Doubts about Climate Change

There are still people who claim that climate change is overrated and at most partially man-made. But what would you do if your daughter had cancer and one doctor says she will die from it if she doesn’t do surgery while another doctor says it is certainly a benign tumor and surgery is superfluous. Would you then have a surgery done? In conversation with the doctor who advised against the operation, you might even notice that the doctor does not even know the word scalpel and has never heard of anesthetics.

It is similar with climate change. Ask people who doubt human-made climate change about a few technical terms, such as the albedo effect or the Keeling curve (see below for explanations). It may then become very clear that they have no clue what they are talking about.

Is science sure about man-made climate change?

The short answer is: Yes. Scientists have been sure that climate change is man-made for many decades: an analysis of some 12,000 scientific articles from 2013 revealed that only 0.7% questioned man-made climate change (but see also below), compared to 6% in 2006 [1]. And scientists have also been sure for decades what the consequences will be. However, some scientists underestimated how fast greenhouse emissions increased and how much climate change has accelerated [2]. Scientist can reliable predict that the temperature will increased by 3°C or 5°C by 2100 (compared to pre-industrial times), if we stay on our current path (business as usual). Only where we will end up within this range is not certain. Measured CO2 concentrations and temperatures show that CO2 is steadily rising in the Earth’s atmosphere and we now have (2019) already exceeded 1°C warming compared to preindustrial times [3].

CO2 concentrationen in the atmosphere until 2019. Measurements from the weather station
at Mauna Loa, Hawai (so called Keeling curve; source: Delorme, Wikipedia, CC BY-SA).

A look into the past can also be taking using ice cores, taken at the poles, as in the polar ice small amounts of air from past times are conserved. As a result, CO2 concentrations could be measured thousands of years back in the past. This gives a fairly clear picture: only since the beginning of industrialization has the CO2 concentration increased sharply.

CO2 conzenzentration in der atmosphere until 2016, measured in ice cores (blue) and atmospheric measurements (red)
(Source: DeWikiMan, Wikimedia, CC BY-SA; data from NOAA and CSIRO).

Are the predictions of the climate models reliable? Again, the answer is yes. Already climate models from the 80s [4,5], predicted pretty well that in 2020 about 1°C global warming would be achieved (which has now indeed been reached). Even though assumptions of earlier models differed from what lateron actually happened (e.g. assumed emissions), the predictions of global warming was quite realistic. By the mid-90s, models were even more accurate predicting almost exactly those CO2 levels and temperatures for 2020 that have now been reached. An evaluation of a number of recent climate models from 2007 [7] concluded that climate models make realistic predictions, especially at the continental and global levels. I.e. the forecasts are generally reliable, only on the small-scale (regional) there are uncertainties. But how is it possible to evaluate if models produce correct predictions without looking into the future? This can be done, for example, by looking back into the past: Simulations with 14 different climate models starting in 1900 practically gave the same temperature curve that was actually measured lateron (Figure below).

Model predictions are generally in good agreement with historical measured temperatures: Temperature predicted from 14 climate models staring in 1900 (orange lines; average: red; in pre-industrial times temperatures were even lower than in 1900) and measured global temperature (black). From Randall et al. 2007 [7].

Furthermore, models were considered realistic as they accurately reflect the relevant physical processes.

Recent studies [8,9] indicate that in current climate models positive feedback mechanisms and tipping points have not been included sufficiently, leading to an underprediction of future global warming. This is supported by current observations, for example thowing of permafrost (and hence release of methane) in Canada has recently been found to have surpassed levels that were predicted by climate models only for the year 2090 [10]. Hence, we may have passed the first tipping points now. By this, processes could be started that result in the next tipping points to be reached. This way a cascade of events may be started by which global warming continues even if we live carbon free in the future.

How come some people doubt climate change?

Since decades, the overwhelming number of scientific studies leaves no room for doubt (see e.g. the comprehensice complete reference list of the IPPC Report 2018). But how is it possible that scientists agree that greenhouse emissions will jeopardize our livelihoods in one or few decades and at the same time many politicians and large sections of the population do not perceive this as a threat? How is it possible that data has been collected over decades but some still claime that it is uncertain whether climate change is man-made? This discrepancy is in fact typical of many areas of science. For example, it was clear lat latest since around 1940 that smoking causes cancer [11]. Lung cancer was previously an absolute rarity. But it took decades and cost millions of lives to be recognized by governments and people. Targeted disinformation campaigns by the tobacco industry also contributed to this (although the companies were well aware that smoking is fatal). For example, the tobacco company Philip Morris initiated the Whitecoat project, which paid physicians to write articles for journals to challenge the harmful effects of smoking [12]. The tobacco industry created seemingly independent organizations to publish pseudo-scientific articles in which the negative consequences of smoking were questioned and which aimed at suppressing health information [13,14]. It was only in 1999 that Philip Morris admitted that there is a scientific consensus that smoking causes lung cancer and other diseases.

The denial of climate change is similar to smoking. For example, in 1977 (11 years before climate change was publicly debated for the first time), oil giant Exxon Mobil already knew that burning fossil fuels had caused a climate change [15]. Exxon even invested US$ 1 million to measure CO2 concentrations on their oil tankers. Exxon employees warned management that doubling their CO2 concentration would result in a warming of 3°C (an estimate shared by scientists today). Exxon, however, decided to keep the warnings secret and spent millions (about US$ 30 million [16]) to disseminate doubts about fossil fuel-induced climate change. Furthermore, the Global Climate Coalition [17] and various think tanks (i.e. industry-funded, pseudo-scientific organisations promoting articles or studies for the benefit of companies, [18]). Many of these think tanks were even the same as those that earlier questioned the carcinogenic effects of smoking [15]. Also the methods were the same: Sowing doubts, distributing untrue information and deliberately placing climate skeptics into public media discussions in order to give the public the impression that climate change was a scientific controversy [17]. To illustrate this approach, an example of an article is given which was recently published by the think tank CATO Institute (funded by the tobacco, pharmaceutical, energy industries and others [20]): In January 2019, CATO Institute published an article entitled “Are climate models overpredicting global warming?” [19]. This article gives the impression that a new scientific study [21] would show that model predictions overestimate climate change. However, when reading the original study, it already becomes clear from the title “Taking climate models to the next level” that the authors never wrote that climate change or global warming was overestimated. The authors instead have been looking for ways to further improve existing models and therefore they focussed on ‘weaknesses’ and identified possibilities on how to improve these. This has been reinterpreted by the author of the CATO Institute, suggesting that the authors indicated climate change was overestimated. However, the wording of the article by CATO Institute is cleverly worded. The titel “Are climate models overpredicting global warming?” suggest that there is an overprediction, but strictly it is only a question, hence legally they didn’t spread wrong information. Another example is the Prague University (PragueU), which is in fact not a university but a think tank financed by the fracking industry [22], which produces YouTube videos that question man-made climate change [22]. There are many more such think tanks. In order to understand which sources are providing neutral information, one can hence not focus on the name of the institution. It may help to stick to well-known scientific or governmental institutions and to always look at the funding of studies and organisations. Usually, when digging into the funding of articles that doubt man-made climate change, one will sooner or later always find ties to industry, in particluar energy or fracking industry. Further information on the topic can be found at Wikipedia [23].

Another aspect that makes dealing with climate change more difficult is that people perceive climate change as a remote threat affecting others [24,25,26,27]. As a result, people feel little motivation to become active. Evolutionarily, we may not be prepared to look further into the future than to the next winter. Most dangers in evolution (predators, attackers) had immediate consequences. Concerning. For most people climate change seems a remote problem, since they are not aware that climate change will (and partly already does now) have direct, personal consequences (food shortages, deterioration in health, migration, wars [3]). Again, there are parallels to smoking. Even a smoker knows that he can die from smoking. But he hopes to be spared in the future and clings to the doubts spread by lobbyists. But how many really old people do you see smoking? Not as many as young people? Do you think all the old people stopped smoking, or may those left have been mostly non-smokers?

Finally, people who doubt human-made climate change are much more frequently represented in the media than scientists: in a study from 2019 [28], the media presence of an equal number of climate skeptics and climate researchers was examined. Based on 200,000 scientific articles and 100,000 publications in digital and print media, it has been found that climate skeptics are featured about 50% more frequently in media than actual climatologists. This means that in the media much more frequently that climate change is not man-made than that it is man-made. This is a huge discrepancy with scientific opinion, according to which only 0.7% of all scientists expressed doubts about man-made climate change [1], i. Almost 100% of all scientists agree with man-made climate change. One reason for this discrepancy is that scientists mostly publish in peer reviewed scientific journals whereas climate skeptics publish mainly in digital and print media (only about 1% of articles in scientific journals was attributed to climate sketptics [28]).


Albedo effect

The albedo effect (from Lat. Albus “white”) describes the reflection of solar radiation by the earth. This is due to atmospheric reflection (e.g. clouds) and reflection of the polar and glacial ice masses. Without this reflection, the earth would heat up. Therefore, the melting off of the polar and glacier masses leads to a feedback effect. Once global warming has reached a level at which the ice masses have largely melted off (tipping point), an automatism sets in, through which the earth heats up even further (even if greenhouse emissions are completely stopped). See Wikipedia article about Albdeo and Albedo feedback.

Keeling curve

The Keeling curve is a graphical representation of the concentration of the greenhouse gas carbon dioxide (CO2) in the Earth’s atmosphere since 1958. Charles Keeling was one of the first scientists to use CO2 measurements with the help of airplanes, weather balloons and ships to notice that the concentration this greenhouse gas is rising steadily. More details in Wikipedia and articles referenced int this link. Figure by Delorme, Wikipedia, CC BY-SA.



  1. Cook et al. 2013. Quantifying the consensus on anthropogenic global warming in the scientific literature. Environmental Research Letters 8: 024024. Online
  2. Hansen et al. 1981. Climate impact of increasing atmospheric carbon dioxide. Science 213: 957-966. Online
  3. Weltklimarat (IPCC) 2018. Global warming of 1.5 °C. Special report. Online (eine komplette Liste aller vom Weltklimarat genutzter Quellen gibt es auch hier)
  4. Hansen et al. 1988. Global climate changes as forecast by Goddard Institute for Space Studies three-dimensional model. Journal of Geophysical Research 93: 9341-9364. Online
  5. Houghton et al. 1990. Climate change. The IPCC scientific assessment. Cambridge University Press. Online
  6. Houghton et al. 1990. Climate change 1995. The science of climate change. Cambridge University Press. Online
  7. Randall D.A. et al. 2007. Cilmate models and their evaluation. In: Climate Change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Online. Einen guten historischen Überblick gibt es auch hier.
  8. Steffen W. et al. 2018. Trajectories of the earth system in the Anthropocene. PNAS 115: 8252-8259. Online
  9. Anthony K.W. et al. 2018. 21st-century modeled permafrost carbon emissions accelerated by abrupt thaw beneath lakes. Nature Communications 9: 3262. Online
  10. Farquharson L.M. et al. 2019. Climate change drives widespread and rapid thermokarst development in very cold permafrost in the Canadian high Arctic. Geophysical Research Letters 46: 1-9. Online
  11. Cummings K.M. et al. 2002. Failed promises of the cigarette industry and its effect on consumer misperceptions about the health risks of smoking. Tobacco Control 11 (Suppl I): i110–i117. Online
  12. Orginal proposal for the organisation of the Whitecoat Project at UCSF: Online
  13. Brownell K.D., Warner, K.E. 2009. The perils of ignoring history: Big tobacco played dirty and millions died. How similar is big food? Milbank Q 87: 259-294. Online
  14. A good summary are availabe as a Video with references.
  15. Hall S. 2015. Exxon knew about climate change almost 40 years ago. Scientific American Oct. 26 2015. Online
  16. List of think tanks and other organizations financed by Exxon at
  17. Article about the Global Climate Coalition auf Wikipedia.
  18. Article about Thinktanks auf
  19. Article “Are climate models overpredicting global warming? ” by the think tank CATO Institute. Online
  20. Article about CATO Institute on Wikipedia.
  21. Eyring et al. 2019. Taking climate models to the next level. Nature Climate Change 9: 102-110. Online
  22. Article about Prager University on Wikipedia.
  23. Artikel über “Climate change denial” on Wikipedia.
  24. Bord R. J. et al. 1998. Public perceptions of global warming: United States and international perspectives. Climate Research 11: 75–84. Online
  25. Lorenzoni I. et al. 2007. Barriers perceived to engaging with climate change among the UK public and their policy implications. Global Environmental Change 17: 445–459. Online
  26. Lorenzoni I. et al. 2006. Public views on climate change: European and USA perspectives. Climate Change 77: 73–95. Online
  27. Brügger A. et al. 2015. Psychological responses to the proximity of climate change. Nature Climate Change 5: 1031–1037. Online
  28. Petersen et al. 2019. Discrepancy in scientific authority and media visibility of climate change scientists and contrarians. Nature Communications 10: 3502. Online

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