What Causes the Greenhouse Effect?

June 13th, 2015 by Roy W. Spencer, Ph. D.

I’ve had a request to (once again) go through an explanation of the (poorly-named) Greenhouse Effect (GHE). Hopefully there is something which follows that will help you understand this complex subject.

The greenhouse effect usually refers to a net increase in the Earth’s surface temperature due to the fact that the atmosphere both absorbs and emits infrared radiation. (Our miniscule enhancement of the natural greenhouse effect with carbon dioxide emissions, and its possible role in global warming, is a separate issue).

This GHE temperature increase is frequently quoted as being around 60 deg. F, thus keeping the Earth from being an ice planet, since its average surface temperature is somewhere around 59 or 60 deg. F.

This 60 deg. F warming attributable to the GHE is actually incorrect; the greenhouse effect on surface temperature, if left to its own devices, would actually be at least twice that strong…more like 140 deg F average surface temperature…but most of that theoretical surface temperature rise is short-circuited by convective heat loss from the surface caused by convective air currents, in turn caused by the greenhouse effect, which also largely creates the weather we experience.

That’s right – without the greenhouse effect, we would not have weather as we know it. The greenhouse effect, energized by solar heating, creates weather.

The GHE is somewhat controversial among some skeptics, probably because we can’t “see it” the way we can see visible sunlight and the resulting heating of surfaces sunlight falls upon – a rather non-controversial cause-and-effect process. It instead involves infrared (IR) light, which we cannot see, but which is an essential part of the energy flows in our climate system…and in most other systems that generate heat. You can actually feel if it is sufficiently strong (e.g. radiant heat from a stove or fire).

I must preface the following discussion with this: The temperature of any object represents a balance between energy gained and energy lost by that object. Temperature is an energy balance issue. Unless phase changes are involved (e.g. melting ice), if more energy is gained than lost, temperature goes up. If more energy is lost than gained, temperature goes down. Understanding this is fundamental to understanding weather and climate, as well as the following discussion.

The atmosphere contains “greenhouse gases” (GHGs), which means gases which are particularly strong absorbers and emitters of IR radiation. In the Earth’s atmosphere, the main GHGs are water vapor and carbon dioxide. Absorption and emission of IR go together because anything that is a good absorber of IR is also a good emitter, although in general the rates of absorption and emission are not the same since absorption is mostly temperature-independent but emission is very temperature-dependent.

In the classical Kiehl-Trenberth global energy budget diagram, the energy flows I have marked with an “X” would not exist without GHGs:


Now, recall I said that temperature is a function of rates of energy gain and energy loss. Thus, those energy flow arrows marked with an “X” in the above diagram represent huge flows of energy which can affect temperature, if they really exist.

So, let’s now think through what happens as sunlight enters the climate system. As the Earth’s surface absorbs sunlight it warms up. As it warms up, it emits more and more IR energy, limiting its temperature rise (remember “energy balance”?).

If the atmosphere could not intercept (absorb) any of that surface-emitted IR energy, the energy would readily escape to outer space and as a result it has been estimated that the Earth’s average surface temperature would be only about 0 deg. F. But we really don’t know exactly because there would be a lot more ice, which would reflect more sunlight, which would make temperatures even colder. Also, we have no idea why kinds of clouds would exist under those conditions. Suffice it to say the Earth would probably be too cold for most life as we know it to survive.

But the atmosphere DOES absorb IR energy. The IR absorption coefficients at various wavelengths, temperature, and pressures have been measured for water vapor, CO2, etc., in laboratories and published for decades.

This absorption means the atmosphere also EMITS IR energy, both upward and downward. And it is that DOWNWARD flow of IR energy (sometimes called “back radiation”) which is necessary for net warming of the surface from the greenhouse effect.

(Technical diversion: This is where the Sky Dragon Slayers get tripped up. They claim the colder atmosphere cannot emit IR downward toward a warmer surface below, when in fact all the 2nd Law of Thermodynamics would require is that the NET flow of energy in all forms be from higher temperature to lower temperature. This is still true in my discussion.)

Now, some will claim the atmosphere’s decreasing temperature with height is also necessary for the greenhouse effect to occur. While this is true, the decrease in temperature with height in the troposphere is ultimately caused by the greenhouse effect itself.

You see, as long as an atmosphere (it doesn’t matter from which planet) has greenhouse gases, the temperature will decrease with height. Without convection, the temperature would decrease drastically with height…the so called “pure radiative equilibrium” case, first demonstrated by Manabe and Strickler (1964). The net effect of GHGs is to strongly warm the surface lower atmosphere temperature, and strongly cool the upper atmosphere temperature, compared to if those gases did not exist. The GHE makes the atmosphere so unstable that convection – weather – results, which restores the atmospheric temperature lapse rate to somewhere between dry adiabatic and moist adiabatic.

Remember, without greenhouse gases, the upper atmosphere could not lose the energy it accumulates from all sources, and would stay warm, and the atmosphere would not destabilize and cause convective overturning (weather).

This net result is not intuitively obvious. I sometimes use the (admittedly imperfect) analogy of insulation in a house in winter (even though heat conduction is a different physical process from radiation). Given the same rate of energy input into the home by its heating system, addition of insulation slows the net rate of heat flow from the warmer interior to the cold exterior, causing higher temperatures inside and lower temperatures outside, compared to if the insulation did not exist.

Again, temperature is the result of energy gain AND energy loss. If you reduce the rate of energy loss, temperature will rise…even if the energy input is the same. Extremely high temperatures can even be created with very little energy input…even from a tiny battery…if you can reduce the rate of energy loss to near zero. You cannot say anything about temperature based upon the rate of energy input alone, any more than you can say what the average level of a lake will be based upon the rate of water input alone. It just ain’t physically possible.

Analogous to insulation in a heated home, greenhouse gases reduce the net rate of infrared energy transfer from the surface and lower atmosphere to outer space, causing the surface and lower atmosphere to be warmer, and the upper atmosphere to be colder, than if greenhouse gases did not exist.

Since the effect is not entirely intuitive, years ago we programmed up the equations ourselves in a 1-D radiative-convective model for me to be convinced this is what actually happens. When the model is initialized with global average sunlight and atmospheric greenhouse gas concentrations, from any initial temperature profile you want (even absolute zero), it eventually equilibrates to the observed average vertical temperature structure of the atmosphere.

And I suppose it is the non-intuitive nature of the process (I required a model demonstration to finally believe it) that breeds so much controversy and alternative ideas. I get that.

Now, I know that this post will cause a few people (you know who you are) to object with hand-waving arguments involving technical jargon that what really happens is something different. But until they put their ideas in the form of physical equations based upon known (and laboratory-measured) processes, which conserve energy, in a time-dependent model that also produces the observed average temperature profile of the atmosphere, I will not believe them.

What those people need to do is go read a book on atmospheric radiation, say Grant Petty’s A First Course In Atmospheric Radiation. I know Grant, and he is a brilliant and careful scientist. If you disagree with him (and the many other experts who agree with him), you’d better have some pretty good evidence to back your case up.

The bottom line, then, is the Greenhouse Effect, due mostly to greenhouse gases, is largely caused by the fact that the atmosphere emits IR energy downward, the so-called “back radiation”. This single component of the whole GHE process basically then determines all of the other features of the greenhouse effect and leads to net GHE warming of the Earth’s surface.

You can measure the greenhouse effect yourself with a handheld IR thermometer pointed at the sky, which measures the temperature change caused by a change in downwelling IR radiation. In a clear sky, the indicated temperature pointing straight up (“seeing” higher altitudes) will be colder than if pointed at an angle (measuring lower altitudes). This is direct evidence of the greenhouse effect…changes in downwelling IR change the temperature of a surface (the microbolometer in the handheld IR thermometer). That is the greenhouse effect.

If I’ve make a mistake in the above, I’ll fix it. I realize some might not like the way I’ve phrased certain things. But I’ve been working in this field over 20 years, and the above is the best I can do in 1-2 hours time. From some of the objections you will see in the comments, you will find it is a complex subject, indeed.

527 Responses to “What Causes the Greenhouse Effect?”

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  1. Dr Spencer;
    I read this and all of it is utter nonsense.
    Where to start?
    Well may as well start here;
    “…the decrease in temperature with height in the troposphere is ultimately caused by the greenhouse effect itself.”
    The thermal gradient in the troposphere is not caused by a ‘greenhouse effect’.

    There cannot be two causes of the lapse rate, (which is what leads directly to the surface thermal enhancement over the effective temperature) and there isn’t. A ‘greenhouse effect’ plays no part in it, or in anything else, because there is no greenhouse gas effect.
    What causes this thermal gradient – more commonly known as the lapse rate, is detailed here.

    First what is temperature? Temperature in a gas is a measure of the average kinetic energy in that gas.

    Auto-compression is well known in underground mining and is used by ventilation engineers to calculate how hot the mine air will get, so that they know how much cooling air to provide at each level. The effect of auto-compression can be calculated by the following relationship;
    Pe = Ps exp(gH/RT)
    Pe = absolute pressure at end of column (kPa)
    Ps = absolute pressure at start of column (kPa)
    g = acceleration due to gravity (m/s)
    H = vertical depth (m)
    R = Standard Temperature (Kelvin)
    T = Final Temperature (Kelvin)

    As can be clearly seen, this effect primarily relies on pressure and gravity, which will be different for each planetary body.
    Mechanism is adiabatic

    Note that we are examining a largely adiabatic process during convection. When a gas parcel expands adiabatically, as it does when rising in a gravitational field, it does positive work and the kinetic energy drops and so the temperature drops.

    However, when a gas parcel is compressed, as it is when it descends adiabatically in a gravitational field, then it does negative work, and its kinetic energy rises and so its temperature goes up. Why does the kinetic energy of the gas rise when descending?

    Its because some of its potential energy is converted to enthalpy, so producing an increase in pressure, specific internal energy and hence, temperature in accordance with the following equation;

    H = PV + U
    H = enthalpy (J/kg)
    P = pressure (Pa)
    V = specific volume (m)
    U = specific internal energy (kinetic energy)

    It doesn’t matter how many climate scientists agree on something – if it conflicts with data then it’s wrong.

    Here is the data which proves that all 97% (or whatever) of them are wrong; Venus/Earth/Titan.

    Relative insolation Earth/Venus = 1.91
    Measured temperature in the Venus atmosphere at 1 bar = 339 Kelvin
    Tplanet 1 bar=∜(TSI relative) x Te

    Te=(Tvenus at 1 bar )/∜(TSI relative)
    Te=(340 )/∜1.91
    Te=289.1 Kelvin

    So temperature of Earth as calculated from the Venus atmosphere = 289.1 Kelvin
    There is no need to know anything about the Earth’s atmosphere.

    Venus has a massive 96.5% “Greenhouse Gas” atmosphere
    Earth has just 2.5% “Greenhouse Gas” atmosphere.

    These numbers are not possible if there is a “Greenhouse Gas Effect”.
    This is final proof that there is no “Greenhouse gas effect” on Venus, Earth or anywhere else.
    This collapses the “Greenhouse effect” hypothesis and proves it does not exist.

    * A very important point; after 40 years of “Greenhouse Effect by our CO2” claims, and 30,000 published climate papers;

    – There is still NOT ONE published paper paper in the literature, which quantifies any warming and attributes it to increasing atmospheric CO₂ concentrations.

    On top of this total lack of any empirical evidence at all for the so-called “Greenhouse Effect” of “Greenhouse Gases”, is my two recent papers published in 2018 and 2020, which each individually invalidate the so-called “Greenhouse Effect” in different ways.

    And previous paper of mine published in the Journal of Physics & Astronomy in early 2018, clarifies the accuracy of the ideal gas law, and what actually causes tropospheric temperatures in thick planetary atmospheres; it is pressure and insolation.
    In short; there is no such thing as a “Greenhouse Effect” and there is no special class of gases called “Greenhouse Gases” which can anomalously warm the atmosphere. The result is; Arrhenius was wrong, and James Maxwell was correct.

    Here are these three papers;

    Holmes, R. I. (2020). On the Apparent Relationship Between Total Solar Irradiance and the Atmospheric Temperature at 1 Bar on Three Terrestrial-type Bodies. Earth, 8(6), 346-351. (doi: 10.11648/j.earth.20190806.15)

    Holmes, R. I. (2018). Thermal Enhancement on Planetary Bodies and the Relevance of the Molar Mass Version of the Ideal Gas Law to the Null Hypothesis of Climate Change. Earth, 7(3), 107-123. (doi: 10.11648/j.earth.20180703.13)

    Robert, I. H. (2018). Thermal Gradients on Planetary Bodies and the Molar Mass Ideal Gas Law. J Phys Astron, 6(1), 134.

    Dr Robert Ian Holmes

  2. Dr. Spencer,

    I appreciate your efforts to educate people about these topics.

    The current essay (“What Causes the Greenhouse Effect?”) is mostly “spot on.”

    Unfortunately, one thing prevents me from wanting to refer people to it. That’s your emphasis on the centrality of back-radiation. While it’s not technically wrong, I think it puts the emphasis in an unfortunate place, and is a “loser” of an argument, in terms of how much unproductive nonsense it stimulates, without being all that helpful (in my view) for illuminating what is fundamentally going on.

    R. T. Pierrehumbert, in “Principles of Planetary Climate”, identifies what he calls the “surface energy budget fallacy” as being a common source of false reasoning about the greenhouse effect. He suggests that when people focus on energy exchange between the surface and the atmosphere, rather than between Earth and space, this generally leads to confusion and false inferences. That matches my experience.

    It is difficult for most people to reason correctly about the surface energy budget because of the complications of needing to consider both radiative and non-radiative energy flows there. You can’t predict what will happen to non-radiative energy flows just by considering what happens at the surface. Things are much simpler at TOA, where only radiative effects matter.

    “Back radiation” is part of the surface-energy-budget narrative about the greenhouse effect. Focusing on that narrative is problematic, and the “back radiation” way of talking about the surface interactions is even more problematic, due to people’s lack of understanding of radiative heat transfer (and its fundamental nature of involving bidirectional thermal radiation flows).

    My preferred way of talking about the greenhouse effect these days is something like this: Greenhouse gases block the upward flow of thermal radiation at low altitudes, and emit thermal radiation to space at higher altitudes. How much thermal radiation gets emitted depends on temperature. There is lots of thermal radiation at the surface, and much less at high altitudes where the air is cold. So, by shifting the location of emissions to space from the surface to a high altitude, greenhouse gases reduce (by 40%) the rate at which thermal radiation is emitted to space. This reduction in the planet’s rate of cooling allows the Sun’s energy to accumulate, raising temperatures until the planet is able to emit enough energy to space to balance the rate of incoming energy.

    You say many of the same things. Yet, the way of expressing things I’ve offered makes no reference whatsoever to the hot-button topic of “back radiation”, is logically irrefutable to anyone who actually understands the argument, and captures the core of how the greenhouse effect warms planets. I have hope that there would be less controversy about the greenhouse effect if the sort of presentation was used more widely.

    (When I do talk about the surface energy budget, I like to emphasize that greenhouse gases suppress radiative heat transfer upward at low altitudes. To me, that’s less stimulating of controversy and confusion than talk about “back radiation”, and it’s describing the same physics.)

    Bottom line: I generally like what you say in this essay, but find the emphasis on “back radiation” unfortunate.

    (Perhaps your way of talking about such issues has already shifted in the years since you published this essay. But, the essay is still prominently featured.)

    I hope these reflections are in any way helpful.

    Robert Wentworth
    Ph.D. Applied Physics