What's a ton of CO₂ anyway?

You've probably seen striking headlines that express climate impact by comparison, e.g. "Training a single AI model emits as much carbon as driving a car around the world five times". To me this always feels intangible: I don't circumnavigate the earth on a daily basis, and I doubt you do either. So I started looking for a way to give these numbers meaning. Most of these comparisons use "carbon dioxide equivalent" (CO₂e) to express global warming potential, which makes it easy to compare emissions regardless of which gas actually causes the warming¹. But how much is a ton of carbon dioxide? Is it a little or a lot? What does it take to undo it?

The physical reality

To paint a literal picture, one ton of CO₂ at atmospheric pressure fills a cube with sides a little over 8 meters². That's awfully close to the size of my house. Of course it won't stay a cube: it joins the atmosphere and spreads out, acting like a thermal blanket: absorbing heat that would otherwise escape to space and re-radiating part of it back down. Over a 100-year horizon, that one ton will trap about 1500 gigajoules of extra heat energy³, enough to bring 3.6 million liters of water (about an Olympic-pool-and-a-half) from freezing to boiling⁴.

A measuring stick

What I needed was a personal measuring stick. The one I landed on is a carbon budget: your individual fair share of CO₂e emissions per year, the amount you could emit without overstraining the planet (assuming, unrealistically, that everyone else does the same).

There are many ways to calculate this budget limit. The Delft University of Technology introduced "2 Tons 2 Thrive", which as the name suggests gives you 2 tons of CO₂e budget each year.⁵ The French Agency for Ecological Transition reached the same number based on its own research.⁶ Oxfam's research lands at 2.1 tons, with the fair caveat that wealthy Westerners should really be content with less.⁷ The Hot or Cool Institute suggests a gradually decreasing budget that goes from at most 3.2 tons now to between 0.7 and 1.4 tons by the year 2050.⁸

The main differences between these calculations stem from whether they assume we'll have large scale carbon capture before it's too late, whether they only include personal lifestyle or also the impact of shared infrastructure, and whether the maximum limit of global warming they try to avoid is 1.5 or 2 degrees.

So we now have a number to compare events and habits to. I'll use 2 tons in this post. Note that the average European currently emits 10.7 tons CO₂e per year⁹, so you can already guess where this is headed.

Budgeting the fundamentals

Let's walk through the categories that dominate a typical lifestyle, from the unavoidable to the fun extras, and see how far this budget gets us.

You'll agree with me that breathing is quite fundamental. Unfortunately we exhale carbon dioxide, about 365 kg per year¹⁰. That's 18% of our budget just from breathing! Rest assured: breathing is part of the natural carbon cycle and doesn't count against the budget¹¹. But it puts things in perspective.

Next: water. Two liters of tap water a day costs about 255 g of CO₂e per year, a rounding error in our budget. Switch to bottled water and that jumps to 148 kg, or 7.5% of the budget.¹²

Food is where things get confrontational. A 200 g serving of red meat twice a week (104 meals across the year) adds up to 2069 kg of CO₂e: your entire budget, with nothing left for anything else. Switching to poultry lets you have two meals a day for 1440 kg, or 72%. And if you love animals and three good meals a day, a plant-based diet gets you there for only 437 kg, or 22%¹³. Way more meals, for a fraction of the budget.

Budget: 2000 kg CO₂e

Tap water (2l/day)  [....................] 0.26 kg (  0%)
Bottled (2l/day)    [##..................]  148 kg (7.5%)

Red meat 2x/week    [####################] 2069 kg (103%)
Poultry 2x/day      [##############......] 1440 kg ( 72%)
Plant-based 3x/day  [####................]  437 kg ( 22%)

Also important is a comfortable place to stay, which starts with a roof over your head. While we aren't building a new house every year, buildings have a significant impact even when spreading it over many years. Building a house emits on the order of 15 to 100 tons of CO₂e¹⁴. Assuming a lifetime of a hundred years, that's anywhere from 7.5 to 50% of your budget per year.

Now there are running costs in our house: we heat and/or cool it and power all kinds of electronic appliances. It's impossible to generalize this to a single number as it depends on area, insulation, the fuel you use to heat, whether you use air conditioning, etc. There are countless calculators online, but to give you an idea: my average-sized (in Belgium), reasonably insulated, gas heated, solar panelled house was good for 760 kg of CO₂e in 2025¹⁵, or 38% of my budget.

Transportation's next. A 5 km bicycle commute every business day is cheap, at 40 to 125 kg of CO₂e (depending on your diet). Let's say 5% of your budget. A 25 km daily commute by train, tram, or subway adds up to 338 to 438 kg, or about 20% of the budget. The same route on a fossil-powered bus or a motorcycle clocks in at 1125 kg, 56%. An electric car does better at about 30%, but a fossil car blows past the budget at 106%.¹⁶

Budget: 2000 kg CO₂e (25 km daily commute)

Bicycle (scaled)    [####................]  410 kg ( 21%)
Train / tram        [####................]  388 kg ( 19%)
Electric car        [######..............]  600 kg ( 30%)
Fossil bus / moto   [###########.........] 1125 kg ( 56%)
Fossil car          [####################] 2120 kg (106%)

Can't live without your cuppa joe in the morning? I sure can't. Two cups of coffee a day will cost you 290 kg of CO₂e¹³, or about 15% of your budget.

Holiday time! The Eurostar from London to Paris and back sets us back 4 kg of CO₂e, basically nothing. Taking the airplane on the same route is 140 kg, or 7% of the budget. Want to go a bit further, say Brussels to Athens? That return flight is 630 kg, more than a quarter of our annual budget. Prefer visiting another continent? Better start saving up: the return trip to NYC is 2.1 tons of CO₂e, 105% of the budget.¹⁶

You might've heard about this new technology called AI. Daily personal use lands in the ballpark of tens to hundreds of kg per year, depending on your usage patterns.^{17 18} The complication is the training cost: between 5000 and 140000 tons of CO₂e per frontier model¹⁹, paid up front and spread invisibly across all users. Even a frugal user is riding on top of an enormous bill that never appears on a personal ledger. Some emissions don't fit cleanly inside personal accounting but we should still be conscious about them.

Let's stack a moderately-conscious European's choices and see where they land. Plant-based diet, gas-heated house with solar, train commute, daily coffee, and one return flight to Athens for holidays:

Budget: 2000 kg CO₂e

Plant-based food     [####................]  437 kg  ( 22%)
+ House (amortized)  [##########..........] 1012 kg  ( 51%)
+ Heating + power    [##################..] 1772 kg  ( 89%)
+ Train commute      [####################]# 2160 kg  (108%)
+ Coffee (2/day)     [####################]#### 2450 kg  (123%)
+ Athens return      [####################]########## 3080 kg  (154%)

The budget was gone before the holiday even started, and this is just the conscious version! The 2-ton budget isn't a target most of us are anywhere close to. It's a measuring stick that shows just how far off we are.

Levers that work

So the picture so far is bleak. The good news is that some of the biggest items on the ledger have levers attached to them, and the levers actually work.

We already saw that an electric car has about a third the budget impact of a fossil one. Solar panels are getting more affordable every year, and adding them to my roof saved me 185 kg of CO₂e yearly, almost 10% of my budget.¹⁵ Swapping out gas-based heating for a heat pump can cut one of your biggest budget hogs in half.²⁰ And as the food paragraph showed, shifting what's on your plate is one of the largest single moves available to most people.

Undoing emissions

Those technologies and choices help us reduce emissions. But what about undoing them? There's a lot of work being done on direct air capture (DAC), giant machines that capture greenhouse gases directly from ambient air. To give you a sense of the state of the art, the largest DAC in the world is a factory-sized installation in Iceland called "Mammoth".²¹ It removes up to 36000 tons of CO₂ each year, at a price of about $1000 per ton.²² That puts a $2000 price tag on your annual budget, if you wanted to undo it that way.

Nature is plenty good at managing carbon too. A 1-hectare forest can absorb roughly 4 to 40 tons of CO₂ per year during the first 20 years after planting, depending on the type of forest and the climate.²³ Once fully mature, their net carbon sequestration drops significantly. The reason is that as trees die (whether naturally, through human intervention, or through natural disasters), much of the carbon they stored as biomass is released again. In that sense, forests act more as temporary carbon storage than as permanent carbon removal.²⁴

One hectare also happens to be about the share of habitable land each person would get if we divided the planet evenly.^{25 26} Living within the budget by reforestation alone would mean turning almost your entire share of the planet into permanent forest (and even then, only temporarily).

Don't count on being bailed out.

Living with the budget

Researching this post changed how real these decisions feel to me. I thought I was already eco-conscious, but learning that one cross-Atlantic flight wipes out a year of careful choices was its own kind of education.

I'll probably get email comments about how Big Industry is responsible and they should clean up the mess. There's something to it, but those industrial emissions aren't separate from us. The gas in your boiler, the kerosene in your flight, the diesel in the truck that brought your dinner: viewed from the demand side, that is the industry's footprint. It's the same ledger, looked at from a different end.

The budget isn't a binary test you pass or fail, but a unit of measurement. Once you have it you can't unsee it, and it becomes an internalized tool. It's not about guilt or perfection but about understanding the numbers that make the next decision a little harder to lie to yourself about.

Footnotes