What do you think caused this beam of light ?

I'm speculating pretty wildly here, but things I can think of that might have caused it:

Exotic weapons testing (space lasers or whatever)

A strange lensing effect from an aircraft or cloud

Some kind of accelerant was poured onto, or generated in, that patch of grass, and the sun was able to ignite it.

A nearby building had a concave, reflective surface, aimed such that light hitting it would all be focused on that one area, and the offending building either was new, or was somehow getting more sunlight than it usually does.

It slipped through a hole in the ozone layer.

Emissions Scenario A2.

Water vapour feedback.

That's one hell up an 'uptick' you've got there.

Scientists at the scene agreed, "It's worse than we thought"

This is what they meant when they said "and if it continues at this rate"

It's climate change! (definitely not weather)

This is exactly what the models predicted.

The Illuminati, obviously.