Having travelled to the future and seen the end of humanity, a misanthropic scientist has one year to change events that lead to world war 3.

misanthropic seem to be a better and shorter description than increasingly pessimistic.
the fact he is in simulated loop is just an artefact of the plot, so I left it out of the Logline.

The original Logline lead me to more questions:
is he really trapped in the simulation?
if it is the loop what is the time constraint?
what and who is the antagonist?

>>>back in time

Back how far?

>>>back in time

Back how far?

Trapped in a simulated loop of Earth's final year, a scientist must relive an increasingly strange sequence of events before trying to change history.

get rid of the words "to be" they break the flow

Why must HE care? how does it personally matters to him?
Why NOW? It seems to lack an Inciting Incident

Also, if it's a loop, then the clock isn't ticking right?
because If they fail to train themselves (i am guessing) "the final year" will start again..