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5. The Thick Mesozoic Atmosphere

Understanding Fluids

Evolution of the Atmosphere

A condensed explanation of how the Earth’s thick atmosphere evolved comes from John D. Fix’s college astronomy textbook Astronomy: Journey to the Cosmic, where he explains the connection between the atmospheres of Earth and Venus:

“The present atmospheres of both Venus and the Earth are thought to have formed through the release of gases from their interiors. The three principal gases that result from outgassing on the Earth are carbon dioxide, nitrogen, and water vapor. Only nitrogen has remained primarily in the atmosphere. Nearly all of the Earth’s outgassed carbon dioxide is locked up in carbonated minerals that form in the oceans and become rocks on the ocean floor. These rocks are eventually subducted into the interior, where the carbon dioxide is released from the rock and recycled back into the atmosphere in volcanic venting. Most of the Earth’s outgassed water now forms the oceans.

For Venus, the principle atmospheric gases are carbon dioxide and nitrogen, both of which presumably were released from the interior. The total amount of outgassed carbon dioxide and nitrogen are remarkably similar for the two planets. If the carbon dioxide now locked up in the Earth’s rocks were released into the atmosphere, the result would be an atmosphere almost identical to that of Venus in thickness and composition.”*

*Because Earth is larger and much more geologically active, Earth’s earlier atmosphere was actually several times thicker than Venus’s present atmosphere. How Earth’s thick atmosphere evolved and how it later became locked up in carbonated rock is a primary topic of the remaining chapters.

It may be hard to imagine that the Earth’s air could be so thick that its density would be comparable to water. Nevertheless, there is no reason why a gas can not be compressed so much that it has properties similar to that of a liquid, and in fact compressing a gas into a liquid is a common industrial process.

In order to compress the air near the Earth’s surface, there has to be a substantial amount of overlapping air pressing down on the ground level air. Thus the high density ground level air is evidence of an extremely thick Mesozoic atmosphere.

Unlike water or other liquids that have nearly constant density between the top and the bottom, the density of a planet’s atmosphere increases as one travels from the darkness of space downward to the planet’s surface. In addition, there is also an increase in pressure as we move downward towards the surface. Close to the planet’s surface both the atmosphere’s density and the atmospheric pressure is the greatest due to the weight of all of the air above compressing the air at the surface.

A good approximation of the pressure at the Earth’s surface can be achieved by using the ideal gas law:

P V = n R T

P is the pressure in N/m2, V is volume in m3, n is the number of moles, R is the ideal gas constant 8.31 J / mol*K, and T is the absolute temperature given in Kelvin. This is the ideal gas law in its standard form often used for closed containers. But our atmosphere is not a closed container so before we can use our ideal gas law equation we need to make the substitution:

n / V = ρ / M.

Where ρ is the density of the atmosphere and M is the average molecular weight of the gas. Making these substitutions gives the equation for calculating the pressure:

P = ρ R T / M.

Earth's Atmosphere

We insert into our equation 667 kg / m3 for the density, 294 K (21 degrees Celsius) for the average Mesozoic global temperature, and 43.0 grams per mol for the molecular weight of the atmosphere. This shows that 150 million years ago the Earth’s atmospheric pressure near the surface was about 370 atmospheres.

An atmosphere is a unit for pressure. The present sea level atmosphere is said to have a pressure of 1.00 atmospheres. Other corresponding measurements of the present sea-level pressure are 1.01 E5 N/m2 and 14.7 PSI or pounds per square inch.

A pressure reading taken within a static fluid is an indication of the weight of fluid above that location. So a Mesozoic sea-level atmospheric pressure of 370 atmospheres would indicate that the Mesozoic atmosphere was 370 times thicker than what it is today.

Before concluding that the Earth’s thick Mesozoic atmosphere would crush all the species on its surface, stop to consider the pressure that currently exist at the deepest depths of the oceans. The average ocean depth is 3790 m and at this depth the pressure is 380 atmospheres. So for all practical purposes, the present day pressure at the average depth of the ocean is the same as the pressure at the bottom of the Mesozoic atmosphere. Yet there are numerous species that live at this depth and many more that live much deeper. Extremely high absolute pressure has no ill effect on our present creatures of the deep that have evolved in these environments; likewise, the extremely high pressure of the Mesozoic era had no ill effect on the terrestrial species of the Mesozoic era.


Understanding Pressure

Underwater reef

To clear up possible confusion over pressure it may be helpful to recognize the distinction between absolute pressure and a difference in pressure.

If both the inside and outside of an enclosed container are at the same absolute pressure, no matter what the absolute pressure might be, there will be no net force on the sides of the container. For example, if both the inside and outside of a closed container are at 370 Atm. the walls of the container will not be under any stress.

In contrast to the example of where an extremely high but equal pressure is on each side of a wall, even a small or moderate difference in pressure between each side of a wall can produce a substantial force. For example, let’s imagine that the difference between the inside and the outside pressure on a typical window is ‘only’ 1/30 of an atmosphere. However, if this were attempted there would no longer be a window. 1/30 of an atmosphere would create a force on the window equivalent to laying the window on the ground and then asking several men to stand as close as possible on top of the glass surface. A typical window would break with a pressure difference of only about 4 E-3 Atm.

The distinction between a difference in pressure and absolute pressure can be further illustrated by comparing the effect on submarines to the effect on marine biology. When a submarine on the water’s surface closes its hatch before its descent, the sea-level air pressure both outside the sub and inside the sub is 1.0 atmosphere. As the sub dives down into the ocean’s depth the water pressure outside the sub greatly increases while the air pressure inside the sub remains at 1.0 atmosphere. After the sub dives down 500 meters, the difference in pressure pushing inward on the sub is nearly 50 atmospheres. Submarines must have thick steel hulls so that they can withstand the crushing pressure at these depths.

Submarine

In contrast to the rigid submarine, the many species that live at the great depths of the oceans experience no ill effects despite living in an extremely high pressure environment. Unlike the submarine, these species do not attempt to maintain a pressure difference between the interior and the exterior of their bodies. Since the pressure inside the bodies of these deep-sea creatures is the same as the outside water pressure, there is no net force or strain on their bodies no matter what the absolute pressure might be.

This is true as long as the animal does not change depth too quickly and in so doing change the exterior pressure too quickly. Most people are familiar with the mildly painful experience of having their ears pop when changing altitude such as flying or driving up or down a mountain, or even just diving into the deep end of a pool. Scuba divers who make the mistake of rising to the surface too quickly may have the much more painful experience and possible death from the decompression sickness known as the bends. This effect is not limited to humans since even whales and possible other marine animals suffer from the bends if they rise to the surface too quickly. Because of decompression problems, marine biologists are still experimenting with different methods for bringing deep sea creatures to the surface without killing them. For clarification, a quick decompression does cause problems for many species, whereas there are no adverse effects for species to continuously exist in a constant-pressure environment.

Sea Turtle

As human beings living on the Earth's surface, we live at the bottom of a sea of air. At sea level this air produces a pressure on our bodies of 1.0 atm., 1.01 E5 Pa, or 14.7 PSI. The area on the face of an average adult's hand is about 0.0116 m2 or 18.0 square inches so there is about 1200 N (270 pounds) of force bearing down on an average adult human hand. Since the pressure is the same for both inside and outside of us, the net forces balance out to zero. Rather than weighing us down, we are indifferent to this force.

Regarding fluid pressure, it does not matter if we are discussing air or water, they are both fluids. The creatures of the Mesozoic era were at the extremely high absolute pressure of 370 atmospheres. Yet just like the deep ocean species of today, this high absolute pressure was on both the inside and outside of their bodies so it produced no stress on their bodies. The extremely high absolute pressure of the thick Mesozoic atmosphere would have had no adverse effect on the dinosaurs.


Light Penetration

In order to imagine the thickness of the Mesozoic atmosphere, the density of the thick Mesozoic atmosphere has been compared to the density of water. Hopefully this comparison to water has also been helpful towards understand how the thick atmosphere provided buoyancy and to imagine what it would be like to move through such a thick atmosphere. However it still needs to be kept in mind that these two fluids are not the same in all respects. One difference between the two fluids is the way light is limited in how far it can penetrate into water.

Charge of Water Molecule

Water is an electric dipole molecule. This means that even though the overall electric charge of the molecule is neutral, one side of the molecule is positive while the opposite side is negative. It is this unbalanced distribution of electric charge that gives water its many unique characteristics.

Scuba Diver

When we microwave our food for a quick meal, the microwave is interacting with the electric dipole molecules of water causing rapid oscillation. This rapid oscillation at the atomic level is what we perceive as thermal energy at our sensory level. It is because the microwaves often penetrate deep within the food before they transfer their energy to the water molecules that a microwave is able to cook so fast. The microwave example illustrates water’s ability to absorb electromagnetic energy, in other words, light.

Light from the sun does not lose energy as it travels to the Earth. But once it penetrates into the depths of the ocean much of its energy is absorbed by the water. Most of the sunlight’s energy is absorbed as it travels down through only the first 200 meters of the ocean water. Beyond this upper lit zone, the epipelagic zone, the light becomes too dim to even support plant life. If we sink even deeper the light continues to diminish until it becomes completely dark at about 1000 m; beyond this there is nothing but darkness into the depths of the sea.

It would certainly be a problem to the dinosaurs and the other species of the Mesozoic world if the thick Mesozoic atmosphere absorbed light in a similar fashion to the water’s of the oceans. But similar to today’s atmosphere, the Mesozoic atmosphere would contain only a small percentage of water vapor in comparison to the total volume of other gases. So similar to the present atmosphere, the vast majority of light would past through the thick Mesozoic atmosphere to reach the surface and even penetrate the upper portion of the ancient oceans.

This is not to say that the Sun and stars of the Mesozoic sky would look exactly the same as they do today. Astronomers are well aware of how atmospheric turbulence deflects starlight causing stars to twinkle. During the Mesozoic era, starlight passing through such a thick atmosphere would be thrown about so much by atmospheric turbulence that individual stars may not have been distinguishable. Likewise because of the extreme thickness of the atmosphere the Mesozoic Sun would probably appear extremely hazy in comparison to how it appears today.


How Solar Radiation Determines a Planet’s Temperature

Without the warmth of the Sun’s illumination the Earth and the rest of the planets would all be extremely cold worlds. Fortunately for us, soon after coming on line our solar nuclear power plant has put out a nearly constant supply of 3.85 E26 watts of radiate power. This value is calculated by using an optical thermometer to measure the Sun’s surface temperature to be about 5770 K, measuring the radius of the Sun to be 3.5 E8 m, and then applying the Stefan-Boltzmann equation regarding radiation:

P = e k A T4

Where P is power, e is the emissivity factor, k is the Stefan-Boltzmann constant equal to 5.67 E-8 W/m2*K 4, A is the area, and T is the absolute temperature given in Kelvin.

The emissivity factor e is a measure of how well a surface will absorb and emit radiation. Depending on such factors as a material’s reflective properties, smoothness, and color the emissivity factor can have a value anywhere between zero and one. The emissivity value of a material is the same for emission as it is for absorption. This means that if an object easily absorbs radiation when light shines on it then it will just as easily give off this energy as radiation to its surroundings. The Sun is considered to be a perfect emitter and absorber of radiation and so its emissivity value is one.

solar system

Only a small fraction of the light radiating away from our Sun is intercepted by the planets of our solar system, yet this is the light that warms these planets. How warm a planet will be is mostly determine by how close it is to the Sun. The closer a planet is to the Sun the greater the intensity of the sunlight and so the warmer the planet. The relationship between distance from the Sun and the average planet surface temperature is extreme, from the hot surface of nearby Mercury to the extreme coldness of the distant Neptune. Just between Earth and Mars the drop in average planet temperature is so great that the average temperature on Mars is the same temperature as the Earth’s South Pole, the coldest location on Earth.

David at a campfire

The equation for intensity is:

I = P / A

I = P / 4 π R2

where I is the intensity, P is the power, A is the surface area of a sphere, and R is the distance away from the source.

This relationship between distance from a radiant heat source and warmth is well understood by anyone that has sat next to a blazing campfire on a cold night. When a person is too near the fire they will be uncomfortably hot while if they are too far away they will feel cold. A person sits the right distance away from the campfire so that they are comfortable, in the same way that the Earth is just the right distance away from the Sun so that it has a comfortable temperature.

Our first step in deriving the relationship between a planet’s distance from the Sun and its average temperature is to recognize that all of the radiant power absorbed by a planet is equal to the radiant power that a planet emits back out to space.


Radiant Power Gain = Radiant Power Lost

By first writing the radiation power gained by the planet, and then writing the equation for the radiation power lost by the planet, we can set the two equations equal to each other to find our relationship between temperature and distance. The radiation power gained by the planet is:

Pgain = Ps e (Af / Ao)

Pgain = Ps e ((π r2) / (4 π R 2))

Pgain = Ps e r2 / (4 R2)

Where Ps is the power output of the Sun, e is the total emissivity of all the atmospheric components and the surface of a planet, Af is the area of the planet intercepting the sunlight, Ao is area of a sphere whose radius is the distance of planet away from the Sun, r is the radius of a planet, and R is the orbital radius of the planet. The radiation power being lost by the planet is:

Plost = e k Ap T4

Plost = e k (4 π r2) T4

Where k is the Stefan-Boltzmann constant equal to 5.67 E-8 W/m2*K4, A p is the planet's surface area, and T is the absolute temperature given in Kelvin. Setting the radiant power lost equal to the radiant power gain and inserting 3.85 E26 watts for the Sun’s radiate power gives the following simple relationship for a planet’s average temperature in degrees Kelvin as a function of its orbital distance from the Sun using meters for distance.

T = 1.08E8 / R1/2

Planet Distance from Sun
(E9 m)
AtmosphereCalculated Temperature
Average or Range
(Kelvin)
Actual Temperature
Average or Range
(Kelvin)
Mercury 57.9 No Atmosphere 449103 - 623
Venus 108 Extremely Thick 329 753
Earth 150 Moderate 279 285
Moon 150 No atmosphere 279 110 - 390
Mars 228 Thin Atmosphere 226 210
Jupiter 778 Gas Planet 122 123
Saturn 1430 Gas Planet 90 93
Uranus 2870 Gas Planet 64 63
Neptune 4500 Gas Planet 51 53

Six of the nine major solar system objects show a good match between the calculated and actual temperatures values. The Earth is about fifteen degrees warmer than what the equation predicts while Mars is about fifteen degrees cooler. The Jovian gas planets consisting of Jupiter, Saturn, Uranus, and Neptune show excellent agreement between the calculated and measured values. Of the three remaining objects in the table, Mercury and the Moon are grouped together as the ‘no atmosphere’ category of objects whose temperature varies considerably depending on whether it is day or night, and finally there is the mysterious Venus that is in a category by itself. Let us address the ‘no atmosphere’ category of Mercury and the Moon first.

Gas planet absorbing and emitting radiation

The derivation of the temperature as a function of distance equation is based on the simplifying assumption that the radiation energy from the Sun is being uniformly distributed around the planet or moon. If this is true then the planet will radiate evenly its thermal energy back out to space from its entire surface. All of the Jovian gas planets along with the relatively thin atmosphere planets of Earth and Mars are able to fulfill this requirement. However Mercury and the Moon have no atmosphere and consequently fail to meet the requirements of our conceptual model. The solar system objects that have no atmosphere are hot on the side that faces the Sun and cold on the side that is dark.

Without an atmosphere all of the Sun’s radiation falls directly on the planet or moon’s surface. Then at night, without an atmosphere, there is nothing to impede this warmth from radiating away from the surface. As a planet or moon with no atmosphere rotates its surface will go from being heated by the sunlight to the darkness of space and once this happens the temperature at that surface location will drop rapidly.

Besides the dramatic difference between day and night time temperatures, terrestrial planets and moons also have a large temperature difference between the low latitudes near the equator and their high latitude Polar Regions. The reason why the lower latitudes receive more heat and reach a higher temperature is that near the equator the surface is perpendicular to the rays from the Sun while near either pole the light rays hit the surface at a steep angle. The difference in latitude means that near the equator the surface receives the full intensity of the light while near the poles the intensity is much less since it is spread out over a much larger area. The low intensity of sunlight hitting the Polar Regions means that the Polar Regions of a planet are going to be colder than the lower latitudes near the planet's equator.

Ideal heating and cooling pattern of Mercury and the Moon

These differenced in temperatures based on either 1) whether it is day or night or 2) according to latitude are going to be the most extreme for the objects that have no atmosphere while less so for the planets that have at least some atmosphere. For Mercury and the Moon the lack of an atmosphere makes the temperature swings between night and day so great that it makes more sense to list these extremes in temperature rather than the average. For planets having a relatively thin atmosphere such as the Earth and Mars it makes more sense to give the planet’s average temperature but then remain aware that variations in temperature exist. On Earth and on Mars the atmospheric temperature goes up and down with the changes from day and night. On these two relatively thin atmosphere planets there are also large difference between the high temperatures near their equators and the lower temperatures of their Polar Regions.


The Mesozoic Paleoclimate Paradox

temperatures profile of the Mesozoic era

During the Mesozoic era a remarkably homogeneous flora of tropical and temperate plant species covered the Earth. Plants such as ferns, laurels, palm trees, and Magnolia that could not withstand freezing, thrived at 70 degrees north and south latitude. Many of the same plants that existed near the equator were also thriving at the Polar Regions of the Earth. Along with the plant life, early crocodiles along with dinosaur footprints and fossilized bones are also found at these high latitudes. Furthermore for nearly all of the Mesozoic era geologists have found no evidence of glaciation near the Polar Regions thus indicating that throughout the Mesozoic there was no ice at the poles.

For decades paleoclimatologists have tried to explain the Mesozoic paleoclimate paradox. If they matched the vegetation of the lower and middle latitudes then their climate models were too cold at the higher latitudes. If they matched the warm temperatures of the Polar Regions then their models projected an unrealistic hot sauna for the rest of the planet that again conflicts with the geological evidence. Their frustration is apparent when they claim it is the geological evidence rather than their simulation models that are wrong. None of the paleoclimate computer simulation models have come close to matching the mild balmy global climate of the Mesozoic era.

The reason their climate software models fail to match the Mesozoic climate is because the paleoclimatologists make the incorrect assumption that the Mesozoic atmosphere was the same thickness as the present. As stated earlier, the variation of temperatures around a planet whether it is day or night or according to latitude, is a function of the thickness of a planet’s atmosphere. Planets or moons with no atmosphere will have the most extreme difference in temperatures, planets such as the present day Earth and Mars that have relatively thin atmosphere will still have these differenced in temperatures but much less extreme, while extreme thick atmosphere planets such as Venus and the Mesozoic Earth show almost no temperature difference at all according to latitude, seasonal changes in sunlight, or variation in temperature between day and night.

Palm leaf impression in Cretaceous rock

At the equator the rays of the Sun are usually shining nearly directly down on the surface so as to heat this area of the Earth more than any other. The air directly above this surface expands after gathering this heat. This hot, low density air is more buoyant than the air around it and so it pushes up and rises up to a higher altitude. As this air mass rises it must push the air above it out of the way and at the same time the nearby air masses are drawn in towards the equator so as to take the place of the departing hot air mass. Once the new air mass moves over the equator it is next in this process of heating up so as to later ascend. The process keeps on repeating with each air mass pushing or pulling the others along so as to form a continuous conveyer belt of air flowing in a circular pattern.

Once convection currents are established they are extremely effective in transporting heat from warm areas to cold areas. The hot air that rises at the equator would travel all the way to the cold poles except that the present atmosphere is too thin to support such elongated convection cells. So instead of reaching either the north or south pole the air from the equator comes back down in elevation at around 30 degrees latitude before heading back to the equator. Below is a diagram produced by NASA showing the idealized three convection cells per hemisphere circulating pattern of the atmosphere.

To finish the job, in each hemisphere there are two more convection cells, one at the middle latitudes and another at the high polar latitudes. The polar convection cell turns in the same direction as the strong convection cell near the equator. While the middle convection cell acts like a middle gear or ball bearing that is forced to turn in the opposite direction. Because the middle convection cell rotates in the opposite direction its interaction with the other convection cells tends to create constantly changing weather patterns in the mid latitudes. While this three convection cell system does transfer heat from the equator to the poles it is not nearly as effective in transferring heat as a single cell system of a thick atmosphere.

Ideal Global Atmospheric Circulation Pattern Provided by NASA

Besides carrying heat from warm locations to the cooler locations this movement of air is important because the air may or may not also carry moisture. The amount of moisture that air can hold depends on its temperature such that warm air can carry much more water than cold air. The air closes to the ground is the warmest and so this air has the potential of holding the most water. But if this air is forced to rise up in altitude then its temperature will drop and likewise it must also lose its moisture; it will rain.

Near the equator, where the air is rising it continuously rains. In these lower latitude locations the only relief from the constant downpour comes with the annual change in the seasons from wet to dry and back again. Unlike the hot summers / cold winters seasons that are more familiar to those who live in the middle latitudes the seasons of the countries near the equator are focused on if it is raining or not. If not for the axial rotational tilt of the Earth many places near the equator would experience a constant downpour all year long. But because of the tilt of the Earth the line between where the north and south convection cells meet tends to wander slightly north or south of the equator. For the countries near the equator, when this meeting line moves overhead then it is raining, when this meeting line is either north or south then it is more likely to be dry.

Regardless if the air is rising because it is near the hot equator or if it is rising because it is being force over a mountain range the results are the same: rain. As the air mass rises it drops its moisture. Now that this air is extremely dry it acts like a sponge in drawing the moisture out of whatever land it is now flowing over. Wherever a moist air mass climbs up and over a mountain range there can are heavy rains on one side of the mountains and a desert on the other. Likewise the air that drops its moisture as it gained altitude near the equator later comes back down to Earth about 30 degrees latitude either north or south and now it is depleted of moisture. As this air travels back towards the equator this dry air absorbs the moisture from the land thus creating deserts. It is because of the Hadley Cells the two strong convection cell patterns near the equator that the great deserts of the world are located about 20 to 35 degrees above or below the equator. Once again examine the image of the Earth to note the location of the brown regions marking the vast deserts of the world.

Cactus in Arizona desert

Deserts are often the best locations for finding dinosaur fossils. Yet the animal and plant fossils that paleontologists find at these locations are usually those that are better suited for a moderately humid forest rather than a desert. One possible solution to this paradox may be that continents themselves may have moved to higher or lower latitude thus changing the regional climate. Even though continental plates usually move only a few centimeters per year over a hundred million years or more this slow movement can produce considerable change in the position of a continent. Over a hundred million years, it is possible that the climate at various locations on a continent can change considerably because the latitude of those locations changed considerably. Another possibility is rising sea levels can sometime create an inland sea and this could certainly raise the humidity of the surrounding land. Yet once we rule out these possibilities there still remains several large deserts, such as Africa’s Sahara desert, where acknowledging this climate paradox can not be avoided. So the question remains: how is it possible that paleontologists are finding fossils of ancient ferns and other evidence of a previous moist environment in the middle of these present-day dry sandy lands?

One cell per hemisphere atmospheric circulating system

To find the answer we look at Venus, the only planet today that comes close to modeling the Earth’s extremely thick Mesozoic atmosphere. Venus’ atmosphere and the Earth’s Mesozoic atmosphere are comparable in thickness since Venus’s is 91 times thicker and the Earth’s Mesozoic atmosphere was a few hundred times thicker than the Earth’s relatively thin present-day atmosphere. Another shared characteristic is the uniformity of the surface temperature regardless of latitude. Like the Mesozoic Earth, on Venus the surface temperature near its equator is only slightly higher than the surface temperature at either pole.

A primary reason there is almost no variation in temperature over the entire surface of Venus is because Venus has an extremely efficient atmospheric convection current system that uniformly distributes the radiation / thermal energy coming from the Sun. With such a thick atmosphere, there is only one convection cell in each hemisphere carrying the heat from the equator to the one or the other pole. This one cell system is much more effective than the Earth’s present-day three cell system in distributing heat from the lower latitudes to the higher latitudes.

Likewise it is reasonable that the Earth’s much thicker Mesozoic atmosphere would also form a one cell convection system that would be much more effective in transporting heat from the equator to the poles. Today’s atmosphere, being hundreds of times thinner, is compacted too close to the surface to maintain a thin one cell per hemisphere convection system stretching from the equator to each pole. So the thin present atmosphere forms a three cell convection system.

The Mesozoic atmosphere’s one cell system had two major effects on the global climate: 1) it was far more efficient in redistributing the heat to produce a nearly uniform temperature over the Earth’s entire surface and 2) it more evenly distribute the moisture around the globe. Rather than having breaks near the 30 and 60 degree latitudes, the lower air currents of the one cell Mesozoic atmosphere made the complete journey from either the North or South Pole all the way to the equator. Consequently during the Mesozoic rainfall was much more evenly distributed all over the Earth.

The Seasons

It may appear that the Thick Atmosphere Solution has solved all the major puzzles of the Mesozoic climate and yet one still remains: if the temperature is nearly uniform all over the Earth, day and night, and throughout the entire year then how is it that some Mesozoic trees still show growth rings? To explain the solution to this question a review of what causes the present seasons would be helpful.

Every year the Earth circles the Sun and as it does this we experience seasonal changes. The reason for the seasonal changes is that the Earth’s rotational axis is tilted by 23.5 degrees. This tilt of the rotational axis means that during the northern hemisphere’s summer months the northern latitudes are receiving more sunlight than on average. In addition to receiving more hours of sunlight the solar radiation during the day is more intense because the noon day Sun is more directly overhead.

It is important to realize that while the northern hemisphere is enjoying its summer the southern hemisphere is experiencing its winter. Likewise six months later the situation is reversed; while it is winter in the northern hemisphere it is summer in the southern hemisphere. It may seem odd to those of us that live in the northern hemisphere, but 'down under' December, January, and February are the hottest months of the year.

The Seasons

Generally these extremes in seasonal changes in sunlight and temperature become greater the farther we are away from the equator. In the low latitudes near the equator there is only a slight change is temperature and sunlight throughout the year. At these latitudes there are no cold winters. As stated earlier, the seasons in the lower latitudes are more focused on whether it is the dry or rainy season. Going from lower latitudes to the highest latitudes we now see that the temperature and amount of sunlight changes dramatically with the seasons. In the high latitudes the changing of the seasons means a transition from balmy summer days to an extremely cold winter environment. Here, during the winter it is not just extremely cold it is also dark. For most days the Sun cast long shadows all day since it barely makes its appearance above the horizon. At the very highest latitudes either north of the Artic Circle or south of the Antarctic Circle there are winter ‘days’ when the Sun never rises above the horizon.

For the present Earth the amount of sunlight a location receives strongly correlates with its temperature, such that the more radiant energy a location receives the more likely it is going to be hot at that location. Yet during the Mesozoic the amount of solar radiation received made little difference in the local temperature because the strong one cell convection pattern was so effective in redistributing the heat. The non-variant temperatures of the Mesozoic world were a direct consequence of the highly efficient one cell convection pattern redistributing the solar thermal energy in comparison to the far less efficient three cells per hemisphere convection pattern that we have now.

To summarize, during the Mesozoic era the north and south latitudes had their seasonal changes in the amount of radiation they receive during the year yet they did not have seasonal changes in regards to temperature. During the early summer months of the year these locations received much more solar radiation than during the winter months of the year. Yet despite the huge difference in solar radiation received in summer compared to winter, there was hardly any difference between the summer and the winter temperatures. The seasonal change in radiation explains why Mesozoic trees growing in the middle and higher latitudes show growth rings, while in these same locations paleontologists are finding plants and animals that could not have tolerated cold winters.



External Links / References

The Earth's Atmosphere

Absolute Pressure and Pressure Difference

The Oceans

Unique Properties of Water

Mesozoic Paleoclimatology

Atmospheric Circulation

Solar Radiation

Desert Climates

Digging for Dinosaurs

Mesozoic Radiation Based on Tree Growth

Infrared Radiation Absorption


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