Test yourself on Climate Change and Hazard Response with exam style questions, marked instantly.
Flashcards that come back just before you would forget them.
A revision plan that remembers what you know and shows what to do next.
See which topics you have nailed and which need another look.
Quiz yourself on Climate Change and Hazard ResponseGet your flashcardsBuild my revision planTrack my progress🌍 Natural vs Human Causes of Climate Change
A critical point for the exam: climate has always changed naturally. The Earth has experienced ice ages and warm periods throughout its 4.5-billion-year history — long before humans existed. You must know the natural causes of climate change for the exam. However, natural causes cannot explain the current rate or pattern of warming. The current phase of warming is happening far faster than any natural process can account for, and the scientific evidence — summarised in the IPCC Sixth Assessment Report (2021) — concludes with "unequivocal" certainty that human influence is the dominant cause of warming since 1950.
Natural Causes
Milankovitch cycles — changes in Earth's orbit
The Earth's orbit around the Sun is not perfectly circular. It varies between more elliptical and more circular on approximately a 100,000-year cycle. The tilt of Earth's axis also varies slightly over a 41,000-year cycle, and the axis "wobbles" on a 26,000-year cycle. Together, these orbital variations — called Milankovitch cycles — alter how much solar radiation reaches different parts of Earth at different times of year. Over tens of thousands of years, these cycles drive ice ages and warm interglacial periods. They are well understood and clearly visible in ice core records. However, Milankovitch cycles operate far too slowly to explain modern warming — they produce changes over millennia, not decades.
The Earth's orbit around the Sun is not perfectly circular. It varies between more elliptical and more circular on approximately a 100,000-year cycle. The tilt of Earth's axis also varies slightly over a 41,000-year cycle, and the axis "wobbles" on a 26,000-year cycle. Together, these orbital variations — called Milankovitch cycles — alter how much solar radiation reaches different parts of Earth at different times of year. Over tens of thousands of years, these cycles drive ice ages and warm interglacial periods. They are well understood and clearly visible in ice core records. However, Milankovitch cycles operate far too slowly to explain modern warming — they produce changes over millennia, not decades.
Volcanic eruptions — temporary cooling
Major volcanic eruptions inject large quantities of sulphur dioxide (SO₂) into the stratosphere. Here, SO₂ reacts with water to form tiny sulphate particles that reflect incoming solar radiation back into space, temporarily cooling the planet. The eruption of Mount Pinatubo in the Philippines in 1991 reduced global average temperatures by approximately 0.5°C for 1–2 years. Importantly, volcanic eruptions cause cooling, not warming — so they cannot explain modern temperature rises. They are also temporary: once the sulphate particles settle out of the stratosphere (within 1–3 years), temperatures return to previous levels.
Major volcanic eruptions inject large quantities of sulphur dioxide (SO₂) into the stratosphere. Here, SO₂ reacts with water to form tiny sulphate particles that reflect incoming solar radiation back into space, temporarily cooling the planet. The eruption of Mount Pinatubo in the Philippines in 1991 reduced global average temperatures by approximately 0.5°C for 1–2 years. Importantly, volcanic eruptions cause cooling, not warming — so they cannot explain modern temperature rises. They are also temporary: once the sulphate particles settle out of the stratosphere (within 1–3 years), temperatures return to previous levels.
Solar output variation — sunspot cycles
The Sun's energy output is not perfectly constant. It varies in an approximately 11-year cycle linked to sunspot activity, with small increases in radiation during solar maximum periods. Over longer timescales, solar output has also varied. However, solar output has actually shown a slight cooling trend since the 1980s — the opposite of what would be expected if the Sun were driving modern warming. Scientists can detect this directly via satellite measurements. Solar variation accounts for less than 10% of the observed warming since 1950.
The Sun's energy output is not perfectly constant. It varies in an approximately 11-year cycle linked to sunspot activity, with small increases in radiation during solar maximum periods. Over longer timescales, solar output has also varied. However, solar output has actually shown a slight cooling trend since the 1980s — the opposite of what would be expected if the Sun were driving modern warming. Scientists can detect this directly via satellite measurements. Solar variation accounts for less than 10% of the observed warming since 1950.
Human Causes
Human causes have dominated the warming signal since approximately 1950, and are almost entirely responsible for warming since the 1970s. They work by increasing the concentration of greenhouse gases in the atmosphere, enhancing the natural greenhouse effect.
Burning fossil fuels — the dominant cause
Coal, oil and natural gas are formed from organic material buried millions of years ago. When burned for energy (electricity generation, heating, transport), they release carbon that has been locked underground for millions of years as CO₂. In 2022, the world emitted approximately 38 billion tonnes of CO₂ from fossil fuel burning — the highest annual total in human history. The industrial revolution, which began around 1750 in Britain and spread globally through the 19th and 20th centuries, is the starting point for the rapid rise in atmospheric CO₂.
Coal, oil and natural gas are formed from organic material buried millions of years ago. When burned for energy (electricity generation, heating, transport), they release carbon that has been locked underground for millions of years as CO₂. In 2022, the world emitted approximately 38 billion tonnes of CO₂ from fossil fuel burning — the highest annual total in human history. The industrial revolution, which began around 1750 in Britain and spread globally through the 19th and 20th centuries, is the starting point for the rapid rise in atmospheric CO₂.
Deforestation — removing carbon sinks
Forests store enormous quantities of carbon in their biomass (wood, leaves, roots) and in the soil. When trees are felled and burned to clear land for agriculture or settlement, that stored carbon is released as CO₂. Deforestation also removes the tree's capacity to absorb CO₂ in the future through photosynthesis — destroying both a carbon store and a carbon sink simultaneously. Deforestation currently accounts for approximately 15% of annual global CO₂ emissions. The Amazon rainforest has lost approximately 20% of its original area since 1970, partly converting from a net carbon sink to a net carbon source in heavily deforested areas.
Forests store enormous quantities of carbon in their biomass (wood, leaves, roots) and in the soil. When trees are felled and burned to clear land for agriculture or settlement, that stored carbon is released as CO₂. Deforestation also removes the tree's capacity to absorb CO₂ in the future through photosynthesis — destroying both a carbon store and a carbon sink simultaneously. Deforestation currently accounts for approximately 15% of annual global CO₂ emissions. The Amazon rainforest has lost approximately 20% of its original area since 1970, partly converting from a net carbon sink to a net carbon source in heavily deforested areas.
Agriculture — methane and nitrous oxide
Agriculture contributes greenhouse gas emissions through several pathways. Livestock — particularly cattle, sheep and goats — produce methane (CH₄) during digestion (enteric fermentation). Methane is approximately 28 times more potent as a greenhouse gas than CO₂ over a 100-year period (and about 80 times more potent over 20 years). Globally, there are approximately 1 billion cattle. Rice paddies produce methane as organic matter decomposes in flooded, oxygen-poor conditions. Nitrogen fertilisers release nitrous oxide (N₂O) — approximately 265 times more potent than CO₂ over 100 years — when broken down in soil by bacteria. Together, these agricultural sources account for roughly 10% of global greenhouse gas emissions.
Agriculture contributes greenhouse gas emissions through several pathways. Livestock — particularly cattle, sheep and goats — produce methane (CH₄) during digestion (enteric fermentation). Methane is approximately 28 times more potent as a greenhouse gas than CO₂ over a 100-year period (and about 80 times more potent over 20 years). Globally, there are approximately 1 billion cattle. Rice paddies produce methane as organic matter decomposes in flooded, oxygen-poor conditions. Nitrogen fertilisers release nitrous oxide (N₂O) — approximately 265 times more potent than CO₂ over 100 years — when broken down in soil by bacteria. Together, these agricultural sources account for roughly 10% of global greenhouse gas emissions.
Industry and cement — process emissions
Beyond energy use, several industrial processes release CO₂ directly. Cement production — the world's most widely used building material — involves heating limestone (calcium carbonate), which releases CO₂ as a by-product. Globally, cement production accounts for approximately 8% of annual CO₂ emissions. Steel production, aluminium smelting and chemical manufacturing also contribute. These process emissions are distinct from the emissions from burning fossil fuels for energy and are harder to eliminate through simply switching to renewable electricity.
Beyond energy use, several industrial processes release CO₂ directly. Cement production — the world's most widely used building material — involves heating limestone (calcium carbonate), which releases CO₂ as a by-product. Globally, cement production accounts for approximately 8% of annual CO₂ emissions. Steel production, aluminium smelting and chemical manufacturing also contribute. These process emissions are distinct from the emissions from burning fossil fuels for energy and are harder to eliminate through simply switching to renewable electricity.
Transport — aviation, shipping and road
Transport accounts for approximately 16% of global greenhouse gas emissions. Road vehicles are the largest contributor, followed by aviation (which has an enhanced warming effect because emissions occur at high altitude) and shipping. Global aviation alone produces approximately 2–3% of CO₂ emissions, but its total climate forcing is higher when contrails and other effects are included. Transport emissions are growing in absolute terms as vehicle ownership and air travel expand in emerging economies.
Transport accounts for approximately 16% of global greenhouse gas emissions. Road vehicles are the largest contributor, followed by aviation (which has an enhanced warming effect because emissions occur at high altitude) and shipping. Global aviation alone produces approximately 2–3% of CO₂ emissions, but its total climate forcing is higher when contrails and other effects are included. Transport emissions are growing in absolute terms as vehicle ownership and air travel expand in emerging economies.
Make this stick
Signing in gives access to all PrepWise features.