Behavioural science suggests that making an option a default increases the likelihood that it is chosen!

Currently the default meal options on most long-distance flights are meals with either fish or meat. Vegetarian meals, however, need to be ordered beforehand and specifically requested. People wanting to choose the environmentally friendlier option are therefore required to put in more time and effort. Many passengers are indifferent between meat or vegetarian meals, but are reluctant to make the extra effort of requesting a vegetarian meal.

Making the vegetarian meal option the default option would therefore greatly reduce the meat consumption on airplanes and help reduce the enormous carbon footprint of air travel without limiting the choices of its passengers.