Doomsday: Local Timelines

The last few blogs focused on the ultimate consequences of continuing to make “progress” by relentlessly using the physical environment to serve humanity as if it were as a limitless resource. I tried to make the case that such efforts (business-as-usual scenarios) push the planet outside the window of habitability, with no place to go. The now fully-anticipated result is global doomsday. The expected timeline is uncertain but can be counted within a few hundred years. For people such as myself, who grew up exposed to thousands of years of history, these prospects are unacceptable – particularly because it is still within our power to prevent or at least considerably postpone this impact. Such an apocalyptic scenario obviously will not take place instantaneously on a global scale. It will start with a slowly expanding area of habitable local environments turning uninhabitable and their residents fleeing to friendlier places. Such a migration creates millions of environmental refugees and affect us all. This is not a speculation on an unknown future. It is already happening. This is the main reason that national security organizations are trying to understand the changes in the physical environment and how they affect its ability to support humans (See the “Global Trends 2035” blog, May 23, 2017).

In this blog I will try to quantify the criteria for local doomsdays with some concrete examples, starting with the concept of Wet Bulb temperature:

Wet Bulb Temperature – Twb

The Wet Bulb temperature is the adiabatic saturation temperature.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a “wet bulb temperature” lower than the “dry bulb temperature” in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation from the wet muslin is reduced when air contains more water vapor.

The Wet Bulb temperature is always between the Dry Bulb temperature and the Dew Point. For the wet bulb, there is a dynamic equilibrium between heat gained because the wet bulb is cooler than the surrounding air and heat lost because of evaporation. The wet bulb temperature is the temperature of an object that can be achieved through evaporative cooling, assuming good air flow and that the ambient air temperature remains the same.

Steven Sherwood and Matthew Huber wrote a paper, “An adaptability limit to climate change due to heat stress,” about the connections between Wet-Bulb temperatures and local limits of habitability. It was published in the Proceedings of the National Academy of Sciences (PNAS). Here is the abstract and some of their conclusions:


Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature TW, is surprisingly similar across diverse climates today. TW never exceeds 31 °C. Any exceedance of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.


We conclude that a global-mean warming of roughly 7 °C would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11–12 °C would expand these zones to encompass most of today’s human population. This likely overestimates what could practically be tolerated: Our limit applies to a person out of the sun, in gale-force winds, doused with water, wearing no clothing, and not working. A global-mean warming of only 3–4 °C would in some locations halve the margin of safety (difference between TW max and 35 °C) that now leaves room for additional burdens or limitations to cooling. Considering the impacts of heat stress that occur already, this would certainly be unpleasant and costly if not debilitating. More detailed heat stress studies incorporating physiological response characteristics and adaptations would be necessary to investigate this.

If warmings of 10 °C were really to occur in next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level. Heat stress thus deserves more attention as a climate-change impact.

The onset of TW max > 35 °C represents a well-defined reference point where devastating impacts on society seem assured even with adaptation efforts. This reference point constructs with assumptions now used in integrated assessment models. Warmings of 10 °C and above already occur in these models for some realizations of the future (33). The damages caused by 10 °C of warming are typically reckoned at 10–30% of world GDP (33, 34), roughly equivalent to a recession to economic conditions of roughly two decades earlier in time. While undesirable, this is hardly on par with a likely near-halving of habitable land, indicating that current assessments are underestimating the seriousness of climate change.

The paper by Eun-Soon Im et al. in Science Advances, 2017, 3(8), “Deadly heat waves projected in the densely populated agricultural regions of South Asia,” describes the evolving situation in South Asia. Here are a few key paragraphs from that paper that describe the main conclusions. They include the physiological criteria of the concept of hyperthermia, as mentioned in the previous paper:


The risk associated with any climate change impact reflects intensity of natural hazard and level of human vulnerability. Previous work has shown that a wet-bulb temperature of 35°C can be considered an upper limit on human survivability. On the basis of an ensemble of high-resolution climate change simulations, we project that extremes of wet-bulb temperature in South Asia are likely to approach and, in a few locations, exceed this critical threshold by the late 21st century under the business-as-usual scenario of future greenhouse gas emissions. The most intense hazard from extreme future heat waves is concentrated around densely populated agricultural regions of the Ganges and Indus river basins. Climate change, without mitigation, presents a serious and unique risk in South Asia, a region inhabited by about one-fifth of the global human population, due to an unprecedented combination of severe natural hazard and acute vulnerability


The risk of human illness and mortality increases in hot and humid weather associated with heat waves. Sherwood and Huber (1) proposed the concept of a human survivability threshold based on wet-bulb temperature (TW). TW is defined as the temperature that an air parcel would attain if cooled at constant pressure by evaporating water within it until saturation. It is a combined measure of temperature [that is, dry-bulb temperature (T)] and humidity (Q) that is always less than or equal to T. High values of TW imply hot and humid conditions and vice versa. The increase in TW reduces the differential between human body skin temperature and the inner temperature of the human body, which reduces the human body’s ability to cool itself (2). Because normal human body temperature is maintained within a very narrow limit of ±1°C (3), disruption of the body’s ability to regulate temperature can immediately impair physical and cognitive functions (4). If ambient air TW exceeds 35°C (typical human body skin temperature under warm conditions), metabolic heat can no longer be dissipated. Human exposure to TW of around 35°C for even a few hours will result in death even for the fittest of humans under shaded, well-ventilated conditions (1). While TW well below 35°C can pose dangerous conditions for most humans, 35°C can be considered an upper limit on human survivability in a natural (not air-conditioned) environment. Here, we consider maximum daily TW values averaged over a 6-hour window (TWmax), which is considered the maximum duration fit humans can survive at 35°C.


To study the potential impacts of climate change on human health due to extreme TW in South Asia, we apply the Massachusetts Institute of Technology Regional Climate Model (MRCM) (24) forced at the lateral and sea surface boundaries by output from three simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) coupled Atmosphere-Ocean Global Climate Model (AOGCM) experiments (25). By conducting high-resolution simulations, we include detailed representations of topography and coastlines as well as detailed physical processes related to the land surface and atmospheric physics, which are lacking in coarser-resolution AOGCM simulations (26). On the basis of our comparison of MRCM simulations driven by three AOGCMs for the historical period 1976–2005 (HIST) against reanalysis and in situ observational data, MRCM shows reasonable performance in capturing the climatological and geographical features of mean and extreme TW over South Asia. Furthermore, the mean biases of MRCM simulations are statistically corrected at the daily time scale to enhance the reliability of future projections (see Materials and Methods). We project the potential impacts of future climate change toward the end of century (2071–2100), assuming two GHG concentration scenarios based on the RCP trajectories (27): RCP4.5 and RCP8.5. RCP8.5 represents a BAU scenario resulting in a global CMIP5 ensemble average surface temperature increase of approximately 4.5°C. RCP4.5 includes moderate mitigation resulting in approximately 2.25°C average warming, slightly higher than what has been pledged by the 2015 United Nations Conference on Climate Change (COP21).

On the basis of the simulation results, TWmax is projected to exceed the survivability threshold at a few locations in the Chota Nagpur Plateau, northeastern India, and Bangladesh and projected to approach the 35°C threshold under the RCP8.5 scenario by the end of the century over most of South Asia, including the Ganges river valley, northeastern India, Bangladesh, the eastern coast of India, Chota Nagpur Plateau, northern Sri Lanka, and the Indus valley of Pakistan (Fig. 2). Under the RCP4.5 scenario, no regions are projected to exceed 35°C; however, vast regions of South Asia are projected to experience episodes exceeding 31°C, which is considered extremely dangerous for most humans (see the Supplementary Materials). Less severe conditions, in general, are projected for the Deccan Plateau in India, the Himalayas, and western mountain ranges in Pakistan.

Many urban population centers in South Asia are projected to experience heat waves characterized by TWmax well beyond 31°C under RCP8.5 (Fig. 2). For example, in Lucknow (Uttar Pradesh) and Patna (Bihar), which have respective current metro populations of 2.9 and 2.2 million, TW reaches and exceeds the survivability threshold. In most locations, the 25-year annual TWmax event in the present climate, for instance, is projected to become approximately an every year occurrence under RCP8.5 and a 2-year event under RCP4.5 (Fig. 2 and fig. S1). In addition to the increase in TWmax under global warming, the urban heat island effect may increase the risk level of extreme heat, measured in terms of temperature, for high-density urban population exposure to poor living conditions. However, Shastri et al. (28) found that urban heat island intensity over many Indian urban centers is lower than in non-urban regions along the urban boundary during daytime in the pre-monsoon summer because of the relatively low vegetation cover in non-urban areas.

We all live “locally,” as do our friends and families. It is not surprising that we are most interested in the projections for climate change in our local environments. Uncertainties in long-term projections for local environments are much greater than those regarding global projections. However, our ability to mitigate and adapt to local environmental changes reflects our skills in making decisions with regard to other, global risks.

Some localities published their projections based on the highest global spatial resolution simulations that they could find, while others are conducting dedicated local simulations. Simulations, local or global, depend on how we run our lives. Usually we follow the IPCC’s practices and make our forecasts based on scenarios similar to those which it publishes.

If we live in big cities, websites like “Climate Central” are good sources to start with. Some of its guidelines are given below, along with a global map of the cities that the site covers:

Summers around the world are already warmer than they used to be, and they’re going to get dramatically hotter by century’s end if carbon pollution continues to rise. That problem will be felt most acutely in cities.

The world’s rapidly growing population coupled with the urban heat island effect — which can make cities up to 14°F (7.8°C) warmer than their leafy, rural counterparts —  add up to a recipe for dangerous and potentially deadly heat.

Currently, about 54 percent of the world’s population lives in cities, and by 2050 the urban population is expected to grow by 2.5 billion people. As those cities get hotter, weather patterns may shift and make extreme heat even more common. That will in turn threaten public health and the economy.

Figure 1

Here’s a short, personal, account of the current circumstances in Phoenix, Arizona and its future prospects:

Sorry to put such a fine point on this, but even without climate change, Phoenix, Arizona, is already pretty uninhabitable. Don’t get me wrong, I spend a fair amount of time there, and I love it—particularly in the fall and winter—but without air-conditioning and refrigeration, it would be unlivable as is. Even with those modern conveniences, the hottest months take their toll on my feeble Southern Californian body and brain. The historical average number of days per year in Phoenix that hit 100 degrees is a mind-bending 92. But that number is rapidly rising as climate change bears down on America’s fifth-largest city.

“It’s currently the fastest warming big city in the US,” meteorologist and former Arizonan Eric Holthaus told me in an email. A study from Climate Central last year projects that Phoenix’s summer weather will be on average three to five degrees hotter by 2050. Meanwhile, that average number of 100-degree days will have skyrocketed by almost 40, to 132, according to another 2016 Climate Central study. (For reference, over a comparable period, New York City is expected to go from two to 15 100-degree days.)

I live in New York City. Periodically (usually following new IPCC reports) the city compiles an up-to-date report on how climate change will impact the city, based on the best available information. The information is published in a full dedicated issue of the “Annals of the New York Academy of Sciences” for everyone to see.

The press has picked this up:

Climate change will come to New York the same way water boils around one of those mythical frogs. The city will be the same old New York in 2050; people won’t be frying eggs on the manhole covers in the summer, or riding gondolas around Times Square. But by then winter will have fewer than 50 days of freezing cold, instead of the average of 72 that was the norm in the late 20th century. There will be less shivering agony on train platforms, plus less salting and shoveling. But the summers will be harsher—half a dozen heat waves instead of the usual two, and those heat waves will be even longer and more sweltering than usual; there will be twice as many plus-90-degree days as there once were. In 2006, a brutal summer led to 140 people dying of heat-related causes; it’s safe to say that that sort of death toll will be routine by 2050.

All of that is according to the estimates in the 2013 Climate Risk Information report from the New York City Panel on Climate Change (I’m using that report’s low or median estimates). As one of America’s wealthiest and most liberal big cities—where even some prominent Republicans are staunch climate hawks—it’s not surprising that New York would commission a report like that, or take other steps toward fighting the effects of climate change. But even with all the resources of the five boroughs, that’s a tall order.

Next week I will leave aside the depressing topic of imminent doomsdays and try to address our options for mitigating or at least postponing them.

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About climatechangefork

Micha Tomkiewicz, Ph.D., is a professor of physics in the Department of Physics, Brooklyn College, the City University of New York. He is also a professor of physics and chemistry in the School for Graduate Studies of the City University of New York. In addition, he is the founding-director of the Environmental Studies Program at Brooklyn College as well as director of the Electrochemistry Institute at that same institution.
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