It's not off the cuff. There's widespread consensus among climate scientists that the temperate zone will shift northward from latitudes of Virginia and Pennsylvania up into the upper Midwest. Parts of the southern US, particularly the southwest, may be so hot nobody will want to live there. Some regions in the southern US will become tropical in terms of the climate, with high heat and humidity throughout more of the year.
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If the wet-bulb temperature exceeds 95 farenheit, it is not possible for humans to survive for any length of time without air conditioning, as the body loses its ability to cool itself - people in third world living conditions forced to live in intensely hot, humid climates typically live short, miserable lives. Even wet-bulb temperatures over 90 can be highly compromising for physical activity.
Off-the-cuff predictions tend to be made on the fly, sensationalist, and, unfortunately, have a poor track record. I’m amazed that most don’t seem to check projections of something as simple as inundation due to rising sea levels. All we have to do is to take the elevation of a location and divided it by the annual projected rise in sea level.
Let’s make sure we’re on the same page here. This isn’t a debate about global warming, anthropomorphic or otherwise. This is about checking the plausibility of predicted effects. It’s simple to see if projected inundation is within the range of possibility. Can we do the same with livable temperatures? It’s more involved, but I think we can.
First we start with maximum livable temperature. That’s the 95° Fahrenheit / 35° Celsius wet bulb limit you cited. Or you can use a heat index of 140° Fahrenheit / 60° Celsius. That gives our upper limit.
Next we look at the average highest temperature and the record high temperature for a location. Then we look at the average low in the period of the average high and the low the morning of the record high temperature. Example: If the average high typically occurs in July, we look at the average morning low for July.
Now we do the same for relative humidity (RH). Ideally, we’re looking for the RH that coincides with both the average high temperature and the record high temperature. Failing that, we look for the average RH at the time of the morning low in that period, or the date of the record high and estimate the RH at the time of peak temperature. Warm air can contain more water vapor than cooler air. Unless there’s a small shower that increases RH without a significant reduction of temperature, the RH at the point of the daily high in the summer is lower than the humidity at the daily low.
If we have RH data coinciding with the average peak and record peak, we’re good to go. We simply increase the temperature by the expected amount and look up the value based on the RH for that time of day to arrive at the wet bulb temperature or heat index. If it’s below the maximum, things are miserable but livable. We look at both the average and the outlier (the highest recorded temperature) to get a sense of what seasonal and unusual conditions would be like.
If we
don’t have RH data coinciding with the high, then we have to estimate the RH at the time of the peak. Here it gets tricky.
RH is the percentage of water vapor pressure / saturation water vapor pressure. Obviously there are equations for this, but it’s been decades since I’ve sat in a chemistry class, so when I did this, I found an online calculator (
Online calculator: Saturation vapor pressure ) and had it calculate the saturation vapor pressure for the temperatures in question. To simplify things, I kept the standard pressure the same. I used the online calculator to get the saturation vapor pressure at the time of the low and the time of the peak. Since I had RH at the time of the low, I multiplied that by the saturation vapor pressure for that temperature to determine the vapor pressure of water. Then I divided that by the saturation vapor pressure at the peak temperature to determine the RH.
You’ll notice the assumption here that RH remains the same despite an increase in global temperatures. That’s because change in RH due to rising global temperatures is an unknown. Keeping it constant assumes that evaporation of water increases with rising temperatures, but since the air can hold more water vapor at higher temperatures, the safest guess for our purposes is that RH will remain the same.
“But you can’t just assume the relative humidity will remain the same,” some might say, and that’s true. Like keeping the barometric pressure the same, this is for simplification. But if RH falls, which would be expected is there’s a decrease in rainfall, that means less relative humidity, and that means a lower wet bulb temperature and heat index.
At least, that’s how I see it.
Now, when I did this, I just wanted to see what the effect would be for the projected rise in temperature for a particular location. But it’s also a way of seeing if projected wet bulb and heat index projections are plausible.
