Natural causes of climate change

Climate change has occurred throughout Earth's history, with significant shifts such as the end of the last Ice Age around 12,000 years ago. Many attribute these changes to natural processes, particularly variations in solar radiation received by the planet due to orbital and axial movements.

Milankovitch cycles and solar radiation variations

• Eccentricity - The shape of Earth's orbit around the Sun changes over time, becoming more or less elliptical. This affects the distance from the Sun and thus the amount of solar energy received.
• Obliquity - The tilt of Earth's axis varies, altering the distribution of solar radiation across the planet's surface.
• Precession - The wobble of Earth's axis as it spins causes changes in the direction the axis points.
• Combined effect - According to Milankovitch's theory, these three factors combine every 100,000 years in a way that significantly reduces solar radiation, triggering glacial periods of cooler global temperatures.

Other natural contributors to climate change

• Volcanic activity - Major eruptions release vast amounts of dust and ash into the atmosphere, which can block solar radiation. This reduction in sunlight can lower global temperatures, potentially leading to temporary glacial conditions.
• Cosmic impacts - Large meteors or asteroids striking Earth can also eject significant debris into the atmosphere, mirroring the effect of volcanic eruptions by reducing solar input. Some theories suggest such an event contributed to the extinction of dinosaurs at the end of the Jurassic period through abrupt climate shifts.

Human causes of climate change and the greenhouse effect

In recent times, human activities have been identified as a primary driver of the current rise in global temperatures. This is largely due to the emission of greenhouse gases, which enhance the natural greenhouse effect, trapping heat within Earth's atmosphere.

Understanding the greenhouse effect

The greenhouse effect is a natural process where greenhouse gases in the atmosphere allow solar radiation to pass through and warm the Earth's surface. These gases also absorb outgoing heat, preventing it from escaping into space. Some incoming solar radiation is reflected back into space, but greenhouse gases permit enough to reach and warm the surface. The outgoing radiation is then absorbed by these gases, maintaining higher atmospheric temperatures.

Increased levels of greenhouse gases from human sources have intensified this effect, trapping more heat and contributing to a gradual rise in global temperatures, known as global warming.

Key greenhouse gases and their human sources

Global temperature trends and predictions

Accurate weather and climate measurements over the past 100–150 years have provided evidence of significant changes in global temperatures, highlighting a clear warming trend in recent decades.

Observed temperature changes

• Historical data - Since records began in 1850, global land temperatures have risen by an average of 1°C compared to the late 19th century.
• Recent trends - Over the last century, an average increase of 0.7°C has been documented.
• Record highs - The 1990s recorded some of the highest temperatures on record, indicating a continuing upward trend.
• Future projections - Leading scientific organisations forecast a potential rise of up to 4.5°C by the end of the 21st century, which could have severe ecological impacts, as historical data from the Permian period (250 million years ago) suggests a 6°C increase led to the extinction of 95% of species.

Sources and sectors of greenhouse gas emissions

Human activities across various sectors contribute significantly to greenhouse gas emissions, with carbon dioxide (CO₂) being the most dominant, accounting for nearly three-quarters of the total emissions.

Annual greenhouse gas emissions by sector

Key observations on emissions

• Dominant contributors - Electricity and heat production, along with agriculture and land use, are the largest sources of emissions, closely followed by industrial activities.
• Geographical impact - Developed nations, particularly the USA, contribute a significant portion, with the USA alone responsible for 36% of global greenhouse gas emissions, though the consequences affect all regions worldwide.
• Correlation with warming - The parallel increase in global temperatures and CO₂ emissions has led many scientists to establish a causal link between human-induced greenhouse gas emissions and global warming.

Debates surrounding the causes of global warming

While there is substantial evidence linking human activity to current climate change, not all scientists agree on the primary causes, leading to ongoing discussions about the relative impacts of natural and anthropogenic factors.

Contrasting perspectives on global warming

• Human activity as primary cause - Many scientists argue that the rapid increase in greenhouse gases, especially CO₂ from burning fossil fuels and deforestation, is the main driver of recent global temperature rises, intensifying the greenhouse effect.
• Natural climate variability - Some researchers contend that current warming is part of Earth's natural climate cycles, pointing to historical fluctuations in temperature over geological and even recent historical periods, with alternating warm and cold phases independent of human influence.
• Broader environmental impacts - Beyond greenhouse gases, other human practices such as air, land, and water pollution, as well as river and coastal management, disrupt natural systems, increasing environmental fragility.