- Climate change
- Greenhouse gas
- Carbon emissions
- Carbon footprint
- Fossil fuels
- Watts and kilowatts (W and kW)
- Kilowatt hours (kWh)
- The ‘Unit’
The Earth’s climate has been changing for millions of years as a result of natural processes on the Earth such as volcanic eruptions and changes in ocean currents as well as external processes such as slight wobbles in the Earth’s axis of rotation and orbit and tiny variations in the “strength” of the sun. However, today, “climate change” usually refers to man-made climate change which is the result of man’s activities in the recent past, principally the burning of fossil fuels and cutting down of forests. These activities have upset the balance of heat trapped by greenhouse gases in the atmosphere. This man-made climate change appears to be happening far faster than any natural climate change has occurred in the past and it is difficult to be sure how serious the consequences will be in future. However, even a rise in average temperatures of just 2oC relative to pre-industrial times is likely to have serious consequences for mankind, and the current scientific consensus is that it is quite likely that temperatures will rise by more than this over the next 50 years. The average temperature has already risen by over 0.7oC.
Greenhouse gases are gases in the Earth’s atmosphere that contribute to global warming. They have the special property of absorbing heat emitted from the Earth’s surface, which is warmed by the sun, and preventing some of that heat from escaping into space. Thus gradually the atmosphere warms up. The commonest greenhouse gases are water vapour and carbon dioxide (CO2). For this reason, “carbon dioxide”, “CO2” and “carbon” are commonly used as shorthand for “greenhouse gases”. Water vapour, although present in large quantities in the atmosphere, on average is naturally maintained at the same level and so does not affect climate change.
The amount of carbon dioxide in the atmosphere has increased from around 280 parts per million (ppm) in pre-industrial times to around 391 ppm today and it is this change which is mostly driving climate change. Carbon dioxide is emitted when hydrocarbon fuels, such as petrol, diesel, coal, natural gas and wood, are burnt; most of the increase in carbon dioxide levels has come from the burning of fossil fuels.
Another important greenhouse gas, methane, is emitted by aircraft, livestock, decaying vegetation and the cultivation of rice. Its warming effect over 100 years is 25 times more powerful than that of carbon dioxide.
“Carbon emissions” and “carbon dioxide emissions” are terms used as shorthand for the amount of greenhouse gas that is emitted into the atmosphere. In fact the chemical element carbon makes up only 3/11 of the weight of carbon dioxide. So, if you read that the carbon emissions of some activity are X kg (and the context does imply carbon emissions, not carbon dioxide emissions), then the actual amount of carbon dioxide emitted is 3.67 times X.
Similarly, because some activities, such as flying, cause the emission of significant amounts of other greenhouse gases, emissions are sometimes more accurately quoted as carbon dioxide equivalent (“CO2-eq” or “CO2e”). This number takes account of the warming effect of greenhouse gases other than carbon dioxide and expresses their contribution to climate change as the amount of carbon dioxide that would have the same warming effect.
The carbon footprint of an individual or organisation is the amount of carbon dioxide attributable to them in a given time period, usually a year. The emissions can be the actual amount of carbon dioxide emitted by them directly or indirectly or, more usually, the amount of carbon dioxide that has the same warming effect as all greenhouse gases for which the individual or organisation is responsible.
The mains “gas” that most of us use to heat our homes is a mixture of hydrocarbon gases, principally methane. It is known as “natural gas”. It is a non-renewable fossil fuel that has accumulated over millions of years from the breakdown of organic matter in sediments deep in the Earth. Given suitable geological conditions, natural gas, like oil, can become trapped under impermeable layers of rock. If such traps are penetrated by a borehole, then the gas can be extracted and piped to our homes.
Natural gas doesn’t contain carbon dioxide, but carbon dioxide is produced whenever natural gas is burnt (2.75 kg of carbon dioxide is produced for every 1 kg of natural gas burnt). For every kilowatt hour (“kWh” or “unit”) of energy generated by burning natural gas, 0.203 kg of carbon dioxide is emitted. Just as important is the fact that methane, the main constituent of natural gas, is a greenhouse gas 25 times more powerful than carbon dioxide. Thus any gas leak can have significant adverse consequences for climate change and this is another reason why leaks should be reported immediately to your gas company.
When estimating the savings you might make from cutting your gas consumption, we have used a price of 3.35 pence per kWh, which was taken from a mid-2011 gas bill.
Mains electricity in the UK is generated at power stations and transmitted over a grid of high-voltage power lines to our homes. Generally power stations burn natural gas (36%), coal (42%) or nuclear fuel (17%). Gas and coal, which are non-renewable fossil fuels, emit carbon dioxide when burnt. Nuclear energy is said to be “cleaner” but has hidden environmental costs (including greenhouse gas emissions) related to mining, processing and transporting the fuel, construction and decommissioning of reactors and disposing of the radioactive waste.
Electricity generation is not efficient; for example, a coal-burning power station is only 35% efficient and much of the rest of the energy is lost as heat. The best gas-burning power stations have an efficiency of just under 60%. There are also losses of around 7% in transmitting electricity over the national grid.
Electricity is measured in kilowatt hours ( kWh” or “units”) which is a measure of the energy delivered to our homes. On average across the UK, for every kilowatt hour (or unit) of electricity that we consume, 0.617 kg of carbon dioxide has been emitted at a power station.
When estimating the savings you might make from cutting your electricity consumption, we have used a price of 12 pence per kWh, which was taken from a mid-2011 electricity bill.
Fossil fuels are so-called because they are the product of natural processes acting over millions of years. Organic carbon in sediments deep within the Earth is slowly turned into a variety of hydrocarbons such as coal, oil and natural gas. A key point about fossil fuels is that they have taken millions of years to form, yet are gone in a moment when we burn them. The rate at which mankind is consuming fossil fuels is far far faster than the rate at which they formed or are forming today. Thus they are said to be non-renewable and our exploitation of them is unsustainable.
The watt, named after James Watt, is a unit used to measure the rate at which energy is consumed or delivered. For example, a 20 watt low-energy light bulb consumes energy at the rate of 20 watts or a 3 kilowatt kettle uses 3000 watts (‘kilo’ signifies a factor of a thousand) when heating water. Watts and kilowatts are abbreviated as W and kW respectively.
A kilowatt hour (or kilowatt·hour) is a measure of energy. In the home it is associated with the amount of electricity and gas that we use and appears on our bills as kilowatt hours (meaning the number obtained by multiplying kilowatts by hours), abbreviated to kWh. 1 kWh means that 1000 watts (1 kilowatt) were consumed for 1 hour, or 2000 watts for half an hour etc.
Electricity and gas bills frequently mention ‘units’. This is a non-standard and unofficial unit intended to be a measure of energy, but it may cause confusion. On electricity bills ‘unit’ refers to kilowatt hours (kWh). On gas bills ‘unit’ refers to a measure of the volume of gas consumed (such as cubic feet). These two ‘units’ are not the same. For example, 1 cubic foot of gas provides around 31.5 kWh of energy. It is better to work in kilowatt hours which is an internationally recognised and agreed unit.
If you would like to give feedback on this site, or have any suggestions for improvement, please contact us or post a comment here. We are particularly keen to hear about other organisations and initiatives in and around Winchester which can help people reduce their carbon footprint.
The information on this page is provided in good faith and reflects our understanding of the underlying science and technology at the time of writing, but we cannot guarantee that it is wholly accurate. All figures for costs, savings and other matters are estimates: the actual figures will depend on your particular circumstances and may differ (perhaps significantly) from those shown. Although we have included links to various organisations, we are not recommending these organisations: it is your responsibility to check that they are suitable for your needs. Nonetheless, if you experience difficulties with any of the links or organisations, or believe that any of the information presented here is inaccurate, please let us know and we will update this page if we consider it necessary.