Animal agriculture generates an ungodly amount of animal suffering and greenhouse gas emissions. Ideally, everyone would adopt a vegan diet^[1], but since this appears to be too much to ask for most people, the next question becomes: which food is the least bad?

Here we run into an uncomfortable observation. Meat from larger animals tends to produce more CO₂, while meat from smaller animals tends to cause more animal suffering.
This makes intuitive sense: when converting animals into a set mass of meat, more individual animals must suffer if they are smaller. We can see this in the data, e.g., we slaughter 309 million cows per year, while the number of chickens slaughtered exceeds 75 Billion per year.

Let’s start ranking

Let's start by comparing greenhouse gas emissions. Which foods have the lowest CO₂ impact?
Here’s how many kilograms of CO₂-equivalized emissions are produced per kilogram of food (some create methane etc, but that’s accounted for by converting it into the same unit), ranked from worst to best:

Beef (beef herd)     99.48 kg
Lamb & Mutton    39.72 kg
Beef (dairy herd)   33.3 kg
Prawns (farmed)   26.87 kg
Cheese                       23.88 kg
Fish (farmed)          13.63 kg
Pig Meat                   12.31 kg
Poultry Meat           9.87 kg
Eggs                             4.67 kg
Rice                              4.45 kg
Tofu                             3.16 kg
Milk                             3.15 kg
Soy milk                     0.98 kg

As we can see, larger animals tend to create more CO₂.

Can we create a similar ranking for animal suffering? Yes we can, if we take the sentience figures from this report and apply it to the production process used to create the meat. Here’s the equivalized days of suffering per kg of food (some have bigger brains etc, but that’s accounted for by converting it into the same unit), ranked from worst to best:

Farmed catfish        290 d
Battery cage eggs   42 d
Farmed salmon       27 d
Chicken                      27 d
Turkey                         15 d
Pork                              3.9 d
Beef                              0.8 d
Milk                              0.049 d
Tofu                              0 d
Soy milk                      0 d

As we can see, the smaller animals tend to produce more suffering. The best products on these lists, both in terms of animal suffering and CO₂, are tofu and soy milk.
For those who don't want to go vegan, milk is still comparatively good in terms of both CO₂ and animal suffering, so you might want to go vegetarian then. But for those who want to keep eating meat, we have a dilemma between CO₂ and animal suffering. Compare beef and chicken again. Beef has ten times the CO₂ impact of chicken meat, but chicken meat has 34 times higher impact on suffering than beef.^[2]

Now, don’t take these exact numbers as gospel. We’re still in the process of researching this so I expect the exact numbers to change over time. If you want to use different numbers you can play with the calculations using this interactive chart.^[3] The important thing is not the exact numbers but rather the orders of magnitude; which clearly point in the direction of meat production with smaller animals creating more suffering.
 

What to pick

Farmed fish is terrible in terms of animal welfare and pretty bad in terms of CO₂, so best avoid that. Beef is terrible in terms of CO₂, but comparatively good in terms of animal suffering (the word "comparative" is doing a lot of heavy lifting there). So if you don't care about animals but do care about CO₂, beef is the most important one to avoid. For those who only care about animals, farmed fish and chickens are the most important to avoid.

For those of you who are having trouble weighing the two against one another, consider this: When choosing between, e.g, chicken and beef, you're effectively choosing between creating the equivalent of 89 kg more CO₂ (with beef) or the equivalent of 26 days more torture (with chicken). To put this into context, a campfire produces about 10 kg of CO₂. Would you rather make 9 campfires or start torturing for 26 days?^[4] Are people who have a hearth morally equivalent to people who torture every day? Or what about a candle? It produces 0,01 kg of CO₂. Is lighting a candle worse than torturing an animal for a couple minutes? I would say: no, clearly not. The enormous quantity of suffering created in factory farms easily trumps the quantity of CO₂ that it creates.
We hear a lot about climate change in the news and online, while animal suffering gets little attention. But the scale of suffering that is produced in factory farms is unimaginably large. Please consider eating fewer small animals.

1. ^{^}

   I’ve heard people object that they wouldn’t get enough protein if they did this. Let me reassure you that there’s more than enough protein available in plant-based foods. For example, here's how much grams of protein there is in 100 gram of meat:

   • Beef, cooked - 16.9 to 40.6
   • Lamb, cooked - 20.91 to 50.9
   • Fish (farmed Atlantic Salmon) 20.4
   • Chicken: 27
   • Pork: 26 to 31

   And here's how much it is for some alternatives:

   • Soy protein isolate (prepared with sodium or potassium): 80.66
   • Egg white, dried: 81.1
   • Spirulina alga), dried: 57.45
   • Baker's yeast: 38.33
   • Hemp husks 30
   • Mock meat (cooked vegetarian preparations): 18.53 to 28.9
   • dry roasted soybeans: 13
   • peanuts: 23.68 to 28.04
2. ^{^}

   If you think that this doesn’t apply to you because you only eat “free-range” chicken meat. I’d hate to brake it to you but the label of "free-range" is often put on treatments that are not substantially better for the lives of the chickens.

3. ^{^}

   As seen in this great post by Bentham’s Bulldog

4. ^{^}

   One possible reason someone could raise for prioritizing climate change, is that its effects are much more widespread and much longer lived. However, I don’t find this too convincing since the effect of one animal is so astronomically small. With 1.000.000.000.000.000 kg of CO₂ we have created 1.1 celsius of warming, so 89 kg is responsible for like 0,000.000.000.000.098 degrees of warming. The most accurate temperature sensors we have are RTDs who're accurate up to 0.1 degrees. Nothing comes remotely close to be able to detect that. Adding 89 kg to 1.000.000.000.000.000 kg is not even equivalent to a drop in the bucket, it's closer to adding another grain to the Sahara. 89kg is equivalent to flying for an hour. To offset 89 kg you need €6,30 in the EU's emission trading system. What would it take to offset 26 days of torture? Not €6,30 I'll tell you that much. And even if the EU is off by an order of magnitude, it's not €63 either.