People per se have almost no impact on climate. It's what people do and how much in aggregate they do it.
Environmentalists are often stereotyped as pessimists, but really most of the people I know who've dedicated their careers are optimistic that technology can address many environmental problems. Sure, they'd like to see the global population stabilized, or even somewhat reduced, because that makes the job of preserving the environment much easier. But they actually believe the sustainability problem can be licked, even without reducing the global population by much.
I'll give you one example of how an actual environmentalist thinks. I was at a meeting with the sustainability director of a major sportswear manufacturer, and he was describing the research they were doing into improving the recyclability of polyester fleece clothing. He made the point that scale is critical to assessing the environmental impact. For a small band of hunter-gatherers, wild animal pelts would be the source of clothing with the least impact; wool would have intermediate impact; a chemical plant that reprocesses coke bottles into polyester resins would have a ridiculously large impact. But if you are making hundreds of thousands of garments, the impacts are actually reversed: the chemical plant has the least environmental impact. Once you turn those bottles into fleece you can continually recycle those molecules into more fleece. He describes recycling as "living off your environmental income instead of your capital."
Environmentalists -- by which I mean the people who are actually working on solutions to environmental problems -- generally believe that even with a large population we can make use of the products of ecosystems without disturbing the equilibria that sustain those systems. As one civil engineering environmentalist I know put it: I = P*S/T ; impact is proportional to population and standard of living but inversely proportional to technology. You can reduce the environmental impact of home heating by reducing the number of people; or you could do it by people getting used to being colder. But you can get the same result by insulating your house and heating it with renewable energy.
It's actually the anti-environmentalists who are the pessimists; they don't believe in people's ability to adapt, and they anticipate nothing but suffering from trying to do anything about problems. Their version of "optimism" is to discount any evidence that problems exist, or to convincing themselves if we do nothing everything will work out for the best.