Theo Wolters at the EIKE Conference

Clintel Foundation
Date: 9 October 2026

Wolters’ presentation therefore concentrated on what he regarded as the evidence for a dominant CO2 effect. He has identified four principal lines of argument: attribution based on climate forcings, the correlation between CO2 and temperature, climate models, and radiation physics. He examined each in turn.

Challenging the attribution argument

The first argument is essentially a “proof by contradiction”. Climate scientists calculate the effects of various warming and cooling forcings and then argue that the observed warming cannot be reproduced unless the warming effect of CO2 is included.

Wolters questioned whether this constitutes proof. For such an argument to work, he said, scientists would have to know the magnitude of all the other relevant forcings with great precision. He particularly questioned estimates of the cooling effect of air pollution, arguing that if aerosols have a smaller cooling effect than assumed, then the calculated warming contribution from CO2 would also have to be smaller.

He also pointed to climate variations before the Industrial Revolution. If the conventional forcing calculation were complete, he argued, temperatures before about 1750 should have been relatively flat because the major anthropogenic warming forcings had not yet appeared. Yet ice-core and proxy evidence shows substantial earlier fluctuations.

Wolters highlighted what he described as warming episodes occurring roughly a thousand years apart. He acknowledged that he could not prove that the current warming is part of such a natural cycle, but argued that it was a plausible possibility that cannot simply be ignored. “It’s plausible. I can’t prove it, but it’s plausible.”

His conclusion was that the attribution argument does not demonstrate that CO2 caused most of the recent warming.

CO2 and temperature: correlation is not causation

Wolters next examined the relationship between atmospheric CO2 and temperature. He stressed that correlation alone cannot establish causation. Looking at the twentieth century, he argued, does not produce the relationship one might expect if CO2 were the dominant driver.

The first four decades of the twentieth century experienced substantial warming even though CO2 concentrations did not increase in the same way as during the second half of the century. Conversely, there was a significant cooling period after the Second World War while CO2 continued to rise.

He then extended his argument to longer time scales. During the ice ages, he noted, temperature changes preceded changes in atmospheric CO2 by thousands of years. He also pointed to geological periods in which atmospheric CO2 varied enormously while temperatures remained comparatively stable. Wolters therefore concluded that, in his view, a consistent CO2-temperature correlation cannot be demonstrated across the various relevant time scales.

Climate models are not proof

The third line of evidence Wolters examined was the output of climate models. His criticism focused particularly on the modelling of the water cycle. Evaporation, convection, cloud formation and precipitation are enormously important components of the climate system, he argued, but are represented in global models through approximations and parameterizations. Consequently, he said, models cannot simply be treated as direct representations of atmospheric physics.

More fundamentally, Wolters argued that climate models already assume that CO2 produces warming. Consequently, their output cannot independently prove the proposition built into them.

He used the so-called tropical “hotspot” as an example. Climate models predict enhanced warming high in the tropical atmosphere as surface temperatures rise. Wolters argued that long-term balloon observations have not shown the predicted hotspot and that this discrepancy remains inadequately explained.

For him, the persistent difference between model projections and observations demonstrates that models should not be treated as definitive evidence of future warming.

What does radiation physics actually prove?

At this point, Wolters moved to what he considered the strongest part of the conventional scientific case: radiation physics.

He explicitly accepted that CO2 is a greenhouse gas. Increasing CO2 changes the Earth’s radiation balance, and a doubling of atmospheric CO2 produces roughly one degree Celsius of warming before additional feedbacks are taken into account. “CO2 causes more or less one degree warming, that’s all.”

Wolters stresses that the roughly one degree of warming associated with a doubling of CO2 is essentially a laboratory value. It describes what would happen if the atmosphere were allowed to respond to the radiative effect of CO2 while nothing else changed. The real climate system, however, is much more complicated. It contains numerous feedback mechanisms, some amplifying an initial warming and others counteracting it. According to Wolters, this is where much of the scientific disagreement begins. The crucial question, therefore, becomes the magnitude of climate feedbacks—especially those involving water vapour, clouds and the water cycle.

He explains the difference between positive and negative feedbacks with a simple example. A negative feedback dampens an initial temperature change: the climate responds to warming in a way that limits further warming. A positive feedback does the opposite, amplifying the original disturbance. Wolters illustrates the latter with a hypothetical chain in which one degree of warming produces another two degrees through feedback; those two degrees then produce four, and four become eight. Such a process would eventually become a runaway effect unless the feedback weakened substantially.

This is particularly important because the IPCC’s estimate of climate sensitivity depends heavily on positive feedbacks. The direct CO2 effect of about 1°C is amplified, especially through increased atmospheric water vapour, which is itself a greenhouse gas. Wolters says the resulting feedback factor is already close to the threshold at which the system would become unstable.

He questions whether this can be correct. Sea-ice loss, for example, can provide a strong positive feedback locally, but a warmer planet also emits more infrared radiation into space, creating a negative feedback. Moreover, a warmer climate intensifies the water cycle. Evaporation and convection remove heat from the surface, while precipitation is also strongly cooling. Clouds can act in either direction.

For Wolters, this raises a fundamental problem. If the IPCC’s estimate represents the sum of all feedbacks, the climate should be much closer to instability than the Earth’s long-term history suggests. He points to the remarkable stability of global temperatures over geological time despite major changes in atmospheric composition and solar output. This suggests that a powerful negative feedback must be operating.

The implication is central to his subsequent “tropical thermostat” hypothesis: rather than assuming that positive feedbacks dominate, scientists should investigate whether the climate contains natural mechanisms that actively limit warming.

The ‘tropical thermostat’ hypothesis

This leads to the most unconventional part of Wolters’ presentation: his discussion of a possible natural ‘thermostat’ regulating Earth’s temperature. Wolters emphasized that this part of his presentation is a hypothesis rather than established science. He  attributed the basic idea to Willis Eschenbach, whose earlier work on tropical thunderstorms he encouraged the audience to examine.

According to the hypothesis, the tropical water cycle—particularly the enormous thunderstorms associated with the Hadley circulation—could act as a natural thermostat. Evaporation transfers heat from the surface into the atmosphere, while condensation and the transport of latent heat to high altitudes allow energy to escape into space. Wolters argued that the relatively stable temperature of large areas of the tropical Pacific despite substantial warming elsewhere provides suggestive evidence for such a mechanism.

If such a thermostat exists and is absent from climate models, calculations of climate forcings and feedbacks could be fundamentally misleading.

From science to climate policy

Wolters’ overall conclusion is not that CO2 has no warming effect. Indeed, he explicitly rejected that interpretation of his argument. “I did not prove that CO2 does not do the warming. I just showed that there’s no proof that it did.”

In his view, CO2 may have contributed somewhere between zero and roughly half a degree to the recent warming, but the exact contribution remains unknown. He considers it entirely plausible that natural factors—including recovery from the Little Ice Age—account for a substantial part of the observed increase.

This uncertainty has major implications for climate policy. If policymakers cannot establish how much warming is caused by CO2, then spending enormous sums to eliminate one particular forcing becomes questionable.

Wolters also challenged an unrestricted application of the precautionary principle. The possibility of climate damage is not enough by itself to justify any policy regardless of its costs. Policymakers must compare the potential benefits of mitigation with its economic costs and with alternative strategies.

He therefore strongly favoured adaptation. Societies can respond to climate risks by improving infrastructure, water management and flood protection. Wolters pointed to the large decline in climate-related deaths over the past century as evidence that societies have become increasingly capable of dealing with environmental hazards.

He cited an estimate by economist William Nordhaus suggesting that attempting to hold warming to very low levels could impose enormous economic costs. Wolters’ policy conclusion was consequently that governments should pursue limited, economically sensible climate measures while concentrating primarily on adaptation. “The sensible choice is adaptation.”

A warmer world may not be a catastrophe

Wolters ended his presentation by turning to what he called the “feel-good part”: what would actually happen to nature if temperatures would rise two degrees?

His answer was that the consequences should not automatically be regarded as catastrophic. He illustrated this by comparing climate zones across Europe and North America. A two-degree shift, he argued, is comparable to moving several hundred kilometres north or south. A warmer Berlin, for example, could acquire climatic characteristics currently found considerably farther south.

From this perspective, climate change is less a transformation of the entire planet than a movement of climate zones. Wolters argued that ecosystems have repeatedly experienced such changes throughout geological history. His final conclusion was therefore deliberately optimistic: “Nature is used to adapt to climate change and so should we.”

For Wolters the real scientific and political questions are how much of recent warming is caused by CO2, how strongly the climate system responds to it, how serious the consequences of further warming would actually be, and whether aggressive emissions reduction produces benefits commensurate with its economic costs. His answer to all four questions is that there remains substantial uncertainty. “Nobody knows how the climate works”.

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