Each one of us (the human body) burns an average of about 100W, ie ~2kWh/day, which is getting on for 30% of my home's household energy use here in London for three of us, roughly the same as our electricity use. And food is not a very efficient way to capture and deliver that energy compared to (say) mains natural gas for heating, so that 100W probably has a disproportionately-high carbon footprint compared to our energy uses.
Food miles are only a small part of the story: eg less energy may be used in growing a tomato in Spain and shipping it to the UK than in growing it in the UK with the assistance of more heating and fertiliser but closer to the consumer.
Chef Barny Haughton said, "If you really want to cut down on the carbon footprint of your food eat seasonally, locally, and limit the amount of meat in your diet. Eat it maybe once a week, but choose locally reared, preferably organic, meat." I have my own views as to whether "organic" is necessarily better for you or the environment rather than just being a sop to the conscience, but Haughton's may well be a good guide.
Also in the Guardian piece it was suggested that: "A good rule of thumb to work out the potential carbon emissions of a fruit is to look at its perishability. If it will easily spoil, then it is more likely to need to be transported by air and cold-storage conditions and be prone to needless wastage."
Humans are omnivores by construction, and in more northern climes storing (or transporting) foods with a high concentration of energy and nutrients such as meat, especially in winter, is probably a good way of sustaining health. Also, meat, fish and non-local fruit and vegetables help maintain diet diversity with the micronutrients/vitamins/minerals/etc that we need, at least over intervals of days or months.
Presumably eliminating superfluous packaging helps also to reduce carbon footprint, and possibly even walking or cycling to a nearby shop to buy it does too.
It's not clear to me if, for bulky shopping, if it is better to have it delivered by the shop, since they presumably work hard to minimise fuel (etc) use by combining multiple nearby deliveries, or to use a private car. We don't own a car, and this is one more reason that we don't need to buy one while still being able have the odd larger-than-usual efficient bulk purchase.
In general, cooking is about getting things hot. About the most climate- (and air-quality-) friendly way overall to do that is with an induction hob and an electric fan oven. (At least on the GB grid circa 2020 as electricity carbon intensity continues to drop.)
In situations where relatively small amounts of energy can be delivered fairly precisely minimising energy loss and heating of containers such as pans, ovens, etc, then other simple electric heat beats natural gas. Two such methods are the microwave and the upright electric kettle.
Euan Murray, general manager for the Carbon Trust's Carbon Footprinting programme, said (2008-04) as regard to cooking method that: "We have learnt some very interesting things, such as there is a clear hierarchy when it comes to emissions from cooking. Baking a potato in an oven generates far more emissions than if you boil it in a pan with a lid on, which in turn generates more emissions than cooking it in a microwave."
(In winter humidity build-up can be a noticeable problem, eg promoting mould, with a significant potential carbon cost in adequate ventilation to control it, but I estimate a fairly modest ~30g water per kWh of gas cooking heat of which we use ~1--2kWh/d on average, vs about 400g (1lb) per day exhaled by one person so maybe 1.5kg/d for us four, and maybe 1--5l/day from drying laundry.)
The Humble Cuppa
I'm trying to work out the carbon footprint of the humble cup of tea. So far I have calculated the CO2 from the ~35Wh (130kJ) to boil the water (efficiently in an upright plastic kettle) producing about 15g CO2 given an average 0.43kgC02/kWh for UK grid-delivered electricity as of 2007 using [defunct: "http://www.defra.gov.uk/environment/business/envrp/conversion-factors.htm"] UK government (DEFRA) figures. (Actually, with our new PV system and a lower-powered 1kW travel kettle then during daylight we should not actually need to import all or any of the energy to boil the water from the grid, though we have still lost the opportunity to export that energy and save someone else on the grid causing that CO2 to be released.) My measurements suggest that if I boil exactly the water I need, and manually stop the kettle just before it comes to the boil, then I use under 30Wh for one normal mug.
At the end of the process my teabag is composted aerobically releasing the CO2 originally absorbed by the tea leaves and whatever made up the paper teabag: ie no further energy nor GHG (GreenHouse Gas, eg methane) costs from sending the teabag through the council-run domestic waste system, eg to landfill. (Though as of 2009 our council now collects food waste separately from landfill thus allowing for composting or incineration, with the GHG savings implied.)
But I don't know about the cultivation, picking, preparation, shipping (etc) energy costs at all. I wrote at the end of January 2008 to Twinings as I drink gallons of their finest to ask if it had numbers, and similarly Typhoo as of the start of February.
Typhoo has sent me two holding replies as at 2008-03-31 (nothing from Twinings), and it seems to be working on the numbers but is not yet confident in them. Here's a snippet from Typhoo's helpful response of 2008-03-31:
Many of the key arguments revolve around questions such as to what  the foot printing covers. Indeed many retailers or [supermarkets] have proposed the simple route of counting the carbon foot print of the stores (but not the delivery of the product into the store or the manufacturer's foot print to make the product).
For tea manufacture we ultimately believe it should cover the whole process i.e. growing the tea bush, processing the tea leaves, shipping the tea, printing the packaging, packing the tea and shipment to the retailer. Although we are quite far down the road of carbon foot printing the Typhoo Tea factory i.e. the total amount of carbon used by the Moreton factory, the carbon foot print of the tea plantations still has to be defined. We will also have to average a value for a true reflection of a standard cuppa, as tea comes from many different regions (Africa, India, Sri Lanka, China and Argentina to name the main areas) and shipment from each area will have a different foot print.
I agree with Typhoo's approach, but I'd still like their best guess so far ASAP!
I also ought to at least consider the energy cost of getting potable mains water and flushing away the result later, and even the cost of (dish)washing my mug after a day's use. At (say) 300ml of water for my tea and to wash the cup (per cuppa), one source suggests that causes the release of ~0.09gCO2, or maybe the equivalent 0.2Wh of grid electricity. (As of 2008-02-07 I sent a request to our local water company, Thames Water, for some energy/GHG figures, and as of 2008-03-19 they wrote back confirming a 0.3gCO2/l average.)
I can make a stab at the dishwashing cost if I ignore any due to the cleaning agent (eg detergent/surfactant/etc): We tend to run our dishwasher once per day at a cost of 1.27kWh (down to 0.8kWh as of 2009--2010). I use one tea mug per day when at home for a daily average (say) 5+ cups, and it probably takes up ~5% of the top rack or maybe 2.5% of the total available space, so that might amortise to ~32Wh/day or ~6Wh per cup of tea.
The tally of computed elements so far:
Boiling the water: 35Wh/cuppa.
Washing the tea mug: 6Wh/cuppa.
Potable mains water for the tea (and washing): 0.2Wh/cuppa.
So, a black tea with just enough water boiled for it is ~40Wh / 21gCO2e.
If you're a heavy solo tea drinker like me, the following minimise energy use and losses.
Use a small enough kettle that you can boil just exactly the water for your cup each time, safely covering the elements, leaving the kettle empty when you fill your cup. Any water left in the kettle to cool down again represents wasted energy. I have found a travel kettle with flat elements. A large kettle may in practice require you to boil 50% or much more than is actually needed for for cuppa, basically wasted. A bigger/heavier kettle may also require more of its own metal/plastic/etc to be heated for a given amount of boiled water.
A (travel) kettle with a lower-power element is kind to your house wiring and the grid, reducing losses on the electricity's route to the element, and at times of high demand is doubly kind by spreading out the load. Additionally, if you have local grid-tied microgeneration, a lower-rated element (eg 650W or 1000W rather than a more-typical 2000W or 3000W) allows more (or all) of the energy to come from your local system rather than the grid, presumably thereby emitting less CO2. A lower-power element should take longer to heat your water, but if you are managing to heat less water to do the same job then it may not be so.
You don't actually need to wait until your kettle automatically turns off; tea is reputed to be slightly better if the water doesn't actually boil, and turning off automatically usually requires a small amount of water to have been turned to steam, which uses significant energy and doesn't make the water hotter.
Possibly drink your tea black to avoid the significant embodied energy of the milk.
Lynn Martinez wrote to me (2009-10) suggesting that I should boil my water in a microwave oven instead. So I tried with my old '800W' (output) oven, and found with my power meter that it consumed 1400W, thus about 60% efficient. I had to run it for 3 minutes to get my mug of water (and mug) hot enough to brew (not actually boiling), and used ~70Wh (the meter showed 0.06kWh) to do so. I redid the test with my travel kettle: according to the power meter it drew just under 1000W, took 2 minutes (I almost always manually cancel it before it turns itself off), and took 0.02kWh (I calculate just under 33Wh in fact from the above). I think my small and cheap and low-tech travel kettle is winning so far!
I note that the energy to make one aluminium beer can, circa 2009 if recycled, is similar to that to boil the water for a cup of tea! (If not recycled, it may be ten times higher starting from scratch.) So tea and beer may be energetically interchangeable...
At the moment we do most of our cooking by gas (stove/cooker/oven), other than an electric kettle and microwave.
As of early 2009 we're contemplating digging out my student-days "slow cooker" (aka "crock pot") to see how it is in terms of energy efficiency and convenience. Given that my pot was for one, and now we'd be cooking for 3 or 4, we would probably put the existing one on FreeCycle and "upgrade" if we like it.
2009-07 we gave it a whirl. Its 1.5l capacity was sufficient for a curry for 4 (albeit two small children), and at a constant 50W and ~7h, 0.36kWh was consumed in the cooking, cf ~6kWh (gas) to cook a roast Sunday lunch at the end of 2009-08.
If the slow cooker is to be beneficial in terms of CO2 emissions then currently it has to use 2 or 3 times less energy for a given meal which is not impossible, but in any case in general I'm happy to move away from appliances/routines that inescapably use fossil fuels (our electricity is nominally all green (zero-carbon), eg from our roof-mounted PV and from Ecotricity).
Grow Our Own
Growing vegetables at home is surely energy (and other resource) efficient provided that we do it without artificial heat, light, or fertilisers/compost for example. Actually we will probably use some bought compost to mix with our own. (There might be other embedded costs to account for, eg for the seeds.)
We have a tiny garden at home, and some is flowerbeds and grass lawn, but we are going to try to expand our vegetable production. If nothing else it will be an education for our children. We had some successes (potatoes in a bin) and failures (beans) in 2008. (In 2009 we had more beans, fewer potatoes, good tomatoes, chard, rocket, leaks still in progress as of end of August...)
We could try for crops through as much of the year as possible, or making best use of growing time and space throughout the year, or other criteria such as nutrients, visual appeal, etc. Most of these are to be grown outdoors in the ground or pots/tubs, but a few may be indoors (and let's see those snails attempt to get in!).
To get some produce most of the year without vast space or complication (and some of these are even candidates as decorative foliage plants and flowers) we're thinking of (or already growing):
The carbon foodprint of 5 diets compared showing a US meat-lover's diet to contribute 3.3tCO2e/y, normal 2.5t, no beef 1.9t, veggie 1.7t, vegan 1.5t, and the intensity of foods ranging from 14.1gCO2e/kcal for lamb and beef, down through 4.6 for fruit, 4.5 dairy, 3.8 chicken fish and pork, to 0.6 sugar...
it is evident that for ~2000kcal/day ~ 2.3kWh/d (or about the mean 100W/person that I assume for home heating gains) human food is something like 2.8kg/kWh CO2e compared with (say) UK electricity at nominally ~0.43kg/kWh COe2 circa 2015, ie food is maybe 6 or 7 times as carbon intense as mains power. Thus, it's better to have an efficient mains-powered machine in many cases than the equivalent manual labour from a carbon point of view.
Or another way: although we have eliminated 6tCO2e/y from gas and electricity, we'd probably need to do at least the same again for our family of four (eg at least another 7kWp+ of solar PV) to match the carbon from our not-very-meat-rich diets, and this doesn't include the other goods and services we consume.