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Managing sunlight

Questions for pompe and anyone else who can answer:
if we used space mirrors so that the nightside of the Earth got the same sunlight as the dayside, would we totally roast? How quickly/badly?

What if we get the same total sunlight as now, but split and redirect it, so that the tropics at noon get less than half what they do now, but the entire planet gets that same level, 24/7? I imagine net thermodynamics wouldn't change much, weather would be massively changed, but I'm most interested in plant productivity -- is it more productive to have a constant moderate level of light everywhere, rather than some times and places with intense light and others with none, or vice versa?

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( 8 comments — Leave a comment )
heron61
Mar. 29th, 2008 09:59 pm (UTC)
I have no idea about the weather, but most plants (there are IIRC some exceptions) have evolved for regular day night cycles, with the day being used to absorb solar energy and the night being used to process it (I don't remember the details). Also, I think plants need a day night cycle to flower.

In short, while I can't remember the details and google isn't turning up anything with more details (although it is confirming my memories). I think you'd have plants growing twice as fast for a while and then dying w/o reproducing, so doing this would be an impressive way to create mass famine in a few months.
pompe
Mar. 29th, 2008 10:27 pm (UTC)
What you basically are saying in scenario one is that we would double the solar infall, right? That's probably a bad idea, but I can't say how fast we'd reach a new balance between incoming and outgoing radiation because I think lots of feedback mechanisms, like cloud cover, would kick in.

But let us make some sort of thought experiment. Double incoming solar radiation would mean if a very rough calculation shows a temperature rise of slightly less than 50 degrees for the global average, which I think would put us into the very dangerous zone of runaway greenhouse heating, because of the increased water vapor content. If the Earth's weather/ocean system still work to redistribute the heat as it does now the oceans might not cook directly in the tropics, but the redistribution would also mean that most or even all of the planet would go too hot for human tolerance. I mean, that increase in solar infall would make the poles receive as much solar infall as the tropics do now, but the poles are from an energy budget view also warmed by a lot of heat transfer and the tropics cooled by the same transfer.

Anyway, if we assume the elimination of sunset, so to speak would give us one real noon and one mirror noon, I'd guess that we might get a - non-feedback-adjusted - temperature rise looking a bit wobbly, because late afternoon (and after-mirror-noon) probably would give a little dip in temperatures due to air mixing. But potentially a ten-degree rise in temperature per 24h-day until new balance for a normal midlatitude place is reached. But you'd get major effects in the soil and atmosphere systems too, like how far into the ground the heat would penetrate.

The second question. Note first that the tropics are a much larger area than the poles, so cutting incoming radiation by half there if perhaps not a good option.

Plants have adjusted to different energy budgets. So it depends. They have adjusted to seasonal and daily variations too, so giving them constant light might mess up the growth cycle or cause photoinhibition. If constant sunlight means a drier surface (no dew) that would also limit productivity. You might get a situation where C4 plants are globally more competitive, which would be a major biogeographical event and certainly affect what you are likely to eat. The basic thing to remember, however, is that photosynthesis if both rather ineffective when you look at incoming radiation and that a lot of things other than sunlight limit how effective photosynthesis is. Like water availability.
mindstalk
Mar. 29th, 2008 10:52 pm (UTC)
Thanks.
Quick reply: tropics are larger than the poles? The poles themselves aren't much, but I thought the tundra/taiga area of Canada and Russia was pretty extensive. Top-heavy planet and all.

Yeah, ineffective photosynthesis is what I was thinking of -- that plus one ecologist saying that plants were actually 20% or more efficient at low light levels. Which is why I often think of CO2 extraction as the limiting factor... and thought that evening out the light supply would be a net productivity boon, after adjustment. Hadn't thought of dew though, never mind what rainfall would be like in this system.
tempter
Mar. 29th, 2008 11:26 pm (UTC)
I remember learning long ago that plants effectively undergo a kind of rest cycle based on when it's dark out. I don't remember the term for the cycle or process, but I remember the experiment to test it: if you take two identical plants (control and test) and interrupt the test subject's dark time with bursts of light every hour or two throughout the night, the plant ends up dying off. I'm pretty sure the result extends to having no dark time at all.

So, my answer's a cop-out -- I don't know how quickly we'd fry, but I don't think we'd last too long without plant life recycling CO2 for us.
pompe
Mar. 30th, 2008 12:19 am (UTC)
It differs between plants. Some plants are shade species as an example.

About fifty percent of Earth's surface lies between 30 degrees north and south.
martianmooncrab
Mar. 30th, 2008 02:09 am (UTC)
there are several plant species that are dependent on a day/night sked, poinsettas are the ones that come to mind, and the night blooming flowers and plants (and some trees). The nocturnal animals would take a real hit.

My concern for sunlight 24/7 would be about the ionization of our atmosphere, if it didnt dissapate/reduce during the evening, would it build up and totally disrupt all electromagectics? The Oceans would evaporate at a higher rate without the night too.
montyy0
Mar. 30th, 2008 02:40 am (UTC)
I have no real answer, but I have some thoughts on things I've read about that would probably be relevant to the calculations:

the mirror geometry could be important, too. When (roughly parallel) sunlight hits close to normal to the surface, the light-energy-per-earth-area is a lot higher than where it's grazing almost tangentially, like at the poles. That's also going on at sunset/sunrise. So if it were truly "2 noons" then things would cycle from "grazing" to "direct" twice a day, roughly as (.5) ( sin ( ( 2pi / 24 ) hour-of-day) + 1) if there were, say, a sun-equivalent light source at the outer Lagrange point (I can never remember how the Lagrange points are numbered... is that L1 or L2?)

Anyway, the mirrors could both be angled differently ("straight over) non-equatorial parts of the planet, or be curved to magnify or spread out the light in all sorts of ways.

It's often also important in this stuff to consider the albedo of the earth in various places: snow reflects rather than absorbs a lot of sunlight, while plants absorb rather than reflect. I forget where water falls on this scale, but my guess is it absorbs pretty well at normal angles, but it clearly reflects a lot at grazing angles, but by the above argument, it absorbs when there's more light. Clouds reflect incident light, but they also stop light radiated from the ground. Since that's mostly IR, though, I think invisible-to-human water vapor blocks that pretty well, but doesn't block incoming visible and UV light much, IIRC.

Of course, the hotter things get, the less snow and more water vapor you have, so there's a feedback system there. While planting a lot of vegetation will pull a lot of CO2 out of the air, it will also decrease the albedo and absorb more heat, which might melt more snow.

In the absence of changes in atmosphere, I'd expect the radiative cooling to increase, since it wouldn't cool down at night, but not by anywhere near as much as the incoming light's heating went up.

montyy0
Mar. 31st, 2008 04:58 am (UTC)
since I was bored, I figured out for reference that about 40% of the earth is in the tropics, and 10% is in the artic/antarctic, and hence the other 50% is neither.
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