After the last post, I couldn't get the topic out of my head, and so decided to dive a little deeper into the mechanisms (climatic or otherwise) behind the projected spatial shifts in climate suitability of various kinds.
The effects of projected climate changes on crops can be fairly neatly separated into two groupings: those due to increases in temperature, and those due to increases in CO2. On the whole the former are expected to cause decreases in yields, while the latter will cause increases (though with many interactions and variations by species and region). The temperature effect, at least to me, seemed counterintuitive — though this is perhaps an artifact of hailing from a place that's always limited by too little heat rather than too much. But major crops, even ones famous for their love of heat like corn and cotton, have yields that fall off precipitously when temperatures climb above 30 or 32 deg C (see left panel below). [Not coincidentally, this maximum tolerance of about 90 deg F matches up well with the climate of the Tehuacan Valley of Mexico, where corn is believed to have been domesticated.] This susceptibility to extremes contributes to an optimal growing temperature somewhat cooler than might be expected, and one that is already surpassed for many of the world's staple crops (see right panel below, and first link).
Hidden in the below charts are a multitude of effects: for example, high temperatures contribute to a higher vapor-pressure deficit of leaves relative to air — causing water stress that reduces photosynthesis rates — as well as to more agricultural pests, more ozone, and greater direct danger from the heat itself. There are also fewer cold extremes, but this factor is apparently outweighed, at least when considering the world's current agricultural regions. Meanwhile, higher CO2 has a fertilization effect on "C3" plants (e.g. the majority which are better adapted to high-CO2 conditions, including wheat, rice, and vegetables; the C4 pathway evolved fairly recently in semi-arid climates as global CO2 levels fell). Therefore, C4 plants like corn and sorghum that pump in CO2 will see relatively less CO2 benefit and more temperature damage. The CO2 benefit will also likely be weaker in the tropics, where nutrients and fertilizer usage are both currently low, because increased CO2 will tend to decrease protein levels without additional nitrogen fertilization. Finally, changes in consistency and/or timing of precipitation will strike the tropics the hardest, both because it will require the most adjustment of practice, and because the agricultural sector there is least-equipped to adapt to the change.
Almost all climate projections, including some I cited in the previous post, suggest that the semi-arid and arid areas of the subtropics will become hotter and drier over the course of this century. But it's not self-evident that this is a major factor driving latitudinal shifts in suitability on a global scale, considering that this may be a minor factor of importance only in sparsely populated regions, and that water is routinely moved hundreds of miles from mountain or lake to city. A recent article from a member of my research group succinctly dispels that notion by first observing that about 1.9 billion people live in areas that depend to some extent on snowmelt runoff to meet their water needs, and then mapping the projected change in runoff by 2080 (see figure below) — the decreases coincide in a number of locations with likely demand growth due to temperature and population increases. The large-scale spatial correlation between basins means that in many cases solutions won't be as simple as pumping water from a neighboring watershed. The impressive water-conservation figures coming out of California fortunately show that this projection is not destiny for those who dream of life in warm sunny locales, and technology from pipelines to desalination always has a role to play, but it does help illustrate the flat or increasing suitability of the high latitudes juxtaposed with the decreasing overall suitability of the expanding subtropics.
One may wonder why cold extremes are expected to retreat at a faster rate than warm ones will expand (see figure below) — effectively increasing the latitudinal band spared from conditions outside of the range of human comfort. For one thing, atmospheric moisture increases at about 7%/K, meaning that in the already moist deep tropics it will even more strongly limit temperature extremes, leading to only modest gains in wet-bulb temperature. The strong high-latitude warming on the coldest night of the year is generally thought to be related to a combination of weakening inversions and the albedo effect of retreating snow cover/sea ice. That study found the primary cause of "Arctic amplification" to be the near-constant high-latitude inversion, meaning that warming is distributed over a shallower layer than in the tropics where heat is quickly mixed up to the tropopause. Summertime humidity and cold meltwater in the North Atlantic will further help hold down the warming of the hottest day at high latitudes. This warming, while more spatially uniform, will be strongest in subtropical and mid-latitude grassland regions that presently have just enough moisture to moderate potential high extremes vis-à-vis those in deserts at the same latitudes, but for which that will not be the case 75 years hence. At the forefront of the disparity between trends in cold and warm extremes, and consequently the increasing frequency of the latter relative to the former, are urban areas, where heat waves are presently more intense, and cold waves less intense, than in nearby rural areas, with a widening gap going into the future. This widening can be further ascribed, in part, to decreases in windiness in urban areas, which tends to exacerbate the urban-heat-island effect while simultaneously making high-latitude cities more livable by lessening the windchill.
So perhaps certain cities that are currently quite inhospitable and quiet should begin preparing to gain new cachet as the poleward fringe of the zone for pleasant human habitation accelerates their way. These benefits, of course, are far outweighed by the negatives of habitat loss, ecosystem shifts, and the accompanying extinction of local traditions. The air may feel mild in Nunavut circa 2100, but what would be the point of visiting if there were no polar bears to see, or sealskins to decorate, or walrus tusks to carve?