What keeps the sky from being lit up by stars at night ?

1  Human generated ambient light. Nearby lit areas are easier for our eyes to catch than more distant light. The nearby light drowns out more distant light which is why many call it light pollution.

2  Time. Light has a speed limit. While some of it is making its way towards us a lot of it is still on its way here (some of the stars that we see light coming from have actually burn out). Every star is being shown to us at a different time period.  We only see light from parts of the universe that are less than 15 billion years old.

3  Olbers Paradox states that the universe expansion is what causes light to be redshifted which lowers its frequency and energy.

Assumption:  A bright light should be seen in every direction.  Objects farther away are fainter but more numerous which should yield the same overall brightness.

Resolved !  The expansion of the universe known the cosmological constant, resolves the paradox. As the universe expands, the light waves are stretched out reducing the energy sent out from galaxy to another. Also, the time to receive the light is lengthened over the time it took to emit the photon. Because luminosity = the energy/time, the apparent brightness is lowered enough by the expansion to cause the sky to be mostly dark.
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4  Light from distant stars is less intense because the photons being emitted are emitted over a larger sphere area, fewer enter people's field of vision. There's also gravity (curves space) and interstellar gas like hydrogen and helium (blocks light).  General relativity shows how gravity is a curvature in spacetime.

5  Distance !  proven by the inverse square law

Climate change enlarges total fertile area while reducing prime locations, C4 replaces C3

  Climate change negatively affects the growing season for areas in the mid latitudes traditionally associated with farming & wheat production (in part due to more super wet regions, change in night temperature, wild shifts in temperature over a short period of time) but it does enlarge the total land area globally able to be used for growing through a warming of the northern latitudes (shifts precipitation over to rain from snow).

In Western Canada for example in 2010 more than half of the lower latitude regions of the key provinces of Manitoba and Saskatchewan received more than two feet of water in the fall alone, that's more than what all of Western Canada usually averages per year. This presents new problems; In Canada for example by 2050 optimal growing conditions will shift to as far north as the Canadian Shield where for the last century the area has been infertile or at the very least, difficult to grow vegetation on. Photosynthesis shifts from C3 to C4 allowing for better water efficiency use and faster carbon dioxide intake (important since CO2 levels are also rising). Under phosphorous stress both C3 and C4 plants show similar biomass production even though the root to shoot ratios are not the same.

C3 - Only enzyme used (rubisco) is also involved in CO2 uptake. Better than C4 under temperate conditions (cool/avg light intensity).

C4 - CO2 initially incorporated into 4 carbon compound. Differs from C4 since it makes use of both rubisco and pep carboxylase (pep is responsible for both the uptake and delivery of CO2). rubisco ends up preocupied with CO2 deliveries from pep and so it does not have the opportunity to also take in o2 for photorespiration. pep is also faster than rubisco at taking in co2 and that means stomata aren't open as long (less water ends up transpired and so C4 is more efficient at photosynthesis in less moist conditions).

-More co2 is absorbed, less water is used, photosynthesis only takes place within a specialized part (kranz cells), an anatomy feature unique to C4 plants; that makes C3 photosynthesis simpler and less energy intensive.
-C4 plants end up with a higher shoot to root ratio (better suited to high light intensity/competition for light becomes more of a factor).

root (supportive tissues)/shoot (growth tissues) ratio - decreases with plant size, changes according to nitrogen availability (fertilizer).