What Are the Real Causes of Climate Change?
Earth's climate has never been static. Over millennia, the planet has gone through ice ages, warmer phases, and numerous climate transitions, long before human societies appeared. However, the warming observed since the late 19th century raises particular questions, both in terms of its magnitude and its speed. According to the Intergovernmental Panel on Climate Change (IPCC), the global average temperature has increased by approximately 1.1°C since the pre-industrial era [1].
This rise, seemingly modest, conceals a complex climate dynamic. It raises several legitimate questions: is climate change solely attributable to human activity? What role do natural cycles still play in the observed changes? And how do scientists manage to distinguish the influence of natural processes from that of human activities?
Earth's Natural Climate Cycles
Climate variations are an integral part of Earth's history. Long before the industrial era, the climate evolved under the influence of complex natural processes, sometimes over very long timescales, sometimes more abruptly. These natural mechanisms do not operate in isolation: they interact with one another and collectively shape the climate variability observed in geological and historical records.
Orbital Variations and Their Millennial Impact
Earth's orbit around the Sun is not fixed. It evolves according to regular cycles, known as Milankovitch cycles, which alter the quantity and distribution of solar energy received at the planet's surface. These cycles include the eccentricity of Earth's orbit (approximately 100,000 years), the obliquity of the rotational axis (41,000 years), and the precession of the equinoxes (approximately 19,000 to 23,000 years). Together, these orbital cycles have triggered the glacial-interglacial cycles of the Quaternary [1], alternating between periods of intense glaciation and more temperate epochs. They explain in particular why Earth naturally alternates between cold and warmer climates, independently of any human influence.
Well-Documented Warm and Cold Periods
On timescales shorter than those of orbital cycles, other climate fluctuations have also been observed, notably over the past few millennia.
During the Holocene, which covers the last 11,700 years, the climate experienced several notable fluctuations. Relatively warm periods, such as the Holocene Climatic Optimum or the Medieval Warm Period (approximately 950–1250), coexisted with cooler phases, including the Little Ice Age (approximately 1300–1850). These episodes illustrate the climate system's capacity to generate significant variations without direct human intervention, at both regional and global scales.
The Variable Role of the Sun
Alongside orbital and internal mechanisms, the very source of the climate system's energy — the Sun — is not perfectly constant.
Solar activity fluctuates in cycles of approximately 11 years, causing variations in solar irradiance of around 0.1% [1]. Although modest, these variations have measurable effects on the climate. Over long periods, prolonged phases of low solar activity can contribute to regional or temporary cooling.
The Little Ice Age coincides notably with extended periods of low solar activity, during which European winters were more severe and certain Alpine glaciers advanced.
Volcanic Eruptions and Short-Term Variability
While the preceding factors act over long to intermediate timescales, volcanoes primarily influence the climate in the short term.
Volcanoes play a role in short-term climate regulation. During major eruptions, enormous quantities of sulfate aerosols are ejected into the stratosphere, creating a veil that reflects a portion of solar radiation. This phenomenon can cause temporary cooling of approximately 0.4 to 0.6°C for 1 to 3 years [1].
These coolings are transient, but clearly detectable in modern climate observations. The 1991 eruption of Mount Pinatubo in the Philippines perfectly illustrates this mechanism: it caused a measurable global cooling that persisted for several years.
Ocean Dynamics
Finally, the oceans play a central role in redistributing heat within the climate system, acting as vast energy reservoirs.
Phenomena such as El Niño, La Niña, and the Pacific Decadal Oscillation redistribute heat between the ocean and the atmosphere, influencing global and regional temperatures over periods ranging from a few years to several decades [1].
The IPCC emphasizes that, over the recent period, the combined contribution of natural factors and internal variability remains limited compared to the observed warming [1].
Chronological graph showing temperature variations over the last 800,000 years (glacial-interglacial cycles)
Source: Wikimedia Commons
The Role of Greenhouse Gases and Human Activity
Since the beginning of industrialization, new factors have been added to natural mechanisms, altering the global energy balance.
The Increase in Atmospheric Carbon Dioxide
Carbon dioxide (CO₂) is the main greenhouse gas of anthropogenic origin. Naturally present in the atmosphere, CO₂ plays an essential role in maintaining a temperature compatible with life, but its rapid increase is disrupting this balance. Its concentration rose from approximately 280 ppm in the pre-industrial era to approximately 410 ppm in 2019 [1].
The Main Sources of Human Emissions
This increase in atmospheric CO₂ is explained by several emission sources linked to modern production and consumption patterns.
In 2016, global fossil CO₂ emissions reached approximately 36.4 billion tonnes [2]. They are distributed primarily as follows:
Combustion of fossil fuels (coal, oil, gas) and associated energy processes: ~73.2%
Agriculture, forestry, and land-use changes (including deforestation): ~18.4%
Industrial processes (cement production, chemical industry): ~5.2%
Waste (landfills, wastewater, solid waste management): ~3.2%
Image source: Our World in Data
This breakdown highlights the central role of energy systems in global emissions, while underscoring the importance of land-use changes and agricultural practices.
Methane and Other Greenhouse Gases
CO₂ is, however, not the only greenhouse gas influenced by human activities.
Methane (CH₄) has a global warming potential approximately 28 times greater than that of CO₂ over 100 years. Its atmospheric concentration has increased by 150% since the pre-industrial era [1]. The main anthropogenic sources include agriculture, activities related to fossil fuels, and waste management.
Deforestation and Its Climate Consequences
Greenhouse gas emissions do not come solely from the burning of fossil fuels: modifications to natural ecosystems also play a major role.
Forest destruction affects the climate through two main mechanisms. On one hand, it releases carbon stored in plant biomass and soils. On the other hand, it eliminates natural carbon sinks capable of absorbing atmospheric CO₂. Deforestation thus contributes approximately 10% of global greenhouse gas emissions (CO₂ + others) [1].
The IPCC states unequivocally in its sixth assessment report that human influence has warmed the atmosphere, oceans, and land surfaces [1]. More specifically, human activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels [1].
Extreme Weather Events: A Historical Perspective
Extreme Events Well Before the Industrial Era
Earth's climate history is marked by extreme events. Prolonged droughts, such as those documented during the North American Dust Bowl in the 1930s, or major storms that occurred well before the modern era, testify to the natural variability of the climate [1]. These phenomena have occurred repeatedly and will continue to do so.
A Context Modified by Global Warming
Current warming is, however, altering the baseline conditions in which these events occur. According to the Clausius-Clapeyron relationship, each additional degree Celsius allows the atmosphere to hold approximately 7% more moisture [1], which can intensify certain precipitation events.
Measurable Changes
Heat waves that previously occurred approximately once every 50 years now occur approximately five times more frequently [3]. These figures illustrate a shift in probabilities, rather than the emergence of entirely new phenomena.
The IPCC concludes that it is virtually certain that heat extremes have become more frequent and more intense across the majority of land regions since the 1950s [1].
From Scientific Understanding to Energy Choices
This nuanced understanding of the causes of climate change — whether natural or linked to human activities — provides an essential foundation for addressing the responses envisioned at the societal scale. Energy systems occupy a central place in these reflections, as they are both influenced by the climate and capable of modifying its future trajectories. It is in this context that the notion of energy transition arises, which aims to adapt our modes of energy production and consumption to current climate realities.
-> To explore this concept further and understand its implications, we invite you to consult our article : What Is the Energy Transition and Why Is It Essential?
Conclusion
Climate change results from a combination of natural and anthropogenic factors. Natural cycles — orbital, solar, volcanic, and oceanic — remain active and continue to influence the climate, as they always have throughout the planet's history.
However, the scientific data indicate that the bulk of the warming observed since the mid-20th century is associated with human activities, in particular with changes in energy systems and production methods. The speed of change, the marked increase in greenhouse gas concentrations, and the converging results of climate models make it possible to clearly distinguish the current trend from past natural variations. These findings raise important challenges for sectors related to energy and natural resources, which are called upon to reflect on the evolution of their practices. It is from this perspective that Ressources & Énergie Squatex is interested in developing energy exploration models aimed, in time, at reducing their carbon footprint and positioning themselves in a transition toward more carbon-neutral activities.
References
[1] IPCC. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 2021. https://www.ipcc.ch/report/ar6/wg1/
[2] Friedlingstein, Pierre, et al. "Global Carbon Budget 2021." Earth System Science Data, vol. 14, 2022, pp. 1917-2005. https://essd.copernicus.org/articles/14/1917/2022/
[3] IPCC. Climate Change 2021: Summary for Policymakers. 2021. https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf
