Earth’s climate is driven by a handful of interconnected forces: energy from the sun, the composition of the atmosphere, the movement of ocean currents, the positioning of continents, and feedback loops that amplify or dampen changes already underway. Some of these operate on timescales of decades, others over millions of years. Right now, the dominant driver of climate change is the rising concentration of heat-trapping gases in the atmosphere, which reached 427 parts per million of CO2 as of December 2025.
Energy From the Sun
The sun is the original engine of Earth’s climate. It delivers the energy that warms the surface, drives weather patterns, and powers the water cycle. But the sun’s output isn’t perfectly constant. It fluctuates on an roughly 11-year cycle, and over longer periods its brightness has shifted enough to leave a measurable imprint on global temperatures.
Space-based instruments have measured these fluctuations directly. Over a typical solar cycle, total solar output varies by about 0.1%, which translates to roughly 0.24 watts per square meter reaching the lower atmosphere. That’s a small number, but over centuries, longer-term shifts in solar brightness played a significant role in pre-industrial climate. Reconstructions of solar output suggest that during the Maunder Minimum, a period in the late 1600s when sunspot activity nearly vanished, solar energy reaching the lower atmosphere dropped by an estimated 2.9 watts per square meter. That helped push Europe and other regions into what’s sometimes called the Little Ice Age.
During the 20th century, however, solar variability accounts for less than half of the observed warming. After about 1970, the gap widens dramatically: temperatures kept climbing while solar output stayed relatively flat. The sun still matters, but it’s no longer the lead actor.
Earth’s Orbital Shifts
On much longer timescales, slow changes in Earth’s orbit around the sun have triggered the advance and retreat of ice ages. These orbital variations, known as Milankovitch cycles, alter how much sunlight reaches different parts of the planet at different times of year. Three orbital properties matter most.
The shape of Earth’s orbit stretches and contracts over a cycle of about 100,000 years. Right now, the orbit is nearly circular and slowly becoming more so. The tilt of Earth’s axis shifts between 22.1 and 24.5 degrees over a 41,000-year cycle; it’s currently at 23.4 degrees and decreasing. And the wobble of the axis, called precession, completes a full rotation roughly every 23,000 years.
A landmark 1976 study using deep-sea sediment cores confirmed that these cycles line up with major climate shifts over the past 450,000 years. Between one and three million years ago, ice ages arrived on a 41,000-year schedule, matching the tilt cycle. Then, about 800,000 years ago, they shifted to a 100,000-year rhythm, tracking the orbital shape cycle instead. These orbital changes don’t dramatically alter total sunlight reaching Earth. They redistribute it, changing which hemisphere gets more warmth in summer, and that’s enough to tip the balance between glacial buildup and melting.
The Greenhouse Effect
Earth’s atmosphere acts like a selective blanket. It lets visible sunlight pass through to warm the surface, but certain gases absorb the infrared heat the surface radiates back upward, trapping energy that would otherwise escape to space. Without this natural greenhouse effect, Earth’s average surface temperature would be well below freezing.
The gases responsible include carbon dioxide, methane, nitrous oxide, and water vapor. CO2 gets the most attention because human activity has pushed its concentration from about 280 parts per million before the Industrial Revolution to 427 ppm today, a roughly 50% increase. Methane is far more potent per molecule but present in smaller quantities. Water vapor is actually the most abundant greenhouse gas, but its concentration responds to temperature rather than driving it independently: warmer air holds more moisture, which traps more heat, creating a powerful amplifying effect.
The result of all this additional heat-trapping is measurable. As of 2025, global mean surface temperature has risen 1.34°C (2.41°F) above the 1850-1900 pre-industrial average.
Feedback Loops That Amplify Change
Climate drivers rarely act alone. Small initial changes get magnified or suppressed by feedback mechanisms built into the Earth system. These feedbacks explain why relatively modest shifts in sunlight or CO2 can produce dramatic climate swings.
The ice-albedo feedback is one of the most powerful. Ice and snow are highly reflective, bouncing incoming sunlight back to space. When warming melts ice, it exposes darker ocean water or land, which absorbs more heat, which causes more warming and more melting. The process works in reverse too: cooling leads to more ice, more reflection, and further cooling. This positive feedback loop helps explain why ice ages, once they get started, can deepen rapidly.
The strength of this feedback depends on atmospheric conditions. In atmospheres rich in CO2, the temperature difference between ice-covered and ice-free surfaces shrinks considerably (the albedo difference drops from 0.35 to just 0.05 for dense CO2 atmospheres around a sun-like star). Even trace gases like methane and ozone can alter how strongly ice cover affects planetary temperature. This means the ice-albedo feedback doesn’t operate in isolation; it interacts with atmospheric composition in ways that can either sharpen or blunt its impact.
Water vapor feedback is another major amplifier. As the planet warms, more water evaporates into the atmosphere. Since water vapor is itself a greenhouse gas, this extra moisture traps additional heat, warming the planet further. This single feedback roughly doubles the warming caused by CO2 alone.
Ocean Circulation and Heat Transport
The oceans absorb, store, and redistribute vast amounts of heat. One of the most important mechanisms is the thermohaline circulation, a global-scale pattern of ocean currents driven by differences in water temperature and saltiness. Near the surface, warm water flows from the tropics toward the poles. As it travels north across the Atlantic and along Europe, it releases roughly 50 watts of heat per square meter of ocean surface into the atmosphere. That’s why Western Europe has milder winters than its latitude would suggest.
When this warm water reaches the far North Atlantic, it cools, becomes denser, and sinks deep below the surface, then flows back toward the equator at depth. This overturning pattern transports about one quadrillion watts of heat poleward, roughly one-quarter of the total heat moved by the combined ocean-atmosphere system. The net effect is a warmer Northern Hemisphere and a cooler Southern Hemisphere than you’d get from sunlight distribution alone.
This circulation is sensitive to disruption. As global warming intensifies the water cycle (more evaporation, more rainfall), extra freshwater dilutes the surface waters in the North Atlantic, making them less dense and less likely to sink. Climate models project that a fourfold increase in atmospheric CO2 could weaken or even collapse the Atlantic portion of this circulation. Such a collapse would reorganize heat distribution between the hemispheres, potentially cooling parts of Northern Europe even as the planet as a whole warms. The shift could be abrupt and difficult to predict.
Plate Tectonics and Deep Time
Over millions of years, the slow movement of tectonic plates reshapes the climate in ways that dwarf anything happening on human timescales. Continental drift determines which landmasses sit at the poles (where ice sheets can form), which ocean gateways are open or closed (redirecting warm and cold currents), and where mountain ranges rise.
Mountain building matters for two reasons. First, large mountain ranges and continental plateaus alter atmospheric circulation, steering wind patterns and precipitation. Second, the weathering of newly exposed rock pulls CO2 out of the atmosphere through chemical reactions, gradually cooling the planet. The uplift of the Himalayas and the Tibetan Plateau, for instance, is thought to have contributed to the long-term cooling trend over the past 50 million years.
Volcanism works in the opposite direction. Eruptions and volcanic outgassing at mid-ocean ridges and subduction zones release CO2 stored in Earth’s interior back into the atmosphere. Over geological time, the balance between volcanic CO2 release and rock weathering acts as a slow thermostat. Plate tectonics also controls the opening and closing of ocean gateways. When Antarctica separated from South America roughly 34 million years ago, it allowed a circumpolar current to form, isolating Antarctica from warm tropical waters and triggering the growth of its ice sheet.
Global atmospheric circulation follows predictable latitude patterns shaped partly by continental positioning. Dry climates tend to cluster around 30° latitude and the poles, where air descends from higher altitudes. Wet climates concentrate near the equator and around 60° latitude, where surface winds converge and push air upward. As continents drift through these zones over tens of millions of years, regional climates shift dramatically, even without any change in greenhouse gas levels.