Climate Change and Diet

The research on diet and climate change is not new, but it's not easy to hear.

Posted Oct 11, 2018

Just a few years ago, I was listening to a psychologist who works on beliefs about climate change telling us that we humans have to keep global warming within 2 degrees Celsius of its present temperature or people will literally die in the equatorial sauna when they try to harvest their food. I don't know if this Mad Max desert-vision version of our future is likely to be true, but it appears we're going to get to find out. Following the UN's Intergovernmental Panel on Climate Change, the director of that panel, Michael Oppenheimer, has now said that avoiding 2 degrees warming "is now totally unrealistic."

So what are we humans going to do about it?  Well, if you ask people what they can do themselves about climate change, most will say "drive less", "turn out the lights", and "recycle".  

Few will mention diet.  

What is remarkable is that diet is probably the single largest and most immediate change that anyone can make. 

In the latest United Nations report, diet is mentioned directly because it is basically tied for the largest contributor to human greenhouse gas emissions with electricity and heat production.

In 2006, the United Nations Food and Agriculture Organization (FAO) estimated that farm animals accounted for between 20% and 30% of global greenhouse gas emissions. This is somewhere between one-fifth and one-third of the human contribution to greenhouse gases. 

A study of 56,000 people's diets conducted by the University of Oxford established that meat-rich diets produced double the CO2 emissions of vegetarian diets, with 7.2kg of CO2 emissions per day for meat-rich diets compared with 3.8kg of CO2 per day for vegetarian diets.

To put the dietary significance of climate change in perspective, a study by Weber and Matthews (2008), two environmental engineers, found that shifting from a red meat based diet to a totally vegetarian diet is equivalent to driving approximately 8000 miles less per year in a standard car. On any given day, this is roughly equivalent to driving an extra 22 miles.  

Weber and Matthews also found that most greenhouse gases are generated in the production phase (~80%), not in the distribution phase. "Buying local" is nice, they point out, but how the food is produced far outweighs the distance it travels. 

This evidence goes on, but it sounds like beating a dead horse with these numbers, and you know horses don't like numbers.  So let's step away from this crapload of evidence that meat matters.

The question is, does eating meat influence your ability to believe in this evidence? And if it does, why? 

Truelove and Parks (2012) did a formal study on what people thought influenced greenhouse gas emissions. This is a fascinating study where, among other things, people ranked a list of 20 items on how effective they were in reducing greenhouse gas emissions.  According to participants, the least effective way to reduce greenhouse gas emissions was to reduce meat consumption. So these people were clearly not well informed.

Based on what else they said, they are not just poorly informed, they are misinformed. "Throwing away garbage instead of littering" was seen as most effective in reducing climate change.  They said this despite the fact that there is no detectable relationship between littering and greenhouse gases.  

We did a more informal study recently that looked at people's diets and their awareness of the relationship with climate change. The results were that people with more plant-based diets were more likely to rate the relationship between diet and climate change as strong.  People with more meat-based diets were more likely suggest this relationship was weak.  

It is reasonable to claim that people who learn about climate change and diet become more aware of their diet choices.  This seems like a good causal narrative. Someone tells you that meat is bad for the environment and you being the cool-minded individual that you are either believe them or do more research and go, "yeah, that's right.  I should stop eating so much meat." But besides sounding somewhat ridiculous, the evidence for this is not so straightforward.

In our study, after we asked people about the relationship between diet and climate change, we then told them about the relationship using all the data I provided earlier in this article.  Then we asked them again, what's the relationship between diet and climate change.

People with meat-based diets were less likely to recognize the new evidence then people with plant-based diets.

You probably recognize this as good old-fashioned cognitive dissonance.  Plainly put, if something doesn't fit with your existing actions and beliefs, and especially if it paints you as the bad guy, then you're going to resist believing it.  

In George Marshall's book on climate change and cognition, Don't even think about it: why our brains are wired to ignore climate change, Marshall basically lays out the many reasons why if climate change is anything like the majority of scientists think it's going to be, we're pretty much screwed. One of his biggest reasons for thinking it's not going to be easy to save ourselves is basically cognitive dissonance. 

As I said in a previous post on the darker side of the Kavanaugh confirmation,  on many issues (and climate change is one of them), "The evidence is no longer about making decisions, it is about figuring out whether the producer of that evidence is on your side or not."

People have an obligation to preserve their self-image and the ingroup that image is part of. How can you blame them? 

Unfortunately, if cognitive dissonance is the problem, then the question we need to start asking may not be how to get the word out about the relationship between climate change and diet.  This is not an information deficit problem. 

The question we need to start asking is how to help people change their identity so they can actually hear and internalize the message.  

It is not clear how to do this, which is why that question is so important.  One potential way may be to treat the climate-change-cognitive-dissonance problem a bit like HIV.  For many years, doctors have used a cocktail of drugs.  

How about a cocktail of information? One with identity-preserving information crystals? 

One way to do this might be to add information that is not about moral values (like saving the world) but about self-interest (saving yourself).  Could this work?  In our informal study, it seemed to help. We found that telling people the following was more likely to lead to intended diet change than simply telling them about the diet-climate relationship.  Here's what we told them:

A meta-analysis based on 124,706 participants established that vegetarians have a significantly lower risk of mortality from ischemic heart disease (29% lower) and overall cancer incidence (18% lower) than non-vegetarians.

A simple explanation is that people are simply willing to make dietary changes to protect themselves.  One might even go so far as to interpret this as a form of identity preservation in a slight variation on cognitive dissonance: I will change my identity, a little, if it means the rest of me gets to hang out for longer.

Thomas Hills on Twitter


Scarborough, P., Appleby, P. N., Mizdrak, A., Briggs, A. D., Travis, R. C., Bradbury, K. E., & Key, T. J. (2014). Dietary greenhouse gas emissions of meat-eaters, fish-eaters, vegetarians and vegans in the UK. Climatic change, 125(2), 179-192.

Weber, C. L., & Matthews, H. S. (2008). Food-miles and the relative climate impacts of food choices in the United States.

Truelove, H. B., & Parks, C. (2012). Perceptions of behaviors that cause and mitigate global warming and intentions to perform these behaviors. Journal of Environmental Psychology, 32(3), 246-259.

Huang, T., Yang, B., Zheng, J., Li, G., Wahlqvist, M. L., & Li, D. (2012). Cardiovascular disease mortality and cancer incidence in vegetarians: a meta-analysis and systematic review. Annals of nutrition and metabolism, 60(4), 233-240.