Life cycle assessment (LCA)

Which is greener: a cotton tote bag, or a thin plastic carrier bag? Almost everyone says the cotton one. But growing and weaving the cotton for one tote takes so much water and energy that it has to be used many, many times before it does less harm than the carrier. So which one is greener depends on when you look, and on how long the bag is kept. Settling the bag question means looking at every stage of each bag's life.

A life cycle assessment, LCA for short, judges a product's environmental impact at every stage of its life, from raw material to disposal. That span is called cradle to grave. There are six stages: raw material extraction, processing, manufacture, distribution, use, which includes maintenance, and disposal, which could be re-use, recycling, landfill or burning. At every stage, the same three things are checked: the energy used, the materials and water used, and the emissions and waste produced. Washing a reusable bottle in a dishwasher: which stage is that? Use. Maintenance, like washing, counts as part of the use stage. An LCA is not the product life cycle. The product life cycle is the hill-shaped sales graph of a product: introduction, growth, maturity and decline. An LCA tracks harm to the environment; the sales graph tracks sales. One examiner's report, on a question asking for the sales graph of a fad product, notes this: A number of candidates got confused and chose to draw a Lifecycle Analysis cycle. The fix is to read which one the question names. Sales over time means the hill; environmental impact means the LCA.

Here is a simple LCA in full, on a reusable stainless steel water bottle, with one impact written in at every stage. Raw materials: iron ore, chromium and nickel are mined, so land is dug up, habitats are lost, and energy is used. Processing: the ores are turned into stainless steel in furnaces at very high temperatures. That takes a lot of energy, much of it from fossil fuels, so carbon dioxide is released. Manufacture: the steel is pressed, welded and polished, using electricity, and the offcuts can be recycled. Now try the distribution stage yourself: what impact does it have? The bottles are shipped and trucked to the shops, so fuel is burnt and carbon dioxide is released. Use: washing it takes some water and energy, but it lasts for years and replaces hundreds of single-use bottles. Disposal: steel is easily recycled, so its metal can be recovered. Which stages hit hardest, and why is the bottle still a good choice? Raw materials and processing, from the mining and the furnace heat. That big start is paid back over years of use, replacing hundreds of single-use bottles. One report for a question asking candidates to complete an LCA of a product of their own choice says many did it well, with detailed descriptions from sourcing and extracting the materials through to disposal. The stages are learnable, and many students learn them. If you can walk them as the bottle did, you are well placed, and the habit that lifts an answer is an impact at every stage. But not every answer gets there. Another examiner's report records this: Many candidates mentioned only the first and final stages of the lifecycle and did not explain or discuss any of the other stages. A different report adds: Other candidates gave good descriptions of the lifecycle stages but little information on the impact on the environment. The fix is what the bottle shows: write an impact for every stage, the middle ones included, before you move on.

An LCA earns its place by finding the stage with the biggest impact, the hot spot, so the designer knows where to make a change. An LCA of an electric kettle finds that most of its carbon dioxide comes from the use stage, boiling water for years, not from making it. What design change should that finding lead to, and why? Cut the electricity used each time it boils. A window with a one-cup line helps people heat only what they need, so less power is used across the kettle's life. Less electricity means less fuel burnt at the power station, and less carbon dioxide. Changing the case material would barely help, because making the kettle is a small share of the total. Now two answers to a different question: give one result of an LCA that could cut a desk lamp's impact. The first writes: an LCA looks at a product's impact from cradle to grave. The second writes: the LCA found most of the lamp's carbon dioxide came from electricity in use, so fitting an LED bulb cuts it. Which answer does what the question asked, and what does the other do instead? The LED answer. The question asked for a result, and it gives a finding and a change. The first only defines an LCA, so it answers a different question. One examiner's report, on a question about a laptop bag, puts it like this: Many candidates offered a definition of a LCA as opposed to an explanation of an outcome of a LCA that could help reduce the environmental impact of the laptop bag. The fix is to start from the finding and end at a change: this stage is the hot spot, so change this.

An LCA can also choose between two products, so back to the two bags, stage by stage. Raw materials: cotton is farmed with large amounts of water, pesticides and land. Polythene comes from crude oil, a finite resource, but each bag uses very little. Processing and manufacture: spinning and weaving cotton takes far more energy per bag than making thin polythene film. Distribution is similar for both. Now try the use stage: how does each bag do there? The tote can be used hundreds of times. The carrier is often used once or twice. The last stage: the tote biodegrades. Polythene can last for centuries, on land and in the oceans. So which bag is greener? Give the verdict, and what it depends on. Per bag made, the tote does more harm, mostly from growing the cotton. Per shopping trip, it only wins if it is used many times. The carrier's lasting harm comes after disposal, as litter. Another examiner's report, on a question about carbon dioxide released over a product's life, says the best responses clearly considered the full life cycle of products. That is what the bag comparison just did: every stage of each bag, not only making it or throwing it away. Take the same habit into any answer about a product's impact. An LCA can weigh people too. Choosing materials on ethical grounds means weighing their ecological footprint, the harm to the natural world, and their social footprint, the effect on the people who grow, mine or make them. For the tote, that means asking who grows and picks the cotton, and in what conditions. Fair trade cotton is one way to lower that footprint, and the video on fair trade covers it.

A last set of questions now, ending where the two bags began. What are the six stages of an LCA, in order? Raw material extraction, processing, manufacture, distribution, use, and disposal. Now another: an LCA of a TV finds most impact is in use. Which design change? Cut the electricity it draws, with a more efficient screen or a switch-off when nobody is watching, so less carbon dioxide over its life. Here's a different one: a hill-shaped graph of sales over time. Is that an LCA? No. That is the product life cycle, a sales graph. An LCA tracks environmental impact, stage by stage. Now back to the bags: is the cotton tote always the greener choice? No. It does more harm to make, so it only wins once it has been used many times.

If you can walk all six stages and turn a hot spot into a design change, you've fully understood it. A thumbs up now crosses this one off for good. If not, keep it saved, because a method like this often lands on a second viewing.

Next in the chapter is Judging a new technology's environmental impact. Hopefully that's next in your feed, but if not, search for ScholaFly D T oh seven, oh seven.

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