Eat seasonal and eat local.
This is about eating food that hasn’t been grown in heated glass houses or kept in refrigeration to prolong its season, and food that hasn’t been air freighted round the world so you can serve fresh strawberries at Christmas. All of these season stretching techniques use energy and so release greenhouse gasses.
A 250g pack of locally grown summer asparagus in season in the UK produces about 125g CO2, compared with the same pack airfreighted from Peru in January which produces 3.5kg… 28 times as much!
And eating seasonal is also eating food at its absolute best and always gives you something to look forward to. Eating local supports local farmers and growers and what grows well near you helps create the unique character of your natural local cuisine.
But I’d suggest giving priority to seasonal. According to Mike Berners-Lee’s excellent book, “How bad are bananas? – the carbon footprint of everything” Buying locally grown tomatoes outside of the natural season (July to October in the UK) means they’ve been grown in heated glasshouses which can release as much as100 times as much CO2. So be ware. In winter tinned tomatoes are best.