Think about it. A stroller is made up of aluminum, plastic, and nylon and polyester.
The aluminum is okay. It's a naturally occurring metal that can be recycled and reused in a number of ways.
But today's strollers are comprised of large quantities of plastic. The wheels are plastic, the handles are plastic, the cup holder is plastic, etc. Plastic is a petroleum based product. I read somewhere recently that plastics account for 8% of petroleum use. Plastic doesn't break down either. The pieces get microscopically small over time, but they are still there.
Nylon is a petroleum based product too and does not break down easily. That's why people will periodically dredge up old fishing line that somebody dumped in yester year. Fishing line, in the modern age, is made from nylon. Those mesh under carriages of strollers are made from nylon.
But lets talk about the part that your baby is sitting in, sucking on, chewing on, etc. That is made from polyester or some other plastic substance. Polyester is a fabric blended from plastic. Lollidoos are made from polyester, but it's recycled. Most strollers are made from virgin or new plastic polyester threads. It also won't break down in a landfill.
Additionally, my research shows that most strollers are made in China. These products are put in a shipping container which is loaded onto a barge. Once at port, the container is loaded onto a truck. That truck is brought to a distribution center where it is put on a different truck with other goods. Then that truck travels however long it takes to get your stroller to your front door or your local big box store. There is only one stroller not made in China. It's made in Italy.
So slings are great and greener. They can be made from organic cotton, silk, or linen. The ring slings can use that aluminum that I mentioned. There are slings made in the US. Although I wish there were more textile factories here in the US, you may be lucky to find fabric milled here and reduce your carbon footprint even further. The same can be true of mei teis and of wraps.
Do your research. Where is your baby carrier coming from? What is it made out of? Is it biodegradable? These are all important questions to ask when buying a baby carrier.