Over one June day, the North Pole can receive more solar energy than the equator and still remain cold over the year. Its annual climate depends on the angle and seasonal timing of sunlight, heat carried poleward by the atmosphere and ocean, and the way snow, ice, and water reflect, absorb, and release energy.

Distance is the wrong scale

Earth is about 149.6 million km from the Sun, while its radius is only 6,371 km. The radius is about 0.0043% of the Earth–Sun distance. The difference between the equator's and the pole's distance from the Sun is therefore far too small to explain the polar climate.

The timing of the seasons also rules out distance as their cause. Earth reaches perihelion, its closest point to the Sun, near 3 January during Northern Hemisphere winter. It reaches aphelion, its farthest point, in early July during northern summer. This changing distance slightly changes the total solar input, but it does not create the seasons or make the poles cold.

A low Sun spreads the same beam out

The solar intensity at a surface depends on the angle at which the beam arrives. The solar zenith angle is measured from the point directly overhead. When the Sun is overhead, a beam covers the smallest surface area. At a solar zenith angle of 60°, the same beam covers twice that area, so each square metre receives roughly half as much energy. A shallow beam also travels through more atmosphere, which gives gases, aerosols, and clouds more opportunity to absorb or scatter it.

Earth's axis is tilted about 23.5°, so the solar angle and the length of daylight both change through the year. Near the June solstice, the noon Sun is about 68.5° above the horizon at 45°N but only 23.5° above the horizon at the North Pole. At the pole, the Sun circles the sky without rising high above the horizon.

The tilt also sets the latitude of the Arctic Circle:

$$90^\circ-23.5^\circ=66.5^\circ\text{N}.$$

Poleward of 66.5°N, the Sun stays above the horizon for at least one full day each year and stays below it for at least one full day. At 80°N, polar night lasts for roughly four months. Summer has continuous daylight, but the Sun remains low.

Why June does not make the Arctic tropical

Around 21 June, representative daily mean solar inputs at the top of the atmosphere are about 524 W/m² at the North Pole and 385 W/m² at the equator. At the pole, twenty-four hours of low-angle sunlight can add up to more energy than twelve hours of stronger sunlight at the equator.

These values describe the top of the atmosphere, not the energy absorbed at the ground or ocean surface. Clouds and the atmosphere remove part of the incoming beam. Snow and ice reflect much of the sunlight that reaches them. When melting ice absorbs energy, that energy changes the ice from solid to liquid instead of raising its temperature. A melting snow or ice surface therefore stays near its melting point even when nearby Arctic air, bare land, and open water rise above 0 °C.

Over a full year, the long winter darkness outweighs the summer pulse. The poles receive only roughly 40% as much top-of-atmosphere solar energy as the equator.

Radiation and transport complete the planetary picture

Every object above absolute zero emits electromagnetic radiation. The hot Sun emits mainly shortwave radiation. Earth's cooler surface and atmosphere emit mainly infrared, or longwave, radiation, and a warmer surface emits more longwave energy than a colder one.

Greenhouse gases and clouds absorb some of the outgoing infrared radiation and then emit infrared radiation in all directions, including back toward the surface. This is absorption and emission, not reflection. The downward infrared emission keeps Earth's average surface much warmer than the planet's effective radiating temperature of roughly −18 °C.

Clouds affect both sides of the radiation budget. By reflecting sunlight, they usually reduce the shortwave energy that reaches the surface. By emitting infrared radiation downward, they increase the surface's longwave input. During polar night, there is no sunlight for clouds to reflect, so only the longwave effect operates. In midsummer, the shortwave cooling effect can be larger.

Earth can balance its energy over the globe and over long periods even though individual latitudes do not balance their own budgets. The tropics absorb more radiation than they emit to space, while high latitudes emit more than they absorb. Atmospheric circulation and ocean currents carry energy from the tropical surplus toward the poles. Near 35° latitude, their combined poleward transport is of order 5 petawatts. Weather systems in the atmosphere carry most of this total. Ocean currents carry a smaller but more geographically concentrated share, including Atlantic heat that enters the Barents Sea.

The surface energy balance

For one square metre of surface, the energy balance depends on the fluxes that cross its boundary:

1. Net shortwave: sunlight arriving minus sunlight reflected;
2. Net longwave: infrared arriving from the atmosphere minus infrared emitted by the surface;
3. Sensible heat: turbulent heat exchange between the surface and moving air, driven mainly by a temperature difference;
4. Latent heat: energy exchanged when water evaporates, condenses, sublimates, or deposits and water vapour is transported;
5. Heat from below: for sea ice, heat conducted upward through the ice and heat delivered from the ocean to the ice base by mixing.

Five terms in the surface energy balance

• Mirror: raises reflection and reduces net shortwave absorption.
• Blanket: slows heat moving upward from the ocean.
• Cap: suppresses evaporation and latent heat loss.
• Buffer: separates the ocean from direct wind stress.

Each term is positive or negative according to the chosen sign convention and the direction of energy flow. When the sum supplies energy to the surface, the energy raises internal energy, melts ice, or does both. When the surface has a net energy loss, it cools or water freezes.

These processes move energy in different ways. Radiation travels as electromagnetic waves and does not need a material medium. Conduction transfers molecular energy through solid ice or snow. Sensible heat flux is turbulent exchange with the air; “sensible” means that it is associated with temperature change, not simply that a person can feel it. Latent heat flux transfers energy as water changes phase and water vapour moves. Surface melting is also a latent-heat process, but its energy is stored in the phase change rather than carried away as an atmospheric vapour flux.

What sea ice changes

Sea ice does not create energy. It changes how much energy is reflected, transferred, and stored in each part of the surface balance.

Snow and ice have a high albedo, so they reflect much of the incoming sunlight and reduce absorbed shortwave radiation. This effect operates only while sunlight is present. §3 traces how the change in absorption feeds back on the ice.

Sea ice also slows conduction between seawater near its freezing point and much colder winter air. For a simple slab,

$$F_c\approx k_i\frac{\Delta T}{h},$$

where $F_c$ is conductive heat flux, $k_i$ is the thermal conductivity of ice, $\Delta T$ is the temperature difference across it, and $h$ is ice thickness. With $k_i\approx2\ \mathrm{W\,m^{-1}\,K^{-1}}$ and $\Delta T=30\ \mathrm{K}$:

• through 0.10 m of new ice, $F_c\approx600\ \mathrm{W/m^2}$;
• through 2.0 m of ice, $F_c\approx30\ \mathrm{W/m^2}$.

These are idealized conductive fluxes, not measurements of the total heat lost by the surface. Two metres of ice does not stop the flux and is not a hard limit on growth. The comparison shows that thin ice conducts heat away quickly and can grow rapidly, while the longer conductive path through thick ice slows further thermodynamic growth. Ridges and ice that survives for several years can become much thicker.

Continuous ice also reduces evaporation from the ocean, so less latent heat and moisture move into the atmosphere. Over an open lead, seawater near −1.8 °C can transfer large sensible and latent heat fluxes to cold, dry air. The escaping moisture can produce fog or sea smoke as the water cools and new ice forms.

Sea ice also changes how wind transfers momentum to the ocean. Wind pushes the ice first, and the drifting ice can transmit force to the water below. Without continuous ice cover, wind acts on the upper ocean more directly and stirs it more strongly.

Why melting can absorb energy without warming

Temperature change and phase change use energy differently. At 0 °C, melting one kilogram of pure ice requires about 334,000 J/kg, while the ice stays at essentially the same temperature. Using a sea-ice density of about $917\ \mathrm{kg/m^3}$, melting a one-metre-thick column over one square metre requires

$$(334{,}000\ \mathrm{J/kg})(917\ \mathrm{kg/m^3})(1\ \mathrm{m}) \approx3.06\times10^8\ \mathrm{J/m^2}.$$

Suppose the ice receives a net $100\ \mathrm{W/m^2}$ for one day and every joule goes into melting:

$$(100\ \mathrm{J\,s^{-1}\,m^{-2}})(86{,}400\ \mathrm{s}) =8.64\times10^6\ \mathrm{J/m^2}.$$

The idealized melt depth is

$$\frac{8.64\times10^6\ \mathrm{J/m^2}} {3.06\times10^8\ \mathrm{J/m^3}} \approx0.028\ \mathrm{m},$$

or about 2.8 cm. This is the result for one day, not one year. In real sea ice, only the energy left after all the other fluxes is available for melting.

Freezing reverses the phase change. Water releases latent heat as it becomes ice, and that heat must leave through conduction or mixing before the ice can get thicker. Ice growth therefore depends on the complete energy balance, not on air temperature alone.

Snow changes both light and heat flow

Fresh snow reflects more sunlight than bare ice and contains a great deal of air, which makes it a poor conductor. It raises the albedo while adding a strong barrier to conductive heat flow. Typical thermal conductivities are about 0.1–0.4 W m⁻¹ K⁻¹ for snow and 2.0–2.2 W m⁻¹ K⁻¹ for sea ice.

These two effects matter at different times of year. In winter, snow insulates the ocean and slows ice growth. In summer, its high albedo can delay melting and protect the ice beneath it. Snow can limit winter growth and preserve summer ice at the same location.

During polar night, net shortwave radiation is zero over both ice and open water. An open lead still loses more longwave, sensible, and latent heat than an ice-covered surface. The upper ocean cools, freezing releases latent heat, and the new thin ice begins to reduce those losses.