I'm Sarah, a Professor at The University of Manchester. I'm using my astrophysics research background to identify ways to help reduce greenhouse gas emissions... from food. Ask me Anything!

If everyone suddenly behaves “perfectly” with their food choices, how much would that go towards becoming sustainable regarding climate change? What I mean is, do we the people have enough power to make a difference when compared to industrial pollution etc

Good question. In short, yes. Food currently causes about 25% of all climate change. Different foods cause very different amounts of climate change, so reducing our food climate impacts by 50% is doable (e.g. that amount of reduction has been found for average vegan diets - but see above that it doesn't have to be so restrictive / simple as going vegan). This would reduce greenhouse gas emissions by 12.5% (50% of 25%), which is significant. Furthermore, if spared land could to be used to combat climate change, e.g. planting trees, then that could offset our remaining food emissions i.e. reducing greenhouse gas emissions by 25%. However, this still leaves 75% of climate change that needs to be addressed i.e. we do also need to stop burning fossil fuels. When we've done that, then food will be the biggest cause of climate change so we need to be thinking about that too

Are we screwed?

I am more optimistic now than I used to be, now I know more information. We have all the technology breakthroughs we need to address climate change ... if everyone were to implement the changes needed... The most promising thing I learnt researching about food and climate change was about land: the foods that cause the most climate change also tend to use the most land, so if we change our diets to eat less of them, then we free up land. We really need land to address climate change e.g. by planting / keeping more trees. To give an example to illustrate this, on average, plant-based foods use 16 times less land than animal-based foods. This means that in the extreme case where everyone in the world went vegan, this would free up 3/4 i.e. the majority of agricultural land for other purposes including forest. Since 37% of the ice-free land on Earth is used for agriculture, that's a lot of land! I'm not suggesting we need to go that far, but it shows there's a lot of potential

What are some practical steps we can take today to prepare for these realities and make an authentic difference in climate change? I think a lot of folks recycle but that seems like the minimal amount one can do and is it really effective? But then the next step is going vegan or have a compost in your yard. These options aren’t the most practical for middle income families with kids.

Thanks for your great question. Recycling is important for lots of reasons - in terms of climate change it reduces greenhouse gas emissions because the energy needed to recycle is lower than that needed to extract + use the raw materials. But for most foods, the contents of the packaging has a bigger impact on climate change than the packaging itself. For the most part, animal-based foods cause more climate change than plant-based foods (per gram of food or per gram of protein), though it depends a lot on farming practices, and e.g. air-freighted fruit and veg can cause similar climate impacts as the lower emissions meats like chicken. There are big differences in climate impacts between different types of food e.g. an 8oz steak and fries dinner causes over 20 times the climate impact of a microwaved potato and beans - so we can make a big difference by changing our choices. Luckily some of these changes don't break the bank (beans are cheaper than steak!), though I know from personal experience that experimenting with new meals doesn't always go down so well with the kids, so that part is tough... For most people, the simplest place to start is quantities: e.g. halving the quantity of the meat, while adding more veggies! You're right that composting helps - food waste sent to landfill causes extra emissions because it decomposes into methane, a potent greenhouse gas - whereas composting decomposes mostly to carbon dioxide. I know its tough with kids to avoid food waste, but again it can also save money. In our house fwiw, people help themselves from cooking bowl/tray, and if you put it on your plate you need to eat it up, or promise to take less next time! Hope that helps!

What type of food would you consider as "ideal" to reduce one's contribution to this problem ?

One study looked at changing the quantities of different foods to meet nutritional requirements and minimise climate impacts https://academic.oup.com/ajcn/article/96/3/632/4576889 They managed to reduce climate impacts by 90% i.e. ten times less climate change from food! But they admit it wasn't a very palatable diet, containing a lot of whole-grain breakfast cereal (without milk!), peas and seeds! They then went on to look at smaller departures from acceptable diets. This got me thinking though... because I had the same question as you... I suspect that it would be possible to formally meet all the macro and micronutrient requirements by combining different foods in a way that minimises climate change - but it might end up looking a lot like some of the meal replacement drinks available e.g. Huel. I don't think any of these meal replacement drinks have calculated their climate impacts, though and I don't know how healthy they are (e.g. uptake of different combinations of micronutrients - and all the nutrition that isn't in micronutrients).

Hi Sarah, thank you for doing this. Do you have an estimate on what's is the impact of food waste on global warming. I am referring to direct impact on CO2 and methane production food waste. And if it is substantial, can we use food waste to generate energy so deal with 2 things at a time?

Yes, food waste is a significant issue. For an average person, food waste decomposition to methane adds about 20% to their daily emissions. If we collect that food waste instead of sending it to landfill then indeed it can be used to produce methane that's used for energy (or if there are methane collectors at landfill sites). But that isn't the most efficient way to produce energy, because producing the food also contributes to climate change - it would be better to waste less, really..

What do you think is the root cause behind a growing trend of ignoring the science and facts on subjects such as climate change?

Gosh, that's a great question and I'm not an expert on that topic so can't speak with any authority. Speaking as a citizen (not as an expert) I have the impression humans become less rational when they feel afraid (e.g. of climate change), and this isn't helping.. Although COVID-19 has made us more afraid, in many many ways, I see a glimmer of hope that there has been much more weight placed on the opinions of scientists/experts - and awe that humans can't always control nature

A person I know has been rumored eating only biscuits and instant noodles for almost a year. He eat one meal a day. One day of biscuits and another day of instant noodles. He called himself depressed and not willing to find a job. He doesn't let anybody see him. What will happen to his body after this year? Is it eco friendly?

That's very sad. Definitely that diet is going to affect his physical, if not also mental health. There are definitely eco friendly diets that are much healthier than this! I hope he can get some help to recover his mental health.

Which meal choices are unexpectedly bad for the environment? I mean, many people are aware that meat, especially beef, is not eco-friendly, but are there other food items that harm the environment that we are unaware of?

Most people are surprised to learn that a large latte (~500ml of milk) causes a lot more impact than a regular tea or coffee containing just a tablespoon of milk - about 10 times more in fact (e.g. see milk chapter of my free book, linked in my intro at the top). On the plus side, most people are surprised that shipping food isn't nearly as bad as they thought. Shipping causes 100 times less emissions than air-frieghting, for the same weight of food. So bringing an apple from the other side of the world isn't a big deal in the grand scheme of things. On the other hand flying e.g. strawberries from another continent brings the climate impact up to be similar to that of a lower emissions animal product like chicken

What are your thoughts on the consumption and farming of insects as a low impact nutrition source? And do you it could ever become part of many people's diets?

I personally think eating insects is quite fun and we often give out free insects at our outreach events for www.takeabitecc.org but in practice it isn't necessary! There are lots of low impact non-insect foods out there! However, farming insects to feed to animals (to replace soy) is a growing industry, and a good thing if the insects are eating things that don't cause a lot of climate change - e.g. food waste or food production byproducts (e.g. leftovers from brewing such as Entocycle)

How can I eat steak and drink milk and not be part of the problem? Because I really like steak and milk.

yeah, some of these higher climate impact foods do taste really good. I feel your pain! (I love slow roast lamb...mmm) There is a big variation in climate impacts between different farming practices e.g. see this beautiful graphic from the fabulous Our World in Data project https://ourworldindata.org/environmental-impacts-of-food#less-meat-is-nearly-always-better-for-your-carbon-footprint-than-sustainable-meat e.g. there's a huge range for beef. Unfortunately this information is mostly not available to consumers. I really want to see climate impact labelling (gCO2e) on each food packet so I can choose the lowest impact food within a category e.g. lowest impact steak. You can also see in the graphic that despite the huge range for beef, it doesn't overlap much with the plant-based foods. So again I'd take it back to quantities and frequency. If you eat a lot of steak e.g. if you ate it daily then its probably your biggest contributor to climate change from your food: an average 8oz steak produced in Europe causes more climate change than a whole day of food for an average person in the world (~10kg emissions cf ~6kg emissions). On the other hand, if you ate steak just twice a year, then the numbers say that this would increase your total food climate impact by 1 percent i.e. not much (2 8oz steaks a year = 210kg of emissions = 20kg cf average food emissions per year of 3656kg = 2190kg so 20/2190 = 0.01 - I'm assuming reddit readers are geeks who want numbers - you can ignore the numbers if you're not that geeky :)

Thanks for this AMA! I have 2 questions:

1) Is there anything that you learned in your research that really surprised you?

2) I’m always a little skeptical of efforts to place the responsibility for action on the consumer rather than the industry. Of course our choices matter, but at the end of the day, even if I recycle every piece of plastic that comes into my house, that’s still nothing compared to big box stores that are throwing away tons of plastic every day with no consequences - there needs to be pressure on both sides. Has any of your research been about changes that should be made on an industry scale? For example, “if 50,000 people went vegan, it would have this effect, but if just one cattle ranch {insert fact here}, that would have the same net effect.” Any facts along those lines that you can share with us so that we can then share them with our elected officials?

Absolutely we shouldn't be burdening consumers with all the responsibility. I'm actually mostly passionate about labelling (all foods with climate impacts) for a different reason: food producers end up seeing their emissions numbers, and when they know consumers see them too, they are motivated to change. For example in the UK they have traffic light colours on the front of packets to show the amount of sugar in a product - red is bad, green is good, amber in the middle. They changed the threshold between red and amber - but they gave the food industry good warning - and it turned out that no food moved from amber to red, even though they changed the threshold - because all the food producers reformulated their products to reduce the amount of sugar in them (before the threshold was changed)! So consumers benefited because they were getting less sugar in their food! It would be brilliant if that sort of thing could happen with climate impacts of food!

I hear a lot of people say they but organic grass fed beef for the enviroment, how does that compare to beyond burger in terms of ghg emissions per calorie?

There is a wide range of climate impacts depending on the way beef is produced. It isn't clear that organic grass fed is better than other methods - the longer a cow lives the more it burps methane, and grass causes more cow methane burps per calorie than more refined foods such as soy - however eating soy causes deforestation so these issues partially cancel out. However, emissions from beef generally are much higher than plant based alternatives like beyond burger e.g. you can see that in this fab graphic I mentioned before https://ourworldindata.org/environmental-impacts-of-food#less-meat-is-nearly-always-better-for-your-carbon-footprint-than-sustainable-meat You can see that processed plant foods like tofu cause much less climate change than average beef, by more than a factor of 10 (per gram of protein)

As a research scientist..what do you mean when you say..."learned about climate change properly for the first time"?

Were you ignoring it before, learning about it improperly before? This phrasing is very odd...

I hadn't given climate change a lot of thought before - I wasn't an eco-warrior growing up - I was interested in astrophysics / the stars. But like most people I was aware of it from hearing people talk / the news etc so I learned about it in a vague way without giving it a lot of my attention. So for me it was a big deal when I got around to sitting down and really learning by asking questions and trying to find out the answers - I guess that's what I meant by 'properly'

Hi Sarah. Thanks for the AMA, it’s been really interesting! I’m interested in getting involved in climate change research, could you talk a bit on how you “learned about climate change properly”? And what others could do to learn more themselves? Thanks!

Great! I started with this fantastic book, written by my former mentor https://www.withouthotair.com/ - this was the inspiration for my work. But for all the latest science, if you're up for a lot of reading, the IPCC reports are amazing - this is the main one https://www.ipcc.ch/report/ar5/syr/ and then there are two really important updates to it here https://www.ipcc.ch/srccl/ and here https://www.ipcc.ch/sr15/ Happy reading!!! (And wikipedia is incredibly helpful for a lot of easy-to-read explanations - if you check the references are good as always)

Often I feel that people forget about other environmental factors that foods impact. Such as the impact of avocados on local water sources. What is your opinion on people using co2 to compare everything?When often its hard to compare the impact of an 8oz steak to the pack of avocados.

Great question - yes I'm personally focussing just on climate change impacts here but there are lots of other factors including health, taste, cost, human and animal welfare, as well as other environmental impacts like land use, water, biodiversity, runoff etc. I'd love to see all these things factored into the discussion too - but with the numbers for climate change included!

What do you think of permaculture and converting suburban lawns into food gardens?

We're limited by land to grow more food and to help with climate solutions (such as growing more trees / reducing deforestation), so if people can spare their lawns to do something more useful with the land then that's going to help the climate. The more biomass is building up (stem, trunk, branches, roots) that is permanent, the more carbon is being stored so the more carbon dioxide is being sucked out of the atmosphere and locked away - so turning lawn into permaculture food production is doubly good for the climate (if it reduces the amount of land needed to produce food outside the home, and if the plants are building up mass from one year to the next). However, not all home gardening is as efficient as doing it professionally - for a lot of people its going to be more efficient and effective to plant trees on the lawn cf growing a very small amount of food

how hard is it to do outreach when Brian is hogging the limelight?

the more the merrier :)

I've heard if you feed seaweed to cows, they emit less methane. Is that true? If it is true, why isn't it used?

Yes, there are various methane inhibitors being developed to reduce the amount of methane from cows, which is great - and even better when they get used widely. At the moment there isn't a huge incentive for farmers to use them - having labelling to show the amount of greenhouse gas emissions = climate impact from each pack of food would provide some incentive because at least consumers would be able to see which beef causes less climate change

Are there any brands you would recommend we use or don't use? (From the U.K btw)

Also my mum has started growing veg in the back garden. How much in greenhouse gas is she saving by doing so? (Carrots, corgettes, potatoes and salads if that makes a difference)

All the numbers I'm quoting are averages across production methods but there have been many great questions about how different production methods cause different emissions - indeed they do. I think we can only make significant progress, and have a good discussion, when we have more transparent up-to-date information about how different food items (including brands) contribute to climate change. Ultimately I want to see mandatory accredited labels on all food packets showing their climate impact (gCO2e). To reach that goal we would need a lot of products already doing that voluntarily ... so I encourage you to support brands that already provide that information. In the UK I believe only Oatly and Quorn currently provide that information but hopefully soon more will follow.

Regarding your question about growing at home: much of the emissions from fruit and veg growing are around transport, storage and refrigeration, and most of the rest are from fertilizer application. So there is a good chance that your mums veg are much better for the climate than shop bought. Having said that, most seasonal fruit and veg cause a small proportion of people's daily food emissions, so its not making a huge difference. However, if your mum previously ate a lot of air-freighted fruit and veg then this makes a much bigger difference. Getting involved with growing-your-own is a great way to learn about seasonal foods and appreciate the hard work put in by farmers, so we waste less food and buy more in season!

Is there somewhere I can support your accredited labels idea?

There are organisations like the Carbon Trust in this country who do accredited labels. You can find a very nice article which influenced me a lot - by a leading researcher in this field, saying we need mandatory labelling (who also made the data for many of the numbers I'm quoting) https://www.theguardian.com/environment/2018/oct/10/we-label-fridges-to-show-their-environmental-impact-why-not-food As for campaining to have it mandated - I don't know of an existing campaign. I did work with the Earl of Caithness to table an amendment to the UK's Agriculture Bill, however unfortunately it didn't pass. In case you want more info on that, here it is!

The proposed GHGE labelling amendment, as tabled, pasted from https://publications.parliament.uk/pa/bills/lbill/58-01/112/5801112-IV(Rev).pdf

``THE EARL OF CAITHNESS BARONESS JONES OF MOULSECOOMB

253A Page 32, line 36, at end insert—

“(o) requiring information about attributed lifecycle greenhouse gas emissions for the product to be available at the point of sale, including on packaging. (2A) For the purposes of subsection (2)(o), regulations may include provision for financial assistance for businesses towards the cost of providing that information.”

Member’s explanatory statement This amendment makes provision for greenhouse gas emissions emitted in the lifecycle of agriculture products to be available to consumers at the point of sale (e.g. on packaging), and allows for the provision of financial assistance for food producers and accreditation bodies to compile this information.''

Watch the proposal here from 21:17
https://parliamentlive.tv/Event/Index/4b68a14d-8050-4135-b78b-7faefe078a3b

Or read on Hansard here https://hansard.parliament.uk/lords/2020-07-23/debates/70188585-39D2-4D41-A353-38191DFDF4C1/AgricultureBill#contribution-AE07D635-34F4-4A4A-ABD8-B590B7A170EB

Hi Sarah, how much would emissions be reduced if we eradicated baked beans consumption?

I did look into this when I was writing my book - I get this question a lot! It turns out that some humans do fart small amounts of methane, but this is genetic and not linked to the consumption of baked beans. Even for the humans that do fart methane, its 100 times less than the methane burped by cows to produce a large steak. However, increasing the amount of fibre in diets (e.g. from more beans) is linked to an increase in the amount of wind, and improved health!

Is composting and gardening a decent way to prevent some food waste? Is there a difference in gas production between a landfill of food waste or personal compost? Like if a million people composted instead of throwing away their banana peels, would it make the same amount of gasses? Also, what impact does the manufacturing of fertilizer have on the environment? Sorry for all the questions. Thanks

Yes, there is a difference between landfill and personal compost. Compost heaps in people's gardens are usually relatively dry, and so carbon in the food waste decays into carbon dioxide. However, landfill sites tend to be wetter, and so the air (oxygen) can't get in to help turn the carbon into carbon dioxide (CO2), so instead the carbon decomposes more into methane (CH4) which is ten times worse at retaining heat under the atmosphere than carbon dioxide (averaged over a 100 year time period). So it is better if people compost banana peels rather than sending them to landfill. Food waste decomposition at landfill sites adds typically 20% to the climate impact of food, so adding up all the types of food waste (not just banana peel) it is a big deal Manufacturing fertilizer, and nitrous oxide from fertilizer application (whether organic manure or manufactured) are the main causes of on-farm climate impacts from producing plant-based foods (whether for human or animal consumption). The manufacture of fertilizer causes climate change because it uses a lot of energy to extract nitrogen from the air and turn it into fertilizer, and usually this energy comes from burning fossil fuels

Thank you for this AMA.

What animal meat has lowest environmental impact? Also, overall would cooked meat be less impactful compared to raw meat (because the facility must have a large oven etc so could cook more efficiently than cooking individually at a house)?

Thank you!

*Sorry English isn't my mother tongue

Chicken, eggs and fish all have about the same low climate impact, on average e.g. take a look at the beautiful graphic I mentioned earlier https://ourworldindata.org/environmental-impacts-of-food#less-meat-is-nearly-always-better-for-your-carbon-footprint-than-sustainable-meat - but again you can see there's a range, and e.g. as they point out intensively farmed chicken is at the lower end of the graphic (though this doesn't always have the best animal welfare). I was surprised to learn that fish has similar impacts whether its caught at sea or farmed (most salmon in the UK comes from fish farms, rather than caught at sea) - its a coincidence that the amount of fuel used to run a ship to go fishing causes a similar amount of climate impact to growing food to feed fish. Its less of a coincidence that farmed fish causes similar impacts to chicken - growing the food is a large part of the climate impacts of each. Very interesting question about cooking at scale! Yes, heating up a large metal box (home oven) is an inefficient way of cooking food compared to bulk cooking e.g. buying pre-cooked. You can also investigate other ways to cook at home that don't involve heating up a large amount of metal (or air) e.g. slow cooking in a well-insulated oven is much better for the climate, even though it takes longer to cook

Hello, this may not be related directly to your field of study but I just wanted to know what the prospects and career of an astrophysicist might be like. I am considering a career in astrophysics but many people around me tell me it's not worth it. So what would your advice be? Thanks in advance.

Astrophysics is a lot of fun and you learn lots of incredibly important skills along the way, like data analysis, approximating calculations, and of course all the underlying physics. However, its true that there aren't a lot of astrophysics-specific jobs out there - but check out our STFC Food Network+ project which links up astro, particle and nuclear physicists with lots of really interesting food-related challenges www.stfcfoodnetwork.org

How can the size of the universe be 93B light years if the its age is only 13.7B?

The size of the universe might not be related to its age. Usually in research we talk about the size of the 'observable universe' which is the only bit we can see - because light hasn't yet had time to reach us from more distant parts of the universe!

Hi Sarah,

If my wife and I are thinking of starting a family would we have a greater impact by doing our best to limit the number of children to one or two (twins happen) or are we in a position with current technology where it would be completely reasonable to have seven like my Grandparents did?

This is a serious life choice question for us now as we are both concerned about the future and our impact on the planet and we keep seeing very mixed information on the question.

Gosh, that's a big question. There's a huge range of climate impacts depending on the person e.g. see https://ourworldindata.org/grapher/annual-ghg-emissions which is also shown as kgCO2e per person per day in my free book Fig 1.1. In short, in some countries an average person causes over 10 times the climate impact of the average person in another country. So there is a lot we can do in our lifestyles. So 10 very low impact children could cause similar climate impacts to 1 average child. But children don't always do what you tell them :)

GHG emissions from food constitute how much towards the overall GHG emissions? Aren't your efforts better spent finding out ways to prevent volcanoes from erupting (or other higher impact events) that can put out orders of magnitude more GHG at once?

Food contributes about a quarter of all climate change. Most of the rest is fossil fuels. Volcanoes aren't a significant contributor e.g. see Myhre, G. et al. (2013). “Anthropogenic and Natural Radiative Forcing”. Climate Change 2013 – The Physical Science Basis: Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 659. doi: 10.1017/CBO9781107415324.018.

Hey Professor!

We recently watched Endgame 2050 on Prime Video. They're conclusion was that if "everyone" became vegan, then we'd stop climate change.

My husband & I have already made it a point to eat less meat & have stopped buying & eating beef. But, we're not vegans.

Our question after watching that documentary is...

What happens to all the farm animals if "everyone" becomes vegan?

I don't agree that if everyone went vegan we'd stop climate change because food causes about a quarter of all climate change. Most animals farmed intensively for meat production live for a maximum of a year or two

I understand the motivation to try to reduce emissions wherever it is practical to do so in any industry, but surely when it comes to quality of life, trying to make really deep cuts by drastically changing diet isn't really a good way to go?

Wouldn't we be better off squarely focusing on key industries - transport, power generation, construction, heavy industry, etc, rather than making people miserable by foisting lots and lots of change upon peoples' diet?

Food, water, and housing are three things that are highly personal, with loads of subjective aspects and I'd hate to see e.g. government boldly intrude in this sphere beyond cutting egregious waste and a few worst practices. Maybe some tinkering around the edges is OK but the quality of life cost will rise dramatically with every few % you try to cut beyond the low hanging fruit.

I agree we need to focus on cutting burning fossil fuels. When we've done that food will be the biggest contributor to climate change, so we're going to need to change that too