Wikipedia:Reference desk/Archives/Science/2020 February 5

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February 5

if the sun were hotter or colder - finding calculations/estimates

Hi friends. I have looked at Earth's energy budget but it is not specific enough for my question. And my google searches keep turning up climate change articles, but I am interested in what would happen if everything on Earth stayed the same but the sun gave off more or less heat. I would like to be pointed to calculations and estimates - for example, if any one has made a model for different scenarios supposing hypothetical changes in solar radiation. I am looking for models/hypotheses/predictions in this sort of form: if the sun dimmed by one per cent, Earth's mean temperature would fall by 10 degrees, etc. Thank you for any help! 70.67.193.176 (talk) 17:05, 5 February 2020 (UTC)

You're probably not going to find much outside of climate change models, since any such change in solar irradiance would need to take into account things like greenhouse gas concentration, albedo, etc. There's a reason that climate change models include all of these, and also include solar irradiance. --OuroborosCobra (talk) 18:07, 5 February 2020 (UTC)

It's difficult: the article Milankovitch cycles shows some of the issues. 2601:648:8202:96B0:0:0:0:E118 (talk) 20:36, 5 February 2020 (UTC)

It's also an impossibility for everything else on Earth to stay the same if the Sun changed. Even ignoring the horrendously complicated web of purely physical and chemical interactions, species of living organisms respond to changes in their environment by Evolution, and it has been suggested that some may in turn effect the physical environment in ways favourable to their survival – see the Gaia hypothesis in its weakest version. Such a homeostatic mechanism may explain why it is that, even though the Sun's radiance has increased by about 40% over the last 3 or 4 billion years, the Earth's temperature has not on average changed very much, remembering that one has to take Absolute zero, not 0° centigrade, as a baseline, so a temperature increase of 40% from, say, 283 kelvin (≡ 10°C) would be 113K/C, not 4°C (taking us to about 123°C). {The poster formerly known as 87.81.230.195} 90.205.58.107 (talk) 00:59, 6 February 2020 (UTC)

Although it's very hard to give an accurate prediction of what's going to happen if the Sun's luminosity were to increase by 1%, it's not very hard to say something useful about it.

Incoming radiation and outgoing radiation on Earth have to be balanced. If the incoming radiation increases by 1%, the outgoing radiation increases by 1% too. As outgoing radiation increases with the fourth power of temperature, this means that the temperature of Earth will increase by about 0.7K. That's a very crude estimate, but it's the right order of magnitude.

The problem is that this increase in temperature will affect the physical properties of the Earth's surface. A change in the albedo in visible light, caused by a change in ice cover (down) and cloud cover (mostly the low-level clouds; really complex), makes that the change in absorbed radiation is different (usually larger) than the change in solar radiation. A change in the albedo in infrared can make that the change in outgoing radiation is different from the change in T⁴, but as it appears that the Earth's albedo in infrared is very low no matter what, this is not such a major effect. A change in the temperature distribution over the Earth's surface will also affect average temperature. A 1K rise in a cold area will increase the outgoing radiation by far less than that same temperature rise in a hot area, so if the temperature distribution becomes more equal (which is likely), the average temperature rises much more than would be expected based on the increase in outgoing radiation. Finally, any increase in temperature can affect levels of water vapour (up), carbon dioxide (probably down), methane (hard to say, probably up) and cloud cover (mostly the high-level clouds; really complex), affecting the greenhouse effect, which sets the ratio between the surface temperature and the temperature dictating the the outgoing radiation.

(Note: I mean that the low-level clouds mostly affect albedo and contribute to cooling, whilst the high-level clouds mostly affect the greenhouse effect and contribute to warming.) PiusImpavidus (talk) 11:13, 6 February 2020 (UTC)

For small changes, the concept you are looking for is climate sensitivity which expresses the expected change in global mean temperature given a change in the amount of radiation hitting the top of the atmosphere. Conventional estimates are about 3 °C / (1 W/m²). The time averaged solar flux at the top of the atmosphere is ~340 W/m², so a 1% change in solar luminosity would predict about a 10 °C change in surface temperature. For details of how that change is distributed over the Earth, or how it affects plants and animals, you would need a climate model or earth system model. Also, changes of that scale, e.g. 10 °C, are already quite large, so the simple concept of a climate sensitivity may not do a great job of characterizing the change you would expect from a 1% change in solar luminosity, so again more detailed models might be needed. Dragons flight (talk) 12:52, 6 February 2020 (UTC)

Dragons flight, you made a small mistake. Conventional estimates for climate sensitivity are about 3 degrees per doubling of CO₂, not per 1 W/m^2. Doubling of CO₂ is roughly 3.7 W/m^2. So a 1% change in solar luminosity would lead to (a bit less than) 3 °C of warming or cooling, and the assumptions for climate sensitivity (usually computed for quadrupling of CO₂) are still valid. Femke Nijsse (talk) 15:02, 6 February 2020 (UTC)

You're right, that's was very careless of me. Dragons flight (talk) 15:18, 6 February 2020 (UTC)

Maybe the original question is a veiled query for the equations that define the planetary equilibrium temperature. This is a physics-based model that assumes black-body radiators - so it isn't able to capture the important nuances and thermodynamics of a complicated planet like Earth. But it is one of the simplest physically-accurate models that provides a direct relationship between a solar energy parameter and a single numerical value that describes the planet's temperature. Just use caution when interpreting the results - this is a dramatically simple model that ignores lots of the details! Nimur (talk)

Hi again friends. Sincere thanks for all the answers. 2601:648:8202:96B0:0:0:0:E118, {The poster formerly known as 87.81.230.195} 90.205.58.107, Nimur, I appreciate the links! Lots of concepts I have never heard of, lots of interesting reading. Dragons flight, Femke Nijsse, PiusImpavidus thank you for the numbers! That was just what I wanted. 70.67.193.176 (talk) 19:19, 7 February 2020 (UTC)

Habitable zone might also be of interest. --47.146.63.87 (talk) 04:26, 8 February 2020 (UTC)