Yes, the Sun does not warm the Earth.
Part of my world view involves “keeping a tidy mind” and that means sometimes clearing out old points of view that are just not accurate. The notion that the Sun warms the Earth is one of those.
Every child knows that when you stand in the sunshine, you get warmed by it. Every child knows that the sunny tropics are much warmer than the polar icy night. It is just Sooooo obvious that “the sun does it”. We don’t think much about the context.
So what is the context of the surface temperature?
The surface has the cold depths of space on one side. Then two working fluids between space and the surface ( air and water ) to conduct, convect, and radiate heat to that cold depth. On the other side, there are a few thousand miles of rock. And that is where the problem arrives. That rock is very very hot.
Earth’s internal engine is running about 1,000 degrees Celsius (about 1,800 degrees Fahrenheit) hotter than previously measured, providing a better explanation for how the planet generates a magnetic field, a new study has found.
A team of scientists has measured the melting point of iron at high precision in a laboratory, and then drew from that result to calculate the temperature at the boundary of Earth’s inner and outer core — now estimated at 6,000 C (about 10,800 F). That’s as hot as the surface of the sun.
On top of that core is the mantle. It is significantly cooler:
The difference in temperature matters, because this explains how the Earth generates its magnetic field. The Earth has a solid inner core surrounded by a liquid outer core, which, in turn, has the solid, but flowing, mantle above it. There needs to be a 2,700-degree F (1,500 C) difference between the inner core and the mantle to spur “thermal movements” that — along with Earth’s spin — create the magnetic field.
1500 C or 2700 F cooler at a minimum.
The mantle runs out at about 3000 K to 1900 K (for all practical purposes using C or K is the same in this context. We don’t know the temperatures to 276 C of precision, so the difference between C and K is in the statistical noise).
The Earth’s internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, and possibly heat from other sources. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.
Much of the heat is created by decay of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements mainly located in the mantle.
We’ve had over 4 Billion years to lose some of that heat of accretion. At this point, it is mostly radioactive decay keeping things hot. We live on the outside of the ‘containment shell’ of a giant nuclear reactor.
The Laws Of Thermodynamics say that heat can only flow from hot to cold. It is our own nuclear reactor heat that warms the Earth. The heat flow is from a 6000 K core to a 3 K dead of space.
At most, warming the surface to 300 K with sunshine can only slow down that process (and that only on the sunny half away from the poles).
The necessary conclusion is that the Sun does not warm the Earth, it only modulates the heat flow from the very hot core / mantle / crust out to the very cold depths of space. Yes, if it were gone we would be much closer to that ‘cold pole’ temperature, and if the Earth were completely unable to cool to space, we would be closer to 6000 K. But both are hypotheticals. The reality is a heat flux from 6000 K to 3 K and we sit near the cold pole. For 1/3 to 2/3 of a rotation (depending on place on the planet at any one time) that flux is briefly slowed by sunshine. But it MUST continue. (If it didn’t, we would rise to core temps over time). All of which says we are warmed by the core, and modulated by the Sun.
I don’t know that this matters much when thinking about practical things like the weather today or the extent of global cooling in an ice age glacial; but it does help to keep a tidy mind. You think differently about solar warming than about solar modulated cooling. “Back Radiation” can not “heat the Earth” (if back radiation does anything at all…) but can only extend the degree to which the Sun modulates heat flow out from the core.
It raises questions, too. Like “Has our million year scale slide into colder been due to cosmic changes, or just the long slow drop of nuclear decay heat flux?” We know the heat flux is dropping over millions year time scales. We know the Sun is not warming the Earth, only modulating that heat flux out. At that point, do we really need to invoke orbital shifts, solar changes, galactic location effects? Similarly, how important is “the faint Sun paradox” if early Earth history had much more warmth from the planet itself? If the Sun is not setting the heat flux at the planet surface, but only providing some photons for plants, it can be much more faint than now and life is still happy. Assuming Venus is similarly heated, do we really need a Greenhouse Venus explanation for the high temperatures, or just a higher initial isotope load so the gasses in the atmosphere have not condensed into oceans and formed oxide and sulfide rocks (yet…)?
How you think about things matters. Even small things. It really does help to keep a tidy mind. The perspective of a hot Earth cooling to space, solar modulated, is closer to reality, and gives a more real understanding. It also leads to asking different questions…