If you could permanently change ONE thing about the world right now what would it be and why?

That humans are actually the rational beings many claim themselves to be, rather than animals that makes decisions based on emotion and then rationalise why they made the decision.

I feel it would solve all the problems other commenters have mentioned.

So, exclusively rational decision making?
- Pretty much.
  
  The co2 we are emitting into the atmosphere is leading to problems.
  
  Rational response: reduce our co2 emissions as fast as possible.
  
  Our constant population expansion and habitat destruction is causing a new mass extension event:
  
  Rational response: Limit population growth. reduce environmental impact and regenerate damage already done.
  
  Etc
  
  etc
  
  etc.

That no matter what scientists and engineers try, when combusting fossil fuel, the exhaust smells like a terrible latrine.

I think that would spur a quick response from everyday citizens as to what they heat their homes with, or how they get around their cities.

Make it red too
I like the direction of this thought. I’ve imagined the same, until I remembered that before fossil fuels, humans used whale oil for lamps, lubricants, soaps, etc, and cut down trees for heat.

This is one of those “be careful what you wish for” scenarios. 8 billion people burning trees for heat and killing whales for lighting creates a whole new hellscape.
That's a clever one.

Get rid of religion.

Organized religion? How do you define religion?
- Free or not, religion is brainwashing

Get rid of greed

Probanly erradicate akrasia. A lot of the world's problems are down to rampant akrasia on the part of most people.

Akrasia - A lack of self-control or acting against one's better judgment.
Thanks for teaching me a new word!
- Any time! I find being able to put a name to it makes it slightly easier to call ourself out for doing it.

I'd change a single bag of Cape Cod party-sized sea salt potato chips so that it would be at my current location.

The part where the ecosystem is in collapse?

Would that be stopping global warming or all the extinctions?
- Ideally, global warming, but it would be fair to view that as pointless when dissipating the extra CO2 doesn't necessarily return the trees and the problem would degrade again in a couple hundred years. You'd have to introduce a new fuel source that is sought after, clean, and eternal. Which would be two wishes.
  
  So you have to define it as both of your options, since the loss of either worsens the other. Turn the whole environment back to where it was in the 1200s, overrun the streets with bears, see if I care. It'll give 'em something to do. Especially the Amazonian avocado farmers.