First, I want to stress that sub-zero temperatures are really cold. And you should know your limits, as well as make conservative decisions while out in the cold, especially with your baby.
There are a many different reasons we take our babies (and by babies I mean babies and toddlers) out in the cold. We need to go out to the store, or to a meeting, or general errands, story time at the library, sending an older sibling off on the bus in the morning or outdoor chores.
More reason we might take our babies out in the cold is that we are out skiing or snowshoeing or just taking a brisk walk around the block a few times to get our blood moving and get out of the house for a bit. It's good for the soul and makes us happy, healthy parents.
For general errands, I don't worry too much about my baby getting cold. I put him in a couple layers, put his little wool socks on him and this amazing wool hat.
Then I put a ring sling on, put my coat on and pop him in and out from his car seat. The same pop-in and pop-out idea works well with nearly any carrier if you tie it loosely and put your coat on over it. If I feel it's breezy or too cold, I do just wrap my coat around him, use the ring sling tail as a wind shield, and kind of hunch over with my back to the wind. If you aren't sure how to make it work, try it at home in the warmth of your living room a few times to get it down.
The reason I don't worry too much about him getting cold on quick trips is because he's up against my body and my body heat keeps him warm. Plus, that little hat is amazing. I feel like it keeps the wind from going all the way to his core. If I feel like it's going to be too cold, I put a little fleece outfit on him, which could be his fleece footie jammies. The issue is also that I don't want bulky clothing on him in his car seat.
Now of course there are those times when you will be out for extended periods of time getting your Christmas tree, cross country skiing, snow shoeing, or whatever you like to do in the winter snow and cold. (You probably want to refrain from things like downhill skiing and ice skating with a baby on your back.)
If you aren't into buying extra baby gear, you still have options. This is a great example:
She put her baby on her back in her Ergo and put her husband's coat on backwards. Of course she needed help to get it zipped up, but it worked and she got out and had a great time in the great outdoors. (Notice also that she's using poles. A great way not to fall down!)
Along the same lines you can wear a too big coat the right way and the baby on your front. Many of us have husbands that are bigger than we are, so that can be convenient. And inexpensive.
Here's another example:
It was a gorgeous day that day and I felt that I didn't need a ton of layers. So I just wore a base layer with a fleece over it and put him in a few fleece layers, including that awesome hat and wool socks and these awesome little wool mittens. (He got all his awesome wool for Christmas.)
When I got back to the car and all back in his seat I felt his hands and feet. He was toasty warm still. He also slept through the whole adventure. (Notice also that I have poles. Again, they are a great way to not fall down with a baby on my back!)
Something else you can do is put your baby on your back and wrap a large blanket around you and the baby. This is basically what many traditional cultures did, even using the blanket as the carrier. You can find some good instructions with photos for a front carry here.
And while we're on the subject of traditional carriers, we can't forget the Amauti. The Amauti is a traditional carrier of the Inuit people. It's a coat that goes on over your head and it has a large pocket in the back. It also has a large hood that goes over both you and the child. On first glance it may look like the child is in the hood, but that is not the case.
Here the baby is pretty bundled because it was so cold. But again, after our trek out to get the Christmas tree I checked his hands and feet and he was toasty warm.
And here's a picture of it with the hood over both mom and baby.
The trick is finding an Amauti. The design is legally protected by the native people and they are not mass produced. But with a little determination and a little help, it's very possible to find one. There are individuals out there who make them or they are available used as well.
There are other great options when it comes to clothing made specifically for babywearing in the cold. There's the Peekaru fleece vest:
This vest can be worn with a front or a back carry over your favorite carrier. On not-so-cold days you can wear it with just a base layer. On colder days where you need a coat, you can put your baby in a front carry and put a coat on. You probably won't get it zipped, but you are both still plenty warm. The same company also makes an amazing coat, The Peekaru Soft Shell, that can be worn in either a front or a back carry.
Another option is the Kindercoat. Made by Childrens Needs, a company out of Utah. It's a great option for both front and back carries with your favorite carrier. And the way it's made, it converts easily into a nice coat when you are not babywearing. Check out their many other outerwear options as well.
If you are looking for something more fashion conscious, the MCoat may be be just what you are looking for. It's an amazing down coat that works as a maternity coat, a babywearing coat, and a regular coat with just a couple zips. It doesn't work in a back carry, but it is a gorgeous and warm piece of clothing.
Yet another option is a simple accessory for a carrier. Catbird Baby makes a carrier cover that tucks around your baby and creates a windproof barrier as well as providing warmth. It's inexpensive and very practical.
If you are more of a do-it-yourselfer, there are patterns for babywearing coats like this one or you can just make something up.
Obviously this is not a complete list of all the cold weather babywearing options, but it ought to be enough to get you started and get you thinking. The whole goal is to keep your baby warm while getting out and about, whether it's a quick trip to the store, outdoor chores that must be done, getting an older kid on the bus, or an all day excursion in the great outdoors. Basically, you can't do it wrong if your baby is warm and comfy and you get done what you want and need to.