Solar radiation and climate change

Solar radiation is the fuel that drives Earth’s climate engine. Global average temperatures respond to variations in both the amount of energy emitted by the Sun and the amount of that energy absorbed and retained by the Earth.

Background

The Sun is a typical yellow dwarf star. It is a mixture of incandescent gases, primarily hydrogen (80 percent by mass) and helium (20 percent by mass) with trace amounts of heavier elements. The Sun has a total mass of 1.99 10³⁰ kilograms and a radius of 6.96 10⁸ meters. The Earth orbits the Sun at an average distance of 1.5 10¹¹ meters.

The surface of the Sun has a temperature of 5,800 Kelvins, the temperature at which thermal radiation peaks at yellow in the visible section of the electromagnetic spectrum. Radiation emitted by the Sun totals 3.93 10²⁶ watts. This tremendous amount of energy emitted each second is replenished by nuclear reactions occurring in the core of the Sun. Hydrogen is converted into helium (four atoms of hydrogen fuse to form one atom of helium), with the excess mass converted to energy.

Structure

At the center of the Sun, the pressure is approximately 3.16 quadrillion kilograms per square meter, 300 billion times Earth’s atmospheric pressure. The temperature is 15 billion Kelvins. These conditions permit the fusion of four hydrogen nuclei (protons) into one helium nucleus with the net loss of 7 percent of the original mass. This mass is converted into energy in accordance with Albert Einstein’s special theory of relativity. The Sun converts 4.5 million metric tons of matter into energy each second.

The core of the Sun is the region where nuclear fusion takes place. It contains 40 percent of the mass of the Sun and produces 90 percent of the energy radiated by the Sun. The core extends outward to one-quarter of the distance to the surface. At this radius, the temperature falls to 13 million Kelvins, which is too cool to sustain the fusion reaction.

The energy escaping the core is predominantly in the form of X-rays, an extremely energetic type of electromagnetic radiation. The material of the Sun from the top of the core out to three-quarters of the distance to the surface is a hot, dense plasma that is opaque to X-ray radiation. This layer slows the transport of energy from the core outward to such an extent that it may take millions of years for the energy from the core to reach the Sun’s surface and escape into interplanetary space.

At the three-quarters mark, the temperature of the Sun is about 1.5 million Kelvins. The X-radiation has been replaced by ultraviolet radiation. The plasma in this region undergoes convection, in which hot, buoyant plasma rises to be replaced by cooler and denser plasma from above. This drastically increases the rate at which energy moves outward toward the surface, with the result that the temperature begins to drop precipitously. Three tiers of ever-smaller convection cells fill the Sun from the three-quarters mark out to the surface. The temperature drops steeply in this region to the relatively cool 5,800 Kelvins at the surface.

The outermost shell of the Sun is called the photosphere. The appearance that the Sun has a definite physical surface is an illusion. The density, pressure, and temperature of the photosphere drop continuously and smoothly with increasing distance from the center, but at a certain point the gas composing the photosphere abruptly changes from opaque to visible to transparent and becomes invisible from that point outward.

The region outside the photosphere is regarded as the atmosphere of the Sun. It is divided into two layers. The chromosphere reaches out to 10,000 kilometers above the photosphere, where the temperature rises abruptly to 1 million Kelvins. This new region of the solar atmosphere is called the corona.

The photosphere is an intensely active region. The tops of the third-tier convection cells are visible and give the surface a granular appearance. Sunspots are large, relatively cool regions that appear dark in contrast to the surrounding hotter material. They are associated with intense localized magnetic fields. About 1 percent of the solar surface is covered by spicules, jets of superheated plasma visually resembling flames. Prominences are loops of plasma trapped by the lines associated with sunspots. Prominences may extend thousands of kilometers out into the chromosphere and occasionally erupt, flinging solar material out into interplanetary space.

Capacity to Effect Climate Change

The Earth receives 1,350 watts per square meter of radiation from the Sun at the top of the atmosphere. This is referred to as the average insolation. The varies over the course of the year due to the eccentricity of the Earth’s orbit. The insolation also varies as the number of waxes and wanes over an eleven-year cycle. Long period variations in insolation associated with recurrent ice ages are due to minute variations in the shape of the Earth’s orbit traceable to interplanetary gravitational influences.

Approximately 36 percent of the radiation received from the Sun is reflected into space, with most of this reflection coming from clouds and ocean water. The ratio of radiation reflected to radiation received is called albedo.

The ultraviolet portion of the solar spectrum ionizes virtually all the atoms at altitudes above 160 kilometers to form the ionosphere; more is absorbed in the stratosphere. Only a small fraction of solar ultraviolet radiation reaches the surface of the Earth. Virtually all the visible and near-visible infrared radiation reaches the surface. With limited exceptions, all the rest of the solar radiation, in the form of infrared radiation and radio waves, is absorbed in the atmosphere. The portion of solar radiation absorbed by the ground is reemitted as infrared radiation to which the atmosphere is opaque. The atmosphere thus acts as a gate that lets the energy of the visible radiation in but does not permit the reemitted infrared radiation to escape. This phenomenon is called the greenhouse effect; it keeps the Earth warmer than it would otherwise be.

Key Concepts

• albedo: the fraction of incident light reflected from a body such as Earth
• greenhouse effect: the tendency of Earth’s atmosphere to transmit visible radiation from the Sun to the ground, where it is absorbed and reemitted as infrared radiation that cannot readily escape to space
• insolation: the amount of radiant energy per square meter per second received by the Earth from the Sun

Bibliography

Friedman, Herbert. Sun and Earth. New York: Scientific American Library, 1986.

Sylte, Allison. "Shading the Sun." Colorado State University, 11 Apr. 2024, source.colostate.edu/could-reflecting-sunlight-away-from-the-earth-buy-more-time-in-the-battle-against-climate-change/. Accessed 17 Dec. 2024.

Prialnik, Dina. An Introduction to the Theory of Stellar Structure and Evolution. New York: Cambridge University Press, 2000.

Tribble, Alan. The Space Environment: Implications for Spacecraft Design. Rev. and expanded ed. Princeton, N.J.: Princeton University Press, 2003.