Climate Model Bias 4: Convection and atmospheric circulation

By Andy May

In part 3 we discussed the relationship between changes in solar activity and climate changes. Exactly how solar changes affect climate is not understood. It isn’t the immediate change in radiation delivered to the Earth, since that is too small to have much of an effect. So, it must be how Earth’s climate system reacts to the changes. The observed impact of solar irradiance changes over the solar cycle on the climate is much larger than the change in delivered radiation can account for.^[1] A likely amplifying mechanism is Earth’s convection and atmospheric circulation system. This post examines that idea. It is yet another important idea that the IPCC and AR6 ignore and brush away as unimportant, vis-à-vis global warming.

The emphasis the IPCC places on global average surface temperature and the use of the phrase “global warming” suggests that atmospheric and oceanic circulation of thermal energy are not important in discussions of global climate change. Theodore Shepherd argues that global climate is driven by thermodynamics, and only regional climate is driven by convection and atmospheric circulation.^[2] He also admits that climate models are much less consistent in their predictions of precipitation than temperature, and that the difference is likely due to atmospheric circulation, which affects precipitation patterns more than temperature. Finally, he acknowledges that our understanding of atmospheric circulation is weak.

The IPCC makes much of the fact that the only way Earth gains thermal energy is through radiation that is absorbed by the atmosphere or the surface from outer space and the only way it loses energy is when radiation is emitted to outer space.^[3] This is true, but the apparent corollary that movement of thermal energy from one place to another on Earth’s surface makes no difference in the overall energy imbalance, is not true. Convection and circulation largely control the residence time of the energy within the climate system.^[4] When the average residence time is short, Earth cools, when the residence time is long, Earth warms.

To demonstrate this, we need to examine six critical areas of climate and geological research. The first is the areal distribution of incoming and outgoing radiation around the globe and the distribution of net radiation flux (incoming-outgoing radiation). Next, we will examine energy transport from the tropics, where there is a surplus of thermal energy, to the polar regions which have a deficit, or net loss of thermal energy to space. This thermal energy transport is called the “meridional transport” of energy. When meridional transport is strong, Earth cools, and when it is weak Earth warms. Figure 1 illustrates strong meridional flow in orange and weak (or “zonal”) flow in red.

Thirdly, tropical temperatures do not vary much over geological time because over the oceans they are limited to less than 30°C by evaporation.^[5] So-called “global warming” happens almost exclusively in the higher latitudes, not in the tropics. Fourth, we examine how the temperature difference between the equator and the poles forms a characteristic equator-to-pole temperature gradient. Fifth, the equator-to-pole temperature gradient today is relatively steep, suggesting the climate of today is unusually cold. Sixth, the temperature gradient powers meridional transport, the steeper it is, all else equal, the colder the Earth is.

While the temperature gradient powers meridional transport, meridional transport has many modulators, and the gradient is only one of them. It is unclear exactly how the temperature gradient and meridional transport interact, but clearly, they are the main drivers of global climate change at all time scales.^[6]

In summary, discussing annual or monthly global average surface temperature as if it represents global climate change is very misleading. Earth’s climate does not behave that way. It circulates excess energy from warmer areas with a strong greenhouse effect to colder areas that have a weak greenhouse effect. Deserts and the polar regions have a weaker greenhouse effect due to their lower humidity (water vapor is the strongest greenhouse gas^[7]) and can more easily send energy to space as a result. The speed of energy transport determines whether the world warms or cools. The world is not a static uniform object that simply receives energy from the Sun and evenly emits it back into space, with a minor delay caused by greenhouse gas emissions, which seems to be how the IPCC views and models our planet’s climate.

Areal distribution of energy

The tropics (roughly 30°N to 30°S) cover half of Earth’s surface. This is the region that contains the location where the Sun is directly overhead at noon. Half of the tropics, 25% of Earth’s surface, is in daylight at any given time, and this 25% of the surface receives 62% of the solar energy that strikes Earth. This, combined with a very large tropical greenhouse effect and a low albedo,^[8] creates an enormous surplus of energy in the tropics.

Because more energy is received in the tropics than is emitted to space, the excess energy must be transported elsewhere. To make the situation even more complicated, Earth has an axial tilt relative to the plane of its orbit around the Sun. The effect of this tilt is illustrated in figure 2, where the Northern Hemisphere winter net energy flux^[9] profile is shown as a heavy dashed line and the Northern Hemisphere summer profile is shown as a light dotted line. The yearly average is shown as a solid line. The X axis is the latitude, positive latitudes are north of the equator and negative are south.

Because Earth is closest to the Sun (perihelion) on January 4^th, more total energy is delivered during the Northern Hemisphere winter. Figure 2 illustrates how complicated the task of regulating the surface temperature of Earth is. The point receiving the maximum energy input from the Sun is constantly moving. Besides this problem, the tropics have the strongest greenhouse effect in the lower troposphere as previously mentioned.

Meridional transport

In contrast to the tropics, the polar regions have the smallest greenhouse effect in the troposphere. This is especially true in the polar winter when the relative humidity is nearly zero due to the lack of sunlight and the low air temperature. This causes the air moving into the polar region to be warmer than the surface. The water under the polar ice is relatively warm (approximately -1.8°C) but it is insulated from the colder surface by ice. Thus, there is more radiant cooling to space from the polar air than from the surface because warmer bodies emit more radiation. In the dry polar regions, most radiation in the winter is from CO₂, and adding more CO₂ there means more radiation to space, which increases the rate of cooling, so we observe a reverse CO₂-human-enhanced greenhouse effect during polar winters.

The transport of energy from the tropics to the poles (aka meridional transport) is very large, exceeding five petawatts,^[10] as illustrated in figure 3. In figure 3 the northward energy flow is positive and southward energy flow is negative.

Figure 3 is an average over the year, it conceals very large differences during the year due to storms, Earth’s axial tilt, and changes in meridional transport.^[11]

Tropical temperature hardly varies

There is one more important point to make regarding surface temperature and incoming solar radiation. The tropical temperature is limited to about 30°C.^[12] As the ocean surface approaches 30°C, very rapid evaporation occurs and “deep convection” commences that drives lower density humid air^[13] very high (to an air pressure of about 200 hPa, or about 38,000 ft or 12 km) in the troposphere where it cools and forms clouds that shield the surface from the Sun.^[14] This deep convection also causes downdrafts of cool, dry air that work to cool the ocean surface.^[15]

The equator-to-pole temperature gradient

Because tropical temperatures are limited over the oceans and cannot exceed 30°C, the global energy balance forces global warming to occur at higher latitudes.^[16] Thus, a temperature gradient is created from the tropics to the poles that drives meridional transport. Since tropical temperatures do not change much,^[17] as the global average temperature decreases the temperature gradient increases in slope and as the average temperature increases, the gradient decreases. This is illustrated in figure 4, it is by Chris Scotese and colleagues.^[18]

Figure 4 is based on a model created by making 100 maps of ancient Köppen^[19] climatic belts around the world, each map represents the estimated paleoclimate of a five-million-year period, so the maps cover the past 500 million years. Scotese’s studies are significant for two reasons. First, the surface temperature ranges over the oceans and any significant body of water are limited, and since most of Earth’s surface is water, this limits the global average temperature, regardless of the greenhouse gas concentration in the atmosphere. Secondly, his work shows that the global average temperature today is unusually cold.

Today is unusually cold

Scotese’s work suggests that the “normal” surface temperature for the Earth over the past 500 million years is about 19-20 degrees, thus our surface temperature today is well below normal for Earth. Today’s average temperatures, by latitude, are marked in figure 4 with small plus signs. They tend to fall between 14- and 15-degrees global average temperature.

The last column in figure 4 is the percentage of the time each gradient shown exists in his maps. Over half the time (59%) in Earth’s recent history the global average temperature was between 19 and 20°C.

The current climate is unusual in Earth’s history, but it is unusually cold, not unusually hot. Scotese’s work is very well accepted in the geological community, yet it is ignored by the IPCC who prefer to proclaim current warming is unprecedented. In fact, although Chris Scotese is famous for his work on paleoclimate in the geological community, a search for his name in AR6 WGI comes up with nothing. In fact, his name is not referenced in any of the AR6 volumes, although his findings are relevant to all three.

Summary

Atmospheric circulation and convection do play a role in global climate change since they affect the speed and efficiency of meridional heat transport, which helps determine the equator-to-pole temperature gradient and the residence time of thermal energy in the climate system. Some would have us believe that global average temperature is only a function of thermodynamics and the global climate can be characterized by this quantity. Yet circulation patterns are very important in regional precipitation patterns, which are very poorly understood, and poorly represented in climate models.

Characterizing the global climate using the best modeled quantity (global average temperature) is not very scientific. The focus of our work should be on what we do not understand. As Shepherd states:

“Every aspect of climate change in which there is strong confidence … is based on thermodynamics. Circulation, on the other hand, is … governed by dynamics. Therefore, the earlier dichotomy can be re-stated as saying there is relatively high confidence in the thermodynamic aspects of climate change, and relatively low confidence in the dynamic aspects.”^[20]

Shepherd, 2014

In other words, if we treat Earth as a static and uniform thermodynamic body, we understand it. If we look at it as a real, dynamic planet with a circulating atmosphere and ocean, we don’t. A rather obvious point, and a part of the climate system that the CMIP models do not model well. This is acknowledged, regarding regional precipitation, in AR6.^[21]

In the next post, we will cover the sixth item in the list above, that the temperature gradient powers meridional transport. We will also cover the topic of storminess, that is extreme weather, is it increasing as the world gets warmer?

Download the bibliography here.

1. 
   

   
   (Wigley & Raper, 1990) and (Lean, 2017). ↑
   

   
   
2. 
   

   
   (Shepherd, 2014) ↑
   

   
   
3. 
   

   
   (IPCC, 2021, pp. 933-934) ↑
   

   
   
4. 
   

   
   (Vinós & May, The Sun-Climate Effect: The Winter Gatekeeper Hypothesis (III). Meridional transport, the most fundamental climate variable, 2022c) ↑
   

   
   
5. 
   

   
   (Sud, Walker, & Lau, 1999), (Newell & Dopplick, 1979), and (Hoffert, Flannery, Callegari, Hsieh, & Wiscombe, 1983) ↑
   

   
   
6. 
   

   
   (Vinós, Climate of the Past, Present and Future, A Scientific Debate, 2nd Edition, 2022, p. 261), (Scotese, Song, Mills, & Meer, 2021), (Liang, Czaja, Graversen, & al., 2018), and (Barry, Craig, & Thuburn, 2002) ↑
   

   
   
7. 
   

   
   (Wijngaarden & Happer, 2020) and (Pierrehumbert, 2011) ↑
   

   
   
8. 
   

   
   The tropics are mostly covered by oceans and oceans absorb almost all incident solar radiation. Their albedo is small, meaning they reflect less energy than land or clouds. ↑
   

   
   
9. 
   

   
   Net energy flux is just the incoming solar energy – the outgoing (to space) energy, averaged by latitude. ↑
   

   
   
10. 
    

    
    A petawatt is 10¹⁵ Watts or a billion megawatts. ↑
    

    
    
11. 
    

    
    (Liang, Czaja, Graversen, & al., 2018) ↑
    

    
    
12. 
    

    
    (Sud, Walker, & Lau, 1999) and (Hoffert, Flannery, Callegari, Hsieh, & Wiscombe, 1983) ↑
    

    
    
13. 
    

    
    Humid air is less dense than dry air due to the low density of water vapor. ↑
    

    
    
14. 
    

    
    (Sud, Walker, & Lau, 1999), (Newell & Dopplick, 1979), and (Hoffert, Flannery, Callegari, Hsieh, & Wiscombe, 1983) ↑
    

    
    
15. 
    

    
    (Sud, Walker, & Lau, 1999) ↑
    

    
    
16. 
    

    
    (Hoffert, Flannery, Callegari, Hsieh, & Wiscombe, 1983) ↑
    

    
    
17. 
    

    
    (Scotese, Song, Mills, & Meer, 2021, Fig. 9B) ↑
    

    
    
18. 
    

    
    (Scotese, Song, Mills, & Meer, 2021) ↑
    

    
    
19. 
    

    
    A Köppen belt is a latitude band around the world that contains a set of diagnostic fossils and rocks that are characteristic of a climatic zone. For example, tropical rainforests have diagnostic fossils and rocks, as do deserts, temperate grasslands, and ice-covered polar regions. By dividing the world into diagnostic climatic zones, a pole-to-equator gradient can be constructed which can be translated into a global average temperature using Figure 4. See (Scotese, Song, Mills, & Meer, 2021) for more details. ↑
    

    
    
20. 
    

    
    (Shepherd, 2014) ↑
    

    
    
21. 
    

    
    (IPCC, 2021, pp. 452-454), especially see figure 3.13. ↑