The Poleward Habitability Shift

We've all heard by now about animals shifting their ranges poleward and/or uphill, which has been seen not just in traipsing large mammals but also in more parochial species like insects and fish. Over the last few decades, this progression has been on the order of 1.7 km poleward and 1.1 m uphill per year. [Using a standard estimation of the lapse rate as about 6.5 deg C per 1000 m, this implies an altitude adjustment rate of 0.72 deg C per century, which is quite similar to the observed rate of warming in the atmosphere since 1900 (using GISS data). Estimating the annual-mean equator-pole temperature difference as 50 deg C (equivalent to about 5 deg C per 1000 km) implies a latitude adjustment rate of 0.85 deg C per century. Always gratifying when these back-of-the-envelope calculations imply something reasonable — in this case, that species are shifting their ranges uphill and poleward at the same rate with respect to temperature, and that this leading edge is keeping pace with the warming.] The complete picture is not quite that neat, though: there is also evidence that the equatorial bounds of ranges are shifting less than the poleward ones, if at all. The figure below shows some evidence that the hot end of a climatic range is better tolerated than we would theoretically expect (whereas from the same study the cold end is, empirically, a harder limit).

A comparison of the lab-measured thermal tolerance of a variety of species (determining their placement on the y-axis, and with their latitudinal ranges shown in green) against the highest temperature recently observed in each latitudinal band. While a gross simplification, it makes the point that most species (if not individuals) have likely experienced temperatures in their native habitats exceeding those they would theoretically be expected to not survive. Source: Sunday et al. 2012, supplement.

Given the intriguing findings above, I was curious about whether these concepts had any applicability to humans. After all, despite all our trappings of civilization and thus limited exposure to the unadulterated elements, humans resemble other animals in key ways: we are thermally sensitive, and we acclimate to the climate around us, so that where it's cold we die more from heat and where it's hot we die more from cold (also see top two figures in the panel below). In the U.S., heat is usually considered deadlier than cold as tallied up by the National Weather Service, but in fact a meta-analysis of 74 million deaths published in The Lancet earlier this year found exactly the opposite: for the regions considered (primarily also in the mid-latitudes), cold has killed many more people than heat in the past 30 years. Another interesting finding was that much of this excess mortality was associated not with the (rare) most-extreme temperatures, but rather with the more-common yet still anomalous ones. This pattern held up geographically as well: it wasn't Canada that had the most deaths from cold, or Thailand that had the most deaths from heat, but rather (of the 13 countries examined) China and Italy respectively.

Together, these findings go against many of our presuppositions about the effect of extreme temperatures on society-wide health. Italy is generally considered an epitome of a benign and temperate climate! One hypothesis that occurs to me is that it is the rate of variability that matters most, more than the absolute temperature: too fast and animals' body temperatures stay low, they can burrow into the soil, avoid activity, etc; too slow and acclimatization cuts into mortality. This could be a partial explanation for why the animal species in the figure above were evidently able to survive extreme heat when they 'shouldn't have.'  A major caveat on the human side is that the limited data from tropical areas makes it unclear if the pattern in Thailand, Taiwan, and Brazil — more deaths from cold than heat! — is accurate, and if so, if it's global. Because of the much greater amount of moisture, the maximum temperatures are lower in tropical environments than in the arid subtropics, though the wet-bulb temperatures are typically higher... More empirical studies are needed because intuition doesn't provide much of a guide here. It's certainly a complex story — not surprising when finely tuned and highly responsive biological and climatic systems are thrown into opposition. But again, the human-mortality data back up the idea that under CO2-driven warming the tropics stay reasonably habitable even as the high latitudes become less uninhabitable.

Percent increase in mortality at 40 deg F vs 60 deg F, with red being the most effect and dark blue the least, and circle size representing statistical significance. Source: Anderson & Bell, 2009.

Same as at left, but for mortality at 80 deg F vs 60 deg F.

Percentage of all recorded deaths attributable to temperatures that were 'moderately anomalous' (simply warmer or colder than optimal) and to extremes, for the period 1985-2012. Note that overall about 7.5% of all deaths were attributable to temperature as the immediate cause. Source: Gasparrini et al. 2015.

Considering the above, all other things being equal we would expect the line of minimum mortality to shift poleward in a warming climate (with many complications for possible changes in temperature variability, storm tracks, moisture levels during the hottest events, etc). U.S. cities with the lowest vulnerability to climate-change impacts are generally in the North, even after taking out the heat-stress component, due in large part to anticipated water shortages. Together with the findings of the Gasparrini et al. study, and factoring in things like the wicked heat observed in parts of the Persian Gulf, maybe the (expanding) subtropics are the most-marginal places currently inhabited, and in that case will be the toughest to remain in. The overall uninhabitability crown will surely be kept by the Antarctic interior, though, which even with exceptional warming will stay bitterly cold. 

Going back to the very first sentence of this post, another one of the truisms about climate change is that it is in effect more of a redistribution — of heat, of moisture; in short, of suitability for life — than anything else. This is of course appropriate given that the same could be said of the entire climate system itself. Without redistribution, there'd be local ('radiative') equilibrium, which would look a lot different than what's observed (in the linked figure, E is for Earth observations). The projections for arable land resemble those for mortality, the only difference being that they are based on physiological stresses on plants rather than animals. Again, increased temperature and its corollary, increased aridity (via the vapor-pressure deficit), are the main culprits in losses of arable land in South America and Africa, but globally arable land will likely increase moderately due to gains in the highest latitudes. In the absence of large-scale latitudinal migrations much larger and unlikelier than those discussed in the previous post, and barring any major technological breakthroughs, this shift in regions of agricultural and hydrological suitability over the 21st century away from regions of high population growth — and toward ones of slow or negative growth — means that the cities of tomorrow will have to be supplied by another resource redistribution of similarly massive magnitude.