600,000,000: End of C3 photosynthesis: One of the three types of photosynthesis will no longer be possible. 99% of all species will die.
I understand their reasoning, but that's ... highly conjectural. [Also, there are more than three types of photosynthesis. Currently-extant
plants use three types of photosynthesis, but plants are by no means the only photosynthetic organisms.]
But let's break that down a bit. Here's a very abbreviated synopsis of photosynthesis in plants.
The great majority of plants alive today use C
3 photosynthesis. As I'm sure everyone knows, in photosynthesis, plants use solar energy to do 2 things. First, captured solar energy is used to split water molecules into hydrogen and oxygen. The oxygen is a waste product and is expelled; the hydrogen will ultimately be used in
carbon fixation. The second thing plants [indirectly] use captured solar energy to do is make
adenosine triphosphate (ATP), which is used to transfer captured solar energy within cells.
Then, using the ATP as an energy source, the plant combines the hydrogen with CO
2 to produce organic molecules such as glucose. This is carbon fixation -- conversion of inorganic carbon dioxide to organic molecules such as glucose.
Now then, in C
3 plants, the primary enzyme that allows carbon fixation is
ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly abbreviated as RuBisCO. RuBisCO-assisted carbon fixation works most efficiently at temperatures of around 25 degrees C. As temperatures rise above that, the reaction becomes less efficient (because RuBisCO can't bind to CO
2 as effectively), and the enzyme will begin to catalyze what is essentially the reverse reaction. That is, instead of catalyzing the building of organic molecules from hydrogen and CO
2, the enzyme begins catalyzing the reverse; breakdown of organic molecules into their inorganic components -- this is
photorespiration.
At temperatures above about 35 degrees C, photosynthesis becomes so inefficient in C
3 plants that they may lose more glucose to photorespiration than they can produce. This is especially true if it's both hot and arid, since a C
3 plant needs a lot of water in order to maintain photosynthesis at high temperatures.
Some plants, known as C
4 plants have a slightly different carbon fixation process, and can continue to photosynthesize at temperatures where photosynthesis in C
3 plants shuts down. C
4 plants still need a good deal of water to do this, but these plants -- including many grasses (including sugar cane and maize) -- do well in hotter environments, so long as they have sufficient water.
A few plants use
Crassulacean Acid Metabolism photosynthesis; these CAM plants are mostly adapted for hot, arid conditions. Like C
4 plants, they can maintain photosynthesis at higher temperatures than can C
3 plants, but they're much better at conserving water than are C
4 plants. (CAM plants include Cacti and many succulents.)
So why do the vast majority of plants still use C
3 photosynthesis then? Well, so long as the temperature doesn't get too high, C
3 photosynthesis is
much more efficient than is C
4 or CAM photosynthesis.
So, why will C
3 photosynthesis supposedly be impossible in 600 million years or so? Well, as the Sun "burns" through its hydrogen reserves, the helium "ash" accumulates in its core. This causes the pressure, and therefore the temperature to slowly rise. Accordingly, the Sun is sloooooowly getting brighter over time.
This, by the way, has essentially
nothing to do with climate change here on Earth. Though the Sun
is getting hotter over time, we're talking on timescales measured in tens of millions of years. The heating of the Earth is
far to rapid and far too drastic to have anything to do with the increasing luminosity of the Sun.
Anyway, it's estimated than in 600 million years or so, the Earth will simply be too warm for C
3 photosynthesis. But 600 million years is a
loooong time, even on an evolutionary timescale. After all, 600 million years ago, the very first multicellular organisms were evolving -- think of how much evolutionary change has occurred since then.
As such, I wouldn't be surprised if more efficient forms of photosynthesis evolved within that timescale. Even if that doesn't happen, what I
would expect to happen is that C
4 and CAM plants will gradually replace C
3 plants as the dominant plants on Earth.
Of course, a point will eventually be reached where even C
4 and CAM photosynthesis can't occur. At that point, assuming no new form of photosynthesis has evolved, then pretty-much everything on Earth other than bacteria and archaea will be thoroughly screwed.
Re: Drive by science
title is not entirely correct. It's just that some simulations have shown that a body 1 - 3 Earths in mass could account for Uranus's unusual rotational axis and other characteristics.
I don't really know myself, so I'm just guessing. It looks as though it's a magnetic fluid being pulled by some magnets behind the backplate. And the magnets are either being moved by some mechanical contraption that moves them closer or further away from the backplate and perhaps up/down as well to lift the fluid up from the bottom, or (more likely) it's an array of electromagnets that can be switched on or off as required. 
Linear Mode
