What exactly is a tonne of CO2?

As a reference unit for climate plans, a tonne of carbon dioxide remains an abstract notion for most of us.

What is CO₂?

Carbon dioxide is an invisible, odourless and colourless gas. This does not prevent it, like the air, from having a mass: 1,964 g/litre or 1.964 kg/m³. These equivalents will help you understand the scales we’re talking about when discussing one tonne of CO₂:

• the average emission of one passenger on a return-flight from Paris to New York
• driving 6000 km with a diesel car
• 4300 kWh power consumption

Where does CO₂ come from?

Quite simply, from combustion: when you burn petrol, gas, wood, etc. All combustible fuels (except hydrogen), whether renewable or fossil, contain carbon.

Inside the flame, the carbon reacts with the oxygen in the air to produce heat and carbon dioxide. Chemists express this in a small formula: C + O₂ = CO₂ + heat

And in terms of the weight of the various components, this represents:
1 kg of carbon + 2.67 kg of oxygen give 3.67 kg of carbon dioxide and heat

How much CO₂ is produced per 1kWh of electricity?

To know that, you have to find out how the energy you use, is generated. In Belgium, for instance, many people would like to abandon nuclear energy. Still, neither CCGT-power plants (or worse: lignite plants, as is the case in Germany) will help to reduce our CO₂ emission level.

The following table shows the amount of CO₂ produced per kWh for specific types of electricity generators:

Production method	g CO2/kWh
wind energy	11
nuclear energy	12
solar energy	27
gas-fired power plant	490
coal-fired power plant	820

In Belgium, 1 kWh of electricity currently 'costs' 220 g of carbon dioxide. In 1998, it was 290 g. Our country has reduced its carbon footprint by developing renewable energy sources and improving the efficiency of gas-fired power plants (gas-steam turbines).

How much CO₂ do the main fuels produce?

Each fuel contains a greater or lesser amount of carbon. Consequently, they emit various amounts of carbon dioxide to produce the same energy:

Fuel	kg CO2/ kWh generated
Coal	0,343
Fuel oil	0,271
Kerosene	0,267
Petrol	0,264
LPG	0,231
Natural gas	0,206
Household waste	0,149

This means that, if someone uses 20,000 kWh of natural gas per year to heat their house, they produce 4,120 kg of carbon dioxide. Unless you opt for a more sustainable alternative, which can be used for exactly the same applications as natural gas: green gas.

How much CO₂ do vehicle fuels produce?

Going strictly by the amount of CO₂ emitted per litre of fuel, LPG comes out as the big winner:

Fuel	kg CO2 / litre
Diesel	2,67
Petrol	2,28
CNG	2,00
LPG	1,66

Careful with taking things at face value. The differences are tiny!

Other data must also be taken into account:

• Weight of the fuel: One litre of LPG weighs 25 to 30% less than petrol. The energy supplied by a litre of this fuel will therefore be less than that of a litre of petrol or diesel
• Engine efficiency: A diesel engine consumes less fuel per 100 km than a petrol engine and therefore emits less carbon dioxide for the same trip. An LPG engine uses 30% more per 100 km than a petrol car.
• Ethanol share of petrol: If we take this element into consideration, the emissions need to be revised downwards. As ethanol is from renewable sources, its contribution to the emission of carbon dioxide is zero.
• Other pollutants: carbon dioxide is not everything. We also need to factor in toxic particulates, nitrogen dioxide, unburnt hydrocarbons, sulphur, etc.

In short, the differences are more subtle than they seem!

How to calculate the CO₂ impact of my holiday? 

Considering only the transport, you can calculate the amount of carbon dioxide emitted according to the means of transport you choose.

Means of transport	kg CO2 /passenger per 100 km
Alone by car	10,4
By plane	28,5
By scooter	7,2
By coach	6,8
By train	1,4

_{Source: European Environment Agency}