Wikipedia:Reference desk/Archives/Science/2018 September 25

= September 25 =

Global Warming in Mexico
Under the various global warming scenarios, does it seem possible/likely that Mexico will become uninhabitable except for mountainous regions?Rich (talk) 00:55, 25 September 2018 (UTC)


 * See here: "A 2010 study concluded that under a worst-case scenario for global warming with temperatures 12 °C (22 °F) higher than 2007, the wet-bulb temperature limit for humans could be exceeded around much of the world in future centuries.[10] A 2015 study concluded that parts of the globe could become uninhabitable.[11] An example of the threshold at which the human body is no longer able to cool itself and begins to overheat is a humidity level of 50% and a high heat of 46 °C (115 °F), as this would indicate a wet-bulb temperature of 35 °C (95 °F).[12]". Count Iblis (talk) 04:01, 25 September 2018 (UTC)
 * However, that's speaking of global effects of a staggering 12 degrees Celsius of warming, which would take centuries even with unabated emissions. In the tropics and subtropics, which includes Mexico, things will start getting dire much earlier. If you believe this analysis, originally published in New Scientist, at 4 degrees of warming much of Mexico would indeed be close to uninhabitable. And depressingly we have a good chance of hitting 4 degrees by the end of the century if we continue with our present policies. If you haven't read "The Uninhabitable Earth", do so, though you won't be upbeat afterward. --47.146.63.87 (talk) 04:31, 25 September 2018 (UTC)


 * Technology can help adapt to the worst conditions. The Inuit adapted to the extreme cold near the north pole, the Tuareg adapted to the extreme heat of the Sahara desert. Walipinis seem a working technological adaption in South America. --Kharon (talk) 04:50, 25 September 2018 (UTC)
 * There is a limit. The problem is not just heat but humidity—wet-bulb temperature reflects both. At a wet-bulb temperature of around 35 Celsius, the atmosphere no longer convects heat away from your body, and you quickly cook to death. The tropics are famously humid, as contrasted with dry deserts (including polar deserts). Also there's the minor issue of sustaining a modern agricultural society. The Inuit and Tuareg are historically nomadic, but to keep alive 7.6 billion people and growing, you need to grow food, you need water supplies, and so on. We could debate the exact definition of "uninhabitable", but it's not really important for analyzing the potential future, because people will start evacuating long before the point of "you drop dead if you go outside during the day". --47.146.63.87 (talk) 07:56, 25 September 2018 (UTC)


 * Not a strictly scientific answer, but an interesting fictional consideration of almost exactly the OP's area of interest is to be found in Paolo Bacigalupi's 2015 science fiction novel The Water Knife. {The poster formerly known as 87.81.230.195} 2.221.81.75 (talk) 08:43, 25 September 2018 (UTC)


 * Yes, that lethal combination does not exist on Earth - the eastern United States comes closest. 92.31.140.53 (talk) 13:07, 25 September 2018 (UTC)


 * Really? What about Arabia and the Persian Gulf when the wind comes from the water? The Upper Midwest can actually get surprisingly close, it can reach the mid-110s in Minnesota and all the crops drying out makes the air ~115 and not very dry. Sagittarian Milky Way (talk) 13:39, 25 September 2018 (UTC)

Lunation and tropical year in 5000 BC
Nowadays, around AD 2000, the value of the mean synodic month is about 29.5306 mean solar days, and that of the mean tropical year, 365.2422 mean solar days, where a mean solar day is about 86,400 SI seconds. I was wondering about their variation over the millennia, and what their values would have been around, say, 5000 BC or AD 5000. — 79.113.236.200 (talk) 13:30, 25 September 2018 (UTC)
 * The Wikipedia article titled year has information on the variation of the length of a year, and even has a handy table which, while it doesn't have 5000 BC listed, does have several different years. There are 4 references just before that table, you perhaps could use those to help you in your research.  -- Jayron 32 13:35, 25 September 2018 (UTC)
 * Year length varies from year to year. I am interested in an average, for a few centuries around the given date. Is that what the table is supposed to represent, or does it merely show the length of the specific year in question ? — 79.113.236.200 (talk) 18:30, 25 September 2018 (UTC)
 * In Year it states "Mean year lengths in this section are calculated...". Reading the cited sources confirms that the values are mean values near the stated year, not the length of the individual stated year. Jc3s5h (talk) 17:10, 27 September 2018 (UTC)
 * Correction: 86,400.002 seconds. Sagittarian Milky Way (talk) 13:43, 25 September 2018 (UTC)
 * Two milliseconds over the course of a year fits within the allowable parameters of "about". -- Jayron 32 13:48, 25 September 2018 (UTC)
 * 86,400 seconds is a day but yeah, it's pretty similar. Sagittarian Milky Way (talk) 13:58, 25 September 2018 (UTC)
 * The Babylonians used a mean synodic month of 29d 12h 44m 03 1/3s.  The modern value is 29d 12h 44m 02.87s.   Although the moon is slowing down (due to the tides) the increase in the length of the day caused by the tides slightly overcompensates.   Thus although the month is getting longer, in terms of mean solar days it is diminishing.   The length of the mean tropical year diminishes by about 1/2 second per century - you can't really differentiate between mean solar days and ephemeris days (those are the ones that consist of 86,400 SI seconds). 92.31.140.53 (talk) 14:06, 25 September 2018 (UTC)


 * Just courious, but how can we know to the third of a second what duration the Babylonians attributed to the synodic month? Or to any month for that matter? Is there any Babylonian text stating such a thing, and if, how did they express "44 minutes and 3.33333 seconds"? Thank you. 194.174.73.80 (talk) 16:05, 27 September 2018 (UTC) Marco Pagliero Berlin


 * I'm not up in Babylonian units of measurement.  However the Jews (who had a long sojourn in Babylonia and still use their calendar) assign the length of 29d 12h 793 halachim to the month.   There are 1080 halachim to the hour and 18 to the minute, so ... 44m 3.33 (recurring) seconds. 86.131.233.235 (talk) 09:46, 28 September 2018 (UTC)


 * More on this . 86.131.233.235 (talk) 10:50, 28 September 2018 (UTC)