Expert: Philip Stahl - 5/31/2009

Question
QUESTION: how cold would it get on earth if the sun went away

ANSWER: Hello,

If the Sun "went away" the temperature would drop to near absolute zero (-273C) within 8- 15 mins.

Fortunately, such a scenario is not at all plausible!

---------- FOLLOW-UP ----------

QUESTION: why would it get cold so fast doesn't the earths atmosphere
act like a blanket

Answer
True, it does, but the way you posed your hypothetical question as that the Sun 'went away'. E.g. vamonos! Disappeared! Hence, NO thermal or other inputs at all. Zero. The Earth is then bathed in the extreme cold of space with nothing coming in - not even one watt of solar energy (which is responsible for cloud cover, driving weather systems) and not one erg of infrared radiation as heat.

The problem then becomes one of thermal and diffusive contact. The analogy would be (somewhat) like dropping a semi-warm penny into a vat of ice. How long would it take the penny to get cold - say to the same temp. as the ice?

Ok, technically the Earth is not a penny, but its crust is composed of silicates and is estimated to have a heat capacity of around the range of silicates.. (Which is not as high as water). Of course, with the Sun GONE....the oceans freeze. You are already then talking about three fourths of the planet's surface at 0C. How much longer will the land hold out? Not long!

Clouds and Rayleigh scattering of radiation trapped in the atmosphere would be next to go, according to the basic gray atmosphere transport eqn: e.g. -dI/dt (1/k rho) = I – J

where k is a mass scattering coefficient, rho is the molecular density (e.g. in cloud cover) and J is the vector source function for a specific intensity I. With the Sun gone, I –> 0,  J ->0 and dI/dt-> 0.

Of course, certain assumptions were made, including that the CO2 concentration is and remains less than 380-400 ppm, or roughly what it is now. If in fact the concentration > 400 ppm, you can extend those times by about fourfold. Maybe more.

You could then say humans might have some small advantage if the planet is on the cusp of a 'runaway greenhouse effect'. At least they might have a tad more time to make arrangements or make peace with their maker before the end!