I'll take a stab at these:
No moon -
While the moon probably stabilizes the movement of the Earth's axis somewhat, it's not at all clear how essential that is to life. Different authorities make different guesses, so take your pick.
There is some thought that tides (and particularly tidal zones) were critical in allowing life to make the transition to living on land. While that's probably true, lunar tides are not the only tides. The sun also causes tides which, though not as large as the lunar tides, would probably be enough to do the trick
Multiple moons -
Most moons in our solar system are nowhere near as large, relative to their planet, as the Moon is to Earth. Our moon is thought to be a statistical outlier, an accident of the later period of planet formation. The theory, which is supported by good evidence, has it that a Mars-sized proto-planet slammed into the proto-Earth late in formation, blasting a spray of debris out of the Earth's crust. Some of that debris was lost to interplanetary space, some fell back to Earth, and some formed the Moon.
This means that an Earth-sized planet is unlikely to have two large moons like our moon. Other worlds in our solar system have large moons, notably Pluto, so the odds of finding one around another Earth are not impossible, but I would not expect it to be common. Smaller moons, like Mars's two captured asteroids, might be common, and multiples of small moons might be common. We have not data yet with which to make an estimate.
If your world has multiple moons, they will interact with each other. For long-term stability (geologic timescales), the moons will have well-separated orbits not in harmonic resonance with each other. If your moons are similarly sized, they will appear as different sizes from the surface of your planet because of different orbital distances.
Of course, in a fantasy world with magic and so forth, you can have as many moons as you like, as large as you like.
Large igneous province - by which I assume you mean a region of active volcanism
Volcanoes affect climate in two ways. They put a lot of gasses into the air and they put a lot of particulates into the air. The gasses - methane, carbon dioxide, sulfur dioxide, and the like - are all strong greenhouse gasses. They enhance the atmosphere's retention of heat. The particulates, in the form of volcanic ash, act in much the same way as clouds do. By reflecting a portion of the sun's energy back to space, they reduce the amount of heat available to be captured at the Earth's surface and thus have a cooling effect. The particulates can be injected into the upper atmosphere and remain there for years.
On the whole, the balance is probably tilted toward cooling. The year after Penetubo (sp?) erupted was a cold and wet one for much of the northern hemisphere. The year after Krakatoa erupted was known as the year with not summer, with snowfall recorded in the summer in many paces int he United States. A prolonged period of eruptions by multiple volcanoes may allow greenhouse gasses to build up to a greater extent than these isolated eruptions did, but the amount of particulates would be correspondingly greater as well.
Life has survived periods of volcanism before, and probably would again. Life is marvelously adaptable. It's not clear that the thing we call civilization would be as adaptable, dependent as it is on a small number of intensely grown crops, but assuming people survived (which seems likely), some kind of civilization would be established in the new world.
Different size of planet -
A planet's gravity at the surface is dependent on the planet's radius and its mass: gravity = mass / square(radius). It's mass is a function of its mean density and its radius: mass = 4/3 pi x density x cube(radius). If you put those two together you see that gravity is proportional to density x radius. If you hold the density constant, then the gravity is proportional to the radius. The bigger the planet, the higher the gravity.
In the solar system, density is not constant. Even if you build the planets out of the same initial ingredients, they will be more compressed in a larger planet and we would expect larger planets, in general, to have higher densities. This is not always true. Mercury, in spite of being a very small planet, has a density almost as great as Earth's. And once a planet is large enough to retain hydrogen and helium in significant amounts, it's becoming a gas giant and its bulk density plummets.
The primary determinant of density may be the region of the solar system where the planet was formed. In our solar system, it is thought the planets were formed more-or-less in the orbits they inhabit today. There is evidence that Neptune has migrated into a wider orbit over time and that harmonic interactions between planets have adjusted other orbits somewhat, but that doesn't affect this argument much. The idea is that the inner planets formed in a region that was much hotter than the region the outer planets formed in. Heat is a measure of how fast molecules are moving. Hotter molecules are moving faster than colder ones and are harder to capture in a gravitational field. Bigger molecules move more slowly than smaller ones at a given temperature and are easier to catch. For this reason, the inner planets are systematically impoverished of the lighter elements, which means their densities are correspondingly higher. In the outer regions of the solar system, water ice behaves much as rock does in the inner system, and the low densities of many of the moons of the outer planets is a reflection of this.
What else would be different about different sized planets? A larger planet, with its higher gravity, would likely have smaller mountains. Mountains are limited by plastic deformation. If you pile stone high enough, it crushes or squishes and can no longer support the weight of the stone above it. It's no accident that the tallest mountain in the inner solar system is on Mars - it has a much lower gravity than does Earth.
The atmosphere might be different too. If the planet is too small, it can't retain an atmosphere. The Moon has a negligible atmosphere and Mars's atmosphere is 1%-2% the pressure of Earth's. A bigger planet, with higher gravity, can hang on to those lighter molecules longer. A planet larger than Earth might well have a thicker atmosphere, which would retain more heat than does Earth.
Venus is about the same size as Earth and has an atmosphere sixty times thicker. What gives there? One theory is that all the inner planets were originally burdened with the same heavy atmospheres composed primarily of carbon dioxide and methane and water. The gravity of Mars was too low to retain the atmosphere, and over a billion or two years the atmosphere of Mars bled off into space. On Earth, chemical reactions involving water and, later, the chemistry of life sequestered much of the carbon from the original atmosphere into the crust. Venus was enough hotter than earth that its water was photodisected and the hydrogen bled off into space. With no water, there was no mechanism to sequester the carbon, and Venus today is a balmy 750 Kelvins.
Earth is, according to many estimates, on the inner edge of the Goldilocks zone. Even five percent closer to the sun, it is thought, and Earth too would look like Venus does today. If Earth was enough bigger than it is, it would have retained heat as if it were closer to the sun, and would not be habitable today. So if you're going to have a much bigger planet than Earth, push it a little further from its sun to be safe - or give it a slightly smaller sun.
No tectonics -
There are three forces that create mountains on Earth, the largest of which is tectonics. The two minor forces are impact cratering and volcanoes. Volcanoes are primarily driven by tectonics, so without tectonics there would likely be no active volcanoes today. Most impact craters in the solar system were created in the first billion years of its existence. Due to erosion, there are very few surviving craters on Earth's surface today. Impact craters survive on the Moon because erosive forces are slight and because its lack of tectonics does not recycle its crust.
There is an additional mountain forming process that is not seen on earth. Planets formed when lots of smaller bits fell to a common center. The heat of impact was not dissipated to space because other bits were piling on top and holding it in. Eventually the heat rose high enough the rocks melted. The earth's center is still molten with the heat of creation and of radioactive decay. That heat drives plate tectonics. The amount of heat originally produced is roughly proportional to the planet's volume. The amount it can radiate to space is a function of its surface area. Volume is proportional to the cube of a planet's radius but surface area is proportional to the square of its radius. This is a long-winded way of saying that bigger planets cool more slowly that smaller ones.
Because of faster cooling, smaller planets of the same age will have thicker crusts than larger ones. When the crust is thick enough, its mechanical strength becomes greater than the currents driving continental drift, and tectonics cease. But solid forms of a material are generally more compact than their liquid forms. (Water ice is a notable exception to this.) In a very small planet, a thick and rigid crust forms, but that's not the end of it. The molten interior continues to cool and to shrink. The crust, no longer supported, cracks and collapses inward, forming long scarps that run for thousands of kilometers.
So even without tectonics, you can still have mountains. On a planet with an atmosphere and especially with water, those mountains will be eroded away fairly quickly (geologically speaking). Continents may not disappear, but they would appear to our eyes flat and featureless. The weather patterns on such a world would be very different. Mountains form rain shadows, for example. Water-laden clouds can't get over the mountains. If they are pushed high enough to get over, they cool enough they drop most of their water as rain. When the Indian subcontinent slammed into Asia and raised the Himalayas, it change the world's weather patterns. I don't know enough to predict how weather would be different in a mountainless world, but we can be sure it would be very different.
Ice Age or Tropical Age -
We don't really know what causes an ice age. One theory that has pretty good traction is that the Earth's climate, a chaotic system (chaotic in the mathematical sense), has regions of relative stability. One of them is a warmer regime more or less like the historical norm, and another is an ice age regime. This theory is bolstered by the cyclic nature of ice ages. A 'tropical age', if there is such a thing, might be another stable regime.
Climate changes over time and we are far from having a complete understanding of it. But we do know that it's primarily a heat engine. The more heat that's in the atmosphere, the more active the climate is. People are often surprised at how close those climactic regimes are. Cool the Earth by 4.5 Kelvins and we'll have an ice age. Warm it by the same amount and the polar ice caps will be all gone. Warm it another 4.5 K and we'll have palm trees at the poles.
As to how well civilization would survive, that's an ongoing experiment. My guess is not too well. Our civilization is dependent on a small number of cereal crops. The best soil for growing those crops is found in the mid-latitudes of the northern hemisphere. Fortuitously, the temperatures at these latitudes are also suitable for these crops. If we shift the band of suitable temperature north or south by very much, which would be the case in an ice age or a tropical age, the best land for growing would no longer have a good climate for growing.
If the shift in climate occurred slowly enough, we might be able to develop new crops that could a) grow in the climate now found in mid-latitudes (but not if they're under ice), or b) grow well in the soils where the climate was now suitable. My guess, given how well we are currently cooperating on climate change, is that it would not happen without a major disruption.
There's another problem. Consider an ice age. Ice now comes down to, say, the forty-fifth parallel. How many people are displaced? Much of Europe is frozen, most of Russia, and the northern tier of states in the United States. Where will all those people go? Will they be welcomed with open arms? If they are accepted into southern countries will the ideas they bring be acceptable or disruptive? How will all those people be fed if the world's food supply is impacted?
Perhaps it's fortunate that the current experiment is with global warming and not global cooling.
Climactic distributions -
You have several topics lumped into this one. I'll start with the easiest to answer, the one about a desert planet with no plants. Even that one is really two questions. First, for much of its history, Earth was a desert planet. Life got along fine in the oceans and didn't venture out of the mother. But of course, there were plants, just not on land. If we define a plant as an organism that can capture sunlight and turn it into energy, and then postulate a planet with no plants, what would it look like? It would be dead. If the planet has plate tectonics, there might be bacterial life that metabolizes sulfur compounds near undersea vents, but that's about it. Without plants, there can be no animals. Animals get their energy from eating plants or from eating animals that eat plants.
Such a planet would not have an oxygen atmosphere. Oxygen is a highly reactive element. Free oxygen (O2) in the atmosphere is ephemeral. Sooner or later it will all react with something and pass out of the air. We have oxygen in the air because we have living beings (plants, mostly) that give off oxygen as a waste product and constantly replenish the oxygen that is lost. If we ever find a planet with free oxygen in the atmosphere, it will be a planet with life on it.
The climate of a location on Earth is determined firstly by water and latitude. The latitude determines how much solar energy you get and hence the temperature. Water determines if you're living in a desert or not. How much water you get depends on ocean currents and geography. Google climate zones and you'll see how they are distributed on Earth. Currents in the northern hemisphere move in clockwise gyres; they go counterclockwise in the southern hemisphere. Places with similar latitudes and similar relations to ocean currents have similar climates. Britain and the Pacific Northwest have climates with much in common. So do the eastern coast of North America and the coast of China. Southeast Asia and the Caribbean Basin are both prone to hurricanes and cyclones. The seaward side of mountain ranges is always wetter than the inland side.
The reason latitude is important is twofold. First, he further poleward you go, the lower the angle of the sun in the sky. A higher percentage of the light reaching Earth reflects off the atmosphere at higher latitudes and the light that reaches the ground is attenuated through more atmosphere. Second, axial tilt changes how great the seasonal changes are. In the tropics, seasonal changes are not very significant. In the polar regions, the difference between a winter with no sun and a summer with no night can be extreme.
Changing the axial tilt on your world will change how latitude interacts with climate. A lesser axial tilt will push the regions of negligible seasonal difference poleward but will not eliminate the other regions: they will be compressed instead. You'll still have a land (or sea) of the midnight sun, but it won't be as extensive. It will only disappear if your axial tilt is very small.
There is a lot more to climate than I've touched on, and a lot more than I'm qualified to speak to, so I'll stop while I'm ahead.
A topic you did not touch on is how the star around which your planet orbits will affect the planet, but perhaps that's a topic for another time.
--HBrown
Reply With Quote
Originally Posted by HBrown
