites Hegerl 2018 “The early 20th century warming: Anomalies, causes, and consequences” that may be found here [LINK] .

This post is a critical review of the claims made in the Hegerl 2018 paper.

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1. PREAMBLE AND CONTEXT: Anthropogenic Global Warming (AGW) is a theory that the observed long term warming trend “since pre-industrial times” (1880) is caused by fossil fuel emissions of the industrial economy in a two stage process. First, CO2 in fossil fuel emissions cause atmospheric CO2 concentration to rise. And second, higher atmospheric CO2 concentration imposes higher surface temperature by way of a absorption spectrum of CO2 described by Arrhenius and others in 1896. The proposed mechanism is that incident solar radiation reaches the surface relatively unhindered and there, though some is reflected, much of it causes the surface to get warmer and radiate long wave infrared back out to space. But some molecules in the atmosphere, notably CO2, absorbs IR at specific frequencies and then re-radiate the energy in all directions with approximately half of it returning to the surface. It is argued that this energy recycle causes the surface to be warmer than it would have been otherwise. And it is further argued that (1) this mechanism is the causal agent that explains the observed warming since pre-industrial times, and (2) that left unchecked the warming trend will have catastrophic consequences in terms of ice melt, sea level rise, and extreme weather as in heat waves, super storms, floods, and droughts.  These arguments are used as the rationale for an expensive overhaul of the world’s energy infrastructure away from fossil fuels as “climate action” to attenuate the effects of AGW.
2. At the heart of the demand for climate action are two essential causation relationships. First, that the observed changes in atmospheric CO2 concentration are driven by fossil fuel emissions and that therefore they can be moderated with the climate action of reducing or eliminating fossil fuel emissions. The second essential causation relationship is the climate sensitivity relationships which holds that surface temperature is a linear function of the logarithm of atmospheric CO2 concentration. This relationship is normally expressed as: climate sensitivity = the temperature increase caused by a doubling of atmospheric carbon dioxide concentration. This is the essential equation that relates warming to rising atmospheric CO2 concentration and is the cornerstone of AGW theory as well as the rationale for the call for climate action. In this equation, climate sensitivity is a universal constant.
3. THE ETCW ANOMALY: Figure 1 presents empirical climate sensitivity values computed from observational data for the period 1880-2018 in a moving 60-year window. Four different datasets are used for these computations and their results are compared. They are three global temperature reconstructions; Hadley Centre (HAD), Berkeley Earth (BRK), and NASA GISS (GIS), and a dataset of the theoretical series RCP8.5 projected by climate models for the “no climate action” condition (RCP). The study period 1880-2018 is is the relevant time span as of this writing for the study of the impact of the fossil fueled industrial economy for global warming since pre-industrial times“. Each chart contains 12 lines colored differently for the twelve calendar months. The calendar months are kept separate and not combined into annual means because it is known that trend behavior varies among them as described in a related post [LINK] . The theoretical shape of these curves would resemble something close to a horizontal line within an uncertainty range described by the IPCC as 1.5<λ<4.5 where λ is climate sensitivity but that is not what we see in Figure 1.
4. Figure 1 shows significant departures from theory in two ways. First, we find that the lines are not even approximately horizontal but that the observed sensitivity varies over a large range. This behavior indicates climate sensitivity varies according to location along the 139-year full span of the data 1880-2018 showing very high values in some 60-yr period and low values in others. High values indicate strong warming at low atmospheric CO2 concentration. Low values indicate weak warming at high atmospheric CO2 concentration. Both of these are anomalous in the context of AGW theory. The other anomaly in these charts is that the range of sensitivity values is much larger in the observational data than in the theoretical RCP8.5 series. A more detailed discussion of these anomalies is presented in a related post [LINK] .
5. The high value of climate sensitivity varies from λ=8 in the BRK series to λ=4.5 for RCP with intermediate values of 6<λ<7 for HAD and GIS. For the observational data (HAD, BRK, GIS), the high value is seen for the 60-year period 1904-1963. In the theoretical series (RCP), the high value occurs twice, first in the 60-yr period 1884-1943, and again in the 1904-1963 period. The high value in the theoretical RCP series is within the IPCC range of 1.5<λ<4.5. The high values are much higher in the observational data particularly so in the BRK series. A high value of λ implies that a strong 60-year warming rate is seen at low atmospheric CO2 concentration.
6. The low value of climate sensitivity varies from λ close to zero for HAD and BRK and values of λ<1 for GIS and RCP. These values are well outside the IPCC range of 1.5<λ<4.5. A low value of λ implies that no warming or a very low rate of warming was seen in a 60-year period with high atmospheric CO2 concentration. These results present a serious challenge to AGW theory which holds that λ should have the same value within a range 1.5<λ<4.5.
7. HEGERL 2018 EXPLAINS THE ANOMALY: The paper cited above [LINK] addresses this issue and proposes explanations for these anomalies in the value of λ found in the observational data. To explain the observed anomaly, the Hegerl paper proposes a retraction of a prior climate science position that surface temperature is completely explained in terms of atmospheric CO2 concentration (Hansen 1981, Hansen 1988, Lacis 2010, Hansen&Lacis 2013, Hansen&Sato 2015) . The paper says that AGW theory that describes CO2 driven warming in terms of λ is seen in the latter part of the theoretical AGW period (described as the industrial economy (1880-2018) after the year 1950, the retraction says that natural factors should be taken into account in the earlier periods where anomalous values for λ are seen.
8. In particular, the paper identifies the high λ period 1901-1950 referred to as Early Twentieth Century Warming (ETCW) as an anomaly not explained in terms of AGW and that therefore natural causes should be taken into account for the portion of the observed warming not explained by the accepted IPCC range of λ values. The author proposes that volcanic activity explains the unexplained warming and concludes that the warming anomaly of 1901-1950 does not violate AGW theory because the excess warming is explained by volcanic activity net of aerosol cooling.
9. THE PROBLEM WITH HEGERL 2018: This study contains the so called Texas Sharpshooter Fallacy. The most serious flaw in this analysis, that effectively nullifies its conclusions, is that a theory derived from data cannot be verified with the same data. Stated differently, the data used to construct a hypothesis can’t be used to test that hypothesis. That kind of empirical test suffers from circular reasoning.
10. THE ONE HISTORY PROBLEM: In climate science, as in economics, there is only one history and if the history is used to construct a theory that theory can only be tested in a different time span than the one used to construct the hypothesis. The essential thesis of the paper that the latter period where acceptable values of λ are found does not require help from natural causes and that the early period where values of λ are anomalous must be explained in terms of natural causes is a case of circular reasoning in which the research methodology and the interpretation of the data are guided by the assumption of the truth of AGW theory. That the climate sensitivity anomaly in the early part of the industrial economy is explained by volcanism is supported by the data used to construct the hypothesis but that support has no interpretation in this case.
11. POSTSCRIPT: A further use of circular reasoning used in the study is that volcanoes can cause both warming and cooling.  Cooling is caused with aerosol backscatter in the stratosphere. This condition allows flexibility in the interpretation of volcanic activity to suit the needs of the researcher. Yet a third flaw in the methodology is that the research question is itself biased. The author does not set out to determine what effect if any volcanic activity has had during the study period but rather to use volcanic data to “explain” the extra warming in specific portions of the study period where anomalies are found. The biased research question together with circular reasoning in the interpretation of results implies that no rational or scientific conclusion can be drawn from this work.

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