Guest Post by Wim Röst
It is said that the Earth’s surface temperature variations are controlled by [human-induced] greenhouse gases1. This is not the case. When cooling systems dominate, surface temperatures are set by the cooling system and not by the system that is warming the surface. On Earth the surface cooling system dominates; temperatures are set by the natural cooling system. The strength of natural surface cooling is set by temperature. Adding greenhouse gases to the atmosphere does not make any difference for surface temperatures. Their initial warming effect is neutralized by extra surface cooling and by a diminished uptake of solar energy. The cooling system dominates.
The Earth was assembled from ‘space debris’ orbiting the Sun. Gravity made objects like ‘space rocks’ and ice comets coalesce. When accretion took place, gravity melted all assembled objects and a big ‘snooker ball’ of molten material was built. The proto-Earth was also warmed by the Sun, but eventually it cooled down until ‘energy in’ equaled ‘energy out.’ Currently, the surface of the Earth is at balance at around 15 degrees Celsius. A similar planet, with no oceans or atmosphere would have stopped cooling at around 5.3 degrees Celsius, if it reflected no sunlight. Why did the surface of the Earth stop cooling at 15 degrees? And why didn’t the Earth’s surface stop cooling at, for example, 50 degrees Celsius?
Answering those questions reveals that it is not greenhouse warming that sets the level of the Earth’s surface temperatures but the mechanisms cooling the surface. The Earth’s additional cooling systems determine surface temperatures: evaporation, convection, and cloud cooling. All are H2O related.
And the main reason the Earth stayed about ten degrees warmer than its 5.3°C ‘rock temperature? ‘ As will be argued, it is the existence of large oceans in combination with their self-produced water vapor greenhouse effect.
The temperature an Earth-like object in space would have if the planet did not have an atmosphere while receiving the same amount of solar radiation as the Earth is 5.3°C. Only radiation would warm and cool the object. This Stefan-Boltzmann calculator shows that such a planet would have a surface temperature of 5.3 degrees Celsius or 278.5K. But our Earth has a higher global surface temperature, 15 degrees Celsius. The atmospheric greenhouse effect partly accounts for this.
On Earth greenhouse effects are huge, but that does not mean they are decisive in setting the final temperature of the surface of the Earth. There is not just one greenhouse effect, there are two, and each have their own surface warming effect. The first greenhouse effect is back radiation. After surface radiation is absorbed by greenhouse gases the atmosphere is warmed. A warmer atmosphere radiates more energy back to the surface: back radiation. Back radiation adds 333 W/m2 to the 161 W/m2 of surface absorbed solar energy. Back radiation is a strong surface warming force.
The second greenhouse effect warms Earth because of the diminished efficiency of radiative surface cooling. When effective cooling is diminished, more energy remains at/near the surface and both the surface and Earth warm. On Earth only about 40 W/m2 out of 396 W/m2 surface radiation directly reaches space: an efficiency of about 10% or a direct emissivity of only 0.1.
Both greenhouse effects each have their own surface warming effect. For each greenhouse effect its consequence for surface temperatures can be calculated. Separately and together the two greenhouse effects result in huge initial surface warming effects.
Without additional cooling: 270.1°C
Figure 1 shows the surface temperature of an Earth-equivalent ‘rock planet’ with greenhouse effects added, but only warmed and cooled by radiation. Our real Earth has additional systems cooling the surface: cooling by evaporation, convection, and clouds. These additional surface cooling systems cool the surface much further than ‘by radiative cooling only.’ From the initial greenhouse temperature of 270.1°C to the actual surface temperature of 15°C. The additional cooling sets the final surface temperatures. Of decisive importance: the strength of Earth’s additional cooling depends upon water and temperature.
Temperature dependency of additional cooling
The Earth’s additional cooling is predominantly H2O related. Surface temperatures determine the quantity of water vapor in the air. And the quantity of atmospheric water vapor determines the total cooling effect. In this way, surface temperatures determine the strength and the dynamics of H2O related cooling.
Figure 2 shows the equilibrium vapor pressure and temperature according to the Clausius-Clapeyron relation. As shown in the graphic a rise in temperature from zero to 30 degrees Celsius multiplies the equilibrium vapor pressure of water vapor by six times.
When surface temperatures go down by one degree Celsius/K, the quantity of water vapor goes down by about 7% and by consequence all water vapor related cooling processes diminish in strength. At a certain temperature level, ‘energy in’ will equal ‘energy out’. When surface temperatures don’t change, H2O related surface cooling will remain constant. But the small rise in temperature by only one degree Celsius (or one K, a 0.3% rise in temperature) will result in about 7% more water vapor. That huge rise in water vapor content empowers all H2O related cooling processes, often with a more than a proportional cooling result (tropical convection, tropical clouds). As shown by figure 2, H2O related cooling is very dynamic, especially in the higher temperature range. Dynamic additional cooling even limits the temperature of open oceans.
Open tropical oceans have a maximum average yearly temperature of 30°C to 32°C. Richard Willoughby reports that less than one percent of the ocean surface exceeds 32 °C for more than a few days at a time. Additional cooling factors limit ocean temperatures to this temperature level. Oceans comprise 71% of the Earth’s surface.
Redistribution of tropical energy
Tropical oceans distribute warm water to the poles in quantities varying over time. The higher the inflow of warm tropical water at higher latitudes, the higher the local quantity of atmospheric water vapor, the main greenhouse gas. Rising water vapor over high latitudes results in a diminished efficiency of local surface radiation in reaching space. Less radiative cooling means that these high latitudes will warm and also that the Earth as a whole will warm. Over time, countervailing processes at the surface (adapting oceans and weather systems) will restore the previous equilibrium temperature (if all other things, like the Milankovitch orbital parameters remain the same). The time frame involved is decades and/or centuries.
Why not 50°C?
Why didn’t the surface of the Earth stop cooling at a temperature level of 50°C? At a surface temperature of 50°C, upward convection of surface energy is huge. High convection will be present over large surface areas. At 50 degrees Celsius oceans will actively be cooled day and night and during the day clouds will reflect most of incident sunlight back to space before it can warm the oceans. Under these circumstances, oceans cool quickly. At the current global temperature of 15°C cooling by evaporation and associated cooling processes diminish enough to balance ‘surface energy in’ and ‘surface energy out’.
Why are oceans at a global temperature of 15°C evaporating exactly the quantity of water vapor needed to equal ‘surface energy in’ and ‘surface energy out’? This temperature level is determined by the intrinsic properties of the H2O molecule. The H2O molecule is very hygroscopic; there is a strong bond between molecules, and it is not easy for an individual molecule to escape from the water surface to the atmosphere. To escape a molecule needs to have a very high kinetic energy. To have enough energy, the surface temperatures has to be high enough and at a global average surface temperature of 15°C enough water vapor molecules can escape to achieve thermal equilibrium.
Intrinsic properties of the H2O molecule set the general global level for surface temperatures. Is there still some role left for greenhouse gases? Well, there is.
Oceans create their own greenhouse
Water vapor is the main greenhouse gas, responsible for about half of the greenhouse effect while clouds (also H2O) count for another 25%. Are oceans able to create a greenhouse effect strong enough to raise their own temperatures? Sure, they are.
At the equator insolation is intense and no ice is possible over the oceans: solar uptake of energy is large and when oceans are still at low temperatures, effective radiative and evaporative surface heat loss is low. Therefore, tropical oceans have to heat up. The small quantity of water vapor released at temperatures just above zero Celsius is high enough to get a strong greenhouse warming effect: the first water vapor molecules are most effective in absorbing spaceward surface radiation. When oceans cannot lose 100% of the solar energy absorbed, they will warm. By the evaporation of water vapor, oceans create their own greenhouse effect: not all surface radiated energy disappears to space and oceans need an additional way to lose their accumulated solar energy. Oceans must warm to the point that rising evaporation and enhanced tropical clouds fully compensate for the strongly diminished efficiency of ocean surface emission. If started at low temperatures, oceans will warm till the Earth has an average surface temperature (for present orbital and continental configuration) of 15 degrees Celsius and ‘energy in’ equals ‘energy out’.
Why surface temperatures are not sensitive to greenhouse gases, except for water vapor
Adding an extra 3.7 W/m2 (for a doubling of CO2) to the calculator only increases the initial surface temperature one degree Celsius/K, from 270.1°C to 271.1°C. Additional cooling then must rise by 1/270.1 or 0.37% to compensate for the extra warming force. What would happen with surface cooling when surface temperatures rise by that one degree Celsius?
- Evaporative cooling (responsible for 78 W/m2 of surface cooling) would speed up by some 7% (Clausius-Clapeyron)
- Convection would speed up to a large degree because of both the higher surface temperature and the higher content of water vapor (+7%) in the warmest and most humid air columns
- As result of higher convection, more tropical clouds will form over larger surface areas and earlier in the day and more sunlight will be reflected to space before it can reach and warm the surface, thus solar absorption diminishes.
Because all surface cooling occurs in concert, the slight 0.37% initial warming following CO2 doubling is potentially more than compensated by the huge cooling resulting from the H2O-related processes. Additional surface cooling easily compensates for any greenhouse warming caused by ‘CO2 doubling’. Whatever the level of greenhouse warming, additional cooling dominates surface temperatures and surface temperatures regulate additional H2O based surface cooling in order to have surface temperatures remaining at the level prescribed by the intrinsic properties of the H2O molecule.
Only a change in orbital and/or continental configuration will change the general temperature level upward or downward. Under unchanged circumstances surface temperatures have a very strong tendency to remain at the same general level because of ‘built-in’ physical properties of H2O molecules involved in additional cooling.
At current temperatures, Earth’s H2O related cooling processes operate at a low level. Strong convective updraft of surface energy is visible above 25°C. Thus, at the surface of the Earth a large capacity to cool is currently dormant. A slight rise in temperature is sufficient to activate multiple powerful cooling systems in a very dynamic way. Most of the time (nights, mornings) and over most locations (all locations below 25°C) H2O related surface cooling is dormant but easy to activate. Any rise in temperatures activates many forms of surface cooling, while at diminishing temperatures H2O surface cooling activities diminish accordingly. The system seems to be made to keep surface temperatures at about the same level.
How to understand present warming?
A change in the distribution of tropical ocean absorbed energy to the North Pacific (El Niño effect) and/or to the Arctic (by warm subsurface inflows into the Arctic Ocean that cause ice melt) enhances atmospheric water vapor over large surface areas at higher latitudes. As argued before, a warming of higher latitudes results in a diminished radiative cooling of the Earth and so in warming. But, on the Earth’s time scale those (and other) changes are only temporary: they can last decades, a century or a bit more. Although not always easy to recognize, warming and cooling periods alternate in cyclic patterns. Those cyclic patterns are irregular by the ever-changing chaotic interactions of the many components of the ocean/atmosphere temperature system. Cooling always follows warming, like the night always follows the day. Sometimes we need more patience to discover how nature regulates and stabilizes surface temperatures – as it has always done.
The Earth cooled from a hot molten mass just after its formation to the present Earth with its solid crust and its lower surface temperatures. Two greenhouse effects (back radiation and blocking surface radiation) were not able to maintain the surface temperature at 270 degrees Celsius. This is the temperature Earth would have if it were only cooled by surface-emitted radiation. Earth’s additional surface cooling systems, all dominated by the various phases of water, kicked in to cool the surface to its average 15 degrees Celsius.
The additional surface cooling systems of the Earth depend on the H2O molecule. H2O related cooling processes are progressively temperature dependent: the warmer the surface, the stronger the cooling. Temperature itself regulates and limits surface temperatures. For a given configuration the level of surface temperatures is set by the intrinsic properties of the H2O molecule and not by the strength of greenhouse warming; additional H2O based surface cooling compensates for any radiative warming. Cooling is dominant. The immediately available H2O related surface cooling is huge, and its reserve capacity is as endless as the oceans.
Decadal and centennial temperature variations around the current global average of 15°C result from a changed distribution of tropical ocean absorbed energy over the latitudes. Natural warming events are temporary, because over time enhanced surface cooling cancels extra surface warming. Cooling always follows warming, but cooling the Earth takes time, often more time than warming. We need to think in timescales of the Earth to see the changes in surface temperatures in the right way. Earth’s warming and cooling periods happen over decades, centuries and sometimes over millennia.
With regards to commenting, please adhere to the rules known for this site: quote and react, not personal. And when commenting, please don’t use abbreviations but words.
About the author: Wim Röst studied human geography in Utrecht, the Netherlands. The above is his personal view. He is not connected to firms or NGOs or funded by government(s).
Andy May was so kind to correct and improve the English text where necessary or helpful. Thanks!
1Lacis, A., Schmidt, G., Rind, D., & Ruedy, R. (2010, October 15). Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature. Science, 356-359. Retrieved from https://science.sciencemag.org/content/330/6002/356.abstract