What is “Zero Carbon”?

Usage of the term

Zero Carbon is used as shorthand to indicate systems that do not increase the concentration of carbon dioxide (or other greenhouse gases) in the atmosphere, and thus do not contribute to climate change.

Preface

In physics nature is described quantitatively, using a special language designed to make nature describable in terms of simple laws.

To understand discussions on climate change is important that you know the language: including the difference between energy and power; the units they are measured in; their symbols; and the general conventions on symbols and units. You also need to know about heat and temperature, radiation, conduction and convection.

There is an Appendix that hopefully helps develop understanding of the vocabulary we use. It includes visualisible examples to help you internalise the meanings of the more abstract terms.

Earth's Thermal Equilibrium

All body surfaces emit electromagnetic radiation. The higher the temperature, the more energy is radiated and the higher the frequency (or shorter the wave length) of the region of the spectrum in which most of the energy is concentrated. [Short, long; far, near; high, low frequency; please see Appendix]

A tiny amount of heat is generated inside the Earth, or arrives from other stars, but almost all of Earth's heat arrives as radiation from the sun. During the day this solar radiation causes local temperature rises.

If that were the end of the story, the temperature would rise a bit more each day and the earth would become uninhabitably hot in about a week. However Earth also radiates energy back into space.

The temperature of the surface of the sun is about 5 800 K. Most of the energy it radiates is in the near-infra-red* and visible regions of the spectrum. [* IR near to visible. See Appendix.]

Earth is in a dynamic thermal equilibrium. If more energy arrives than leaves, the temperature rises, making the rate of outward radiation go up, until the arrival and loss of energy balance.

The average temperature of the Earth's surface, that balances received solar radiation with the earths emitted radiation, is about 300K . Bodies at 300K radiate in the far, or low frequency, end of the infra-red region of the spectrum.

The Glass Greenhouse

A greenhouse does more than just shelter its contents from the wind, while letting the sunshine in. The glass allows in the sunshine's visible and near infra-red radiation, which warms the ground. However glass absorbs the far infrared radiation that the ground sends back.

The energy absorbed by the glass is re-emitted, half to the outside, but half back into the greenhouse. Because some of the heat is being returned, the ground inside has to rise to a higher temperature, and emit more radiation, than similar ground outside, to get enough radiation out through the glass to balance that coming in.

The Carbon Dioxide Greenhouse Effect

About 0.04 % ( or 400 parts per million ) of the atmosphere is carbon dioxide (CO₂). Carbon dioxide molecules do not absorb sunlight, but do absorb the far or long wavelength, infra-red radiation emitted by the surface of the earth. The glass re-radiates most the energy absorbed from outgoing radiation, about half of it towards space and half of it back towards the Earth's surface. (A little of the energy could be lost by convection)

If the concentration of carbon dioxide increases, more outgoing radiation is returned to the surface. Hence the surface temperature rises until the emissions rises enough to compensate for that which is turned back, and the energy radiated to outer space balanced that arriving from the sun.

Other Greenhouse Effect Drivers

Methane (Natural Gas) is another greenhouse gas. Its effect is many times greater than that of the same mass of CO₂, but it decays naturally, meaning its concentration does not reach that of CO₂. Decaying vegetable matter, including that decaying in the digestive systems of animals, produces new methane. To get the protein they need, cattle feeding on grass have to digest vast amounts, and hence are significant methane producers.
Also, fossil methane escapes into the atmosphere - naturally, and from leaks in man's Natural Gas extraction systems. Extreme extraction techniques, such as 'fracking' are especially prone to leakage.

Nitrous Oxides and CFCs are also very effective greenhouse gases, but are only present in low concentrations.

Cloud reflects both incoming solar radiation and radiation from the earth. Aircraft produce soot and water vapour at high altitudes. In falling evening temperatures this seeds cloud. Evening cloud arrives too late to reflect back radiation from the sun, but it acts as a night blanket, reflecting back radiation emitted by the earth. Thus while strictly not the greenhouse effect, reflection, from aircraft produced evening cloud, adds to global warming.

Carbon Intensity of Energy Production, and Carbon Equivalent

Carbon Intensity (in the context of energy) is the mass of carbon dioxide produced for each unit of usable energy produced. Usually it is measured in grams of CO₂ per kilowatt hour; or kilograms of CO₂ per megawatt hour. Because a kg is 1000 g and a MW is 1000 kW, ^1g / _{kW h} is the same as ^1kg / _{MW h}.

One can have other carbon intensities, for example kg of CO₂ per ton of cement produced.

Rather than listing separately the amounts of each greenhouse gas involved in a process, we tend to work in terms of the equivalent of mass carbon dioxide that would produce the same degree of Greenhouse effect as the actual mixture.

Hence we might see say 50 g CO_{2 eqivalent} / kW h. When we talk loosely about Carbon Intensity we normally mean the carbon dioxide equivalent intensity. Hence the above night just be written as 50 g / kWh.

A simple Outline of the Carbon Cycle

Animals eat food containing carbon compounds such as fats and sugars. They breath in oxygen, which reacts with the compounds, ultimately to give energy plus carbon dioxide, water, and other products. The carbon dioxide is breathed out.

In plants, solar energy is used to convert the carbon dioxide and water back into more complex carbon compounds, releasing the oxygen back into the atmosphere. The cycle goes round again, with animals using the oxygen and plant material to give them energy and carbon dioxide. [The above is only an outline of what happens to the carbon. These reactions involve other elements, and take part in many stages.]

So:- plants grow by absorbing solar energy and carbon dioxide, and give off oxygen;

animals gain energy from food by breathing in oxygen and breathing out carbon dioxide.

The net result is that animals capture solar energy via plants. Plant and animal populations influences the concentration of atmospheric carbon dioxide in opposite directions. A balance between plant and animal life makes the concentration of carbon dioxide in the atmosphere, and hence the mean temperature of the earth change only very slowly, in normal conditions.

Climate Change

It is normal for the Earth's climate to change gradually over millions of years. The nature and distribution of life evolves with the changing climate. Five times in the history of the earth, various natural events have made the climate change too fast for nature to adapt, and mass extinctions have occurred.

We are trying to go Zero Carbon, to prevent another major extinction.

We now have significant climate change being produced by human activity. As well as digesting food to producing heat and the energy to do mechanical work, unlike the rest of the animals, man learnt to produce heat by using fire, and to use that heat, to run engines, to do mechanical work. Natural systems can adapt to burning wood and other vegetable matter, as long as replacement vegetation utilises the carbon dioxide emitted.

At the time of the industrial revolution, man started using timber, as a fuel, faster than it could be regrown locally. Man started using fossil fuels:- coal, oil and gas, which comprise carbon and/or carbon compounds, mostly formed, hundreds of millions of years of years ago, by about 60 million years of sunshine making plants from carbon dioxide. Over a couple of hundred years of fossil fuel burning, man has released carbon dioxide that had been removed from the atmosphere by sunshine fuelled growth of vegetation over millions of years. Our vegetation cannot absorb it all. Hence concentration of carbon dioxide in the atmosphere is increasing, and the average global temperature is increasing,

Currently, mankind's actions are taking us towards toward conditions that would make another mass extinction unstoppable. If that happens, human life is likely to be one of the forms that become extinct.

Accounting

Our fuels vary in composition from hard coal, which is mostly carbon, to natural gas, which contains four hydrogen atoms for every carbon atom. When they are fully burnt, energy is released, by each carbon atom combining with 2 oxygen atoms to form a carbon dioxide molecule (CO₂), and by the hydrogen atoms pairing and combining an oxygen atom to a water molecule (H₂O). Both processes release energy.

An atom of carbon is 12 times the mass of one of hydrogen and a CO₂ is 44 times the mass of a hydrogen atom. That make a CO₂ molecule 3.67 times heavier than a carbon atom. No matter what the composition of the fuel, each ton of carbon within in produces 3.67 tons of CO₂ if fully burnt Hence by checking the carbon content of fuel we know how much CO₂ it will produce. A tax on carbon in fuel is thus a tax on the CO₂ it produces.

The more hydrogen there is in the fuel, the more the additional energy released by the formation of water. Thus more the more the proportion of hydrogen to carbon in a fuel, the more energy is generated for the same amount of CO₂ formation.

Hydrogen is a zero carbon fuel. Unfortunately we do not have natural supplies of hydrogen we can tap into. [But when 'spare' low carbon generated electricity is available it can be used to produce hydrogen].

Mains "Natural Gas" is mainly methane, CH₄; that is 4 hydrogens per carbon.

Off-mains liquefied petroleum gas, "LPG", is mainly propane, C₃H₈, that is 8/3 = 2.67 hydrogens per carbon. (Liquefied butane, C₄H₁₀, is also used for lighters and camping)

Oil has much larger molecules (which is why it does not have to be compressed to make it liquid). The ratio of hydrogen atoms to carbon is only about 2 to 1.

Thus for every unit of carbon diode emitted, we get more energy from oil; than from coal, more still from LPG, and even more from natural gas.

Zero Carbon

Strictly a Zero Carbon process is one in which no there is no net release of Carbon Dioxide into the atmosphere. However it is generally taken to mean Zero Carbon equivalent. That is a process produces releases no net release of greenhouse gases.

A Zero Carbon Britain would emit some greenhouse gases into the atmosphere, but would also have processes to remove an equivalent of greenhouse gas.

In a Zero Carbon World, there would be no net change of concentration of greenhouse gases, and hence there would be no global warming due to the greenhouse effect.

High and Low Carbon - first steps to a Low Carbon economy

Most of our greenhouse gases come from our use of fossil fuels for heating, transport and electricity generation.

To move to a very low carbon economy we need to reduce energy consumption, including by: using better insulation, better waste avoiding controls, and more efficient engines and boilers; and by finding alternative energy sources that produce less carbon dioxide.

Coal is the highest carbon fuel. Oil diesel and petrol are very high carbon fuels. They also produce other forms of pollution including sulphur compounds and particulates.

No fossil fuels are low carbon, but gas is less high carbon than solid and liquid fuels. As a bonus, gaseous fuels generally emit less other pollution.

To move towards a very low carbon economy we need to stop using fossil fuel. Given that this cannot be achieved immediately, we need to work hardest on the elimination of the use of coal, heavier oils, diesel, and petrol, in that order of priority.

Once the use of oil and coal is almost eliminated, we will need to make reducing gas use the priority.

While we recognise that we will have to continue to use gas for some time, we should be trying to directly replace very high carbon coal and oil directly with very low carbon energy; rather than investing in a short term increase in high carbon gas use.

Green Lib Dems policy is to seek to ensure the Liberal Democrats have a continuously updated, realistic information on a Critical Path towards a Zero Carbon Britain, to guide their decision making and to use in campaigning.

Steve Bolter Vice Chair Green Liberal Democrats Gestingthorpe January 2017

Appendix

These are not Physics lesson depth treatments. They are an attempt to help you understand these terms and build mental pictures of them, to help you internalise the terms.

We use standard symbols, not freestyle abbreviations, for quantities and units - Roman type for units and Italic type for quantities.

Work, symbol W, unit J It requires work to push a TV across the floor. It requires work to lift it up to viewing height, but it does not require work to keep it there - a table could do that. If you do work pushing you hand across a surface it gets hot. It requires work to turn a dynamo. The more electricity is taken from the dynamo, the more work needed to turn it.

Energy, symbol E, unit J To do work, we need energy. We have to use energy doing work to compress a spring. Later that spring could do work perhaps turning a clock mechanism. We say that the compressed spring was storing energy. The energy we gave it came from our breakfast.

We have an accounting system in which we measure all forms of energy and work in a common unit.

The joule,symbol J We measure work and energy in joules. The symbol is an upper case J because it is named after a person (James Joule)

Power, symbol P, unit WIt requires the same amount of energy to get a one ton milk float or a one ton high power car up a hill. The powerful car does it faster. Power is rate of doing work. That is power is work done divided by time taken.

The watt, WA power of one watt is one joule per second. 1 W = 1 J/ s

Example 1) Work done W = 1₂ 000 J , Time taken t = 1 minute = 60 s

Power P = ^W/ _t= ^{12 000 J} / _{60 s} = 200 J/s = 200 W

Example 2) How much energy is used if 2000 W of power is used for 3 hours?

P = 2 kilowatts = 2000 W, used for 3 hours = 3 × 3 600 s = 10 800 s

P = W / t thus W = P times t

Work done = energy used = 2000 W × 10 800 s = 21 600 000 J , or 21.6 MJ

Because quite a moderate power used for a few hours used millions of joules of energy

we tend to simply calculate 2 kW × 3 h = 6 kW h.

Alternative Energy Unit, kilowatt hour Our system of measure is WYPIIWYGO (W_hat Y_ou P_ut In Is W_hat Y_ou G_et O_ut) the unit is simply kW × hours, which we write as kW h

The kW h is a recognised energy unit. 1 kW h = 1 kW × 1 h

= 1000 W × 3600 s = 3 600 000 W s = 3.6 million J = 3.6 MJ

Internal Energy, Temperature and Heat Matter has "internal energy", Part of which is energy of internal motion of its atoms (For example vibration of atoms inside a brick, but not the motion of a brick thrown at a window).

The higher the concentration of that energy, the higher the higher the temperature.

When there is an available path, energy is transferred from high temperature bodies to low temperature ones. The energy transferred is referred to as heat.

Heat flows from high temperatures to low ones. A closed system would eventually reach a uniform temperature.

Thermal Conduction and Convection

Heat can be transmitted through solids by conduction. Energy of vibration of the atoms is transferred from high temperature regions to cooler regions by the bonds that hold the solid together, just as person who jumping up and down, holding hands with a still person, sets the still one in motion, while the still one slows the active one.

Molecules in gases are in random motion. Heat can be conducted through gases by some molecules from hot regions migrating towards cool regions and colliding with other molecules. Liquids have a mix of both forms of conduction.

Fluids (gases and liquids) can also convey heat by convection, this by bulk motion of the fluid. Liquid moves in bulk to another taking its internal energy with it. We can have pumps to drive forced convection, as in a pumped central heating system. There can also be gravitational, or natural convection. Fluids expand on increase in temperature, making their density decrease. This makes hot fluids rise above cold. [Cold water is exceptional - reducing its temperature between 4°C and 0°C makes it expand again. At night, as temperature falls towards freezing, 4°C water falls to the bottom of the pond and the top freezes.]

For small temperature differences the rate of flow of heat between two bodies is directly proportional to the temperature difference.

Thermal Radiation

Conduction and convection require matter, but radiation can convey radiation through a vacuum. Electromagnetic radiation travels unimpeded through free space, but is absorbed to various degrees when passing through matter. Electromagnet radiation has both wave and particle like properties. There is an infinite spectrum of possible wavelengths.

Just as the speed of motion of a saw chain is the length of chain going past in a second, which equals the number of links coming past per second times length of each link: so the speed of a wave is the number of waves passing per second, times the length of each wave, That is - speed, or velocity = frequency ×wave length, or v = f × λ. [ λ is a Greek l for length]

For a wave of a particular speed, f × λ must always have the same value, so high frequencies correspond to short wavelengths and

low frequencies correspond to long wavelengths.

The kelvin and Radiation

The kelvin, named after Lord Kelvin, is an absolute temperature scale. The size of the unit is the same as the °C, but the scale starts at absolute zero instead of having zero at the freezing point of water. The freezing point of water is 273.2 K and the boiling point is 373.2 K.

All bodies emit electromagnetic radiation which increases in power, P, as temperature, T. increases . For a black body (one without a coloured surface) P is proportional to T⁴ . (Double the absolute temperature and the power gets 16 times larger.

The temperature, the higher the frequency (or shorter the wavelength) of the peak of the spectrum. f _peakis proportional to T

Our eyes have evolved to be sensitive to the wavelengths (or frequencies) close to the wavelength at which the sun's radiation is most abundant.

Electromagnetic waves Each region of the spectrum is named. We may refer to parts of the region by their relative wavelengths, OR frequencies, OR, in the cases of IR and UV by their nearness or farness from the visible.

All radiation from the sun is heat, including the light we see by, Infra-Red and even the UV that is absorbed in the upper atmosphere.. The largest contribution to heat flow from the sun to the earth is in the near (higher frequency, shorter wavelength) infra-red and visible regions.

The earth is very much cooler than the sun, Most of the radiation emitted by the earth is in the (long wave, low frequency) far infra-red.