The Earth’s greenhouse effect is much larger than suggested so far. If surface radiation and the greenhouse effect set surface temperatures, our oceans would be boiling. Fortunately, they don’t. Water Earth has a strong water-vapor-based evaporative surface cooling mechanism that effectively sets and stabilizes surface temperatures at a much lower level than cooling by surface radiation emissions can do. Thanks to water vapor our temperature system is far more stable than admitted by the consensus, and thanks to water, water vapor, and clouds surface temperatures are favorable for present life.
Early Earth consisted of hot molten lava covered by an extreme greenhouse atmosphere: hardly any surface radiation could reach space, if any. Nevertheless, its surface cooled. Upward convection brought sensible and latent heat from hot surfaces to elevations on the very edge of the atmosphere from where energy effectively could be radiated into space. Despite the near maximal greenhouse effect the surface of Early Earth cooled down and at a certain moment the first oceans developed. Those boiling oceans still resulted in a huge upward convective transport of energy, further cooling the surface. Until now, convective upward transport of energy plays the main role in surface cooling. Convection sets and regulates surface temperatures at actual level. Without evaporative-convective-cloud-cooling, our actual greenhouse atmosphere would theoretically result in a surface temperature of 202.3°C. On the real Earth the greenhouse effect warms the surface, but greenhouse warming does not set and control final surface temperatures. Earth’s H2O-based cooling system does.
Theoretical greenhouse effect
We can calculate the warming effect of present greenhouse atmosphere for a theoretical planet in the case where its surface is cooled just by radiation. Without a greenhouse atmosphere and if optimally cooled by radiation the temperature of such a theoretical planet is minus 42.3 degrees Celsius. But a greenhouse atmosphere makes a huge difference. Initially.
Present Earth’s greenhouse atmosphere is still ‘a near perfect’ greenhouse atmosphere. As shown in Figure 1, only 22 W/m2 of surface radiated energy (396 W/m2) can reach space without being absorbed. A surface cooling efficiency of only 5.556%.
Figure 1: The Earth’s radiation budget. Of all 396 W/m2 surface radiated energy 22 W/m2 reaches space without being absorbed (Added: red oval). Source Trenberth and Fasullo 2011
The efficiency of cooling by surface radiation is very low: after absorption, nearly all surface emitted energy returns to the surface as downwelling radiation or (without convection) and stays as sensible heat in the lower atmosphere. Knowing the cooling efficiency of the Earth’s surface radiation, we can calculate the greenhouse surface temperature in the case where the surface of our imaginary planet is only cooled by radiation, as shown in Figure 2.
Figure 2: Surface temperature for a theoretical planet, calculated for the case of surface cooling by radiation only. Input: solar radiation absorbed by the surface and surface emitted longwave radiation of 161 W/m2, the effective emissivity is 5.556% (‘Emissivity 0.05556’). Calculation by Stefan-Boltzmann Law calculator.
With Earth’s present greenhouse effect, the surface of our imaginary planet would have had a temperature of 202.3 degrees Celsius, if only cooled by surface radiation. Total initial greenhouse warming is huge, see Table 1.
Table 1: The greenhouse warming effect per Stefan-Boltzmann for a theoretical planet only cooled by radiation. Calculated for a planet without greenhouse atmosphere and for a planet with Earth’s present greenhouse atmosphere. Radiated power equal to solar absorption 161 W/m2.
Given the high initial greenhouse warming effect, on our relatively cool Earth other factors than surface radiation must control the level of surface temperatures; probably H2O-related surface cooling.
The level of Earth’s surface temperatures
Where in the range of ‘greenhouse temperatures’ do we find Earth’s surface temperatures? On Earth, surface temperatures are best indicated by the surface temperature of ocean water, covering 71% of the Earth’s surface. The maximum average yearly temperature is 30°C while the minimum temperature is minus 1.8°C, shown in green in Figure 3.
Figure 3: The level of the Earth’s actual surface temperatures shown within the range for theoretical greenhouse warming as calculated for the theoretical situation of cooling only by surface radiation in the case of a theoretical planet. The greenhouse effect would warm the surface from – 42.3°C to + 202.3°C. Earth’s actual ocean temperatures are shown in green: from + 30°C to – 1.8°C. Blue shows the temperature range that is too cold for life, red shows the temperature range too hot for life.
‘Radiation only’ would have stopped cooling the planet’s surface at 202.3°C. Actual Earth yearly average surface temperatures are much lower, about 15°C. On real Earth, additional cooling by evaporation, conduction, convection and clouds has lowered surface temperatures far below the level ‘radiation only cooling’ would have resulted in. Why? The answer is that H2O-related cooling (evaporative, convective and tropical cloud cooling) is very strong, very dynamic, and very effective in the temperature range above 15°C.
Evaporation rises by 6-7% (Clausius-Clapeyron) per degree of temperature rise, a huge percentage. In the higher temperature range evaporative cooling cools extremely: think about boiling water of 100°C. In case of temperatures lower than 15 degrees Celsius, evaporative cooling diminishes by the same high percentage of 6-7%. At some point H2O related surface cooling and warming resulting from surface solar absorption came into balance at 15°C.
Convection in the atmosphere is the upward transport of latent and sensible heat from the surface to higher elevations. Convective removal of surface heat effectively cools the surface and brings energy to elevations lacking most of the main greenhouse gas, water vapor. At these elevations emission to space is more effective than surface emission. Convection is highly stimulated by the low-density water vapor molecules resulting from evaporation. Evaporative-convective cooling is huge in the higher temperature range and produces large quantities of solar reflecting tropical clouds. When tropical clouds develop, evaporative surface cooling is combined with diminished surface solar warming: very effective.
Strong convection firmly enhances wind over the surface and brings in drier and colder air from elsewhere, resulting in higher conductive surface heat loss.
Diminishing H2O-based cooling
The whole evaporation-based cooling machine is very dynamic. All H2O-based surface cooling is fueled by rising evaporation, as temperatures rise. But evaporation also strongly diminishes when temperatures fall, even by just one degree, ending further cooling of the surface. At present, the Earth’s total surface cooling and total surface warming are balanced at a yearly average of 15 degrees Celsius.
The oceanic uptake of solar energy is very dependent on the presence/absence of tropical clouds. As temperatures go down, lower-level tropical clouds diminish strongly, and more solar energy is able to reach and warm the surface. Surface warming causes a rise in evaporation. Rising evaporation, thunderstorms, and related processes ending in tropical clouds soon end the extra solar warming. Hence the incredible stability of the Earth’s surface temperatures.
At 15°C there is a balance between surface warming by solar uptake and surface cooling. Any further surface cooling results in higher solar uptake, neutralizing initial cooling. And any surface warming results in higher evaporative-convective-cloud cooling, neutralizing any initial surface warming.
Initial warming by extra greenhouse gases is fully neutralized like all other surface warming. Neutralizing warming happens at different time scales, sometimes seconds (radiation) or hours, a day, or by season, but often over decades (by longer-term ocean oscillations) and sometimes over even longer periods like the recovery from the cold Little Ice Age which might take centuries.
Why 15°C and why not 202.3°C?
Radiative cooling is less dynamic than H2O based cooling. For one degree of difference in surface temperature, radiative cooling goes up or down by only 1.4%, but H2O- based cooling by 6-7%. Early Earth started hot and then cooled down after its creation. At a current surface temperature of only 15°C (the temperature level for this geological period and for this orbital setting) the H2O related surface cooling has balanced surface solar absorption.
Early Earth was hot and steamy. Heat of accretion did melt all the colliding material coming from space that formed the Earth. A nearly perfect sphere formed and its atmosphere was the perfect greenhouse atmosphere: an atmosphere with a superhigh water vapor content, very rich in carbon dioxide, and a sky covered by clouds. Hardly any surface radiation could reach space without being absorbed. Convection had to transport surface energy to the edge of the steamy atmosphere where spaceward emission could take place. For early Earth surface cooling depended on the strength of convection. As temperatures fell, convective cooling continued but continuously diminished in strength. Tropical cloud coverage also diminished, allowing the Sun to warm the tropical oceans. Despite Earth’s huge greenhouse effect, surface temperatures have never been dependent on the strength of the greenhouse effect but on the temperature set by where H2O-related surface cooling balances warming by rising surface solar uptake.
The fascinating H2O molecule has many intrinsic properties. One of its properties gives the molecules a strong cohesion resulting in a strong surface tension which creates ‘tight’ surfaces some insects can even walk on. Strong surface tension makes it difficult for a surface molecule to escape into the atmosphere, which raises the temperature at which enough water vapor will be released to cause ‘super-convection’. Another intrinsic property sets the freezing temperature at zero degrees Celsius and not at +10, +20 or minus 20 degrees. Binding one oxygen atom to two low-density hydrogen atoms results in a low-density water molecule, so very humid, low density, air easily rises. H2O’s intrinsic properties determine all essential elements of Earth’s main cooling system, which is dominated by H2O. The properties are intrinsic to the molecule itself: they don’t change over time. Therefore, the surface temperature of the Earth could have remained at the same level over billions of years if Earth’s orbital settings and the distribution of oceans and continents over its surface hadn’t changed. H2O’s intrinsic properties set the level of Earth’s surface temperatures for every specific orientation and surface arrangement of the Earth. The H2O molecule, nothing else.
Faint young Sun paradox
During the first years of early Earth, the Sun’s output must have been about 30 percent less intense as nowadays. Less solar energy reached the Earth. Nevertheless, the surface of the Earth has never been much colder than present Earth. This is called the faint young Sun paradox. Knowing the role of H2O-related surface cooling, that paradox is solved. As total insolation reaching the surface is controlled by tropical clouds and the Earth’s surface temperatures are controlled by H2O-related surface cooling, the Earth’s surface temperatures don’t simply depend on the intensity of solar irradiation reaching the Earth. In the case of a faint Sun, evaporation diminishes, less tropical clouds cover tropical oceans and enable more (but weaker) solar rays to reach and warm a larger surface area. End result for tropical oceans: about the same.
No Snowball Earth
Because the quantity of insolation reaching the surface is controlled by tropical clouds and because of H2O-controlled surface cooling, no complete snowball Earth has probably existed. A slightly colder surface strongly diminishes H2O related surface cooling. Diminishing tropical clouds result in a higher uptake of solar energy by tropical oceans. The final result is tropical oceans still remaining warm. On water Earth no full snowball Earth is possible. Even when all present land (29% of the total surface) is concentrated on both poles, the result is just a partial snow- and ice-covered surface. Most of the other 71% of the surface will be covered by relatively warm oceans, oceans that are redistributing tropical absorbed solar energy over mid-latitudes, not hindered by any continent.
The Earth’s greenhouse effect is huge, much higher than normally assumed. If cooled by ‘surface radiation only’ the surface of a theoretical planet would have had a surface temperature of 202.3°C. But the Earth’s surface temperatures are not set by the strength of Earth’s greenhouse effect. Additional H2O-based cooling systems keep the surface at a much lower temperature, balancing rising surface radiation uptake. At present, that balance is reached at a yearly average of 15 degrees Celsius.
Thanks to H2O-related surface cooling the Earth’s surface temperatures are bound to a narrow range, at a temperature level well suited for life on Earth. Due to its stability, life developed over many hundreds of millions of years.
Temperature regulates the cooling system; the cooling system regulates temperature.
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About the author: Wim Röst studied geography in Utrecht, the Netherlands. The above is his personal view. He is not connected to firms or NGO’s or funded by government(s).
Andy May was so kind to correct and improve the English text where necessary or helpful. Thanks!
- Calculations are for a theoretical planet fully responding to Stefan-Boltzmann Law. The theoretical planet is a perfect absorber/emitter (blackbody) and consists of an ‘infinitely thin shell’ not able to store any energy. Its surface is superconducting, resulting in the lowest emission temperature possible.
- Calculated for actual solar surface absorbed (161 W/m2) assuming maximal absorption and maximal emission and assuming all surface radiation is directly radiated to space, meaning: without being absorbed. Efficiency of surface emission is 100% or an effective emissivity of ‘1’. Calculation by Stefan-Boltzmann calculator.
- This 2011 version of the graphic is the corrected one. The caption on the image: “The global annual mean earth’s energy budget for 2000–2005 (W m−2). The broad arrows indicate the schematic flow of energy in proportion to their importance. Adapted from Trenberth et al. (2009) with changes noted in the text”.
- The present state of the Earth includes the Earth’s orbital configuration and the location, size, and topography of continents and oceans. The total state results in a specific distribution and redistribution of solar energy over latitudes. Weather patterns depend on the distribution and redistribution of solar energy. Climate by definition is the average of 30 years of weather. Changes in climate are the result of changes in the distribution and redistribution of solar energy over the Earth’s surface.