As I write this, the outside temperature in Toronto, Ontario’s biggest city, is plus-one. This means that every furnace and space heater in that area is fighting against a temperature difference of around 19° in order to maintain an indoor temperature of 20°. The ease or difficulty with which it is winning that battle depends on the walls, windows, ceilings, and floors that confine the indoor space. If those building elements are flimsy, or if they are good conductors of heat, or if there are gaps between them which allow air to pass freely between inside and outside, then the heater will have to work harder.

Most heaters in Toronto run on natural gas. Right now, the majority of these heaters currently working to maintain 20° in indoor spaces in Toronto are collectively producing somewhere in the neighborhood of 11,900 megawatts. If the outdoor temperature in and around Toronto remains 1° for the next hour, then those heaters will have produced 11,900 megawatt-hours of energy. And because their heat came from combustion of natural gas, each of those megawatt-hours came with roughly 200 kilograms of carbon dioxide (CO 2 ). Which means that in one hour gas-fired heaters in Toronto will have produced 2,381 tons of CO 2 .

Have a look at Table A1 on the right. That shows the amount of energy contributed to the Ontario electricity grid by various types of generators. As I write this, the table shows that last hour the generators feeding the grid made 18,426 MWh of electrical energy. Those 18,426 MWh came with 593 tons of CO 2 .

So, 18,426 MWh and 593 tons of CO 2 for electricity in all of Ontario, versus 11,900 MWh and 2,381 tons of CO 2 for heat in Toronto.

And here’s a further, and more uncomfortable bit of reality. Since I started writing this, the temperature in and around Toronto has dropped to -3°. There is now a difference of 23 degrees in temperature between a 20° inside room and the outside. Toronto’s heat load is now around 14,400 MW. If that temperature holds for an hour, and the total energy for that hour equals 14,400 megawatt-hours, then the CO 2 price will be more than 2,800 tons.

Toronto could be getting all its heat from a much cleaner source. Have a look again at the electricity table. Note the amount of energy that is being produced in the nuclear plants: 11,036 MWh in the last hour. Note the amount of CO 2 that accompanied those 11,036 MWh: zero. That is less than the megawatt-hours of heat that Toronto needed last hour.

Which is to say, we could have had a nuclear fleet twice the size of the current one, dedicated to doing nothing other than providing heat for Toronto over a plus-one hour, and that still would not be enough heat.

As it is, had everybody in Toronto an hour ago decided to switch to cleaner heat and plugged in and turned on every available electrical heater, or in the absence of a dedicated heater just set their electric oven to 500 and opened the oven door, that could have added literally thousands of megawatts to Ontario’s electrical load. It would likely have pushed grid capacity into the red zone and caused a crisis. Only the hydro and gas generators could have increased output to keep up with demand, and as there are around 10,000 megawatts of gas-fired capacity and each kWh of gas-fired electricity comes with 400-500 grams of CO 2 we would see our electricity CIPK spike to around 158 grams.

Now, the heaters currently supplying gas-fired heat to Toronto right now have, collectively, a CIPK in the ballpark of 200 grams (I am being very generous with my estimate of their collective efficiency). You could look at the counterfactual CIPK of 158 and say that that is an improvement, but that would be wrong. You have to look at the CIPK of the sources that provided the extra 11,500 MWh for heat (that efficiency is close to 100 percent). That CIPK, assuming that 1,500 MWh came from hydro and 10,000 from gas plants, is about 347 grams. Meaning that if Toronto were to get 11,500 megawatts of heat from electricity for one hour, the city (and planet) would be better off just getting that heat from gas-fired heaters—as it is actually doing.

The only scenario in which getting an additional 11,500 MW from electricity for use in space and water heating makes environmental sense is one in which those 11,500 MW come from zero-emitting sources.

And the only zero-emitting source that we can expand is nuclear.

It is becoming well known that Ontario’s big emitting sectors are now transportation and heat. As I have said before, we would wipe out a major chunk of space-heat-related CO 2 within a single decade by adding enough nuclear generating capacity to meet our demand for space heat and hot water.

How much capacity would that require? That is, quite literally, a multi-billion-dollar question. As we have seen, during a plus-one-degree hour on a sleepy Sunday Toronto needed more than 11,000 MWh of gas-fired heat. That was about as much energy as the entire provincial nuclear generating fleet made as electricity. As I mentioned, in that hour we could have easily made use of a nuclear fleet twice the size of the current one.

That was just for Toronto. For a plus-one-degree hour.

Ontario is at a crossroads. We must decide whether our energy future is with nuclear, or with methane (natural gas). We have an emissions crisis, and we cannot solve it by adding more methane to our energy mix. We must decarbonize, fast, and there is only one way we can do it.

Luckily, we have proven our own way forward.