As the Sun ages, its output increases. This is a standard finding in stellar evolution theory. A question arises: How has the Earth's temperature stayed tolerable as Sun warmed? We might think that the Earth used to be MUCH colder when Sun was 30% weaker 4 billion years ago. According to some very old zircons, the temperature was about the same 4 billion years ago as it is now. All indications are that the Earth's temperature has been remarkably stable over the eons. [Yes, I know about global ice ages, but even then the overall average temperature did not change by as much as increase in solar output predicts. ]
One commonly overlooked factor in the stability of Earth's climate is the reduction of solar influx is the migration of the Earth's orbit to greater distance from the Sun. There are a few reasons for this.
As a prelude: Since the solar influx is inversely proportional to the square of the radius of the Earth's orbit, the input of solar radiation is strongly dependent upon distance. It is therefore a fundamental error to make computations of solar influx while ignoring the radius of the Earth's orbit.
Some reasons for the Earth's migration away from the Sun, not always distinct:
1. Radiation of sunlight reduces the mass of the Sun, thereby reducing the attraction of gravity and permitting the Earth's orbit to grow larger. As fusion occurs, hydrogen is converted to helium with a reduction in mass which is converted into photons that slowly migrate outward and are ultimately radiated into space, reducing solar mass.
2. Sun ejects mass as solar wind and occasional coronal solar ejections, also reducing the mass of the Sun and letting the Earth slip farther away.
3. Influx of sunlight exerts an outward push against the Earth, increasing orbital radius.
4. Influx of the solar wind plus coronal solar mass ejections also pushes the Earth outward, but much more than a naive calculation based on the Earth's diameter would indicate. This is because the Earth's magnetic field greatly increases the effective cross-section of the Earth with respect to charged particles. Since the radius of the Earth's magnetosphere is roughly ten times the diameter of the Earth, the effective cross-section increases by a factor of ten times ten = one hundred. This is not negligible, but very difficult to estimate since the histories of the Sun's solar wind, coronal ejection, and the strength of the Earth's magnetic field over geological time are hard to determine.
5. What is known is that the rotation of the Earth has been slowing down. This is known not only from orbital calculations, but from daily layers in certain old rock deposits. The Earth rotated about every 6 hours at the beginning, gradually slowing until the present sidereal day of 23 hours 56 min 4 sec. The Earth's magnet field is generated by its rotation. When the Earth was young, the greater rotation speed meant that the magnetic field was much stronger and therefore the effective radius of the magnetosphere, which controls the outward push on the Earth from the Solar wind, coronal mass ejections, etc. Therefore, in the time of the early Earth, when both the magnetic field was more intense and the distance to the Sun was less, the Earth was being pushed away from the Sun much faster than it is today.
6. Over the eons, the orbital interaction between Venus and Earth caused a gradual increase of Earth's orbital radius, along with a comparable decrease in Venus' orbital radius.
7. I estimate a net increase in the Earth' orbital semi-major axis 30 million kilometers, so the original value was about 150 - 30 = 120 million kilometers. When the estimated decrease of Solar output was 40% combined with the increase of the Earth's radius, the net result would be that the solar irradiance of the Earth was then 1/0.6 * (120/150)^2 = 1.066666667, which is nearly the same as it is today.
Hence, the increasing radius of the Earth's orbit significantly offset the increase in solar intensity as the Sun matured. This is at least a partial explanation for the climatic stability of the Earth over the eons.
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