How much human-caused global warming should we expect?

By Andy May

OMG! The world is going to end, and we caused it. This story, in one form or another, goes back to biblical times. According to Genesis (6:9 to 9:17) God decided that humans had sinned too much and must be punished, so he called up a great flood to destroy the world. A similar story also appears in the earlier Epic of Gilgamesh. End of the world predictions are very popular and recur regularly in human history.

More recently, prognosticators have predicted climate change disasters that are due to human actions (sins?). During the Little Ice Age (see Figure 3 in the link), the European public blamed the cold weather on witches and Jews, over 50,000 “witches” and tens of thousands of Jews were killed because they supposedly caused the cold weather and glacial advances. Thus, the idea that humans somehow control climate change is very old. We have no more proof that this is the case today than people had in 800AD, which is about when Archbishop Agobard of Lyons, France said:

“In these parts nearly everyone – nobles and common folk, town and country, young and old – believe that human beings can bring about hail and thunder … We have seen and heard how most people are gripped by such nonsense, indeed possessed by such stupidity…” (Behringer 2010, p. 69)

Not much has changed since. It does seem odd that while old single women and Jews were killed, often in horrible ways, for supposedly causing the cold of the Little Ice Age, we are now facing demands from politicians and celebrities to “prevent” warming of 1.5 to 2.0°C above the same “preindustrial” temperatures. To emphasize the point that humans have always blamed other humans for bad weather, we show a 1486AD woodcut of a sorceress conjuring a hailstorm with the jawbone of an ass.

Figure 1. A woman conjuring up a hailstorm with a jawbone of an ass. Behringer’s caption reads: “Anthropogenic Climate Change.” Source: (Behringer 2010, p. 129).

In the 1960s human overpopulation would supposedly cause hundreds of millions to starve to death in the 1970s, according to Paul Ehrlich. In 2013, Sir David Attenborough said humans were a “plague on the Earth.” However, none of these predictions came true. Now, we are faced with the IPCC predictions that we are facing 1.5 to 2°C of warming in this century and that this will cause irreversible damage to the planet and humankind unless we act now. According to the IPCC, this warming is caused primarily by burning fossil fuels and, therefore, we must curtail or quit all fossil fuel use.

The United Nations Framework on Climate Change (UNFCCC) hosted the Paris Climate Convention in 2015 that agreed to keep the global average temperature increase to less than 2°C above preindustrial levels and to pursue efforts to limit the increase to 1.5°C. This agreement is often called the “Paris Agreement.” President Trump pulled the United States out of the agreement in November 2019 and the withdrawal will take effect in November 2020. As explained by Roger Pielke Jr. in 2005 the UNFCCC defines “climate change” as something caused by humans and totally ignores the possibility of natural climate change. This “misdefining” of climate change by the UNFCCC has hurt their credibility and undermines their arguments (Pielke 2005).

Connolly, Connolly, Carter and Soon have written a new paper (Connolly, et al. 2020) examining the IPCC predictions the UNFCCC used to construct the Paris Agreement. We will refer to their paper as “C3S20.” While the paper has just been published, it has been a work-in-progress for some time. Dr. Robert M. Carter passed away on January 19, 2016, but he did a considerable amount of work on an earlier version of the paper.

C3S20 asks, how much human-caused warming will occur if we do nothing, that is, continue “business-as-usual?” It’s unfortunate, but the IPCC, for all their work, do not adequately answer that question, their projections are all based on abstract “scenarios.” C3S20 break this overall question into five parts:

  1. What would greenhouse gas emissions (GHG) be, if we did nothing and continued normally?
  2. For each GHG, how do emissions relate to the change in atmospheric concentration?
  3. What would the global temperature be if GHG concentrations were at “preindustrial” levels?
  4. How sensitive are global temperatures to GHG concentrations?
  5. How much warming should we expect if we do nothing?

C3S20 tell us the Paris Agreement conclusion that we need to limit global warming to 2°C above preindustrial levels suffers from several unknowns.

  1. The preindustrial period is not formally defined. The preindustrial temperature and greenhouse gas level are not specified. In fact, several time periods, temperatures and GHG levels are used as “preindustrial” in the latest IPCC AR5 report.
  2. The assumptions that warming is bad and increasing levels of CO2 are bad, are not supported with any data. Numerous studies have concluded that some warming is good for humankind and additional CO2 is good for plants.
  3. The penultimate draft of AR5 identified the period 1850 to 1900 as the preindustrial baseline for CO2 and temperature. This was the end of the Little Ice Age, the coldest period in the last several thousand years. Why use that period as a baseline (Luning and Vahrenholt 2017)? This is not explained, and the final draft of the report removed the reference to the 1850 to 1900 baseline.
  4. If the UNFCC and the Paris Agreement assume “climate change” and “Human-caused Climate Change” are synonymous, how do they explain that climate has changed much quicker and much more dramatically many times in the past 13,000 years before human civilization began and well before industrialization?

The first five questions are dealt with in C3S20. To answer question five, they adopt the UNFCCC assumption that all climate change is due to human activities. Or, perhaps a better way of saying this, is that they only considered the human-caused portion of global warming, using IPCC assumptions. Then they use various estimates of GHG climate sensitivity (ECS) to compute projected temperatures to 2100AD, as shown in Figure 2. ECS is defined as the temperature rise expected from a doubling of the atmospheric CO2 concentration, the other major greenhouse gases are assumed to increase in concert. Their estimates of the surface temperature in 2100 are similar to the IPCC estimates, shown in Figure 3, because they used similar assumptions. That is, they only considered human influences on climate and the human contribution was computed with the same or similar parameters. The projections differ in detail because the IPCC projections vary the human greenhouse emissions and C3S20 only vary the climate sensitivity to greenhouse gases. See the paper for the details of the C3S20 calculations and the IPCC AR5 report for the details of the IPCC calculations (IPCC 2014, Chap. 12). The AR4 (see Figure 10.4 in AR4) estimates are similar. Basically, the “magical” CMIP5 computer models don’t add much, the uncertainty in their projections is fully explained by the uncertainly in ECS. Further, ECS only applies to the human-caused component of global surface warming. It does not account for natural variability.

C3S20 do note that empirical estimates of ECS, in particular those by Lewis and Curry (Lewis and Curry 2018) and Lindzen and Choi (Lindzen and Choi 2011), are less than 2.5 and as low as 0.4. Soon, Connolly and Connolly computed an ECS of 0.44 using a correlation to solar variability and Shaviv (Shaviv 2012) calculated an implied ECS of 0.69 to 1.26°C. The estimates higher than 2.0°C/2xCO2 are nearly all model results from hypothetical IPCC “scenarios” and not based on measurements. The empirical results are greatly preferred. A reasonable data-based maximum ECS is around two degrees, and as you can see in Figure 2, this shows a maximum human-caused warming of less than two degrees by the year 2100AD while carrying on “business as usual.” Shaviv’s 2012 estimate for warming of between 1 and 1.5°C by 2100AD for a business-as-usual scenario (Shaviv 2012).

Figure 2. Human-caused warming, using IPCC assumptions and making no changes in greenhouse gas emissions. Source: (Connolly, et al. 2020).

Figure 3. The IPCC AR5 global surface warming predictions for 2100 are very similar to those in C3S20, since they are based on nearly the same parameters and assumptions. Source: (IPCC 2014, p. 1037).

The preindustrial period

Since the Paris Agreement does not define a preindustrial period, one can reasonably ask “1.5° to 2.0°C above what?” They often refer to the IPCC AR5 report, but unfortunately, there is no definition of “preindustrial” in that report either (Hawkins, et al. 2017). The whole purpose of the Paris Agreement is thus cast into doubt. Early AR5 drafts tried to define preindustrial as 1850 to 1900, but this definition was removed from the final draft of the report (Hawkins, et al. 2017). And, in fact, within the AR5 report, many different definitions of “preindustrial” are used. AR5 uses the difference between the average global temperature from 1850 to 1900 and the average from 1986 to 2005 (0.61°C) as their “observed” 20th century warming value. These periods are noted in Figure 4.

In C3S20, they do not even attempt to select a period to label as preindustrial, nor do they define a preindustrial temperature. They simply use the greenhouse gas levels from Antarctic ice cores to establish a baseline greenhouse gas concentration, then use that as a starting point for their greenhouse gas concentration warming trend, the trends are shown in Figure 2. They have no starting point, they simply show warming that can be attributed to greenhouse gases, using a set of simple assumptions, described in the paper. The vertical axis in Figure 2 is not a real temperature anomaly, it is more of an index of warming that might be attributable to human greenhouse emissions. The vertical axis bears no relationship to the real world; it incorporates no estimate of natural warming or climate oscillations, like the AMO or ENSO. If the assumptions are correct and it is understood to be only the greenhouse gas caused warming, only the “human-caused warming,” if you prefer, then maybe the estimates in Figure 2 tell us something useful. Figure 2 certainly highlights the oversimplification of climate change that is endemic in the IPCC and UNFCCC. But actual warming or cooling that we experience over the next century may be higher or lower, depending upon natural forces and the true value of ECS. The largest unknown is the impact of solar variability. For a more detailed look at solar variability and climate change see here.

The real problem is that the UNFCCC and the IPCC have conflated human-caused global warming with actual warming and so has much of the world. Human-caused warming may be occurring, it is probably one component of the warming that we are observing. But it is very poorly understood. As Figures 2 and 3 show, the estimates of the potential human component in 2020 spread over one degree Celsius, this is more warming than the world has seen (using the HadCRUT 4 record) since 1850. Thus, the error in one component (the human part) of our current warming is more than we have measured since 1850!

Figure 4. The HadCRUT 4 surface temperature record. Periods of warming and cooling are highlighted with color and trends. The early IPCC draft preindustrial period is identified as “A.” The average used to compute the modern warming baseline is identified as “C.” The difference, 0.61°C is the IPCC AR5 “modern warming.” “B” is the preindustrial baseline recommended by Luning and Vahrenholt (Luning and Vahrenholt 2017).

The reason the period 1850-1900 was dropped from the report is not known, but perhaps it was because many other “preindustrial” periods were used in the report, for example 1750AD was the preindustrial baseline for radiative forcing (Hawkins, et al. 2017). Hawkins et al. suggest that the IPCC adopt a consistent and precise preindustrial period and use the same period throughout the next report. The period they suggest is 1720-1800, however a robust global surface temperature estimate is not available for that period.

Luning and Vahrenholt (Luning and Vahrenholt 2017) suggest a reference period of 1940 to 1970 because this temperature (see Figure 4, average “B”) is closer to the average preindustrial temperature of the past two thousand years. This is in sharp contrast to the period 1850-1900, which is much colder than average for the preindustrial era, this is particularly true for the northern hemisphere.

We have seen that the impact of greenhouse gas concentrations on surface temperature (ECS) is poorly understood and decades of very expensive research has not narrowed the uncertainty The Charney Report (Charney, et al. 1979) estimated that ECS was between 1.5 and 4.5°C in 1979, which is exactly the range determined by the IPCC in AR5 in 2013. We are no closer in estimating the climatic impact of greenhouse gases today than we were in 1979, despite spending billions of dollars on research.

Finally, it is not clear why warming of two degrees, if it even occurs, will be a problem for humankind. Additional CO2 causes plants to grow larger and faster, it also improves crop yields. Warming disproportionately occurs at the poles, there is far less warming at the equator. Thus, warming has the effect of opening land for farming in the far north, especially in Northern Europe, Siberia and Canada, but it does not affect tropical countries much. Some claim that warming is accelerating and will “take off” and burn us all up, but the data shows warming has slowed in the last twenty years or so. This can be seen in Figure 4, especially if we ignore the strong El Nino in 2016. Accelerating warming is sometimes called “going runaway.” This is very unlikely to happen, especially since the Earth has oceans, as explained here. If you have heard the Earth may become like Venus, see here for why it will not happen. For more on ocean oscillations and global warming, see here.


As we see by reading C3S20, the 2.0°C limit idea in the UNFCCC Paris Agreement is very flawed. The agreement does not define the preindustrial starting point, either as a temperature or a greenhouse concentration. It is not even defined in time. The effect of human-emitted greenhouse gases is not known accurately enough. This is clearly seen in Figure 2. The error in warming estimates for 2020, is larger than the total surface warming since 1850. Finally, there have been no observed problems with warming or greenhouse gas emissions. The net impacts of warming and higher greenhouse gas concentrations have been positive to date according to the economists that have studied the issue. Further, the impacts are likely to remain positive for some time to come.

The 2.0°C limit is arbitrary and subjective (Mahony 2015) and has no starting point. There is no way to accurately project the effects of the emissions the UNFCCC seeks to control, and two degrees of warming is just as likely to be beneficial as harmful. Further, we have no idea how natural forces will affect future climate, will they contribute to warming or work in the opposite way? There are studies that point both ways. See here and here.

Given how little we know about future climate, it would seem wise to give up on the idea of revolutionary changes like eliminating or curtailing fossil fuel use. This sort of change will undoubtably cost more than it saves. The impacts of climate change, whether natural or human-made, are different for every location. A coastal city may be impacted by either local or general sea level change. Changes in precipitation may cause flooding in some areas and droughts in another. These problems are better fought on a local level, as they always have been in the past. People must choose to adapt or move; it should be their choice. A global change, so-called “mitigation,” will be extraordinarily expensive and will likely have serious and unpredictable side effects. See the discussion here for more details.

Adaptation is the way to go, because we do not know what will happen. Keep our energy sources, we may need them. Free our builders from the severe and often foolish environmental constraints that cause construction projects to be delayed for years. The Galveston Seawall, that protected the island for 97 years was completed in eleven years after the devastating 1900 Great Galveston Hurricane in 1900. It was not topped by a hurricane until Hurricane Ike in 2008. After Ike, an “Ike Dike” was proposed to improve the island’s protection, but it has been mired in politics and environmental surveys ever since. We need to get rid of most of the national regulations that strangle local solutions, the local communities know what must be done to protect their community, national and global “policies” simply get in the way.

Works Cited

Behringer, Wolfgang. 2010. A Cultural History of Climate. Cambridge: Polity Press.

Charney, J., A. Arakawa, D. Baker, B. Bolin, R. Dickinson, R. Goody, C. Leith, H. Stommel, and C. Wunsch. 1979. Carbon Dioxide and Climate: A Scientific Assessment. National Research Council, Washington DC: National Academies Press. doi:

Connolly, Ronan, Michael Connolly, Robert M. Carter, and Willis Soon. 2020. “How Much Human-Caused Global Warming Should We Expect with Business-As-Usual (BAU) Climate Policies? A Semi-Empirical Assessment.” Energies 13 (6).

Hawkins, Ed, Pablo Ortega, Emma Suckling, Andrew Schurer, Gabi Hegerl, Phil Jones, Manoj Joshi, Timothy J. Osborn, and Valerie Masson-Delmotte. 2017. “Estimating changes in global temperature since the pre-industrial period.” Bulletin of the American Meteorological Society 1841-1856.

IPCC. 2014. Fifth Assessment Report (AR5). 4 vols. Cambridge: Cambridge University Press.

Lacis, Andrew, Gavin Schmidt, David Rind, and Reto Ruedy. 2010. “Atmospheric CO2: Principle Control Knob Governing Earth’s Temperature.” Science 330: 356-359.

Lewis, Nic, and Judith Curry. 2018. “The impact of recent forcing and ocean heat uptake data on estimates of climate sensitivity.” Journal of Climate.

Lindzen, Richard, and Yong-Sang Choi. 2011. “On the Observational Determination of Climate Sensitivity and Implications.” Asia-Pacific Journal of Atmospheric Sciences 47 (377).

Luning, Sebastian, and Fritz Vahrenholt. 2017. “Paleoclimatological Context and Reference Level of the 2 degree and 1.5 degree Paris Agreemnet Long-Term Temperature Limits.” Frontiers in Earth Science.

Mahony, Martin. 2015. “Climate change and the geographies of objectivity: the case of the IPCC’s burning embers diagram.” Transactions of the Institute of British Geographers.

Pielke, Roger. 5005. “Misdefining ”climate change”: consequences for science and action.” Environmental Science and Policy 8: 548-561.

Shaviv, N. J. 2012. “The Role of the Solar Forcing in the 20th century climate change.” Edited by A. Zichichi. International Seminar on Nuclear War and Planetary Emergencies. 279-286.

Ziskin, Shlomi, and Nir Shaviv. 2012. “Quantifying the role of solar radiative forcing over the 20th century.” Advances in Space Research 50: 762-776.

Published by Andy May

Petrophysicist, details available here:

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