by Javier Vinós & Andy May

“No philosopher has been able with his own strength to lift this veil stretched by nature over all the first principles of things. Men argue, nature acts.”
Voltaire (1764)

6.1 Introduction

Climate is a thermodynamic process determined by the energy flux from its entry point, mostly at the top of the atmosphere (TOA) of the tropics on the day side of the planet, to its exit point distributed across the TOA of the entire planet. The Earth’s infrared emission depends on the absolute temperature scale, and on this scale the planet’s surface temperatures occupy a narrow range. The average outgoing longwave radiation (OLR) emission of the planet is c. 240 W/m2 and the all-sky average for most of the surface is in a relatively narrow 200–280 W/m2 range (Dewitte & Clerbaux 2018). OLR is determined more by the irregular distribution of atmospheric water (cloud and humidity) than by surface temperature. The cloud effect on OLR can reach –80 W/m2 (negative values mean cooling) in some equatorial areas. Thus, while 62 % of the energy enters the climate system over 25 % of the Earth’s TOA area (the 30°N-S daytime side), its exit is much more evenly distributed over the entire TOA area.

From a thermodynamic point of view the main feature of Earth’s climate is the transport of energy. Energy transport is the cause of all weather. Most of the solar energy that is not reflected is stored in the oceans, where most of the climate system energy resides. But the oceans are not good at transporting energy (see Fig. 3.4). Differences in water temperature tend to cause vertical movements through altered buoyancy, not lateral movements, and the oceans are temperature stratified, seriously limiting vertical energy transport. Most of the energy in the climate system is transported by the atmosphere, and even a great part of the energy transported by surface ocean currents is wind driven, as ocean circulation is not thermally, but mechanically driven (Huang 2004). The flux of non-solar energy at the atmospheric-ocean boundary (including across sea-ice) is almost always, almost everywhere, from the ocean to the atmosphere (Yu & Weller 2007; Schmitt 2018).

In a simplified form the climate can be understood as solar energy being received and stored by the ocean, and then transferred to the atmosphere for transport and ultimately discharged to space. This energy transfer powers the water cycle creating clouds, rain, snow, storms, and all-weather phenomena. The system is never in equilibrium, nor can it be expected to be. Over the course of a year, the Earth’s surface warms by c. 3.8 °C and cools by c. 3.8 °C (see Fig. 3.1), varying from year-to-year about 0.1–0.2 °C. The Earth is constantly warming or cooling at all timescales.

Thermodynamically, climate change involves changes in energy gain, energy loss, or both. A rapid change in energy distribution within the climate system can also be a cause for climate change, and it has been known to happen in the past under special circumstances, like the abrupt release of meltwater from the Lake Agassiz outburst 8,200 years ago (Lewis et al. 2012), or the Dansgaard-Oeschger events, when ocean-stored energy was abruptly released to the atmosphere in the Nordic Seas basin during the last glacial period (Dokken et al. 2013). These changes were temporary because climate can only change long-term through a change in the energy budget of the system.

The modern theory of climate change understands climate thermodynamics but fails to understand the role of energy redistribution. When studying climate variables, scientists normally work with what are called “anomalies;” they are the residual of subtracting the “climatology,” or the average changes over 24-hour days and seasons in the variables studied. This point of view magnifies small interannual variabilities but conceals the much larger seasonal changes. The result is that important seasonal changes in atmospheric and oceanic energy redistribution are usually ignored. The error is compounded because net energy transport within the climate system, if integrated for the entire planet, is zero (energy lost at one place is gained in another). Redistribution of energy by transport processes doesn’t matter to most scientists in terms of changing the global climate. To them, the TOA over the dark pole in winter is no different than the daylight tropical TOA, except in the absolute magnitude of the annually averaged energy flux. This narrow view obstructs a proper understanding of climate change.

Changes in atmospheric greenhouse gases (GHGs) alter TOA energy fluxes and constitute one cause of climate change. Conceptually, climate change is assumed to be due either to an external cause (forcing), or to internal variability. Fig. 6.1 shows a schematic representation of the climate system with many important subsystems and processes. Anything that is not affected by Earth’s climate system is considered external, although the distinction is not absolute. For example, volcanoes are often external to the climate system, however it is known that their frequency responds to changes in sea level and icesheet unloading during deglaciations (Huybers & Langmuir 2009). Forcings cause climate change, and feedbacks can cause the amplitude of the changes to increase or decrease. If the feedback amplifies the forcing effect it is positive, if it dampens the climate change, it is negative. It becomes confusing because the same factor can be both a feedback, if produced naturally in response to climate change, and simultaneously a forcing if produced by humans. Several GHGs are like that.

Fig. 6.1. Simplified schematic representation of Earth’s climate system. Different subsystems are shown with different background colors. Climatic phenomena and processes affecting climate are in white boxes. Subsystems and phenomena within the central pearl-colored box are generally considered internal to the climate system.

In Figure 6.1, everything outside the “Internal” pearl colored box is normally not affected by climate (with some exceptions) and is considered external. Some important properties or phenomena at the interface between subsystems are placed in the outside boxes. The Latitudinal (Equator-to-Pole) Temperature Gradient is a central property of the climate system that changes continuously and defines the thermal state of the planet (Scotese 2016). For simplification, lines joining related boxes have been omitted. Bold names in red are variables affecting the radiative budget and are almost exclusively responsible for Modern Global Warming according to the IPCC. The figure is from Vinós 2022.

The most important GHG due to its abundance is water vapor. Unlike CO2 or methane, water vapor is a condensing GHG and it is not well mixed. Water vapor is very unevenly distributed around the planet, and its distribution changes with time. The lowest concentration of water vapor occurs in the polar regions during winter. The radiative properties of different regions of the planet cannot be the same if their GHG content is different. It follows that transporting energy from a higher GHG-content region to a lower one increases outgoing radiation efficiency, and therefore, changes in transport must alter the global energy flux budget at the TOA and, as a result, cause climate change. At present that cause is not being considered. Evidence suggests it is the main cause for climate change at all timescales from decades to millions of years. Planetary thermodynamics requires that energy transport is mostly from the equatorial region toward both poles in the direction of the meridians, so the flow is termed meridional transport (MT).

6.2 Meridional transport is geographically determined and gradient powered

The energy that the atmosphere gains from the oceans is mainly in the form of latent heat. Longwave radiation transfer is roughly half as large, and sensible heat flux is an order of magnitude less (Schmitt 2018). Atmospheric transport of that energy is greatly diminished by the presence of continents and mountain ranges through precipitation and wind speed reduction. As a result, MT takes place mainly over the ocean basins and is, therefore, geographically determined. This has huge implications for weather, climate, and climate change.

In the physical universe processes tend to happen spontaneously along gradients, whether they are gradients in mass, energy, or any manifestation of them, like gravity, pressure, or temperature. The gradient that powers MT is the latitudinal temperature gradient (LTG), its primary cause. The LTG is a product of the latitudinal insolation gradient (LIG, the unequal distribution of incident solar radiation by latitude), modulated by the effect of geographic and climate determinants. The LTG is steeper towards the South Pole (see Fig. 3.3b), despite an annually symmetrical LIG with respect to the equator. Antarctica’s unique geographic and climatic conditions, and the large area covered by the Southern Hemisphere oceans, make the southern LTG steeper than the northern. Cionco et al. (2020a; 2020b) discuss neglected changes to the LIG at different latitudes during the Holocene, and high frequency variations in LIG due to the 18.63-yr lunal nodal cycle that are likely to affect climate.

Milanković’s 1920 proposal that the climate of the Earth is altered by orbital changes has its basis in differences in the amount of energy received by the planet (eccentricity), but more importantly on differences in the latitudinal and seasonal distribution of the energy (obliquity and precession). These changes in the distribution of the energy alter the LIG, which changes the LTG, which changes the MT of energy. It has long been debated how the obliquity signal that paces interglacials (Huybers & Wunsch 2005), affects the tropics (Rossignol-Strick 1985; Liu et al. 2015) where the energy changes due to obliquity are very small. The answer appears to be that obliquity-induced changes in the LIG (Bosmans et al. 2015) affect MT.

Summer LIG is affected by changes to Earth’s axial tilt caused by the 41 kyr obliquity cycle and by the 18.6 yr lunar cycle. The winter LIG varies with the level of insolation falling on low latitudes, since high latitude insolation, near the winter pole, is minimal (Davis & Brewer 2011). The changes in the level of insolation at low latitudes are due to Earth’s wobble (21 kyr precession cycle), the distance to the sun (95 and 125 kyr eccentricity cycles) and by changes in solar activity (11 yr and longer solar cycles). Davis and Brewer (2011) have shown that the LTG is very sensitive to changes in the LIG. It is unknown why this hypersensitivity exists. The authors discuss the Kleidon and Lorenz (2005) proposal that MT adjusts itself to produce maximum entropy (Fig. 6.2).

Fig. 6.2. The proposition that meridional transport adjusts itself to produce maximum entropy. The latitudinal temperature gradient, resulting from the difference between tropical (continuous line) and polar (dashed line) temperatures is represented by the gray area. Entropy production (dotted line) is minimal when there is no transport of energy (left side of abscissa), or when transport is so efficient that there is no temperature difference (right side of abscissa), and maximal at some point in between. After Kleidon and Lorenz (2005).

Kleidon and Lorenz (2005) claim that MT dependence on maximum entropy production has been confirmed by simulations with general circulation models. They are obviously wrong, as computer models only constitute scientific evidence of human programming skills. That MT adjusts automatically to maximum entropy production requires a very large number of degrees of freedom (possible outcomes), and as reviewed in part V (Sec. 5.2) MT is modulated by multiple factors that are not well represented in computer models, which reduces the degrees of freedom. It is very likely that the adjustment of the LTG to the LIG is driven in part by entropy, but the Winter Gatekeeper hypothesis (WGK-h; see part V) explains how the LIG can affect the LTG by directly acting on MT. It is important to keep in mind that if the LTG can change MT, the opposite must also happen, so the causality of the changes might be difficult to determine.

The WGK-h provides an explanation for the hypersensitivity of the LTG to changes in the LIG due to changes in solar activity, but not by other causes such as lunar or orbital changes. Within the evidence that the LIG responds to the lunar 18-yr cycle and the solar 11-yr cycle (Davis & Brewer 2011), it is interesting to see that the stadium-wave multidecadal oscillation in MT could be pulsating at the rhythm marked by the interference between the lunar 9-yr half-cycle and the solar 11-yr cycle (Vinós 2022; see Fig. 4.8f). If real, changes in the LIG resulting from this interference provide a mechanism by which the stadium-wave period and strength are determined, i.e., the changes in the LIG result in changes in MT that ultimately shape the stadium-wave.

While the LIG determines the distribution of the energy input to the climate system at the TOA, 29 % of that energy is returned to space by atmospheric and surface albedo. Reflected solar energy is highest during Jan-Mar due to SH cloud albedo, while OLR is highest Jun-Aug due to higher emission during NH summer (Fig. 6.3). The result of these differences is that the planet is colder during the boreal winter, when it is closest to the sun and receiving 6.9 % more energy (see Sect. 3.1 & Fig. 3.1).

There are very important differences between the hemispheres regarding climate energy and transport. As figure 6.3a shows, outside the tropics OLR essentially follows temperature. Within the tropics OLR and temperature show inverse correlation, as higher temperature leads to increased cloud cover and less emission. According to the modern theory of climate change the increase in GHGs results in the same IR emission to space taking place from a higher, colder altitude, requiring surface warming to maintain the energy balance. The Earth must emit the same energy it receives, not more, unless it is cooling. Under this model inter-annual OLR from the TOA should not change unless there is a change in incoming solar energy or in albedo.

Albedo has been very constant since we have been able to measure it with sufficient precision, with an inter-annual variability of 0.2 Wm–2 (0.2 %; Stephens et al. 2015), and solar energy, termed the solar constant, varies by only 0.1 % (Lean 2017). Yet, OLR inter-annual changes are ten times higher than GHG radiative forcing changes. What is worse, the inter-annual changes in OLR are neither global, nor follow temperature changes (Fig. 6.3b). While extratropical SH OLR shows no trend over the last four decades, and tropical OLR shows a small and insignificant trend, the extratropical NH OLR displays a very strong increase. Is this increase due to the higher warming experienced by the NH? According to the data it is not, because during the 1980s and 90s when accelerated warming took place OLR did not increase significantly, while between 1997-2007, when the Pause was taking place, extratropical NH OLR underwent most of the increase of the past four decades (Fig. 6.3b grey area). It logically follows that the negative anomaly in extratropical NH OLR before 2000 contributed to the warming, while the positive anomaly afterwards contributed to the Pause. Obviously, the increase in GHGs cannot explain any of this, but the changes in MT that took place at the 1997-98 climate shift have no problem explaining the coincident changes in OLR at the extratropical NH (see Part IV).

Fig. 6.3. Outgoing longwave radiation yearly and inter-annual changes.

In Figure 6.3, (a) shows monthly changes in TSI (dotted orange curve without scale); the data are from Carlson et al. 2019. The monthly changes in average temperature are shown using red curves (left scale); global average temperature is the thick continuous red curve, the NH is the thin continuous red curve, and the SH is the thin dashed red curve. The data are from Jones et al. 1999. The monthly changes in OLR are shown as black curves and use the right scale; the global outgoing radiation is the thick continuous black curve, 30–90°N is the thin continuous black curve, 30–90°S is the thin dashed black curve, 30°S–30°N is the thin dotted black curve. The data are from KNMI explorer. The grey area is the NH winter period.

Figure 6.3 (b) shows the 1979–2021 changes in the OLR anomaly for the 30–90°N (thick continuous black curve), 30–90°S (thick dashed black curve), and 30°S–30°N (thick dotted black curve) regions. Corresponding thin lines are their least-squares trends. The grey area corresponds to the 1997–2006 period that displayed accelerated Arctic climate change as described in section 4.5. The data are from the KNMI explorer section for NOAA OLR.

One of the most puzzling aspects of climate is that, despite very different land, ocean, and snow/ice surface extensions, both hemispheres have essentially the same albedo. This phenomenon is known as hemispheric albedo symmetry (Datseris & Stevens 2021). Models fail to reproduce such a crucial aspect of the climate, because nobody knows how it is produced and maintained (Stephens et al. 2015). Datseris & Stevens (2021) have identified cloud asymmetries over extratropical storm tracks as the compensating factor of the surface albedo asymmetries. Storm tracks are MT highways over already MT-favored oceanic basins. Storms are the product of baroclinic instability along the LTG and transport a great amount of energy as latent heat. They are also responsible for a significant part of global cloudiness, linking MT to cloud cover. Changes in MT must necessarily result in changes in cloudiness, altering the climate. If the albedo of the Earth is kept symmetrical by changes in storm track cloudiness, albedo is probably another fundamental climate property linked to the strength of MT.

6.3 ENSO: The tropical ocean control center

The climate system is composed of the oceans, land surface, biosphere, cryosphere, and atmosphere (Fig. 6.1). These different components exchange mass and energy, but for the climate system as a whole, energy gains and loses take place at the TOA. Parts of the TOA where the energy gain/loss ratio is positive, mainly above the tropics, constitute an energy source for the climate system, while the rest of the TOA acts as an energy sink. The biggest energy sink is the TOA above the winter pole. On average, energy enters the system at the source and is passed from climate component to climate component as it is transported towards the sink. The flux of energy through the climate system is characterized by both temporal and spatial variability. As a result, the amount of energy in transit through any element of the transport system changes over time, altering the enthalpy (total “heat” content) of the element, often observed as a change in temperature. We infer the regulation of MT by certain control centers that constitute energy gateways into and out of the climate system. These MT control centers are the polar vortex (PV), ENSO system, and the ozone layer. Their conditions change in response to changes in the main modulators of MT, resulting in changes in global energy transport.

The absorption of solar energy in the tropics is spectrally differentiated. The 200–315 nm part of the spectrum is absorbed in the stratospheric ozone layer, while the 320–700 nm part is mainly absorbed in the photic layer of the tropical oceans. The energy absorbed by the ocean is transported poleward in three different ways (Fig. 6.4). Part of it reaches the stratosphere through convection and constitutes the ascending branch of the Brewer-Dobson circulation, another part is transported in the troposphere by the Hadley circulation, and the last part is transported by the ocean. The ENSO state dictates the relative distribution of the energy to be transported. La Niña favors oceanic transport, while ENSO Neutral increases atmospheric transport. At certain times the amount of energy to be transported exceeds capacity and an El Niño is triggered.

El Niño directs a great amount of energy towards the stratosphere and troposphere, extracting it from the ocean and warming the surface of the planet in the process. During the Holocene Climatic Optimum (9–5.5 ka) the planet was warmer, MT was reduced as a consequence, and it resulted in a very reduced frequency of Los Niños (Moy et al. 2002). During the Neoglacial Period (since 5.2 ka) the frequency and intensity of Los Niños increased. In periods of planetary cooling, more energy must be transported poleward as part of the cooling process, which explains the increase in Los Niños from 1000–1400 AD as the world descended into the Little Ice Age (LIA; Moy et al. 2002). Over the past two centuries El Niño frequency has been low and trending lower because the planet is warming, and this is accomplished by reduced MT. At present El Niño conditions are produced by accumulation of subsurface warm water (the main El Niño predictor, see Fig. 2.4c) or by a decrease in the Brewer Dobson circulation in response to a stronger PV during the first boreal winter after tropical or NH stratospheric-reaching volcanic eruptions (Kodera 1995; Stenchikov et al. 2002; Liu et al. 2018).

Fig. 6.4. Northern Hemisphere winter meridional transport outline.

A shown in Figure 6.4, the energy gain/loss ratio at the TOA determines the maximal energy source at the tropical band and the maximal energy sink at the Arctic in winter. Incoming solar energy is distributed in the stratosphere and troposphere/surface where it is subjected to different transport modulation. Energy (white arrows) ascends from the surface to the stratosphere at the tropical pipe (left dashed line) and is transported towards the polar vortex (right dashed line) by the Brewer–Dobson circulation. Stratospheric transport is determined by UV heating at the tropical ozone layer, that establishes a temperature gradient affecting zonal wind strength through thermal wind balance, and by the QBO. This double control determines the behavior of planetary waves (black arrows) and determines if the polar vortex undergoes a biennial coupling with the QBO (BO).

At the tropical ocean mixed-layer ENSO is the main energy distribution modulator. While the Hadley cell participates in energy transport and responds to its intensity by expanding or contracting, most energy transport in the tropics is done by the ocean. Changes in transport intensity result in the main modes of variability, the AMO, and PDO. Outside the tropics most of the energy is transferred to the troposphere, where synoptic transport by eddies along storm tracks are responsible for the bulk of the transport to high latitudes. The strength of the polar vortex determines the high latitudes winter climate regime. A weak vortex promotes a warm Arctic/ cold continents winter regime, where more energy enters the Arctic exchanged by cold air masses moving out. Jet streams (PJS, polar; TJS, tropical; PNJ, polar night) constitute the boundaries and limit transport. Figure 6.4 is from Vinós 2022.

It is clear that ENSO strongly affects the MT of energy. It is therefore surprising that it is considered a climate fluctuation (Timmermann et al. 2018). Its location at the entry point of most of the energy into the climate system makes it a control center for MT that is modulated by solar activity (see Fig. 2.4). It is well known that ENSO responds to stratospheric conditions (e.g., volcanic eruptions) and subsurface conditions (warm water volume), thus linking MT at different levels. Paleoclimatology shows it responds to planetary thermodynamics, i.e., it is related to how the planet warms and cools. As Moy et al. (2002) say:

“We observe that Bond events tend to occur during periods of low ENSO activity immediately following a period of high ENSO activity, which suggests that some link may exist between the two systems.”

Moy et al. (2002)

Bond events are century-long cold periods, like the LIA, that are brought about in part by strongly increasing ENSO activity (frequent, strong Niños). After the planet stops cooling ENSO activity decreases.

6.4 Ozone: The tropical stratosphere control center

The 200–315 nm part of the solar energy spectrum is absorbed in the stratospheric ozone layer, where it has a large effect on temperature and circulation. Although the energy at that wavelength range only amounts to slightly over 1 % of the total (Lean 2017), it varies with solar activity ten times more than the >320 nm range and is responsible for the radiative and dynamic changes that take place in the stratosphere during the solar cycle. UV energy absorption in the stratosphere is on average 3.85 W/m2 (Eddy et al. 2003; one fourth of 15.4 W/m2). This is not a small amount. It constitutes 5 % of the solar energy absorbed by the atmosphere (Wild et al. 2019). The ozone control center handles a significant part of the energy received by the climate, despite being just the UV energy portion.

The stratosphere is c. 5 times larger than the troposphere and contains c. 5 times less mass. With a density over an order of magnitude lower, the effect of the absorbed solar energy on stratospheric temperature is huge. Without ozone the stratosphere would be 50 K colder and the tropopause would not exist (Reck 1976). The ozone layer is a peculiarity of the Earth, as a result of atmospheric oxygenation, that probably developed during the Ediacaran or Cambrian, some 600–480 Ma.

Ozone absorption of solar energy in the stratosphere allows the formation of a stratospheric LTG that depends on UV energy, ozone amount, and ozone distribution. The gradient forms through shortwave heating of ozone and radiative longwave transfer involving mainly CO2 and ozone. Along this gradient the zonal wind circulation is established by the balance between the pressure gradient and the Coriolis factor (geostrophic balance). As a result, stratospheric circulation is opposite in both hemispheres, with the winter hemispheric circulation characterized by westerly winds and the formation of a polar vortex (see Fig. 3.7).

Planetary waves generated at the troposphere can only propagate upwards when stratospheric winds are westerly and of a certain velocity range (Charney-Drazin criterion). These conditions are present in winter, and as a result winter stratospheric circulation is more perturbed (Haynes 2005), resulting in higher MT. Planetary waves are generated more efficiently by orography (the location mountains) and land/ocean contrasts, they are more frequent in the boreal winter. Planetary waves deposit energy and momentum in the stratosphere when they break, and occasionally are deflected downward towards the troposphere affecting circulation there. Their effect in the stratosphere is to drive meridional circulation, reduce westerly circulation, and weaken the polar vortex. As a result, stratospheric MT, known as the Brewer Dobson circulation, depends on the wave flux.

In extreme cases planetary waves reduce winter westerly circulation so much as to make the zonal circulation easterly, causing sudden stratospheric warming as air is forced down and warms adiabatically, while the vortex splits or is displaced away from the pole. This happens about every other year in the NH, but rarely in the SH, and has great repercussions for tropospheric weather. Changes that take place in the winter stratosphere affect weather on the surface on a longer timescale due to stratospheric-tropospheric downward coupling. Unambiguous observations of stratospheric variability affecting the surface show up in the Arctic Oscillation (Northern Annular Mode), North Atlantic sea-level pressures, extreme weather events, the frequency of winter cold spells, the position of the tropospheric mid-latitude jet, and low frequency variations in the Atlantic thermohaline circulation (Baldwin et al. 2019). Stratospheric variability partly controls the tropospheric heat flux into the Arctic (Baldwin et al. 2019), showing that ozone response to solar radiation in the stratosphere acts as a major control center for MT.

Stratospheric circulation and variability are the result of ozone and its response to solar energy. Furthermore, the stratosphere, itself, is the result of ozone. Solar UV energy has two separate roles in the stratosphere. Through photolysis of oxygen and ozone it regulates the amount of ozone, and through radiative heating it regulates the stratospheric LTG which sets up stratospheric circulation and its response to planetary wave flux. The effect of wave flux on the Brewer Dobson circulation (i.e., stratospheric MT) has been termed the “extratropical pump” (Haynes 2005). As a result, the ozone control center participates in the modulation of MT of energy and is sensitive to changes in solar activity through photolysis and shortwave radiative heating rates (Bednarz et al. 2019). The body of evidence on the impact of solar variability on tropospheric climate through changes in the state of the stratosphere has significantly grown in the last few decades (Haigh 2010).

6.5 The polar vortex control center

Together with sea-ice, the PV constitutes a negative feedback to planetary cooling. It forms due to strong cooling in the polar autumn because of very low insolation and sea-ice formation. Atmospheric cooling increases air density, and as the cold air sinks it creates a low-pressure center with cyclonic (counterclockwise in the NH) circulation around the pole. As the westerly winds become stronger, they isolate the interior of the vortex where radiative cooling continues. The strong winter temperature contrast drives the zonal wind circulation that stabilizes the vortex until the sun returns. Without a PV (and sea-ice) the planet would lose a lot more energy every winter. It is thus trivially evident that a strong PV favors planetary warming, and a weak PV favors planetary cooling. The PV is a product of winter zonal circulation. Since MT is driven by meridional circulation that takes place at the expense of zonal circulation, the PV constitutes one of the main MT control centers. It regulates energy access to the biggest energy sink in the planet, the winter polar TOA (see Fig. 3.2).

The discovery of the PV response to the equatorial Quasi-Biennial Oscillation (QBO; Holton & Tan 1980) shows that the PV is not solely the result of high latitude atmospheric conditions. It was later found that PV conditions also responded to the solar cycle (Labitzke 1987), even though the sun doesn’t shine above the pole in winter. After the Pinatubo eruption it became clear that the PV was also affected by stratosphere-reaching volcanic eruptions (Stenchikov et al. 2002; Azoulay et al. 2021), resulting in volcanic winter warming at mid-high latitudes instead of the expected cooling due to solar energy reduction from stratospheric aerosols. It is clear now that the PV responds to changes in the stratospheric LTG and to changes in the propagation of planetary waves in the stratosphere. Planetary waves deposit energy and momentum close to the vortex in the winter stratosphere which weakens the strong potential vorticity gradient of the vortex. Vortex dynamics cause wave perturbations to travel downwards making the vortex more susceptible to successive lower altitude waves and propagating the effect to the troposphere (Scott & Dritschel 2005). This provides an explanation for the stratosphere-troposphere downward coupling at high latitudes.

Thus, PV strength is the result of equatorial-polar gradients in temperature, zonal wind speed and potential vorticity that determine planetary wave effect on the zonal flow (Monier & Weare 2011). PV strength also depends on upward wave activity (Lawrence et al. 2020). As we have seen (Sects. 4.7 & 5.4; Christiansen 2010), PV strength experiences inter-annual and multidecadal oscillations that affect the Arctic Oscillation and surface weather events, like the frequency of severe winter cold air outbreaks (Huang et al. 2021).

Multidecadal changes in PV strength have confused atmospheric scientists for a long time (Wallace 2000). Multidecadal periods when the polar vortex is stronger than average result in the Arctic, Atlantic, and Pacific sectors behaving as a true Northern Annular Mode (NAM; Fig. 6.5a & c), with a seesaw relationship between the Aleutian and Icelandic Lows (Shi & Nakamura 2014), restricting heat and moisture transport into the Arctic. In contrast, multidecadal periods when the polar vortex is weaker than average result in a situation best described by the North Atlantic Oscillation (NAO; Fig. 6.5b), with weak Aleutian Low interannual variability and less restricted Arctic transport. The scientific literature discussions about whether the NAO or the NAM paradigms better describe the main NH extra-tropical atmospheric mode of variability (Wallace 2000), appear to ignore that its changing nature is linked to climate regime shifts (see Part IV) that characterize climate change.

Fig. 6.5. The shifting nature of the Northern Annular Mode/North Atlantic Oscillation.

The three maps in Figure 6.5 are the first empirical orthogonal function of winter-mean SLP anomalies over the extratropical Northern Hemisphere (poleward of 20°N) for three 25-yr periods, whose central years are noted above the maps. Color interval is for 1.5 hPa (positive in red), and zero lines are omitted. The polarity corresponds to the positive phase of the Arctic Oscillation. A true northern annular mode requires the coordination of the three centers of action, otherwise it can be better described as a North Atlantic Oscillation. Figure 6.5 is after Shi and Nakamura 2014.

The PV regulates the exchange of air masses, moisture, and energy between the mid-latitudes and the polar latitudes. It responds to tropospheric climate shifts, stratospheric conditions, and is affected by the propagation and reflection/absorption of planetary waves. It is modulated by solar activity, ENSO, the QBO, and volcanic eruptions, constituting a control center for MT.

6.6 Multidecadal modes: The state of meridional transport

Nearly all the energy and all the moisture transported poleward takes place in the troposphere and upper ocean. As the intensity of this transport varies geographically over time it gives rise to what has been termed modes of climate variability. These modes of variability have fluctuated in the 20th century with a c. 65-yr multidecadal oscillation that produces observed shifts in climate regimes. This oscillation, termed here the stadium-wave (Wyatt & Curry 2014), was detected in global sea-surface temperature (SST), and has been observed in North Atlantic sea level pressure and winds (Kushnir 1994), North Pacific and North American temperature (Minobe 1997), length of day and core angular momentum (Hide et al. 2000), fish populations (Mantua et al. 1997; Klyashtorin 2001), Arctic temperature and sea ice extent (Polyakov et al. 2004), the relative frequency of ENSO events (Verdon & Franks 2006), and global mean sea level (Jevrejeva et al. 2008).

The stadium-wave reflects global MT system variability. The oscillation mostly affects the two ocean basins that communicate directly with both poles, particularly from the equator (ENSO) to the NH high latitudes, and it affects the rotation of the Earth through changes in the angular momentum of the atmosphere (Hide et al. 2000; Klyashtorin & Lyubushin 2007), showing the coupled response of the ocean and the atmosphere. The multidecadal oscillations in SST (Atlantic multidecadal and Pacific decadal oscillations, AMO and PDO) are simply a reflection of the MT energy flux through these elements. Since the amount of energy entering the climate system on an annual basis is nearly constant, the warm phases of the AMO and PDO simply reflect a slowdown in MT causing an energy flux traffic jam. More energy resides at that time in those elements, perhaps due to a reduced ocean-atmosphere flux caused by a predominantly zonal wind pattern in the mid-latitudes. The AMO spatial pattern, obtained by regression of North Atlantic SST anomalies after subtracting the global SST anomalies, reveals that the AMO is the Atlantic portion of a global MT system that moves heat poleward. The global system includes the Pacific and Indian basins, as shown in Fig. 6.6. It shows that the NH SST oscillation of the AMO is phase-locked with other global SST oscillations, reflecting coordinated changes in the global MT system.

Fig. 6.6. Atlantic multidecadal oscillation spatial pattern. Unitless (°C/°C) regression pattern of monthly SST anomalies (HadISST 1870–2008), after subtracting the global mean anomaly from the North Atlantic SST anomaly.

Fig. 6.6 displays the °C of SST change per °C of AMO index. Besides displaying the AMO pattern, it shows that AMO is linked to the global surface MT system that extracts heat from the tropics in the main ocean basins. Figure 6.6 is after Deser et al. 2010.

This global MT system is the complex result of the geographically determined coupled atmosphere-ocean circulation in a rotating planet with its axis tilted in relation to the ecliptic, that receives most of its energy in the tropics. Since the transport intensity varies through time and space, authors typically focus on describing its regional variability, and talk about teleconnections and atmospheric bridges to try and explain what are, in essence, elements of a single very complex process (Fig. 6.7). The importance of MT for the planet’s climate cannot be overstated and multidecadal changes in MT are an important and overlooked factor in climate change. It is a common assumption that the sum of multidecadal variability effects over time trends to zero. Studies on the change in the AMO amplitude over the past six centuries (Moore et al. 2017) show this assumption is ill-conceived.

Fig. 6.7. Meridional transport is the overlooked climate factor.

Meridional transport is both the elephant in the room that everybody ignores as an explanation for climate change, and the elephant from the Indian tale that blind people describe as a different animal when touching different parts of it.

The stadium-wave has a period long enough to have made an important contribution to Modern Global Warming. According to Chylek et al. (2014) one third of the post-1975 global warming is due to the positive phase of the AMO, and models overestimate GHG warming but compensate for it by overestimating aerosol cooling. Regardless of the evidence, the IPCC does not consider that internal variability has contributed significantly to climate change between 1951– 2010 (see Fig. 5.1). An alternative view is that a combination of solar activity and a 65-yr oscillation, if allowed an unconstrained contribution, can explain a great part of the increase in the global warming rate over the 20th century, with residual changes attributable to the CO2 increase and volcanic activity. That view requires the admission that our current estimate of climate sensitivity, to the different known forcings, is erroneous. This is a possibility supported by dynamical systems identification (de Larminat 2016).

As shown in the Fig. 5.2 flow diagram, solar activity affects stratospheric transport directly, and tropospheric transport indirectly, mainly through ENSO. The stadium-wave governs tropospheric transport as an emergent resonant phenomenon. When both act in the same direction the effect is maximal, as happened during the 1976–1997 period when both worked to reduce MT and warm the globe. During the 1890–1924 period both worked to enhance MT, which caused global cooling. But at times they are out of step and in these periods the stadium-wave has a bigger effect because tropospheric transport is much stronger. During the 1924–1935 period, solar activity was low, but the stadium-wave was in the warming portion of its cycle, resulting in early 20th century warming. During the 1945–1976 period, solar activity was high, but the stadium-wave was set on cooling, and cooling resulted due to high MT. In those periods where solar activity and the stadium-wave have an opposite effect, the stadium-wave effect predominates because it is larger, but the effect isn’t as strong as when they cooperate in increasing or decreasing MT. MT is the real “control knob” of climate change.

During the 20th century, the stadium wave 65-year oscillation had two warming periods, for a total of about 65 years in the warm mode. Solar activity displayed the c. 70-year long Modern Solar Maximum (1935–2005). This means that both natural forcing and internal variability spent most of the century contributing to the observed warming. The unusual coincidence of such long periods of natural contribution helps explain why the early 20th century warmed in the absence of significant GHG emissions, and why so much warming was observed in that century as to raise alarm. The natural contribution to observed warming comes at the expense of reducing the anthropogenic contribution.

6.7 Meridional transport as the main climate change driver

The search for the solar effect on climate leads us to an unexpected conclusion about how the climate changes. For solar variations to influence climate change, it is necessary that the climate control knob be MT. The two gigantic polar cooling radiators of the Earth are fed energy through MT. As a result, MT is responsible for most climate change at all timescales. The drivers of MT change depending upon the time frame being considered.

  • At the inter-annual scale, the noise is high, but change is governed by ENSO and short-term phenomena like volcanic eruptions through their effect on PV strength and MT.
  • At the multidecadal scale climate change is governed by the stadium-wave and all its parts, causing climate regime shifts in MT.
  • The centennial to millennial scale is the solar realm. The sun reigns in climate change through its secular cycles in solar activity, acting through long-term changes in MT, particularly during solar grand minima, but also during extended maxima like the modern solar maximum.
  • In the multi-millennial scale Milankovitch rules. The orbitally induced changes in the LIG cause changes in MT. As obliquity decreases, it increases insolation in the tropics and decreases it at the poles. This steepens the LIG during the summers, increasing MT, which drives the required heightened moisture to the high latitudes. The moisture will remain locked there, as ice and snow, until the process reverses. This is how the necessary moisture reaches the high latitudes during glaciations (Masson-Delmotte et al. 2005). Later, when obliquity increases, MT becomes more restricted, contributing to the mid-latitudes warming during deglaciations. Obliquity’s strong climatic signature in the tropics has been linked to meridional transport (Bosmans et al. 2015).
  • At the largest time scale, it is plate tectonics that governs climate change by facilitating or restricting tropical heat access to the two polar radiators. Multi-million-year Earth cooling results when ocean-atmosphere meridional circulation is favored, and zonal circulation is restricted. Zonal wind restrictions are caused by the position of continents, ocean gateways, and mountain ranges, that increase poleward (meridional) heat transport. Multi-million-year Earth warming results when the opposite happens.

It is generally accepted that MT keeps the poles warmer than they should be otherwise. Without MT the poles would be 100 °C colder than the equator on average, instead of 40 °C (Lindzen 1994). But in part III (Sec. 3.2) we reviewed the “low gradient paradox,” and said a possible solution would be offered in this part. This paradox arises from the climate of the early Eocene, the Cretaceous, and early Paleogene, characterized by a warm world with a reduced LTG and low seasonality (Huber & Caballero 2011). Such equable climates cannot be explained by modern climate theory without resorting to extreme CO2 levels and implausibly high tropical temperatures. At the root of the equable climate problem lies the low gradient paradox (Huber & Caballero 2011). For the poles to be warm all year around more energy from the tropics is required. Yet since the poles are warm all year, the LTG must be very flat, which results in less energy transport.

The paradox is only apparent because, as we have seen in parts III to V, the more energy directed toward the poles the colder the planet gets, so it was actually the low gradient that kept the planet and the poles warm during equable climate eras. The planet has been in the Late Cenozoic Ice Age for the past 34 Ma because it is hemorrhaging heat at the winter pole from two gigantic cooling radiators. In the early Eocene, heat loss at the winter pole was limited by an intense cloud-, fog-, and water vapor-GHE during the polar night. Warm polar conditions were not the result of more heat transported from the tropical band. The transition from the early Eocene equable climate to the Pleistocene icehouse climate can be explained by changes in MT that resulted in an increase in the amount of energy directed to the poles.

In the early Eocene (52 Ma) the world geography was very favorable to zonal circulation. There was a well-developed circumglobal seaway formed by the Tethys Sea, the Panama Gateway, and the Indonesian Passage (Fig. 6.8a). Connections to the Arctic were through shallow water seaways and across continents which severely restricted MT toward a warm Arctic above freezing all year around. MT toward Antarctica was unimpeded, but it was free of ice and covered by vegetation, with a stronger GHE due to abundant water vapor and clouds, due to global warm conditions.

The Arctic Gateway, between the North Atlantic and the Arctic Oceans, began opening about 55 Ma allowing increased MT toward the North Pole (Fig. 6.8c red box; Lyle et al. 2008). This opening has been proposed as the cause of the long Eocene cooling (Vahlenkamp et al. 2018). As the planet cooled the LTG deepened, driving more energy toward both poles, and was a positive feedback to global cooling. The Tasman Gateway opened between 36 and 30 Ma. At 34 Ma several low amplitude obliquity oscillations coincided in a very unusual configuration (Fig. 6.8d, box) promoting cool summers for 200 kyr. Antarctica had already developed several ice sheets at higher elevations. A tipping point was reached when low eccentricity promoted ice growth at a time when low obliquity amplitude facilitated summer ice survival, triggering Antarctic glaciation in just 80 kyr (Coxall et al. 2005). The glaciation was completed 400 kyr later during another period of low eccentricity. See the Fig. 6.8d grey bands.

Antarctica had an extensive ice sheet for most of the Oligocene, but after the Mid-Oligocene Glacial Interval c. 26 Ma, and until the end of the Mid-Miocene Climatic Optimum at c. 14 Ma (a 12 Myr interval) the planet entered a warm period that apparently nobody can explain. At the same time CO2 levels collapsed, according to proxies (Beerling and Royer 2011), from 450 to 200 ppm. See the Fig. 6.8c blue triangle. This very low CO2 level remained for the entire period except during the time of the Columbia River Flood Basalt flows, which occurred at the CO2 peak at 16–15 Ma. So, during the Late Oligocene to the Mid-Miocene warm period, CO2 changes do not explain temperature changes. Recent research suggests most of this period was characterized by a strongly reduced LTG (Guitián et al. 2019), indicative of reduced MT.

The Drake passage opened around the beginning of that warm period, between 30 and 20 Ma (Lyle et al. 2008), allowing the development of the Antarctic Circumpolar Current and the Southern Annular Mode. The climatic isolation of Antarctica must have hindered MT of heat from the tropics causing regional cooling, yet globally the planet was warming due to reduced MT, and although the Antarctica ice sheet continued to exist, it entered a long period when it waxed and waned following orbital changes (Liebrand et al. 2017). So as the planet warmed, isolated Antarctica developed a warmer and more variable state than during the Middle Oligocene. MT changes can explain the multimillion‐year Late Oligocene to Mid-Miocene warming within the long‐term Cenozoic cooling.

Fig. 6.8. Meridional transport as the main determinant for climate evolution.

Panel (a) in Fig. 6.8 shows how mountain ranges and ocean gateways affect meridional transport in the Cenozoic. Black boxes show the active, well-developed geological features affecting meridional transport. Red boxes show features undergoing development. Vertical arrows indicate meridional transport (global cooling) is favored, and horizontal arrows zonal transport (global warming) is favored.

In Fig 6.8 (b) we see that the Pleistocene world has developed significant geological features that favor meridional transport. The Himalayas reached modern elevation by about 15 Ma. The Indonesian Passage is still open, but significant restrictions developed about 11 Ma. The Bering Strait began its existence about 5.3 Ma, while the Panama Gateway completely closed around 3 Ma. The figure is after Lyle et al. 2008. Red boxes indicate geological changes affecting meridional transport.

In Fig. 6.8 panel (c) the black curve shows global deep-sea δ18O data as a temperature and continental ice proxy. When the upper bar is full it represents ice volume >50% of present, and when dashed, ice volume is ≤50%. The figure is after Zachos et al. 2001. The red curve shows average CO2 data after Beerling & Royer 2011. The blue triangle shows 14 Myr of warming, while CO2 levels are decreasing.

In panel (d) high resolution δ18O changes in benthic foraminiferal calcite, in black, show that Antarctic glaciation took place faster than previously thought and in two steps. The box marks a period of low obliquity amplitude oscillations. The grey bars are periods of low eccentricity during Antarctica glaciation. This panel is after Coxall et al. 2005.

Changes to the global MT state easily explain the climate changes that took place from the Early Eocene to the late Pliocene that CO2 changes cannot. The isolation of Antarctica with the opening of the Tasman and Drake passages was bad for Antarctica but good for the planet, as it limited energy loss at the South Pole by creating a strongly zonal circulation around Antarctica. As a result, the planet warmed. Even today less energy is lost at the South Polar region, despite much colder temperatures and a steeper LTG, than in the Arctic (Peixoto & Oort 1992). From the Early Miocene a series of events took place driving the planet towards its present severe icehouse climate. The Arctic Gateway continued opening and in c. 17.5 Ma the Fram Strait deepened enough to allow deep-water circulation (Jakobsson et al. 2007). The Himalayas reached modern elevation by about 15 Ma, the Indonesian Passage underwent significant restrictions 11 Ma, the Bering Strait appeared about 5.3 Ma, and the Panama Gateway closed around 3 Ma (Lyle et al. 2008). The result was a transformation from a planet characterized by zonal circulation (Fig. 6.8a) into one characterized by meridional circulation (Fig. 6.8b), where more energy is lost from the poles.

6.8 Epilogue

Climate is one of the most complex phenomena to become a subject of popular scientific debate. Feynman (1981) once said of science that:

“we don’t know what’s true, we’re trying to find out, everything is possibly wrong.”

Feynman (1981)

This is especially true for climate science, a very long-term phenomenon, and where a great deal of the critical data is only available for a few decades. The immaturity of climate data is demonstrated by the periodic changes to temperature datasets, that invariably increase the registered warming over time, despite being based on the same original data.

As an example, Fig. 6.9 shows three different releases of the Met Office Hadley Centre global surface temperature datasets over the past 10 years (HadCRUT 3, 4 & 5) for the period 1997-2014 (13-month averaged). While HadCRUT 3 showed no increasing trend, each iteration displayed a bigger trend, and the changes have resulted in almost 0.2 °C of additional warming in just 17 years from the same raw data. It gives a new meaning to anthropogenic warming. At the end of that period the older datasets are outside the confidence limits of the newest and, therefore, no confidence can be placed on those limits. We don’t know how much the planet has warmed even over such a short modern period, much less over the past century.

Scientific studies done with that data expire the moment the old data is periodically superseded and deprecated. This is a situation without precedent in science, a systematic enterprise that builds on solid, not fluid, data. The reliance of climate science on computer models produces a similar effect, as they also expire and are replaced every time a new “improved” model is released. Once the new models come out, the old projections and the “findings” they once supported become invalid.

Fig. 6.9. Three temperature reconstructions from the same temperature data. They are all 13-month centered averages of monthly global average surface temperature anomalies from three datasets for the July 1996–May 2014 period. HadCRUT 3 data is shown as a thick continuous curve with a least-squares trend (thin continuous line). The HadCRUT 4.6 data is shown as a thick dashed curve, with a least-squares trend as a thin dashed line. The HadCRUT 5.0 data is shown as a thick dotted curve, with a least-squares trend as a thin dotted line. The data are all from the UK Met Office Hadley Centre.

No doubt the situation keeps climate scientists employed as the studies need to be done over and over again with new data and computer models. The constantly evolving models and ever-increasing temperature trends do nothing to improve the standing of climate studies among the more serious sciences, where repeating past experiments produce the same result.

Modern climate science has allowed itself to be contaminated by activism without protest. Activist climate scientists are doing a great disservice to science by abandoning Karl Popper’s goal of objective knowledge and allowing themselves to get emotionally involved with their subject and married to a chosen result. The history of science is not kind to scientists that allow themselves to become misguided servants of social or political goals. Lysenkoism and eugenics come to mind as dark examples. As Joel Hildebrand (1957) said of the scientific method,

“there are no rules, only the principles of integrity and objectivity, with a complete rejection of all authority except that of fact.”

Joel Hildebrand (1957)

The question is: Does research in climate science meet the standards of scientific objectivity? This is increasingly important in framing public debates about science and science policy (Tsou et al. 2015).

Over this series, we have presented some of the evidence that solar activity has an outsized effect on climate change, together with a proposed explanation for the observed effect. The scientific literature is full of additional evidence for a solar effect on climate. To deny that evidence can only delay progress in climate science. The search for a solar-climate effect has had the unexpected result of showing that modern climate theory is missing a crucial component. Changes in the poleward transport of energy cause the planet to change its climate state. It appears to be the main climate change driver.

Opposite to what is generally believed, when less energy is transported poleward the planet gets warmer. The planet warmed after 1850 due to a reduction in MT, followed by the increase in GHGs since the mid-20th century. While global warming is likely to continue over most of the 21st century, the rate is unlikely to increase, and might even decrease, disproving nearly every climate projection. Recent warming appears to be multicausal, caused by changes in solar activity and MT, besides GHGs. It is thus very unlikely that the decarbonization of the economy will have any significant effect on climate, although it could have a great effect on the transfer of wealth from some agents in the global economy to others, even if its total effect on wealth creation is negative.

Note:

The book “Climate of the Past, Present and Future: A scientific debate, 2nd ed. by Javier Vinós will be published on September 20th, and it is now available for pre-orders. Kobo has a preview inside the eBook. At the time of this writing, both Barnes & Noble and Amazon offer the eBook at the discounted price of $2.99.

References

Glossary/abbreviations

The earlier parts of this series on Meridional transport and the Winter Gatekeeper hypothesis:

  • Part 1: The search for a solar signal.
  • Part 2: Solar activity and climate, unexplained and ignored.
  • Part 3: Meridional transport of energy, the most fundamental climate variable.
  • Part 4: The unexplained climate shift of 1997.
  • Part 5: A role for the Sun in climate change.

This post originally appeared on Judy Curry’s website, Climate, Etc.