One of the most helpful frameworks for thinking about sustainability is the concept of 'environmental space' - how much there is altogether, how much we've used up and what's our fair share, with respect to things like CO2 emissions, wood, water, minerals, wilderness, etc. Recent analyses tend to suggest that on several of these counts we in the rich North are several times over the limit and need to reduce our occupation of 'space' by 50-80% in order to achieve a situation which is both sustainable and equitable.

This looks challenging, but it's not impossible. The main question that comes out over and over again is: can it be done without affecting our standards of living? Or, to put it in a slightly different way, will it involve substantial lifestyle changes? The respectable establishment wing of the sustainability movement argues that major lifestyle changes are a political impossibility, so for reasons of sheer practicality we must put all effort into technology, regulations, taxes, treaties and other such mass instruments. In contrast the more radical hands-on wing argues that 'technical fixes' will be nowhere near enough, and that we have to bite the lifestyle bullet sooner or later - so why not now?

At CAT we have assumed that the best strategy would be a combination of both approaches. Although we have tended to specialise on the lifestyle side of things, looking back over the past 25 years it looks as if the Big has achieved more than the Small. Since the mid-70s many, perhaps most, environmental indicators in the rich North have improved, and this is not because householders have changed their behaviour. It is largely through far more efficient technology, and through industry being forced to clean up its act by strict environmental standards. It's us consumers that now lag far behind, and it's about time we got our act together.

Simple environmental actions such as recycling bottles, composting, and installing double-glazing are widely practised. These are fine, but is there any way to know what good they are doing, if any? How do we measure the 'sustainability value' of such an activity compared with others we might undertake? Can we make up coherent packages of measures that deliver the most for the least cost or effort? Without some kind of evaluation, the process risks degenerating into 'eco-chic' - the green equivalent of charming inessentials like quadrophonic hi-fi or medieval calligraphy.

doing the numbers

To cut through the clutter of inessentials needs a hard-headed, quantitative, sceptical mode of thought which constantly demands evidence and which questions the answer you first thought of, then goes back and questions itself. We hasten to add that this is not the only mode of thought required! But it is one which serious greenies must have in their repertoire to help us overcome our fanciful illusions of appearance or wishful thinking. We often call it 'Doing the Numbers'.

Let's take an example. Low-flush WCs reduce water consumption to around 2.5 litres per flush, potentially saving around 20% of total household consumption. Therefore they are A Good Thing. But environmental enthusiasts might feel that dry/composting toilets are An Even Better Thing, since they use no water at all and could save 30% of household water consumption, not to mention various other environmental benefits. Should we therefore recommend dry toilets? Let's feed in a few numbers. What do they cost? Assuming there is an existing sewer connection, posh continental low-flush WCs cost about £200 to buy and install, while a dry toilet with the equivalent user-friendliness would be at least £1000, probably more if extensive rebuilding were required. How much do they save? Let's assume that the household uses 100 cubic metres (m3) of water a year, that the WC saves 20m3 and dry toilet 30m3, and that they last 25 years. These are all middle-of-the-road assumptions. Crudely, over 25 years the WC will have saved 500m3 at an investment of 30p per m3, while the dry toilet will have saved 750m3 at about £1.30 per m3. In this case the WC has been more cost-effective in that it has left us with spare cash to invest in other environmental improvements. The calculations alter a bit if the water is metered, because these installations are then saving money as well as water. Ignoring the complications of interest and inflation, if water costs £1.50 per m3 then the WC pays for itself nearly four times over and 'earns' a surplus of £550 over 25 years. The dry toilet covers its costs with a surplus of only £125.

Now you can go back over this train of elementary calculations putting in different figures (what if you make your own? What if there’s no sewer connection? What about the benefits of nutrient cycling? etc).

That's the householder's view. What about from the perspective of society as a whole, and what we would support as a desirable environmental policy? We must surely admit that where composting toilets are concerned, take-up is likely to be patchy even with the strongest encouragement. It would be surprising if more than a few hundred thousand British households were both willing and physically able to install a dry toilet. Say a million at the outside: that's about 2% of the population. The overall effect is then

30% x 2% = 0.6% reduction in national water use.

Contrast this with ultra-low-flush toilets. Since these toilets are almost indistinguishable from the standard pattern, it is quite plausible to suppose that they could become standard and gradually replace 50% or more of existing toilets. Although they save less per toilet the net effect would be

20% x 50% = 10%, sixteen times better.

We can multiply such examples endlessly. A well-conceived mainstream approach is often more replicable and so outperforms gung-ho greenie perfectionism. So in such cases, why not let the mainstream get on with what it's good at? The difference between our bit of society and the mainstream is that we are culturally much more flexible, and free to explore a far greater range of options, some of which may turn out to be a vital part of the solution in ways that we cannot yet guess. But the logic suggests that this should be approached in a special way. Since we do not know all the answers, at this stage of the game it might be more important to explore options that generate useful information than to be personally ever-so green. There are so few of us that our personal eco-rectitude makes a negligible difference physically, but clear results widely reported could amplify our efforts by an enormous factor. A result is worth 1000 recycled bottles!

going for the jugular

As to the relative time to be spent on actual topics, it's probably sensible to relate this to their general significance in environmental impact, and to what can be done at the 'household level' to influence them. In our experience the most satisfying way to approach all this is to specialise: pick a particular area of life which you are content to be mildly obsessed by, then give it the Treatment. Start by collecting as many public statistics as you can about this bit of your life and how it fits into the overall picture. Then start collecting your own data and relating it to other things in your life. It quickly starts to put things in perspective, saves you money and effort, and helps you gauge if you’re actually getting anywhere towards your own definition of sustainability. Let's look at some more examples.

transport

This is important because it's likely to be a big a chunk of your ecological footprint. If you don't have a car, you're already miles ahead of nearly everybody else. If you do have one, you will be embroiled in the oddity that small improvements on the car can outrank many more-eco-sounding actions elsewhere on the energy front. Say the car uses 30 gigajoules of energy a year. Good tuning, tyres, cap and leads, timing, plugs, taking off roof-rack when not needed, good road atlas so you don't get lost (say £50 extra a year) could improve performance by 10% - saving 30GJ over 10 years, cost 17 pence per GJ saved. Compare a commercial solar water-heating system, which might save the same amount at an installation cost of £3000: £100 per GJ saved: over 5 times less cost-effective. Weirdly, if you’re a car driver, learning how to clean plugs, to change and set points, to change oil and air filters, and to check tyres and battery could lead to greater environmental improvements than many other things you could do.

But cars and personal transport are a wonderful source of eco-numbers. Get a picture of your household annual mobility by keeping a log of trips and mileage, who went, how long it took, fuel etc. (keep it in the dash, with a pencil on a string). Also record costs of spares and repairs, tax and insurance etc. Just the baseline data are often sobering enough, revealing the effects of, say school runs, but observe differences as a result of, say, changing the model or better servicing. Or compare with a friend who has different circumstances. What are the critical variables?

Then there's the environmental cost of your annual holiday in the sun. A return flight to Majorca uses 10GJ, similar to average annual water heating. A single to Melbourne uses 72GJ, equal to all other annual household energy. Moral: it makes little sense to bust a gut trying to save a few megajoules here and there when you blow ten times as much in one throw and hardly notice. If you come up with a nice example from your life, be sure to publish it!

other sectors of domestic life you could have a go at:

Water: Get a meter fitted. Usually they'll fit you one free, and after that bills will be cheaper if you are remotely water-conscious. Take readings every week and compare changes as result of various changes in the house.

Solid waste: Sort into categories and weigh them for a period, log how much and what happens to it, how much goes to the dustman, re-use, recycling, composting. A set of digital scales is very useful, and would allow you to measure material inputs as well as outputs, although this takes a lot of discipline!

Fuel and electricity: The quarterly gas and electricity bills can tell you quite a lot, but you can read the meters at any time you like. For electrical appliances there’s a neat device called the 'Costplug' which measures the instantaneous consumption of an appliance, then tots it up over a period of use.

Food is tricky. Although it's fairly obvious that changes in diet and purchasing habits are more cost-effective than growing your own, food statistics are harder to collect. Worth a try though: record weights and prices of stuff from various sources to build up a rough picture of your food system. How much local? How much organic? How much fair-traded?

recording and displaying the numbers

A general-purpose hard-backed logbook is best for simply dumping numbers and observations into. You can come back later to make sense of them. If you have a home computer, the sky's the limit. Spreadsheet software such as Excel makes the recording and analysis quick and easy. It's also another potential hook to get the kids involved. Full colour charts can appear at the drop of the hat, allowing easy interpretation of the pros and cons of any eco-tweaks. It's often more important to see the rate and direction of change than the actual values themselves. Spreadsheets also allow ballpark estimates of the potential gains if everyone in the street, town or country followed your examples.

It would be a shame if the Big Players succeeded in delivering sustainability entirely through top-down measures. We suspect that rational lifestyle changes can get us very close to our fair shares in environmental space faster, and with a higher quality of life. But we need the figures to prove it. So come on, let's get started!

Paul Allen enjoys playing about with quadraphonic hi-fi while Peter Harper likes medieval calligraphy and has a dry toilet. Neither of them have cars.