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The year is 2100. While climate change has wrought serious damage to the biosphere, humanity has at last managed to become carbon neutral, and has even developed technology that can be used to reduce the level of carbon in the atmosphere. This has the effect of allowing humanity to set the thermostat, as it were, for the average temperature of the Earth.

Supposing that most of the world will be fed with farming practices similar in nature to what exist today, and that cities and habitation patterns will be built in a similar manner (i.e. not megastructures), what's the optimum temperature for Earth? If humans can control our average temperature, are looking to make the globe as habitable as possible, and aren't overly worried about further damage to the biosphere (because it's already been thoroughly wrecked), how warm would we want the Earth to be?

Climate control in 2100 can only affect the average temperature of the Earth. The gradient of temperatures from the equator to the poles will otherwise settle naturally, and any extreme weather that we'd expect to "naturally" form at a given temperature will be unimpeded. Furthermore, the governments of the future are open to helping populations move about the globe: the goal is to make the Earth, as a whole, as habitable as possible, without regards to maintaining or increasing food production in current population centers.

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Frame challenge: a reset to normal would make the Earth most habitable.

The question asserts that future humans:

aren't overly worried about further damage to the biosphere (because it's already been thoroughly wrecked)

However, 80 years is not nearly enough time to destroy even 10% of global species. According to the WWF, "between 0.01 and 0.1% of all species will become extinct each year." If we assume a worst-case-scenario of 0.1% extinction per year, we'll still have 92% of today's species by 2100. That's terrible, but still minimal enough to make saving what's left worthwhile.

After all, further damage to the biosphere would directly cause damage to humans.

  • Loss of biodiversity threatens the very agriculture that increasing temperatures is intended to promote, because diversity keeps soils productive.
  • Biodiversity ensures that adequate nutrients are available in different geographic regions, so killing more species might feed wealthy agrarian countries, but it would starve less developed ones.
  • Increasing temperature changes the movement of disease vectors. As climate change progresses, we are seeing a plethora of new diseases (and old diseases in newly-warm places) even today.

Not to mention the other physical effects of injecting more energy into the Earth's atmosphere / hydrosphere:

  • More tropical storms
  • More floods
  • More droughts
  • More forest fires

These events are not only deadly, but expensive. They would threaten humanity as it expanded; less efficient soils, scarcer nutrients, worse weather, and worse diseases would combine to yield no net benefit.

Furthermore, the assertion that increasing temperatures would yield more productive land is inherently flawed. A warmer Earth may mean hospitable poles - but it would also mean far less land near coasts, as well as inhospitably hot land near the equator. I will concede that the poles are warming faster than the equator, so you might see a minimal net gain in arable land, but sea level rise may reverse even that. You would be better off engineering plants to live in salty soils or hard permafrost than to disturb ecosystems that have been stable for millions of years.

The bottom line is, we have largely only seen negative environmental impacts since the beginning of human-driven climate change. If humans are offered the technology to fix the Earth's climate in 2100 - at which point the ecosystem will still be salvageable - they will begin the process of rebuilding instead of worsening the damage.

TL;DR The optimum temperature of Earth for humans is the one we evolved with, because we are dependent on the ecosystems around us.

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    $\begingroup$ What does a "reset to normal" even mean? I for one would vote for the Holocene thermal optimum. $\endgroup$ – AlexP Feb 20 at 0:07
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Around 14.4o Celsius

Looking through a massive amount of data before typing this up, there are a lot of details that come to hand and the one thing it is fair to say is that crop yields are increasing as the years progress. This is only partly related to temperature; better fertilisers, farming practices and the like are having a massive effect on how much food you can pull out of a given portion of land.

If you take a look at this site you gt a very good description of what has been going on since around 1950 in terms of food production per hectare, and it's clear we're better at getting food out of land. If you do a search on largest agricultural countries, you get countries like China, India, the Netherlands, and USA - looking at this one might infer that you actually want a cold climate generally speaking in that all but India in this group sits around the 6-10 degree C average temp range. That's quite cold. But, India is an outlier, with nearly 24o C as an average temp. The trouble with these figures is that these countries are doing more farming, not necessarily getting better crop yields and the sheer force of numbers in terms of population (and a lot of land to use) means that they are going to do quite well in the food production stakes.

Ultimately, you have 3 factors to consider;
. Human comfort / survival
. Crop comfort / survival
. Extreme Weather Events

If you can control the temperature globally, you can probably prevent or at least drastically mitigate extreme weather events, so that becomes a non-issue. Plants as a general rule are hardier than humans in that trees can grow in temps hotter than we find easy to survive in, provided there is also a lot of rainfall. That means, that if the temp you set globally is comfortable to us, then it's probably comfortable for the crops we want to grow, meaning we are the most fragile factor.

So; you want to set a global average temperature, and let the variance between the equator and the poles sort itself out? Well, let's look at what the average temperature is across the world today, and it's not very high.

Looking at this site, you can see that the average global temp is around 14.8o C today, and was 14o C back in 1880 according to our records. While it doesn't sound like much, that small variance in temp has had significant environmental impacts, but then we are able to pull more crops out of less land today despite this.

So, let's split the difference; let's call it 14.4o C as a static global temperature, but don't let it end just there. You also have to make sure there is regular rainfall and a host of other environmental factors are preserved in order to maintain your artificial utopia. But, if you keep the temp at around that 14 and a half degrees C, you should be fine.

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Looking at the historical record, during the European Warm Period (roughly 1000-1400 AD), Vikings had croft farms on Greenland, England was a wine growing nation and the European population was rising according to preserved parish records and so on. Since it is currently too cold to raise grapes in England or have croft farms in Greenland, we can safely conclude that the Earth's average temperature should be between 2-5 degrees warmer than currently to replicate the European Warm period.

Warmer weather is generally advantageous to agriculture, which in turn makes it easier to feed and support a larger human population. There are no preserved records of extreme weather events, so we can infer that the people living during that time had weather generally similar to what we see today.

The real problem comes when the climate cools, such as the "Little Ice Age" (roughly 1400-1700), leading to crop failures and catastrophic population decline due to harsher weather, and disease claiming larger numbers of ill nourished people. Given the Sun is currently entering a "Grand Minimum", similar to what early scientists described in the 1600's, so humanity will be feeling a great deal of stress wherever agriculture is unable to access large amounts of inexpensive energy as Western farmers do.

Making up the lost insolation may possibly be done through orbiting platoons of mirrors around the Earth and carefully controlling the amount of sunlight available to the atmosphere, hydrosphere and biosphere. Less ambitions plans could include hunkering down with lots of greenhouse agriculture and other intensive farming techniques, especially small "urban" farms which can exist in yards and balconies.

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