The Thermostat Hypothesis

Guest Essay by Willis Eschenbach



The Thermostat Hypothesis is that tropical clouds and thunderstorms actively regulate the temperature of the earth. This keeps the earth at a equilibrium temperature.

Several kinds of evidence are presented to establish and elucidate the Thermostat Hypothesis – historical temperature stability of the Earth, theoretical considerations, satellite photos, and a description of the equilibrium mechanism.

Historical Stability

The stability of the earth’s temperature over time has been a long-standing climatological puzzle. The globe has maintained a temperature of ± ~ 3% (including ice ages) for at least the last half a billion years during which we can estimate the temperature. During the Holocene, temperatures have not varied by ±1%. And during the ice ages, the temperature was generally similarly stable as well.

In contrast to Earth’s temperature stability, solar physics has long indicated (Gough, 1981; Bahcall et al., 2001) that 4 billion years ago the total solar irradiance was about three quarters of the current value. In early geological times, however, the earth was not correspondingly cooler. Temperature proxies such as deuterium/hydrogen ratios and 16O/18O ratios show no sign of a corresponding warming of the earth over this time. Why didn’t the earth warm as the sun warmed?

This is called the “Faint Early Sun Paradox” (Sagan and Mullen, 1972), and is usually explained by positing an early atmosphere much richer in greenhouse gases than the current atmosphere.

However, this would imply a gradual decrease in GHG forcing which exactly matched the incremental billion-year increase in solar forcing to the present value. This seems highly unlikely.

A much more likely candidate is some natural mechanism which has regulated the earth’s temperature over geological time.

Theoretical Considerations

Bejan (Bejan 2005) has shown that the climate can be robustly modeled as a heat engine, with the ocean and the atmosphere being the working fluids. The tropics are the hot end of the heat engine. Some of that tropical heat is radiated back into space. Work is performed by the working fluids in the course of transporting the rest of that tropical heat to the Poles. There, at the cold end of the heat engine, the heat is radiated into space. Bejan showed that the existence and areal coverage of the Hadley cells is a derivable result of the Constructal Law. He also showed how the temperatures of the flow system are determined.

“We pursue this from the constructal point of view, which is that the [global] circulation itself represents a flow geometry that is the result of the maximization of global performance subject to global constraints.”

“The most power that the composite system could produce is associated with the reversible operation of the power plant. The power output in this limit is proportional to


where q is the total energy flow through the system (tropics to poles), and TH and TL are the high and low temperatures (tropical and polar temperatures in Kelvins). The system works ceaselessly to maximize that power output. Here is a view of the entire system that transports heat from the tropics to the poles.


Figure 1. The Earth as a Heat Engine. The equatorial Hadley Cells provide the power for the system. Over the tropics, the sun (orange arrows) is strongest because it hits the earth most squarely. The length of the orange arrows shows relative sun strength. Warm dry air descends at about 30N and 30S, forming the great desert belts that circle the globe. Heat is transported by a combination of the ocean and the atmosphere to the poles. At the poles, the heat is radiated to space.

In other words, flow systems such as the Earth’s climate do not assume a stable temperature willy-nilly. They reshape their own flow in such a way as to maximize the energy produced and consumed. It is this dynamic process, and not a simple linear transformation of the details of the atmospheric gas composition, which sets the overall working temperature range of the planet.

Note that the Constructal Law says that any flow system will “quasi-stabilize” in orbit around (but never achieve) some ideal state. In the case of the climate, this is the state of maximum total power production and consumption. And this in turn implies that any watery planet will have an equilibrium temperature, which is actively maintained by the flow system. See the paper by Ou listed below for further information on the process.

Climate Governing Mechanism

Every heat engine has a throttle. The throttle is the part of the engine that controls how much energy enters the heat engine. A motorcycle has a hand throttle. In an automobile, the throttle is called the gas pedal. It controls incoming energy.

The stability of the earth’s temperature over time (including alternating bi-stable glacial/interglacial periods), as well as theoretical considerations, indicates that this heat engine we call climate must have some kind of governor controlling the throttle.

While all heat engines have a throttle, not all of them have a governor. In a car, a governor is called “Cruise Control”. Cruise control is a governor that controls the throttle (gas pedal). A governor adjusts the energy going to the car engine to maintain a constant speed regardless of changes in internal and external forcing (e.g. hills, winds, engine efficiency and losses).

We can narrow the candidates for this climate governing mechanism by noting first that a governor controls the throttle (which in turn controls the energy supplied to a heat engine). Second, we note that a successful governor must be able to drive the system beyond the desired result (overshoot).

(Note that a governor, which contains a hysteresis loop, is different from a negative feedback. A negative feedback can only reduce an increase. It cannot maintain a steady state despite differing forcings, variable loads, and changing losses. Only a governor can do that.)

The majority of the earth’s absorption of heat from the sun takes place in the tropics. The tropics, like the rest of the world, are mostly ocean; and what land is there is wet. The steamy tropics, in a word. There is little ice there, so the clouds control how much energy enters the climate heat engine.

I propose that two inter-related but separate mechanisms act directly to regulate the earth’s temperature — tropical cumulus and cumulonimbus clouds. Cumulus clouds are the fluffy “cotton ball” clouds that abound near the surface on warm afternoons. Cumulonimbus clouds are thunderstorms clouds, which start life as simple cumulus clouds. Both types of clouds are part of the throttle control, reducing incoming energy. In addition, the cumulonimbus clouds are active heat engines which provide the necessary overshoot to act as a governor on the system.

A pleasant thought experiment shows how this cloud governor works. It’s called “A Day In the Tropics”.

I live in the deep, moist tropics, at 9°S, with a view of the South Pacific Ocean from my windows. Here’s what a typical day looks like. In fact, it’s a typical summer day everywhere in the Tropics. The weather report goes like this:

Clear and calm at dawn. Light morning winds, clouding up towards noon. In the afternoon, increasing clouds and wind with a chance of showers and thundershowers as the storms develop. Clearing around or after sunset, with an occasional thunderstorm after dark. Progressive clearing until dawn.

That’s the most common daily cycle of tropical weather, common enough to be a cliché around the world.

It is driven by the day/night variations in the strength of the sun’s energy. Before dawn, the atmosphere is typically calm and clear. As the ocean (or moist land) heats up, air temperature and evaporation increase. Warm moist air starts to rise. Soon the rising moist air cools and condenses into clouds. The clouds reflect the sunlight. That’s the first step of climate regulation. Increased temperature leads to clouds. The clouds close the throttle slightly, reduce the energy entering the system. They start cooling things down. This is the negative feedback part of the cloud climate control.

The tropical sun is strong, and despite the negative feedback from the cumulus clouds, the day continues to heat up. The more the sun hits the ocean, the more warm, moist air is formed, and the more cumulus clouds form. This, of course, reflects more sun, the throttle closes a bit more. But the day continues to warm.

The full development of the cumulus clouds sets the stage for the second part of temperature regulation. This is not simple negative feedback. It is the climate governing system. As the temperature continues to rise, as the evaporation climbs, some of the fluffy cumulus clouds suddenly transform themselves. They rapidly extend skywards, thrusting up to form pillars of cloud thousands of meters high in a short time. These cumulus are transformed into cumulonimbus or thunderstorm clouds. The columnar body of the thunderstorm acts as a huge vertical heat pipe. The thunderstorm sucks up warm, moist air at the surface and shoots it skyward. At altitude the water condenses, transforming the latent heat into sensible heat. The air is rewarmed by this release of sensible heat, and continues to rise.

At the top, the air is released from the cloud up high, way above most of the CO2. In that rarified atmosphere, the air is much freer to radiate to space. By moving inside the thunderstorm heat pipe, the air bypasses most of the greenhouse gases and comes out near the top of the troposphere. During the transport aloft, there is no radiative or turbulent interaction between the rising air and the lower and middle troposphere. Inside the thunderstorm, the rising air is tunneled through most of the troposphere to emerge at the top.

In addition to reflecting sunlight from their top surface as cumulus clouds do, and transporting heat to the upper troposphere where it radiates easily to space, thunderstorms cool the surface in a variety of other ways, particularly over the ocean.

1. Wind driven evaporative cooling. Once the thunderstorm starts, it creates its own wind around the base. This self-generated wind increases evaporation in several ways, particularly over the ocean.

a) Evaporation rises linearly with wind speed. At a typical squall wind speed of 10 mps (20 knots), evaporation is about ten times higher than at “calm” conditions (conventionally taken as 1 mps).

b) The wind increases evaporation by creating spray and foam, and by blowing water off of trees and leaves. These greatly increase the evaporative surface area, because the total surface area of the millions of droplets is evaporating as well as the actual surface itself.

c) To a lesser extent, surface area is also increased by wind-created waves (a wavy surface has larger evaporative area than a flat surface).

d) Wind created waves in turn greatly increase turbulence in the atmospheric boundary layer. This increases evaporation by mixing dry air down to the surface and moist air upwards.

e) As spray rapidly warms to air temperature, which in the tropics is often warmer than ocean temperature, evaporation also rises above the sea surface evaporation rate.

2. Wind driven albedo increase. The white spray, foam, spindrift, changing angles of incidence, and white breaking wave tops greatly increase the albedo of the sea surface. This reduces the energy absorbed by the ocean.

3. Cold rain and cold wind. As the moist air rises inside the thunderstorm’s heat pipe, water condenses and falls. Since the water is originating from condensing or freezing temperatures aloft, it cools the lower atmosphere it falls through, and it cools the surface when it hits. In addition, the falling rain entrains a cold wind. This cold wind blows radially outwards from the center of the falling rain, cooling the surrounding area.

4. Increased reflective area. White fluffy cumulus clouds are not tall, so basically they only reflect from the tops. On the other hand, the vertical pipe of the thunderstorm reflects sunlight along its entire length. This means that thunderstorms shade an area of the ocean out of proportion to their footprint, particularly in the late afternoon.

5. Modification of upper tropospheric ice crystal cloud amounts (Linden 2001, Spencer 2007) . These clouds form from the tiny ice particles that come out of the smokestack of the thunderstorm heat engines. It appears that the regulation of these clouds has a large effect, as they are thought to warm (through IR absorption) more than they cool (through reflection).

6. Enhanced night-time radiation. Unlike long-lived stratus clouds, cumulus and cumulonimbus often die out and vanish as the night cools, leading to the typically clear skies at dawn. This allows greatly increased nighttime surface radiative cooling to space.

7. Delivery of dry air to the surface. The air being sucked from the surface and lifted to altitude is counterbalanced by a descending flow of replacement air emitted from the top of the thunderstorm. This descending air has had the majority of the water vapor stripped out of it inside the thunderstorm, so it is relatively dry. The dryer the air, the more moisture it can pick up for the next trip to the sky. This increases the evaporative cooling of the surface.

In part because they utilize such a wide range of cooling mechanisms mechanisms, cumulus clouds and thunderstorms are extremely good at cooling the surface of the earth. Together, they form the governing mechanism for the tropical temperature.

But where is that mechanism?

The problem with my thought experiment of describing a typical tropical day is that it is always changing. The temperature goes up and down, the clouds rise and fall, day changes to night, the seasons come and go. Where in all of that unending change is the governing mechanism? If everything is always changing, what keeps it the same month to month and year to year? If conditions are always different, what keeps it from going off the rails?

In order to see the governor at work, we need a different point of view. We need a point of view without time. We need a timeless view without seasons, a point of view with no days and nights. And curiously, in this thought experiment called “A Day In the Tropics”, there is such a timeless point of view, where not only is there no day and night, but where it’s always summer.

The point of view without day or night, the point of view from which we can see the climate governor at work, is the point of view of the sun. Imagine that you are looking at the earth from the sun. From the sun’s point of view, there is no day and night. All parts of the visible face of the earth are always in sunlight, the sun never sees the night time. And it’s always summer under the sun.

If we accept the convenience that north is up, then as we face the earth from the sun, the visible surface of the earth is moving from left to right as the planet rotates. So the left hand edge of the visible face is always at sunrise, and the right hand edge is always at sunset. Noon is a vertical line down the middle. From this timeless point of view, morning is always and forever on the left, and afternoon is always on the right. In short, by shifting our point of view, we have traded time coordinates for space coordinates. This shift makes it easy to see how the governor works.

The tropics stretch from left to right across the circular visible face. We see that near the left end of the tropics, after sunrise, there are very few clouds. Clouds increase as you look further to the right. Around the noon line, there are already cumulus. And as we look from left to right across the right side of the visible face of the earth, towards the afternoon, more and more cumulus clouds and increasing numbers of thunderstorms cover a large amount of the tropics.

It is as though there is a graduated mirror shade over the tropics, with the fewest cloud mirrors on the left, slowly increasing to extensive cloud mirrors and thunderstorm coverage on the right.

After coming up with this hypothesis that as seen from the sun, the right hand side of the deep tropics would have more cloud than the left hand side), I though “Hey, that’s a testable proposition to support or demolish my hypothesis”. So in order to investigate whether this postulated increase in cloud on the right hand side of the earth actually existed, I took an average of 24 pictures of the Pacific Ocean taken at local noon on the 1st and 15th of each month over an entire year. I then calculated the average change in albedo and thus the average change in forcing at each time. Here is the result:


Figure 2. Average of one year of GOES-West weather satellite images taken at satellite local noon. The Intertropical Convergence Zone is the bright band in the yellow rectangle. Local time on earth is shown by black lines on the image. Time values are shown at the bottom of the attached graph. Red line on graph is solar forcing anomaly (in watts per square meter) in the area outlined in yellow. Black line is albedo value in the area outlined in yellow.

The graph below the image of the earth shows the albedo and solar forcing in the yellow rectangle which contains the Inter-Tropical Convergence Zone. Note the sharp increase in the albedo between 10:00 and 11:30. You are looking at the mechanism that keeps the earth from overheating. It causes a change in insolation of -60 W/m2 between ten and noon.

Now, consider what happens if for some reason the surface of the tropics is a bit cool. The sun takes longer to heat up the surface. Evaporation doesn’t rise until later in the day. Clouds are slow to appear. The first thunderstorms form later, fewer thunderstorms form, and if it’s not warm enough those giant surface-cooling heat engines don’t form at all.

And from the point of view of the sun, the entire mirrored shade shifts to the right, letting more sunshine through for longer. The 60 W/m2 reduction in solar forcing doesn’t take place until later in the day, increasing the local insolation.

When the tropical surface gets a bit warmer than usual, the mirrored shade gets pulled to the left, and clouds form earlier. Hot afternoons drive thunderstorm formation, which cools and air-conditions the surface. In this fashion, a self-adjusting cooling shade of thunderstorms and clouds keeps the afternoon temperature within a narrow range.

Now, some scientists have claimed that clouds have a positive feedback. Because of this, the areas where there are more clouds will end up warmer than areas with less clouds. This positive feedback is seen as the reason that clouds and warmth are correlated.

I and others take the opposite view of that correlation. I hold that the clouds are caused by the warmth, not that the warmth is caused by the clouds.

Fortunately, we have way to determine whether changes in the reflective tropical umbrella of clouds and thunderstorms are caused by (and thus limiting) overall temperature rise, or whether an increase in clouds is causing the overall temperature rise. This is to look at the change in albedo with the change in temperature. Here are two views of the tropical albedo, taken six months apart. August is the warmest month in the Northern Hemisphere. As indicated, the sun is in the North. Note the high albedo (areas of light blue) in all of North Africa, China, and the northern part of South America and Central America. By contrast, there is low albedo in Brazil, Southern Africa, and Indonesia/Australia.


Figure 3. Monthly Average Albedo. Timing is half a year apart. August is the height of summer in the Northern Hemisphere. February is the height of summer in the Southern Hemisphere. Light blue areas are the most reflective (greatest albedo)

In February, on the other hand, the sun is in the South. The albedo situation is reversed. Brazil and Southern Africa and Australasia are warm under the sun. In response to the heat, the clouds form, and those areas now have high albedo. By contrast, the north now has low albedo, with the exception of the reflective Sahara and Rub Al Khali Deserts.

Clearly, the cloud albedo (from cumulus and cumulonimbus) follows the sun north and south, keeping the earth from overheating. This shows quite definitively that rather than the warmth being caused by the clouds, the clouds are caused by the warmth.

Quite separately, these images show in a different way that warmth drives the cloud formation. We know that during the summer, the land warms more than the ocean. If temperature is driving the cloud formation, we would expect to see a greater change in the albedo over land than over the ocean. And this is clearly the case. We see in the North Pacific and the Indian Ocean that the sun increases the albedo over the ocean, particularly where the ocean is shallow. But the changes in the land are in general much larger than the changes over the ocean. Again this shows that the clouds are forming in response to, and are therefore limiting, increasing warmth.

How the Governor Works

Tropical cumulus production and thunderstorm production are driven by air density. Air density is a function of temperature (affecting density directly) and evaporation (water vapor is lighter than air).

A thunderstorm is both a self-generating and self-sustaining heat engine. The working fluids are moisture-laden warm air and liquid water. Self-generating means that whenever it gets hot enough over the tropical ocean, which is almost every day, at a certain level of temperature and humidity, some of the fluffy cumulus clouds suddenly catch fire. The tops of the clouds streak upwards, showing the rising progress of the moisture laden surface air. At altitude, the rising air exits the cloud, replace by more moist air from below. Suddenly, in place of a placid cloud, there is an active thunderstorm.

Self-generating means that the thunderstorms arise spontaneously as a function of temperature and evaporation. Above the threshold necessary to create the first thunderstorm, the number of thunderstorms rises rapidly. This rapid increase in thunderstorms limits the amount of temperature rise possible.

Self-sustaining means that once a thunderstorm gets going, it no longer requires the full initiation temperature necessary to get it started. This is because the self-generated wind at the base, plus dry air falling from above, drive the evaporation rate way up. The thunderstorm is driven by air density. It requires a source of light, moist air. The density of the air is determined by both temperature and moisture content (because curiously, water vapor at molecular weight 16 is only a bit more than half as heavy as air, which has a weight of about 29).

Evaporation is not a function of temperature alone. It is governed a complex mix of wind speed, water temperature, and vapor pressure. Evaporation is calculated by what is called a “bulk formula”, which means a formula based on experience rather than theory. One commonly used formula is:

E = VK(es – ea)


E = evaporation

V= wind speed (function of temperature difference [∆T])

K = coefficient constant

es = vapor pressure at evaporating surface (function of water temperature in degrees K to the fourth power)

ea = vapor pressure of overlying air (function of relative humidity and air temperature in degrees K to the fourth power)

The critical thing to notice in the formula is that evaporation varies linearly with wind speed. This means that evaporation near a thunderstorm can be an order of magnitude greater than evaporation a short distance away.

In addition to the changes in evaporation, there at least one other mechanism increasing cloud formation as wind increases. This is the wind-driven production of airborne salt crystals. The breaking of wind-driven waves produces these microscopic crystals of salt. The connection to the clouds is that these crystals are the main condensation nuclei for clouds that form over the ocean. The production of additional condensation nuclei, coupled with increased evaporation, leads to larger and faster changes in cloud production with increasing temperature.

So increased wind-driven evaporation means that for the same density of air, the surface temperature can be lower than the temperature required to initiate the thunderstorm. This means that the thunderstorm will still survive and continue cooling the surface to well below the starting temperature.

This ability to drive the temperature lower than the starting point is what distinguishes a governor from a negative feedback. A thunderstorm can do more than just reduce the amount of surface warming. It can actually mechanically cool the surface to below the required initiation temperature. This allows it to actively maintain a fixed temperature in the region surrounding the thunderstorm.

A key feature of this method of control (changing incoming power levels, performing work, and increasing thermal losses to quelch rising temperatures) is that the equilibrium temperature is not governed by changes in the amount of losses or changes in the forcings in the system. The equilibrium temperature is set by the response of wind and water and cloud to increasing temperature, not by the inherent efficiency of or the inputs to the system.

In addition, the equilibrium temperature is not affected much by changes in the strength of the solar irradiation. If the sun gets weaker, evaporation decreases, which decreases clouds, which increases the available sun. This is the likely answer the long-standing question of how the earth’s temperature has stayed stable over geological times, during which time the strength of the sun has increased markedly.

Gradual Equilibrium Variation and Drift

If the Thermostat Hypothesis is correct and the earth does have an actively maintained equilibrium temperature, what causes the slow drifts and other changes in the equilibrium temperature seen in both historical and geological timese?

As shown by Bejan, one determinant of running temperature is how efficient the whole global heat engine is in moving the terawatts of energy from the tropics to the poles. On a geological time scale, the location, orientation, and elevation of the continental land masses is obviously a huge determinant in this regard. That’s what makes Antarctica different from the Arctic today. The lack of a land mass in the Arctic means warm water circulates under the ice. In Antarctica, the cold goes to the bone …

In addition, the oceanic geography which shapes the currents carrying warm tropical water to the poles and returning cold water (eventually) to the tropics is also a very large determinant of the running temperature of the global climate heat engine.

On a shorter term, there could be slow changes in the albedo. The albedo is a function of wind speed, evaporation, cloud dynamics, and (to a lesser degree) snow and ice. Evaporation rates are fixed by thermodynamic laws, which leave only wind speed, cloud dynamics, and snow and ice able to affect the equilibrium.

The variation in the equilibrium temperature may, for example, be the result of a change in the worldwide average wind speed. Wind speed is coupled to the ocean through the action of waves, and long-term variations in the coupled ocean-atmospheric momentum occur. These changes in wind speed may vary the equilibrium temperature in a cyclical fashion.

Or it may be related to a general change in color, type, or extent of either the clouds or the snow and ice. The albedo is dependent on the color of the reflecting substance. If reflections are changed for any reason, the equilibrium temperature could be affected. For snow and ice, this could be e.g. increased melting due to black carbon deposition on the surface. For clouds, this could be a color change due to aerosols or dust.

Finally, the equilibrium variations may relate to the sun. The variation in magnetic and charged particle numbers may be large enough to make a difference. There are strong suggestions that cloud cover is influenced by the 22-year solar Hale magnetic cycle, and this 14-year record only covers part of a single Hale cycle.

Conclusions and Musings

1. The sun puts out more than enough energy to totally roast the earth. It is kept from doing so by the clouds reflecting about a third of the sun’s energy back to space. As near as we can tell, this system of cloud formation to limit temperature rises has never failed.

2. This reflective shield of clouds forms in the tropics in response to increasing temperature.

3. As tropical temperatures continue to rise, the reflective shield is assisted by the formation of independent heat engines called thunderstorms. These cool the surface in a host of ways, move heat aloft, and convert heat to work.

4. Like cumulus clouds, thunderstorms also form in response to increasing temperature.

5. Because they are temperature driven, as tropical temperatures rise, tropical thunderstorms and cumulus production increase. These combine to regulate and limit the temperature rise. When tropical temperatures are cool, tropical skies clear and the earth rapidly warms. But when the tropics heat up, cumulus and cumulonimbus put a limit on the warming. This system keeps the earth within a fairly narrow band of temperatures.

6. The earth’s temperature regulation system is based on the unchanging physics of wind, water, and cloud.

7. This is a reasonable explanation for how the temperature of the earth has stayed so stable (or more recently, bi-stable as glacial and interglacial) for hundreds of millions of years.

Further Reading

Bejan, A, and Reis, A. H., 2005, Thermodynamic optimization of global circulation and climate, Int. J. Energy Res.; 29:303–316. Available at

Richard S. Lindzen, Ming-Dah Chou, and A. Y. Hou, 2001, Does the Earth Have an Adaptive Infrared Iris?, doi: 10.1175/1520-0477(2001)082<0417:DTEHAA>2.3.CO;2

Bulletin of the American Meteorological Society: Vol. 82, No. 3, pp. 417–432.

Available online at

Ou, Hsien-Wang, Possible Bounds on the Earth’s Surface Temperature: From the Perspective of a Conceptual Global-Mean Model, Journal of Climate, Vol. 14, 1 July 2001. Available online here (pdf).

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The sun puts out more than enough energy to totally roast the earth. It is kept from doing so by the clouds reflecting about a third of the sun’s energy back to space. As near as we can tell, this system of cloud formation to limit temperature rises has never failed.
I guess I’ll have to do a blog post. Thanks Willis!

Fluffy Clouds (Tim L)

good one, a long read!


Thanks a lot for that article. It is really an eye opener. Every AGWer should have a look at it. I already was a skeptic, but this new point of view “from the sun” makes me doubt even the temperature records provided by GISS and the like. We have no idea of what the planet’s temperature is. Even with satellites, we are only deciding if a place is warming or cooling by checking its maximum and minimum temperature during a day and how the numbers evolve in time. We know nothing about how fast it rises, how fast it falls, for how long it mantains each temperature, all of which affect emisivity of the Earth. And clouds have a lot to say about that. And then there is the UHI effect, of course.
For me this has been the very best article I have read in WUWT since I follow it.


Well, there can be no doubt that the atmosphere was richer in CO2 billions of years ago. That is where all of today’s coal, oil, limestone, and marble come from. If you take ALL of the coal, ALL of the oil, ALL of the limestone and ALL of the marble on the entire planet and convert it all back to CO2, you have a situation much like what we had back then. Then add the fact that there was much more volcanism at that time. Anyone who would claim that the atmosphere at the time when the sun was 30% dimmer wasn’t much richer in CO2 is not doing the math.


A very good essay Willis! Truly fascinating remarks with regard to the importance of wv and clouds.

“The globe has maintained a temperature of ± ~ 3% (including ice ages) for at least the last half a billion years”
What is this in degrees?
[REPLY – Degrees Kelvin. The only way you can do degree percentages ins starting from absolute zero. ~ Evan]


I’m a little puzzled by this paragraph:
“(Note that a governor, which contains a hysteresis loop, is different from a negative feedback. A negative feedback can only reduce an increase. It cannot maintain a steady state despite differing forcings, variable loads, and changing losses. Only a governor can do that.)”
But a negative-feedback amplifier, within operating parameters, can maintain a desired output in the face of all of the disturbances you list. Certainly the mechanism of negative feedback as applied in electronics can in principle compensate for any change in forcing, not increases only. And of course a governor mechanism uses negative feedback to operate. Perhaps you mean something different by the term?

UK Sceptic

O/T Paul McCartney is advocating people go veggie on Mondays in order to save the planet from global warming. A cynical person might point out that, given Macca has allegedly suffered tens of millions wiped off his portfolio due to the recession, many people might turn to the McCartney brand of vegetarian food as a convenient option in order to boost his flagging profits.


re: “(Note that a governor, which contains a hysteresis loop, is different from a negative feedback. A negative feedback can only reduce an increase. It cannot maintain a steady state despite differing forcings, variable loads, and changing losses. Only a governor can do that.)”
A “climate governor” suggests an open loop system (no feedback), whilst “temperature stability” suggests a closed loop system (negative feedback).


Interesting reading. Since it seems that Willis was correct about the Tuvalu sea level not rising (latest data here, maybe this hypothesis is correct too.


David Montgomery (01:08:11) : The earth’s surface air temperature is currently around 287 K. The 3% of that is +-9 K. I don’t know what the average is, but these numbers indicate a 18K temperature spread from the warmest to the coldest. — John M Reynolds


Ahhh, now I see why models without clouds can have tipping points, yet no such tipping points have occurred in the past.
Take that Gavin.


RE this posting: If this did not happen we would not be here In general most life systems (including weather if we could call it a life system!), 1- 1 = 0,
BTW on anotehr note…Me thinks that in general, the Scandinavians have decided to no longer go on with the charade and are adjusting realistically..
also DMI lets see in the USA ice monitors realize this the sooner the better.

anna v

OK, this is coherent and plausible.
Now I would be interested to see in this model whatis the effect of the different absorption by the oceans of different frequencies of incoming sunlight; for example, what percentage of those 90 watts/m2 is due to UV. The 8% change in ultraviolet energy from sun minimum to sun maximum might be effective in modulating the heat engine with no amplification needed.

One thing this post leaves out is the important part that life has played in sequestering most of the early terran atmosphere in limestone deposits. Earth’s atmosphere was, at one point, 52 times more dense than today, with a large CO2 component. Even during the cretaceous and jurassic periods, CO2 levels were significantly higher than today by several times and atmospheric density overall was significantly higher. Over time, as the sun has heated up, CO2 levels have gone down and overall atmospheric density has gone down, as more and more CO2, O2, and H2O have been locked into limestone by coral. It is true that much CO2 has also been locked into fossil fuels like oil, natural gas, and methane hydrates in sea bottoms that subducts over time beneath the continent, and that a lot of this CO2 returns to the atmosphere via volcanic erutpions, however this merely gives the CO2 a second shot to be sequestered permanently into limestone via coral deposits.


I thought a trace gas regulated the earths climate? Seriously though, excellent post! Love the comment on clouds causing warming, some people are unable to look out the window and make a scientific observation!
I have come to a similar conclusion where the earths climate is regulated by the ocean, convection, cloud cover and atmospheric mass with greenhouse gasses playing only a small role in the overall scheme of things. The more heat in the system the more the climate will work to remove this heat, increasing outgoing longwave energy observed over the last few decades demonstrates this system of self regulation.
We cant even start to assess any anthropogenic effect on climate until we fully understand the natural climate system – and so far we are not even close!
Good work


Just when might this thermostat kick in? It’s been an extraordinary hot May at the planets surface and that is post La Nina ( We will almost certainly see the largest positive monthly temperature anomaly every observed by man at the earth’s surface in the coming months.
It’s not going to be a pretty sight next year as we go post El Nino and have a warming sun. Watch for a big step up in sea level, a sharp decline in sea ice, and the hottest year on record.

John Wright

The parallel with cruise control is intriguing. Personally I never use it because it is reactive rather than anticipatory. Any experienced driver will tell you that he starts to step on the gas just before reaching the start of an up grade. By the time the cruise control kicks in, the vehicle has already slowed down. You lose speed, and I am sure waste fuel in the long run. However if a similar process takes place in the earth’s temperature control system, this could surely provide an element in weather forecasting precisely because of what I what drives me mad with cruise control i.e. delay in reacting.
Just an idea I throw in.

Very nice Willis. Re: Malcolm, It doesn’t mater if the system is open or closed if the thermostat has control of sufficient energy to maintain its set point. In this case the thermostat is cooling only so it can only maintain its temperature range if there is enough heat input to call for cooling. Interestingly, this thermostat appears to have a glacial setback control setting.


In another thread on this site ref:
based on
“Despite the rather mind-boggling rate at which Earth is losing atmosphere — 5×1025 molecules per second — scientists say there is no cause for alarm. If the loss rate stays the same, the planet’s atmosphere will last for several more billion years.”
If Earth has had an atmosphere for lets say 2 billion years at least.
Is it not then possible then, if this draining of our atmosphere has been going on since, that the atmosphere was thicker and warmer going back in time?
Theoretically for every 100 meter more air you add in the bottom(ground) of the atmosphere moist air will increase by 0.5C/100m and dry air will increase by 1C/100m in that colum of atmosphere.
And if the Sun was only 75 % of todays output earlier in time and the Earth at the same time most of the time 10 deg C warmer than today this could be one way to make sense off it?
Off topic would not atmosphere above desert with dry air be the best place to validate or falsify the UNFCCC/UNEP/IPCC doctrine ?


My “eureka” moment came about three quarters of the way through this splendid article. Especially as it opens up a rather neat slot for Svensmark’s theory to drop into.


I was wondering if the mass of the earth and its velocity in orbit are considerations here. The theories on formation of the moon suggest a collision of a smaller earth and a mars size object formed the moon. This collision must have changed the mass of the earth-moon system and its velocity, which would have changed its orbital position and its distance from the sun… theoretically moving the earth away from the sun and partially compensating for the increase in solar output… any astronomers out there got comments?


And maybe the draining of atmosphere is increasing with a more active Sun?

Chris Wright

Another fascinating article about climate feedback. That the earth has remained hospitable to life despite the sun becoming 25% hotter strongly suggests that negative feedbacks dominate. It seems that AGW depends on the assumption of strong positive feedbacks and this is where it is particularly vulnerable. If feedbacks assumed by AGW to be positive can be shown to actually be negative then the whole ridiculous theory will fall apart – or it should if climate science was completely honest.
One question: if there were no positive or negative feedbacks how much warming would be caused by a 25% increase in solar output?


Thanks Willis for an excellent report on what I also think is one of the main processes which regulate our climate. Lots of pro-AGW propagandists seem to consider average global temperature (whatever that means) as the indicator of change, when total it is total system energy budget which is important.
I also agree that the small ‘drifts’ we see over time are probably made up of several components, as you detail. In chaotic systems even small changes, such as the rate of cloud formation, can result in small changes to the systems ‘balance’ point.

Steve Schapel

Thank you, Willis, for this excellent work, and so carefully explained. I have learned a lot from it.
With the obsessive narrow focus on atmospheric CO2 as the be-all-and-end-all of all things climate, which so many people still adhere to, it is so wonderful to see another example of significant steps towards a broader understanding of what’s really going on.

jmrSudbury (02:40:37) :
David Montgomery (01:08:11) : The earth’s surface air temperature is currently around 287 K. The 3% of that is +-9 K. I don’t know what the average is, but these numbers indicate a 18K temperature spread from the warmest to the coldest. — John M Reynolds

The long-term average temperature of the Earth during the Phanerozoic has been a fairly steady ~22C during “hothouse” or “greenhouse” periods and ~12C during the four “icehouse” periods or “ice ages”.
Hothouse = 22C…295K
Greenhouse = 12C…285K
Median = 17C…290K
+/- 3% (~9K) is more than adequate to account for the average differences between “hothouse” and “icehouse” periods and the warm anomalies at the end of the Permian and in the early Tertiary.

crosspatch (01:04:01) :
Well, there can be no doubt that the atmosphere was richer in CO2 billions of years ago. That is where all of today’s coal, oil, limestone, and marble come from. If you take ALL of the coal, ALL of the oil, ALL of the limestone and ALL of the marble on the entire planet and convert it all back to CO2, you have a situation much like what we had back then. Then add the fact that there was much more volcanism at that time. Anyone who would claim that the atmosphere at the time when the sun was 30% dimmer wasn’t much richer in CO2 is not doing the math.

Atmospheric CO2 was much higher early in the Phanerozoic; but it has not simply gradually declined from 7,000ppmv in the Cambrian to its current level. CO2 levels have followed a sort of declining saw-tooth pattern…Dropping from ~7000ppmv in the mid-Cambrian to ~200-400ppmv in the Pennsylvanian and then rising to ~2500ppmv in the Jurassic. Since then CO2 levels have gradually declined to current levels. Most of the declines appear to have had a declining saw-tooth pattern, with periodic minor increases.
Phanerozoic Temp/CO2
As far as volcanism goes, there’s no clear evidence that the Earth’s volcanic activity has been declining over the Phanerozoic. Volcanism has been episodic, with periods of intense volcanic activity (i.e. late Permian Siberian Traps and late Cretaceous Deccan Traps)…But intense volcanism is associated with cooling and not warming. So, if the Earth was more volcanic “back then” it would have been cooler, not warmer.


I just looked it up, for those who do not have the mass of the planets at the tip of their cerebral cortex. Mars’ mass is about 10% of earth. Then any significant difference in velocity changed the mass and velocity of the earth-moon system.

Steve Keohane

Thanks for this article Willis and Anthony. This is a clear mechanism for controlling the climate. I always think of the poles as cryopumps, like in a vapor deposition system, especially Antarctica where H2O is sequestered, and the areas of permafrost and icecap in the north. Water has a one-way ticket to the south pole, yes there is sublimation, but that is a small fraction of the incoming water.


OT, but I can’t find a way to e:mail you on the site.
An interesting example of Gavin Schmidt and Michael Mann being ‘Economical’ with the truth in the current thread at RC. See (and RC) for detail.


Are you suggesting that the hotter it gets the more clouds we will see?
But surely the hotter the atmosphere becomes the more water vapour the atmosphere holds before clouds will form (e.g. in the UK it is very common for clouds to be burnt off by the sun’s heat as the day progresses in summer).
One has to also ask about the effect of clouds in winter where clouds are associated with warmer weather and clear skys with cooler.
Then of course there are places where there is insufficient vapour in the air to cause clouds (deserts). Fry during the day – freeze during the night.
For cloud cover to increase the water vapour must increase (or the temperature decrease). Increase in this GHG will lower radiation losses and increase temperature.
So albedo will change as cloud cover changes (Day time Clouds are a negative feedback – more clouds=cooler temp=lower water vapour=less clouds=lower temp ~and~ fewer clouds= higher temp=more clouds)
But for clouds to form you need more water vapour
But Water vapour is a positive feedback (higher temp= more water vapour = higher temp ~and~ lower temp =less water vapour = lower temp)
And night time clouds act as a blanket more clouds = warmer =? more clouds
NOTE that feed back is defined above
positive feedback gives less stability
negative feedback gives more stability
I also note that you are suggesting that water vapour/clouds are a feedback and not a forcing
Whilst thunderstorms will transport heat upwards some will be re-radiated to space but some will radiate to the atmosphere and some will be transported down again as rain and downdraughts and lightning.
As you are suggesting that the poles radiate some of this heat to space then perhaps the lack of O3 in the antarctic is the reason for it not following modelled temperatures..
Ahh! so many known unknowns and unknown unknowns!

Willis. A fascinating study – thank you very much indeed. Are you, by any chance the same Willis Eschenbach I knew in Fiji? If so… Bula!!!


I have been a faithful (daily) reader here for more than two years. This contribution is, in my judgment, one of the best. Surely in the top 5. And, not being a PhD in anything, one of the most readable.

Paul R

Interesting article and it makes me wonder if the weakened magnetic field somehow makes the this process more efficient?


It is all starting to come together for one huge almost(~+/-80%) perfect picture of the process that makes the Earth’s climate livable and by large or average homeostatic. Never completely neutral, but always trying to be. Seems like a type of system only a God could create.

A negative feedback can only reduce an increase. It cannot maintain a steady state despite differing forcings, variable loads, and changing losses.
This is only true of strictly proportional negative feedback. If there is an integrator in the system or the control loop a steady state is in theory possible if the loop tuning is correct and the noise level does not keep things bouncing around.
And the system does have an integrator. The oceans.

Lindsay H

Mike Lorrey (02:51:36) :
One thing this post leaves out is the important part that life has played in sequestering most of the early terran atmosphere in limestone deposits. Earth’s atmosphere was, at one point, 52 times more dense than today, with a large CO2 component.
can you give a reference for the 52 times more dense quote ??

Thanks, Willis.
A jaw-dropping synthesis with a sound of feasibility to it. Your clear, accessible exposition includes some fascinating information. I hope the hypothesis survives. It’s almost too good to be true.
Richard Treadgold,
Climate Conversation Group.


“Anyone who would claim that the atmosphere at the time when the sun was 30% dimmer wasn’t much richer in CO2 is not doing the math.”
Nobody is claiming that.

Chris Schoneveld

Now I am confused. On the one hand he says: “clouds control how much energy enters the climate heat engine” and a paragraph further on he says: “the cumulonimbus clouds are active heat engines”. They can’t be both.

Gary Pearse

This would appear to be the E=MC(sqd) of Climate. A big light went on when the viewpoint shifted to sun’s eye view. One can now see that, as usual in the progress of science, we start by looking at the micro picture and it takes someone to find the macro view for the elucidation of a phenomenon. Most are still caught up in the micro-picture – counting tree rings, carbon dioxide in ice cores, centimetres in advancing or shrinking of glaciers, fractions of a milimetre in sea-level, changes of 0.5C over a century. In light of the wonderfully simple (like Einstein’s equation looking at the whole universe) sun’s eye view and the simple physical parameters of the “engine”. The 0.037% solution in the atmosphere theory of climate is the flat earth hypothesis and geocentric theory of the universe rolled into one.
However, I note an almost ho-hum tinge to several of the posts, even though they have a generally positive view of the thesis. Why is it that when shown something truly elegant and inspiring we don’t seem to be very moved by it? Heck, we were animated, enthralled, angry, combative over a misinterpretation of a tree ring count or proper selection of a species of tree to do the count, or whether a countable sunspot had appeared or not. This is an accurate weather retrospect for the past couple of billion years and a forecast for the next several billion!! Perhaps it opens the door to linking the “CO2 cycle” to it – Maybe Geese fly high on the temperate cell and low on the Hadley Cell going south and reverse this going north taking advantage of the engine…. Willis, I’m very impressed (for one thing, the engineer in me likes the idea of carnot cycle running the climate).

(Note that a governor, which contains a hysteresis loop, is different from a negative feedback. A negative feedback can only reduce an increase. It cannot maintain a steady state despite differing forcings, variable loads, and changing losses. Only a governor can do that.)
Like others, I have a significant quibble with this (the rest of the essay more than makes up for this).
A governor can be a pure negative feedback system. James Watt’s governor is like that, see . The automotive cruise controls up to the 1970s or so use this mechanism and add a reference point – setting the control triggers a solenoid that holds the position of the governor’s output. The control system then tries to keep the governor’s output to match the reference, and that’s a simple negative feedback control system.
Hysteresis entails a system whose internal state has memory of the past. Not quite like I describe above, which may be the source of confusion, but more like soft iron magnets. In facl Merriam-ebster says hysteresis is “a retardation of an effect when the forces acting upon a body are changed (as if from viscosity or internal friction) ; especially : a lagging in the values of resulting magnetization in a magnetic material (as iron) due to a changing magnetizing force.”
This is best shown in magnetic tape recorders. Applying and removing a small magnetic force to iron will shift its internal structure and then recover back to its original state. A strong force will shift it so far that it doesn’t bounce back. Old
computer fogeys like me will suggest “core memory” as in example of being serious about using hysteresis for its memory effect.
There’s nothing quite like that in climate that I can think of at the moment, though frozen ponds and ice cap come close. (They’re more like a control system with a non-linear response.)


Thank you Willis Eschenbach. I have taken the liberty of putting a copy of your essay onto my harddrive for further study.
For others interested in water and the atmosphere and are battling with the science,
(I am anyway 🙂 ) I can recommend a series of essays by Patrick J. Tyson:
they are all quite small text only files and very easy reading.


Are there any climate scientists with a computational climate model (and enough hardware to run it) with the motivation to include these considerations in the next design iteration of the model?
If these ideas are right then they deserve a larger set of analysts chewing on them. Even if they are only approximately right.


I love this work.
I especially like the work on thermostat theory – it turns out that the very mathematics of adaptive systems is entirely general and does not distinguish between mechanical, electronic, biological, or social systems. Yes, thermostats, blood sugar levels in the body, and economics have precise the same mathematics. And as this post shows, so does the earth.
Check out:
Is quite an eye-opener.


Does this analysis automatically include hurricanes/typhoons or would they be an additional heat engine?
Were there any hurricanes on the earth during the time the GEOS photos were taken?

Chris Schoneveld

Excuse my ignorance, but when you say that “some scientists have claimed that clouds have a positive feedback. Because of this, the areas where there are more clouds will end up warmer than areas with less clouds”, what mechanism could possibly cause clouds to increase temperature. I can only see them keeping night time temperatures higher, like an insulating blanket.

Stephen Wilde

The essence of this helpful post is very similar to that of my various articles published over the pasr 12 months.
However I think it unnecessarily emphasises the Tropics and fails to treat the net latitudinal position of ALL the air circulation systems as the critical issue.
I believe that the entire air circulation system in each hemisphere shifts poleward or equatorward as a direct result of net global warming or cooling.
That initial net warming or cooling seems normally to be a direct result of oceanic changes in net global emission or absorption of solar energy.
The air circulations as a whole work to maintain equilibrium between sea surface and surface air temperatures.
From time to time factors other than the oceans alter the overall energy budget but the response of the air circulations is just the same. Following substantial volcanic eruptions we see a similar equatorward shift of the air circulation systems during the short period of cooling that follows such eruptions.
I propose that if there is a change in the GHG content of the air then any air temperature effect from that change would again be dealt with by an imperceptible change in the latitudinal positions of the air circulation systems so as to change the rate of energy flow to space and thereby prevent any destabilisation of the existing equilibrium between oceans and air.
The background equilibrium which the system always works back towards is set by the level of solar input combined with the length of time the solar energy stays in the oceans as a whole. That is where Tyndall et al are wrong. They assumed that the chemical characteristics of the components of the air set the equilibrium temperature of the planet.
In fact the oceans set it in conjunction with solar input and the air is forever fated to maintain that equilibrium between sun and sea whatever happens in the air.
In fact the net latitudinal position of all the air circulation systems is a diagnostic indicator as to whether the globe is warming or cooling at any given moment.
We urgently need to divert resources to finding a more precise a method of reading that indicator.

David L. Hagen

BRAVO! Excellent clear description.
Ferenc Miskolczi developed a planetary greenhouse theory for a semi-transparent atmosphere. Miskolczi’s 1D theory finds that energy conservation and minimization (entropy maximization) results in an effectively constant optical depth of water and carbon dioxide absorption.
Combining Eisenbach’s Thermostat/Lindaen’s “Iris” model)/ Bejan’s constructal method / Svensmark’s cosmoclimatology/ and Miskolczi’s planetary greenhouse theory promise to provide strong empirical and theoretical basis for a relatively elegant model of earth’s climate and how it varies (or how little it varies) with changes in carbon dioxide concentration.