Understanding the Earth’s Climate System: Key Influences

Earth’s climate has never been static. Over billions of years, it has swung between scorching warmth and deep ice ages – and everything in between. What drives these shifts? The answer isn’t simple. Earth’s climate system is shaped by a complex web of forces: some originate deep within the planet, others come from space, and today, increasingly, from human activity. Understanding each of these factors is the first step toward making sense of both ancient climate history and the rapid changes happening right now.

Internal forcing and climate variability 🔗

Not all climate change is driven by external forces. Much of Earth’s natural climate variability comes from processes within the climate system itself – particularly the interactions between the ocean and the atmosphere.

The ocean is a massive reservoir of heat, water, and carbon dioxide. As the Woods Hole Oceanographic Institution explains, the ocean stores and redistributes these elements across the planet, exchanging them with the atmosphere at rates comparable to atmospheric transport. This tight coupling between the ocean and atmosphere is what creates internal climate oscillations – recurring patterns that can shift global temperatures for years or even decades.

El Niño-Southern Oscillation (ENSO) 🔗

One of the best-known examples is the El Niño-Southern Oscillation (ENSO). During an El Niño event, unusually warm water surfaces in the eastern tropical Pacific, disrupting normal wind and precipitation patterns across the globe – bringing heavy rains to some regions and drought to others. Its counterpart, La Niña, has the opposite effect. According to the National Academies of Sciences, ENSO events typically persist for 6 to 18 months and have well-documented global connections, including effects on hurricane frequency and rainfall across the Americas.

Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) 🔗

On longer timescales, the Pacific Decadal Oscillation (PDO) operates as an ENSO-like pattern of sea surface temperature variability across the North Pacific, shifting between warm and cool phases over periods of decades. The NOAA National Centers for Environmental Information notes that these phase shifts are associated with widespread changes in Pacific Basin and North American climate. A similar phenomenon – the Atlantic Multidecadal Oscillation (AMO) – drives decadal swings in North Atlantic sea surface temperatures, influencing rainfall patterns in Africa, North America, and Europe. These internal oscillations can temporarily accelerate or slow the rate of global warming at the surface, making them important for climate prediction.

Human influences and climate change 🔗

While natural internal variability has shaped climate for millennia, the dominant driver of climate change today is unambiguously human activity. The IPCC Sixth Assessment Report states clearly that greenhouse gases emitted through human activity have had the greatest effect on the observed increase in global average surface temperature since pre-industrial times.

Fossil fuels and greenhouse gas emissions 🔗

The burning of coal, oil, and natural gas releases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) into the atmosphere. According to the US EPA, electricity and heat production accounts for the single largest share of global emissions, followed by agriculture, transportation, and industry. Earth’s atmosphere now holds roughly 50% more CO₂ than it did at the end of the pre-industrial era – levels not seen for millions of years. These gases trap outgoing heat radiation, warming the lower atmosphere in what is known as the enhanced greenhouse effect.

Deforestation and land use change 🔗

Forests absorb CO₂ during photosynthesis and store vast amounts of carbon in their biomass and soil. When they are cleared or burned, that stored carbon is released back into the atmosphere. The United Nations estimates that deforestation alone contributed approximately 10% of all human-induced greenhouse gas emissions in 2018. Beyond carbon release, the loss of forest cover changes local albedo (how much sunlight a surface reflects), affects water cycling, and disrupts regional weather patterns. In tropical regions, where deforestation rates are highest, these effects can significantly alter rainfall and temperature regimes.

The cumulative effect of these human activities is a climate system being pushed beyond the range of natural variability. According to the British Geological Survey, while natural climate change has historically occurred over thousands or millions of years, the current rate of change is far more rapid than anything visible in the geological record.

Orbital variations and solar output 🔗

Zoom out far enough in time and a different set of forces comes into view – ones that operate over tens of thousands to hundreds of thousands of years. These are the astronomical controls on Earth’s climate, most systematically described as Milankovitch cycles.

The three Milankovitch cycles 🔗

Serbian mathematician Milutin Milanković proposed that three regular variations in Earth’s orbit around the Sun collectively drive long-term climate shifts, particularly glacial and interglacial cycles. As described by Energy Education (University of Calgary), these three cycles are:

• Eccentricity: Earth’s orbit shifts between more circular and more elliptical shapes over cycles of roughly 100,000 years, changing how much total solar energy Earth receives at different points in its orbit.
• Obliquity (axial tilt): Earth’s axial tilt varies between approximately 22° and 24.5° over about 41,000 years. Greater tilt increases the contrast between seasons, especially at higher latitudes.
• Precession: Earth “wobbles” on its axis like a spinning top over a cycle of roughly 23,000 years, altering which hemisphere is tilted toward the Sun during certain seasons.

These cycles don’t change the total amount of solar energy Earth receives as dramatically as they redistribute it across latitudes and seasons. Research from Florida Atlantic University’s Climate Science Investigations explains that ice ages begin when the cycles align to produce cool summers in the Northern Hemisphere – snow accumulates rather than melting, and ice sheets gradually build up. Over the past 500,000 years, glacial and interglacial cycles have followed a clear, recurring pattern consistent with these orbital rhythms.

Solar output variations 🔗

The Sun itself is not a perfectly constant energy source. Solar intensity varies over an approximately 11-year sunspot cycle, during which changes in magnetic activity alter the Sun’s total energy output slightly. Historical periods of reduced sunspot activity – such as the Maunder Minimum (1645-1715) – have been linked to cooler climate episodes like the Little Ice Age. However, as the US EPA makes clear, solar variations have played little role in recent decades of warming. Since the 1970s, satellite measurements have shown no increasing trend in solar output, even as global temperatures have continued to rise sharply.

Volcanism and plate tectonics 🔗

Earth’s geological activity is another powerful – if slower-moving – force on the climate system. Both volcanic eruptions and the long-term movement of tectonic plates can fundamentally alter atmospheric composition and global circulation patterns.

Volcanic eruptions and short-term cooling 🔗

During major explosive eruptions, volcanoes inject large quantities of sulfur dioxide (SO₂) into the stratosphere. This gas rapidly converts to sulfuric acid aerosols, which form a reflective haze that partially blocks incoming sunlight. As NOAA’s National Centers for Environmental Information explains, these aerosol particles cool the climate by reflecting sunlight back into space. The effect is real but short-lived – typically lasting two to three years per eruption. A well-documented example is the 1815 eruption of Mount Tambora in Indonesia, which caused the “year without a summer” in 1816, bringing snow in June to parts of North America and triggering widespread crop failures.

It’s important to note the scale difference between natural and human sources of CO₂. According to the US Geological Survey estimates cited by Wikipedia, human activities currently generate 100 to 300 times the amount of CO₂ emitted by volcanoes annually. Volcanic eruptions are climatically significant in the short term but cannot explain the sustained warming trend of the industrial era.

Plate tectonics and long-term climate reshaping 🔗

Over millions of years, the movement of tectonic plates rewrites the geography of the planet – and with it, the climate. Continental positions determine how ocean currents circulate heat around the globe, how mountain ranges deflect winds and precipitation, and where ice sheets can form. As the British Geological Survey notes, large mountain chains influence global air circulation by deflecting warm air toward cooler regions, directly affecting regional climates.

One of the most studied examples is the formation of the Isthmus of Panama. As the Central American landmass connected North and South America through tectonic movement, it cut off the ocean connection between the Atlantic and Pacific, redirecting warm water northward into what became the Gulf Stream. This single geological event had profound consequences for North Atlantic circulation and may have contributed to the onset of Northern Hemisphere glaciation. On even longer timescales, tectonic activity regulates atmospheric CO₂ through volcanic outgassing and the chemical weathering of rocks, acting as a planetary thermostat over geological eons. As research on geology and climate highlights, the deep carbon cycle – balancing volcanic CO₂ release with weathering-based sequestration – sets Earth’s baseline climate state across millions of years.

Putting it all together 🔗

Earth’s climate is the product of forces operating across vastly different timescales – from year-to-year ocean oscillations, to 100,000-year orbital cycles, to hundred-million-year tectonic shifts. Natural internal variability, orbital mechanics, solar fluctuations, and volcanism have all shaped climate throughout Earth’s history. But the critical distinction today is pace and cause. None of these natural factors can account for the speed or magnitude of the warming observed since the Industrial Revolution. The IPCC’s assessments consistently confirm that the current episode of climate change is driven primarily by the accumulation of greenhouse gases from human activity – a forcing that operates on a timescale unlike anything in the natural record.

Recognizing all these influences together – natural and human, internal and external – is essential for accurately interpreting climate data, building reliable models, and designing effective policy responses.

What do you think? Given that Earth’s climate responds to so many forces across such different timescales, how do you think scientists go about distinguishing natural climate variability from human-caused warming? And if tectonic shifts and orbital cycles have driven dramatic climate changes in the past without human involvement, does that change how you think about the current pace of change?

References

1. https://www.whoi.edu/oceanus/feature/oceans-climate/
2. https://www.nationalacademies.org/read/23552/chapter/5
3. https://www.ncei.noaa.gov/access/monitoring/pdo/
4. https://www.congress.gov/crs-product/R47082
5. https://www.epa.gov/ghgemissions/global-greenhouse-gas-overview
6. https://www.un.org/en/climatechange/science/climate-issues/land
7. https://www.bgs.ac.uk/discovering-geology/climate-change/what-causes-the-earths-climate-to-change/
8. https://energyeducation.ca/encyclopedia/External_forcing
9. https://www.ces.fau.edu/nasa/module-4/causes-2.php
10. https://www.epa.gov/climatechange-science/causes-climate-change
11. https://www.ncei.noaa.gov/products/paleoclimatology/climate-forcing
12. https://en.wikipedia.org/wiki/Climate_variability_and_change
13. https://climate.sustainability-directory.com/question/how-does-geology-affect-climate/
14. https://www.nrdc.org/stories/greenhouse-effect-101