The two factors with the greatest effect on climate are solar radiation (the energy Earth receives from the Sun) and the composition of the atmosphere, specifically greenhouse gases. Solar radiation is the primary energy source driving Earth’s climate system, while greenhouse gases determine how much of that energy gets trapped near the surface rather than escaping back into space. Together, these two factors control the planet’s overall temperature and shape every climate pattern on Earth.
Solar Radiation: The Energy That Powers Climate
Everything about Earth’s climate starts with the Sun. Roughly 340 watts per square meter of solar energy reach Earth on average, though this energy is not distributed evenly. The equator receives far more direct sunlight than the poles because of Earth’s curved surface and its tilt on its axis. This uneven heating is what creates wind patterns, ocean currents, and the basic temperature differences between tropical and polar regions.
Not all of that incoming energy stays in the system. About one-third of it bounces back into space, reflected by clouds, ice, snow, and light-colored surfaces. Scientists measure this reflectivity as “albedo,” and Earth’s average albedo is about 0.30, meaning 30% of sunlight is reflected. The remaining 240 watts per square meter gets absorbed by land, oceans, and the atmosphere, warming the planet. If Earth were entirely covered in ice, albedo would jump to about 0.84, reflecting most sunlight and making the planet far colder. If it were covered in dark forest, albedo would drop to about 0.14, absorbing much more heat.
How much solar energy Earth receives also shifts over very long timescales. Earth’s orbit around the Sun isn’t perfectly stable. It wobbles in three predictable cycles: the shape of the orbit changes over roughly 100,000 years, the tilt of Earth’s axis shifts over about 41,000 years, and the direction the axis points rotates over about 26,000 years. These orbital variations, known as Milankovitch cycles, are the primary trigger for ice ages and warm periods throughout Earth’s history. When the tilt decreases, winters get milder but summers get cooler, allowing snow and ice to accumulate at high latitudes. That extra ice reflects more sunlight, which promotes further cooling in a self-reinforcing loop.
Over shorter timescales, the Sun’s own energy output fluctuates slightly. Satellite measurements spanning more than 40 years show the Sun’s output has varied by less than 0.1% during that period. While solar variability has played a role in past climate shifts, it is far too small to explain the warming happening now.
Greenhouse Gases: The Atmosphere’s Heat Trap
Solar energy heats the Earth’s surface, which then radiates that heat back upward as infrared energy. Greenhouse gases in the atmosphere absorb some of that outgoing heat and re-emit it in all directions, including back toward the ground. This process, the greenhouse effect, is what keeps Earth warm enough to support life. Without it, average surface temperatures would be well below freezing.
The strength of this heat-trapping effect depends on the concentration of greenhouse gases in the atmosphere. Carbon dioxide is by far the most important one in terms of driving climate change. As of 2024, CO2 contributes about 2.33 watts per square meter of extra heat-trapping energy compared to pre-industrial levels, accounting for 66% of the total warming influence from long-lived greenhouse gases. Methane is the second largest contributor at 0.57 watts per square meter, responsible for about 16% of the total. Other gases like nitrous oxide make up the rest.
Water vapor is actually the most abundant greenhouse gas in the atmosphere, but it works differently. Its concentration is controlled by temperature: warmer air holds more water vapor, which traps more heat, which warms the air further. This makes water vapor a powerful amplifier of warming caused by other greenhouse gases rather than an independent driver.
Why Greenhouse Gases Dominate Today’s Climate
For most of Earth’s history, natural processes controlled greenhouse gas levels. Volcanic eruptions added CO2 to the atmosphere, while rock weathering and ocean absorption slowly removed it. These shifts played out over millions of years. Since the Industrial Revolution, human activities, primarily burning fossil fuels, have added CO2, methane, and nitrous oxide to the atmosphere at a pace that dwarfs natural cycles.
The scale of human influence compared to solar changes is striking. Since 1750, the warming driven by greenhouse gases from fossil fuel burning is over 270 times greater than the slight extra warming from changes in the Sun’s output over that same period. The Intergovernmental Panel on Climate Change concluded in its Sixth Assessment Report that human influence is “unequivocal” as the principal driver of changes observed across the atmosphere, ocean, ice sheets, and ecosystems.
One way scientists quantify this relationship is through “climate sensitivity,” which estimates how much global temperatures would rise if atmospheric CO2 doubled from pre-industrial levels. The best estimate is 3°C (about 5.4°F), with a likely range of 2.5°C to 4°C. That range matters enormously: the difference between the low and high end translates to vastly different outcomes for sea levels, ecosystems, and weather extremes.
How the Ocean Absorbs the Impact
The oceans play an outsized role in regulating how these two factors affect the climate you actually experience. Water has an enormous capacity to absorb and store heat. More than 90% of the excess heat trapped by greenhouse gases has been absorbed by the oceans rather than warming the air directly. This is why surface temperatures haven’t risen as fast as the total energy imbalance might suggest, but it also means the oceans are steadily accumulating heat that affects marine ecosystems, sea levels, and weather patterns for decades to come.
Ocean currents redistribute this heat around the globe, moving warm water from the tropics toward the poles and cold water back again. This circulation shapes regional climates in ways that go far beyond what latitude and sunlight alone would predict. Western Europe, for example, is significantly warmer than other regions at the same latitude because warm Atlantic currents carry tropical heat northward.
How These Two Factors Work Together
Solar radiation and atmospheric composition don’t act in isolation. They interact through feedback loops that can amplify or dampen climate changes. When increased greenhouse gases warm the planet, ice and snow melt, reducing Earth’s albedo. Less reflected sunlight means more absorbed heat, which causes more warming and more melting. Meanwhile, warmer oceans release more water vapor into the atmosphere, strengthening the greenhouse effect further.
These feedbacks explain why relatively modest changes in either factor can produce large climate shifts. The Milankovitch cycles, for instance, cause only small changes in total solar energy reaching Earth, but they’re enough to tip the balance between ice ages and warm periods because greenhouse gas concentrations and ice cover shift in response, amplifying the initial change. Today, the same feedback mechanisms are amplifying the warming caused by rising greenhouse gas levels, which is why even fractional increases in CO2 concentration translate into meaningful temperature changes at the global scale.