Climate change is driven by a complex mix of natural processes and human activities, both of which influence Earth’s climate systems over short and long periods.
Natural causes of climate change
Natural factors have influenced Earth’s climate for millions of years. Although they operate over longer time scales than human activities, they have historically led to dramatic changes in global temperature and weather patterns. Three key natural causes include orbital changes, volcanic activity, and variations in solar output.
Orbital changes
The Earth's position and movement relative to the Sun change over time. These slow changes, known collectively as the Milankovitch cycles, affect how much solar radiation reaches different parts of the Earth at different times of the year. The three main types of orbital variations are:
Eccentricity
This refers to changes in the shape of the Earth’s orbit around the Sun. The orbit shifts between more circular and more elliptical on a cycle of about 100,000 years.
When the orbit is more elliptical, the difference in solar energy received during different times of the year is more significant.
When the orbit is more circular, seasonal differences are less extreme.
Axial tilt (obliquity)
The tilt of the Earth’s axis varies between 22.1 degrees and 24.5 degrees over a cycle of about 41,000 years.
A greater tilt results in more extreme seasons, with warmer summers and colder winters.
A smaller tilt leads to milder seasonal variations. This affects the growth and melting of ice sheets and therefore influences global climate
Precession
This is the wobble of the Earth on its axis, occurring roughly every 26,000 years.
This wobble alters the timing of the seasons, which can either enhance or reduce the contrast between seasons depending on where the Earth is in its orbit.
These cycles have played a key role in triggering ice ages and interglacial periods throughout Earth's history. While they do not explain current climate change, they help scientists understand long-term natural climate variations.
Volcanic activity
Volcanoes can influence the Earth's climate through large-scale eruptions, which inject ash, sulfur dioxide (SO2), and other gases into the upper atmosphere. These substances form aerosols that reflect solar radiation back into space.
This can lead to short-term global cooling lasting from several months to a few years.
The 1991 eruption of Mount Pinatubo released about 20 million tons of SO2 into the stratosphere and caused global temperatures to drop by approximately 0.5 degrees Celsius for over a year.
However, volcanic activity does not produce enough greenhouse gases to explain long-term global warming. Its effects are typically temporary and lead to cooling rather than warming.
Variations in solar output
The Sun does not emit a constant amount of energy. Changes in the Sun’s activity can affect Earth’s climate. These include:
Sunspot cycles, which occur approximately every 11 years.
When there are more sunspots, solar output is slightly higher, leading to marginal warming.
Fewer sunspots mean reduced solar energy, which can contribute to minor cooling.
A historic example is the Maunder Minimum (1645–1715), a period with very few sunspots. This coincided with the Little Ice Age, when temperatures in Europe were significantly cooler.
Despite these variations, solar output changes are too minor to account for the rapid temperature increases observed since the late 20th century.
Human causes of climate change
While natural processes influence climate over long timescales, recent and rapid changes are largely the result of human activities. These include the burning of fossil fuels, agricultural practices, and deforestation, all of which significantly increase the concentration of greenhouse gases (GHGs) in the atmosphere.
Fossil fuels
The combustion of fossil fuels—coal, oil, and natural gas—releases vast amounts of carbon dioxide (CO2) into the atmosphere.
These fuels are used for electricity production, transportation, industry, and domestic heating.
CO2 is the most prevalent greenhouse gas emitted by humans and contributes to the enhanced greenhouse effect by trapping heat in the atmosphere.
Before the Industrial Revolution, atmospheric CO2 levels were about 280 parts per million (ppm). As of the 2020s, this has exceeded 420 ppm. This unprecedented increase is closely linked with human energy use.
In addition to CO2, burning fossil fuels also releases:
Methane (CH4) from oil and gas extraction.
Nitrous oxide (N2O) from combustion processes and industrial activity.
Both CH4 and N2O are far more potent than CO2 in trapping heat—methane is about 25 times and nitrous oxide about 300 times more powerful over a 100-year period.
Agriculture
Modern agriculture contributes to climate change in several ways, particularly through the release of methane and nitrous oxide.
Methane emissions
Livestock, especially cows and sheep, produce methane during digestion, known as enteric fermentation.
Rice paddies emit methane due to anaerobic conditions in flooded fields, where organic matter breaks down without oxygen.
Nitrous oxide emissions
The use of nitrogen-rich fertilizers leads to N2O being released from soil through microbial processes such as denitrification.
This gas remains in the atmosphere for over 100 years and has a strong warming effect.
Land use changes
Expanding farmland often involves clearing forests, which releases stored carbon and reduces the land's ability to absorb CO2.
Monoculture farming and overgrazing also degrade soil, reducing its role as a carbon sink.
Deforestation
Forests are essential carbon sinks, removing CO2 from the atmosphere through photosynthesis and storing it in biomass. When forests are cut down or burned:
The stored carbon is released as CO2.
The capacity of the environment to absorb future CO2 is reduced.
The carbon cycle becomes unbalanced, leading to more CO2 in the atmosphere.
Causes of deforestation
Agricultural expansion, especially for crops like soy and palm oil.
Cattle ranching, which is a major driver of Amazon deforestation.
Urban development and infrastructure projects.
Illegal logging and timber trade.
Deforestation contributes to about 10% of global CO2 emissions. The loss of tropical rainforests, such as those in the Amazon Basin, has particularly severe consequences due to their biodiversity and carbon storage potential.
Feedback mechanisms and amplifying effects
Climate change can trigger positive feedback loops that accelerate warming and make it more difficult to reverse:
Melting ice caps and glaciers reduce the Earth’s albedo (reflectivity). Ice reflects sunlight, but when it melts, darker ocean or land surfaces absorb more heat, increasing temperatures further.
Thawing permafrost in Arctic regions releases methane and CO2 that have been trapped in frozen soil for thousands of years.
Drier conditions caused by warming increase the likelihood of wildfires, which emit carbon and destroy trees that otherwise remove CO2 from the atmosphere.
These feedbacks amplify the effects of human-caused emissions and highlight the urgency of reducing greenhouse gas output.
Industrialization and urbanization
Since the 18th century, industrialization has transformed societies and economies, but it has also dramatically increased carbon emissions.
Factories and power stations burn fossil fuels to produce goods and electricity.
The construction sector, especially cement production, emits CO2 during both fuel combustion and the chemical reaction of converting limestone (calcium carbonate) into lime (calcium oxide). The chemical reaction is:
CaCO3 (s) → CaO (s) + CO2 (g)
Urban areas
Urban regions are heat islands, where temperatures are higher due to human activity, dense buildings, and minimal vegetation.
Transport systems, including roads and airports, increase emissions through high fuel consumption and encourage vehicle use.
Transportation
The global transport sector is a major source of greenhouse gas emissions:
Cars and trucks mostly run on gasoline or diesel.
Planes and ships burn large quantities of fuel for long-distance travel and shipping.
Air travel has grown rapidly in recent decades, contributing a growing share of CO2 emissions.
Although solutions like electric vehicles (EVs) and public transportation can help reduce emissions, most vehicles still rely on fossil fuels.
Industrial processes
Industries are responsible for around 20% of global greenhouse gas emissions, much of it from:
Cement and steel manufacturing, which emit CO2 during both energy use and chemical processes.
Chemical industries, which release a range of GHGs including methane, nitrous oxide, and hydrofluorocarbons.
Oil refining and natural gas processing, which release GHGs during both extraction and processing.
Efforts to decarbonize industry include improving energy efficiency, using renewable electricity, and developing carbon capture and storage (CCS) technologies.
FAQ
Cement production is one of the most carbon-intensive industrial processes due to both the energy required and the chemical reactions involved. Beyond burning fossil fuels to heat the kilns, a significant portion of CO2 emissions comes from the chemical process called calcination. During this process, limestone (calcium carbonate) is heated and breaks down into lime (calcium oxide) and carbon dioxide (CO2). This reaction alone accounts for about 50% of the emissions from cement production. The overall process emits around 0.9 tons of CO2 per ton of cement produced. Additionally, cement is widely used in construction, meaning global demand is high and growing, especially in developing economies. Unlike some sectors, emissions from calcination cannot be avoided simply by switching to renewable energy sources, making cement production particularly challenging to decarbonize. Innovations such as carbon capture and storage (CCS) and low-carbon cement alternatives are being developed but are not yet widespread.
Permafrost is permanently frozen ground found mainly in polar regions like Siberia, Alaska, and northern Canada. It stores vast amounts of organic material that has remained frozen for thousands of years. As global temperatures rise, permafrost begins to thaw, exposing this material to decomposition by microbes. This process releases methane (CH4) and carbon dioxide (CO2)—both potent greenhouse gases—into the atmosphere. Methane is especially dangerous because it traps around 25 times more heat than CO2 over a 100-year period. This release creates a positive feedback loop: warming leads to thawing, which leads to more emissions, which in turn causes further warming. Scientists estimate that permafrost contains nearly twice the amount of carbon currently in the atmosphere, so its widespread thawing could significantly amplify climate change. Furthermore, the infrastructure in these areas—like pipelines and buildings—can collapse as the ground destabilizes, adding economic and safety concerns to the environmental impact.
While methane from cattle digestion is a major concern, meat production affects the climate in several additional ways. First, the entire supply chain—from feed crop cultivation to processing and transportation—consumes massive amounts of fossil fuels, contributing to CO2 emissions. Growing animal feed like soy and corn often requires deforestation, particularly in tropical regions like the Amazon rainforest. This destroys carbon sinks and releases stored carbon into the atmosphere. Additionally, manure from livestock releases both methane and nitrous oxide (N2O)—a greenhouse gas with about 300 times the global warming potential of CO2. Intensive animal farming also depletes natural resources like water and often relies on energy-intensive artificial environments. Overall, the climate impact of meat production is far greater than just the methane emitted by cows. Reducing meat consumption or switching to more sustainable farming methods can help lower the sector’s environmental footprint significantly.
Anthropogenic greenhouse gas emissions are those directly caused by human activities. These include the burning of fossil fuels for energy and transportation, deforestation, industrial processes, and intensive agriculture. They have increased rapidly since the Industrial Revolution, contributing significantly to the enhanced greenhouse effect. Common anthropogenic greenhouse gases include carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), each with varying global warming potentials and atmospheric lifespans.
In contrast, natural greenhouse gas emissions occur without human influence. These include methane released by wetlands, CO2 from volcanic eruptions, and emissions from forest fires caused by lightning. While natural emissions have always played a role in Earth’s climate system, they are generally balanced by natural sinks like oceans and forests. The key difference is that anthropogenic emissions have disrupted this balance, causing GHG concentrations to rise to levels not seen in hundreds of thousands of years. The rapid pace and scale of these human emissions are what make modern climate change so severe and urgent.
Nitrous oxide (N2O) is present in smaller quantities than CO2 or methane, but it is extremely potent. Over a 100-year period, N2O has around 300 times the global warming potential of CO2, making even small emissions highly impactful. It also remains in the atmosphere for over 110 years, which allows it to exert warming effects for an extended period. N2O is primarily released through agricultural activities, especially from the use of synthetic nitrogen-based fertilizers. When these fertilizers are applied to soil, microbial processes convert some of the nitrogen into nitrous oxide. Additional sources include manure management and fossil fuel combustion. N2O not only contributes to the enhanced greenhouse effect but also damages the ozone layer, which protects life on Earth from harmful ultraviolet radiation. Its dual role as a climate and atmospheric pollutant makes it one of the most dangerous yet under-recognized greenhouse gases associated with human activity.
Practice Questions
Explain how human activities contribute to the enhanced greenhouse effect.
Human activities, particularly the burning of fossil fuels like coal, oil, and natural gas, release large amounts of carbon dioxide into the atmosphere, which traps heat and contributes to global warming. Agriculture also adds greenhouse gases, such as methane from livestock digestion and rice cultivation, and nitrous oxide from fertilizers. Deforestation reduces the number of trees that absorb CO2, increasing its concentration in the atmosphere. These activities intensify the natural greenhouse effect by adding more heat-trapping gases, causing Earth’s temperatures to rise faster than would occur through natural processes alone. This contributes directly to climate change.
Describe how volcanic eruptions can lead to short-term climate change.
Volcanic eruptions can release large volumes of ash and sulfur dioxide into the upper atmosphere. These substances form aerosols that reflect incoming solar radiation away from Earth, leading to temporary global cooling. For example, the 1991 eruption of Mount Pinatubo caused global temperatures to drop by about 0.5°C for over a year. Although the effects are short-lived, they demonstrate how natural processes can influence climate. However, volcanic eruptions do not contribute significantly to long-term climate change since they don’t release enough greenhouse gases to compete with human-induced emissions over time.
