What does it mean to knit green? The word 'green' is useful because it concisely bundles all kinds of environmental and social values. But with this convenience comes imprecision: my definition of green (natural, organic, local) may not match someone else's definition (vegan, fair trade).
Here's a list of values that can mean green. To see the yarns that match yours, check out my yarn chart.
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'Sustainable'
A sustainable activity is one that could be carried on indefinitely, one that doesn't degrade or use up the resources on which it relies. When applied to yarn, we might mean that its production doesn't significantly erode the soil, deplete water resources, release toxic pollutants, or have high petroleum inputs. This is what I fundamentally think of when I talk about something being "green" or "eco-friendly." It's a useful way of framing our yarn evaluations. But remember: every product consumes resources in some way, and you really have to be thoughtful when comparing products that consume different resources. Which do you think is more sustainable: organic cotton (high water inputs, low chemicals) or bamboo (lower water inputs, high chemicals)? Local, conventionally-processed wool (inputs of antibiotics, insecticides, detergents, etc.) or organic wool from across the world (petroleum inputs for transport)?
'Natural'
A natural fiber is one that's derived from a plant or animal, as opposed to synthetic sources. Many people equate this with "sustainable" or "eco-friendly," but this is a dangerous conflation. A natural fiber is not necessarily a green fiber. Cotton is natural, but as a crop it's incredibly water- and pesticide-intensive. Wool is natural, but myriad toxic chemicals go into growing, processing, and dyeing it. Cashmere is natural, but its production is contributing to massive desertification in China. You get the point - natural fibers can be grown and processed in highly sustainable and chemical-free ways, but it's a misconception that they're inherently green. Even with natural fibers, you'll need to investigate their environmental footprint.
'Organic'
USDA organic certification* is one of the clearest tools we have for identifying eco-friendly yarns, but it's not perfect. To be certified organic, fibers must meet the same standards that we have for food: crops are grown without synthetic agro-chemicals, while livestock can't receive routine antibiotics or parasite dips and must be raised on organic pasture and feed. But the catch is that current USDA organic standards only cover how the plants or animals are raised. Yarn sold as organic could have been chemically scoured, bleached, spun with petroleum-based oils, and dyed with metallic dyes. So although the organic label can be deeply meaningful, it's not the end of the story . For a yarn made with organic fiber, you'll still have to consider the processing, dyeing, and shipping. And because getting organic certification can be quite expensive, you'll find lots of environmentally-friend yarns out there - artisan wools, alpaca, hemp - that aren't certified organic.
*Internationally, look for certification by the Soil Association (U.K.), IVN (Germany), KRAV (Scandanavia), JOCA (Japan), and EKO or Skal(Netherlands)
'Vegan'
Vegan yarns contain no animal or insect product. Anything that uses even a small amount of an animal, such as dye from cochineal insects, is not vegan. It doesn't matter whether the animal is killed in the process or not. For example, some wild silks are sold as 'vegan' silk because the silkworms aren't killed in the process. But since the fiber is derived from an animal/insect, it's not accurate to call it vegan.
Personally, I have misgivings about giving the green light to a fiber just because it's vegan. A crop grown with lots of chemicals (conventional cotton), derived from petroleum (acrylic), or processed with toxic chemicals (bamboo) doesn't seem to me to be a sustainable choice. This isn't to say that choosing a yarn just because it's vegan is wrong; it's just that it's not necessarily green.
'Recycled'
There are two ways to think about recycled yarns. The first way - reused yarn - includes yarn from unraveled projects (e.g. thrift store sweater) or creatively fashioned from other existing products (e.g. strips from plastic bags or blue jeans). The second way - yarn spun from recycled fibers - usually comes from pre-consumer materials, like leftover fabric from mills. Probably the most popular example is recycled sari silk yarn, but there are also ones made with recycled cotton scraps. Using recycled materials is environmentally-friendly because it takes usable fiber out of the waste stream. But it's overall impact is very small, as it would have to be done on a large scale in order to reduce overall demand for new yarns.
'Socially-conscious'
I usually take this to indicate that a yarn was produced in a way that furthers peace, justice, or human well-being. For example, there are yarn companies that try to support indigenous fiber cooperatives, maintain traditional fiber arts, and bring together historic enemies with a common economic activity. The Fair Trade movement covers a specific agenda and certification within this category. It promotes fair compensation for producers that have been historically marginalized, often with additional attention to gender equality, labor conditions, and environmental protection.
This is a tricky category for those of us who think of 'green' as an environmental issue. But there's a strong argument to be made that a robust handicrafts or agricultural industry can give people the resources they need to protect their environment.
'Alternative-fiber'
This term covers fibers derived from non-traditional sources, such as soy, corn, milk, bamboo, seaweed, nettles, and wood cellulose. Sometimes the rarer natural fibers, such as hemp, quiviut, or yak, are lumped into this heading as well. These fibers are often presented as if just being outside the mainstream, and thus having the potential to displace cotton or wool, somehow makes them more sustainable. Not true! Being "alternative" doesn't say anything about the environmental impact. The yarns that use an industrial byproduct, such as soy (derived from a tofu byproduct) or some lyocells (can be derived from lumber scrap) are making a usable product out of waste, but they often require toxic chemicals for processing. And corn? milk? The crops themselves are as mainstream and resource-intensive as it gets.
'Local'
Buying local can be eco-friendly in three ways. First, it can reduce emissions from transportation (you'd be amazed how many yarns criss-cross the world for processing, dyeing, etc before they reach your local yarn store). Second, it may help preserve agriculture in your community and thereby promote awareness of local environmental issues. Third, it can give you more of a view, and sometimes a voice, into the fiber production. That said, buying local often means making a trade-off with other values. You may not find the specific fibers you want, and they may not be organic, chemical-free, etc.
To find local fibers, I'd recommend visiting farmers' markets, inquiring at your local yarn store, and checking out websites like Local Harvest (for all kinds of products) and Alpaca Nation (for local alpaca farms).
'Humane' or 'Cruelty-free'
I think the issue of cruelty most often comes up with regards to the killing of silkworms in silk production and the practice of mulesing in merino wool production. Cruelty can be a slippery concept, though. To start with, definitions of cruelty can vary widely (do you think killing is always cruel, for example?). Then there's the fact that cruelty doesn't happen in a vacuum; unsettling practices often represent a tough call between suboptimal outcomes, and in that context it's difficult to categorically condemn a practice as cruel.
That said, I think there's some reason for environmentalists to pay attention to claims about cruelty. Cruelty and greenness aren't always connected, but there is often a relationship mediated by the issue of health. Fiber animals that are raised in healthy, happy conditions seem to me to be less likely to harbor diseases that could spread to other animals and disrupt the ecosystem. Factories that reduce or carefully control their toxic chemical use will be safer for both the workers and the environment.
'Naturally-dyed'
Natural-, plant-, or vegetable-dyed means thats the dyes used for the yarns are derived from leaves, flowers, bark, roots, lichens, mushrooms, and sometimes insects like cochineal. There's a substantial debate over whether naturally-dyeing is greener and safer than synthetic dyes, especially the low-impact acid dyes. It's hard to generalize about its greenness because natural dyeing is as eco-friendly as the dyer wants it to be. At its worst it can involve toxic mordants, overharvested dyestuffs, and lots of energy and water. At its best it can be done with non-toxic mordants (e.g. alum, tannins, and cream of tartar), sustainable harvesting, water recycling, and solar heating. The latter description is how I do my own dyeing. If you're ever in doubt about a particular product, just ask the dyer. In my experience, most artisan natural dyers are proud of their craft, were drawn to its connection with nature, and are happy to talk to you about it.