What should we eat?  It seems there is no simple answer. Supermarkets have created an (over)abundance of choice.  Our food supply relies on heavily subsidized growing of corn in ‘industrial-scale’ agriculture – this has huge effects on almost all the foodstuffs available.  Today’s agriculture is dependent on adding fixed Nitrogen to soils – making that ‘fertilizer’ requires huge amounts of energy from fossil fuels. Nitrogen fertilizers are incredibly efficient, but they make climate change a lot worse.

Then, beef is a very large component of the modern American diet.  Most of that beef comes from steers fed corn in feedlots.  At every stage, large volumes of greenhouse gases (CO2, Nitrous Oxide and Methane) are released into the air.

Carbon cost of protein sources

Environmental costs of meat-based and plant-based burgers
(costs per quarter-pound, to nearest integer)

Source: ‘Sierra’magazine, March/April 2020. Sierra Club, Oakland, CA

This is all described in detail in Michael Pollan’s classic book The Omnivore’s Dilemma: A Natural History of Four Meals (2007)
— From publisher’s description:  What should we have for dinner?  When you can eat just about anything nature (or the supermarket) has to offer, deciding what you should eat will inevitably stir anxiety, especially when some of the foods might shorten your life. Today, buffeted by one food fad after another, America is suffering from a national eating disorder. As the cornucopia of the modern American supermarket and fast food outlet confronts us with a bewildering and treacherous landscape, what’s at stake becomes not only our own and our children’s health, but the health of the environment that sustains life on earth. The surprising answers Pollan offers have profound political, economic, psychological, and even moral implications for all of us.
Wikipedia has a very extensive and thorough review.
Amazon has a preview that allows you to read a substantial part of the book.
And it’s at Richmond Free library.

Other Sources:

What is the Lowest Carbon Protein?

How many people does synthetic fertilizer feed? 

Climate Change and Agriculture – Union of Concerned Scientists, has good general background information.

The food to avoid if you care about climate change – Vox, May 2020 – a  5-min video, based on Reducing food’s environmental impacts through producers and consumers – Science,  June-2018.
The study and video show that distribution/transportation is a relatively small contributing factor in the agricultural system. In general, buying local has a much lower impact than reducing consumption of beef, lamb, cheese/dairy, and other high-impact foods. (Of course, there are other good reasons to buy local.)

How the food system can be improved to be more sustainable: https://www.4p1000.org/  The idea is to increase the carbon sequestration in soils through better agriculture practices — while increasing food security and improving water quality.

According to the Guardian, giving up beef would have a bigger climate impact than giving up cars. We still need to give up cars, but giving up meat might be a lot easier to do – we could do it tomorrow.

A documentary film, about costs of “Eating Animals“.