EDIT: To help with the idea of the story, it might help if I put the full title, which I forgot to do.

T.R.U. Stories
Roswell

“How did people live like this?” Sammy asked, his high-pitched, whiny voice carrying over the sound of the engine. “I mean, fossil fueled ground transport, really? How could these people not know they were destroying our planet?”
“It’s not so bad Sammy,” Katarina answered, her gravelly, slightly accented voice a contrast to his. “Remember all of the smog we had to breathe in smack-in-the-middle-of-Industrial-Age London? Forties U.S.A is much better in comparison.”
“I’ll agree as far as air quality is concerned. But the people could be a little more helpful.”
Katarina rolled her eyes. “You always have to have something to whine about, don’t you Sammy?”
“Katarina, would you please, for the one-millionth time, stop calling me that? My name is Samuel. Not ‘Sam,’ ‘Sammy,’ or

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“How did people live like this?” Sammy asked, his high-pitched, whiny voice carrying over the sound of the engine. “I mean, fossil fueled ground transport, really? How could these people not know they were destroying our planet?”
“It’s not so bad Sammy,” Katarina answered, her gravelly, slightly accented voice a contrast to his. “Remember all of the smog we had to breathe in smack-in-the-middle-of-Industrial-Age London? Forties U.S.A is much better in comparison.”
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There's a lot of info crammed into these sentences. Perhaps too much. It makes them slightly clunky. Firstly, why do we need so much detail about their voices?

You also go on to say Sammy whines in the next paragraph, so we don't that info twice.

I'm interested in the ideas, but there doesn't seem to be an imminent plot point here. I can gather they're time travellers straight away, which is great; I just want a story point to latch onto.

But then I think it set me up for a different story. This is what I think your story will contain based on the opening.

Two time-travelers environmentalists reflecting on the history of global warming.

What are they looking at that sparks this conversation? Where are they? Having Roswell in the title automatically made me think of aliens yet they are not alien to this world so were the aliens that crashed actually us in the future or do aliens have nothing to do with the story. If they are us from the future coming back and reflecting on this area then did they also live in the past having been in London during the industrial revolution? It seems like they lived there not just visited and observed. I am more confused after reading this opening, having way more questions and no real answers.

However; 13 lines is tough for me. I always want to see more. I don’t think all stories work for 13 lines and this is one of them because I am still interested. So if I haven’t offended, which was not my intention, send the story.

DC

The characters feel like your puppets, not like they have their own motivations or feelings.

Try just cutting down on unnecessary descriptions. Katrina rolling her eyes signifies that Sammy is a complainer, so there's no need to have her talk about it. Instead, have Sammy react to her eye roll. And there's plenty of time to figure out where or when our time-travelers are - no need to dump it in the middle of dialogue so unnaturally.

This is how I would cut it down: