Greenhouse effect, carbon cycle and ocean acidification
On a cold January night in Ireland, a turf or peat fire warms the kitchen, and the smoke goes up the chimney into the air. Where does that carbon dioxide end up, and why does it matter that there is a little more of it in the atmosphere each year? In this page you will see how the atmosphere keeps the Earth warm, how to compare different gases, how carbon moves around the planet and what happens when the sea absorbs some of the extra CO₂.
The natural and enhanced greenhouse effect
Without an atmosphere, the average surface temperature of the Earth would be about $-18\,^{\circ}\text{C}$. In fact it is about $15\,^{\circ}\text{C}$. The difference comes from the natural greenhouse effect.
The process has three stages:
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The Sun emits mainly visible light and some ultraviolet. Most of this passes through the atmosphere and warms the ground and the sea.
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The warm Earth emits energy back out as infrared (IR) radiation, which has a longer wavelength.
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Greenhouse gases absorb some of this infrared radiation. They then re-emit it in all directions, including back down towards the surface. The lower atmosphere stays warmer than it would otherwise be.
The natural greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide (N₂O, which is not one of the NOₓ gases) and ozone. Without them, Earth would be too cold for liquid water to be widespread.
Other gases, such as the nitrogen oxides (NOₓ) and sulfur oxides, also affect the climate. Nitrogen oxides and some sulfur-containing gases are released by burning fuels. N₂O is a greenhouse gas; NOₓ (NO and NO₂) are not greenhouse gases themselves, but they affect the climate indirectly by helping to form ozone in the lower atmosphere, whereas sulfur oxides mainly form acid rain and tiny particles that can have a cooling effect.
The enhanced greenhouse effect is the extra warming caused by human activity. Burning fossil fuels, farming and clearing forests add more CO₂, CH₄ and N₂O to the atmosphere. More gas means more IR is absorbed and sent back, so the average temperature rises. This is global warming. Its effects include rising sea levels, melting ice, changes in precipitation (rainfall patterns, with wetter winters and more flooding in places such as Ireland) and ocean acidification.
The natural greenhouse effect keeps the average surface temperature of the Earth at about $15\,^{\circ}\text{C}$ rather than about $-18\,^{\circ}\text{C}$. a) Name two…
The natural greenhouse effect keeps the average surface temperature of the Earth at about $15\,^{\circ}\text{C}$ rather than about $-18\,^{\circ}\text{C}$.
- a) Name two naturally occurring greenhouse gases other than carbon dioxide.
- b) Explain, in terms of radiation, how greenhouse gases warm the lower atmosphere.
- c) State the difference between the natural greenhouse effect and the enhanced greenhouse effect.
Greenhouse gases and global warming potential
Some gases warm the planet far more than others, mass for mass. The global warming potential (GWP) of a gas is a measure of how much heat a given mass of the gas traps over a stated time (usually 100 years), compared with the same mass of CO₂. CO₂ is the reference, so its GWP is 1.
| Gas | Main sources | GWP (100 years) |
|---|---|---|
| $\mathrm{CO_2}$ | burning fossil fuels, deforestation | 1 |
| $\mathrm{CH_4}$ | cattle, rice fields, landfill, natural gas leaks | 28 |
| $\mathrm{N_2O}$ | fertiliser, industry | 265 |
You can combine different gases into one figure using the $\mathrm{CO_2}$-equivalent:
$$\text{CO}_2\text{-equivalent} = \text{mass of gas} \times \text{GWP}$$Why do gases differ so much?
Two things decide the GWP of a gas.
1. How strongly it absorbs infrared radiation. A molecule absorbs IR only if the radiation makes it vibrate in a way that changes its dipole moment.
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$\mathrm{N_2}$ and $\mathrm{O_2}$ are diatomic molecules with two identical atoms. When the bond stretches, the molecule stays non-polar. There is no change in dipole, so no IR absorption.
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$\mathrm{CO_2}$ is linear and symmetrical, so it has no permanent dipole. But its asymmetric stretch (one C=O gets longer while the other gets shorter) and its bending vibration both create a temporary dipole. These vibrations match IR frequencies, so $\mathrm{CO_2}$ absorbs IR.
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$\mathrm{CH_4}$, $\mathrm{N_2O}$ and $\mathrm{H_2O}$ also have vibrations that change the dipole moment.
2. How long it stays in the atmosphere (its lifetime). A gas that is removed quickly has its effect for a short time only. Methane lasts about 12 years, because it reacts in the atmosphere. $\mathrm{N_2O}$ lasts over 100 years, and part of the $\mathrm{CO_2}$ stays for centuries.
This is why the time period matters. Methane's GWP is about 84 over 20 years but only 28 over 100 years. In the first 20 years it is very powerful. After a century most of it has gone, so the average effect per kilogram over the longer period is lower. The result: cutting methane gives a fast reduction in warming, while cutting $\mathrm{CO_2}$ matters most for the long term.
The table gives the 100-year global warming potential (GWP) of three gases, relative to $\mathrm{CO_2}$. | Gas | GWP (100 years) | |---|---| | $\mathrm{CO_2}$ | 1 | |…
The table gives the 100-year global warming potential (GWP) of three gases, relative to $\mathrm{CO_2}$.
| Gas | GWP (100 years) |
|---|---|
| $\mathrm{CO_2}$ | 1 |
| $\mathrm{CH_4}$ | 28 |
| $\mathrm{N_2O}$ | 265 |
- a) Explain what is meant by the term global warming potential.
- b) Which of the three gases has the greatest warming effect per kilogram emitted over 100 years?
- c) Calculate the $\mathrm{CO_2}$-equivalent, in kg, of an emission of $10.0\,\text{kg}$ of methane.
A farm in County Cork has a herd of 120 dairy cattle. Each animal emits on average $110\,\text{kg}$ of methane per year. The 100-year GWP of methane is 28. a) Calculate…
A farm in County Cork has a herd of 120 dairy cattle. Each animal emits on average $110\,\text{kg}$ of methane per year. The 100-year GWP of methane is 28.
- a) Calculate the total mass of methane emitted by the herd per year, in kg.
- b) Calculate the $\mathrm{CO_2}$-equivalent of this emission, in tonnes.
- c) A car emits $120\,\text{g}$ of $\mathrm{CO_2}$ per km. Calculate how many kilometres a car would have to travel to emit the same mass of $\mathrm{CO_2}$ as your answer to b).
Carbon dioxide absorbs infrared radiation, but nitrogen and oxygen do not. a) Explain, in terms of molecular vibration and change in dipole moment, why $\mathrm{CO_2}$…
Carbon dioxide absorbs infrared radiation, but nitrogen and oxygen do not.
- a) Explain, in terms of molecular vibration and change in dipole moment, why $\mathrm{CO_2}$ absorbs infrared radiation while $\mathrm{N_2}$ does not.
- b) Explain why the atmospheric lifetime of a gas as well as its infrared absorption affects its GWP.
- c) Over 20 years the GWP of methane is 84, and over 100 years it is 28. Suggest why the value is lower over the longer period.
- d) A policy-maker states that methane reduction is more urgent for short-term warming than for long-term warming. Evaluate this statement using the data.
The carbon cycle
Carbon moves between the atmosphere, living things, soil, rocks and the oceans. The main processes are:
| Process | What happens | Effect on atmospheric $\mathrm{CO_2}$ |
|---|---|---|
| Photosynthesis | plants and algae take in $\mathrm{CO_2}$ | decreases |
| Respiration | living things release $\mathrm{CO_2}$ | increases |
| Decomposition | microbes break down dead matter | increases |
| Combustion | fuels burn | increases |
| Dissolution in oceans | $\mathrm{CO_2}$ dissolves in sea water | decreases |
The two key equations are:
$$6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} \xrightarrow{\text{light}} \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2}$$ $$\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}$$Photosynthesis is endothermic (it takes energy from sunlight). Respiration and combustion of glucose are exothermic. Overall, each is the reverse of the other.
Why fossil fuels are different
Peat, coal and natural gas are carbon stores that took millions of years (peat: thousands) to form. Burning them moves carbon that was locked underground into the atmosphere within a very short time. Nothing takes it back out quickly, so the concentration rises.
Wood from a sustainably grown forest is approximately carbon neutral. The CO₂ released when it burns is about the same amount the trees took in while growing, and replanting takes up CO₂ again. The cycle is closed over a few decades.
Deforestation adds to the enhanced greenhouse effect in two ways. Fewer trees means less photosynthesis, so less CO₂ is removed. Also, burning or rotting the cleared trees releases the carbon they stored.
The carbon cycle involves photosynthesis, respiration, combustion, decomposition and dissolution in the oceans. a) Write the balanced equation for photosynthesis using…
The carbon cycle involves photosynthesis, respiration, combustion, decomposition and dissolution in the oceans.
- a) Write the balanced equation for photosynthesis using glucose, $\mathrm{C_6H_{12}O_6}$, as the product.
- b) Write the balanced equation for the complete combustion of glucose.
- c) Explain why burning peat, coal and natural gas increases the atmospheric carbon dioxide concentration, whereas burning sustainably grown wood is described as approximately carbon neutral.
- d) Explain why deforestation contributes to the enhanced greenhouse effect.
Calculating carbon dioxide from fuels and in the atmosphere
You can find the mass of $\mathrm{CO_2}$ produced by a fuel from the equation $\mathrm{C} + \mathrm{O_2} \rightarrow \mathrm{CO_2}$. One mole of carbon (12 g) gives one mole of $\mathrm{CO_2}$ (44 g). So every 12 g of carbon gives 44 g of $\mathrm{CO_2}$.
Parts per million
Atmospheric concentrations are given in ppm (parts per million), meaning moles of $\mathrm{CO_2}$ per million moles of air. To change a ppm figure into a number of moles, multiply by the total moles of gas in the atmosphere and divide by $10^6$.
The percentage increase from an old value to a new one is $\dfrac{\text{new} - \text{old}}{\text{old}} \times 100$. Comparing the CO₂ that remains in the air with the total emitted shows how much the land and oceans have absorbed.
The mass of $\mathrm{CO_2}$ released when a fuel burns can be calculated from its equation. A household in Galway burns $50.0\,\text{kg}$ of peat briquettes per week.…
The mass of $\mathrm{CO_2}$ released when a fuel burns can be calculated from its equation. A household in Galway burns $50.0\,\text{kg}$ of peat briquettes per week. Assume the briquettes are $60.0\%$ carbon by mass and that all the carbon is converted to $\mathrm{CO_2}$ ($A_r$: C = 12, O = 16).
- a) Calculate the mass of carbon burned per week.
- b) Calculate the mass of $\mathrm{CO_2}$ produced per week, in kg.
- c) Calculate the mass of $\mathrm{CO_2}$ produced over a 26-week heating season, in tonnes.
Atmospheric $\mathrm{CO_2}$ concentration rose from $280\,\text{ppm}$ before industrialisation to $420\,\text{ppm}$ today. Assume the atmosphere contains…
Atmospheric $\mathrm{CO_2}$ concentration rose from $280\,\text{ppm}$ before industrialisation to $420\,\text{ppm}$ today. Assume the atmosphere contains $1.77\times10^{20}\,\text{mol}$ of gas in total, and $M_r(\mathrm{CO_2}) = 44$.
- a) Calculate the percentage increase in $\mathrm{CO_2}$ concentration.
- b) Calculate the increase in the amount of $\mathrm{CO_2}$ in the atmosphere, in mol.
- c) Calculate the corresponding mass, in gigatonnes ($1\,\text{Gt} = 10^{15}\,\text{g}$).
- d) Humans have emitted about 1,600 Gt of $\mathrm{CO_2}$ over this period. Calculate the fraction that remains in the atmosphere and suggest where the rest has gone.
Ocean acidification
The oceans absorb roughly a quarter of the $\mathrm{CO_2}$ we emit. This slows atmospheric warming, but it changes the chemistry of the sea.
$\mathrm{CO_2}$ dissolves in water and forms carbonic acid, a weak acid:
$$\mathrm{CO_2(g)} \rightleftharpoons \mathrm{CO_2(aq)}$$ $$\mathrm{CO_2(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{H_2CO_3(aq)}$$The carbonic acid then dissociates in two steps:
$$\mathrm{H_2CO_3(aq)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{HCO_3^-(aq)}$$ $$\mathrm{HCO_3^-(aq)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{CO_3^{2-}(aq)}$$More dissolved $\mathrm{CO_2}$ means more $\mathrm{H^+}$ ions, so $\mathrm{pH} = -\log_{10}[\mathrm{H^+}]$ falls. Sea water is slightly basic (about pH 8.2 before industrialisation, about 8.1 now). Even a small change in pH is big in terms of concentration, because pH is logarithmic. A fall of 0.1 means $[\mathrm{H^+}]$ is up by about 26%.
The effect on shells
Corals, mussels, oysters and tiny plankton build shells and skeletons of calcium carbonate:
$$\mathrm{Ca^{2+}(aq)} + \mathrm{CO_3^{2-}(aq)} \rightleftharpoons \mathrm{CaCO_3(s)}$$Use Le Châtelier's principle step by step:
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More $\mathrm{CO_2}$ increases $[\mathrm{H^+}]$.
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The extra $\mathrm{H^+}$ disturbs $\mathrm{HCO_3^-} \rightleftharpoons \mathrm{H^+} + \mathrm{CO_3^{2-}}$. The system opposes the increase by moving to the left, using up $\mathrm{H^+}$ by reacting with $\mathrm{CO_3^{2-}}$ to make $\mathrm{HCO_3^-}$.
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So $[\mathrm{CO_3^{2-}}]$ falls.
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In the shell equilibrium, less $\mathrm{CO_3^{2-}}$ moves the position to the left. Less $\mathrm{CaCO_3}$ forms, and existing shells can dissolve.
Marine organisms then find it harder to build and keep their shells, which threatens food chains and coral reefs.
Carbon dioxide dissolves in sea water and reacts with water to form carbonic acid. a) Write the equation for the reaction of carbon dioxide with water to form carbonic…
Carbon dioxide dissolves in sea water and reacts with water to form carbonic acid.
- a) Write the equation for the reaction of carbon dioxide with water to form carbonic acid.
- b) Write an equation showing the dissociation of carbonic acid into hydrogencarbonate ions and hydrogen ions.
- c) State what happens to the pH of the oceans as more carbon dioxide dissolves.
The first dissociation of carbonic acid in sea water is: $\mathrm{CO_2(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{HCO_3^-(aq)}$ and…
The first dissociation of carbonic acid in sea water is:
$\mathrm{CO_2(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{HCO_3^-(aq)}$
and hydrogencarbonate ions are in equilibrium with carbonate ions:
$\mathrm{HCO_3^-(aq)} \rightleftharpoons \mathrm{H^+(aq)} + \mathrm{CO_3^{2-}(aq)}$
Marine organisms build shells of calcium carbonate: $\mathrm{Ca^{2+}(aq)} + \mathrm{CO_3^{2-}(aq)} \rightleftharpoons \mathrm{CaCO_3(s)}$.
- a) Using Le Châtelier's principle, explain how increased atmospheric $\mathrm{CO_2}$ lowers the concentration of carbonate ions in sea water.
- b) Explain the consequence for shell-forming organisms such as oysters and corals.
- c) The average pH of surface sea water has fallen from 8.2 to 8.1. Calculate the factor by which $[\mathrm{H^+}]$ has increased.
- d) Explain why a fall of only 0.1 pH units is chemically significant.
Back to the turf fire: the carbon in the turf was stored for thousands of years. Burning it puts $\mathrm{CO_2}$ into the air, where it absorbs infrared radiation, and some of it dissolves in the sea and makes it slightly more acidic.
Summary
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The natural greenhouse effect keeps Earth at about $15\,^{\circ}\text{C}$: gases such as water vapour, $\mathrm{CO_2}$, $\mathrm{CH_4}$ and $\mathrm{N_2O}$ absorb infrared radiation from the Earth and re-emit part of it downwards. The enhanced effect is the extra warming caused by human emissions.
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A gas absorbs IR only if a vibration changes its dipole moment ($\mathrm{CO_2}$ yes, $\mathrm{N_2}$ no).
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GWP compares the warming of a gas with that of $\mathrm{CO_2}$ over a stated time; $\mathrm{CO_2}$-equivalent = mass × GWP. Lifetime matters: methane's GWP is 84 over 20 years and 28 over 100 years.
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The carbon cycle: photosynthesis removes $\mathrm{CO_2}$; respiration, decomposition and combustion add it. Fossil fuels release long-stored carbon; sustainably grown wood is approximately carbon neutral; deforestation removes a sink and releases stored carbon.
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Mass of $\mathrm{CO_2}$ from carbon: multiply by $44/12$. For ppm, moles = ppm × $10^{-6}$ × total moles of air.
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Dissolved $\mathrm{CO_2}$ forms carbonic acid and lowers ocean pH. By Le Châtelier's principle $[\mathrm{CO_3^{2-}}]$ falls, so $\mathrm{CaCO_3}$ shells form less easily.