I talk a lot about why we should cover our cities with plants. They’re good for our health in a number of ways, and through transpiration, they tend to cool off their surroundings. Urban heat islands are a big problem that is getting bigger as the planet warms, and plants are regularly proposed as at least a partial solution. I continue to believe that we should have as much urban greenery as possible, but some recent research has touched on a concern I’ve had for a while now.

As I said the other day, we urgently need to be rebuilding our society to deal with a coming heat that can no longer be avoided. That means that we need to account, as best we’re able, for conditions unlike anything our species has ever encountered. I want us to actually be proactive about this. We should be moving cities away from low-lying coastal areas, or rebuilding them to withstand rising seas. We should be moving our agriculture indoors, to the greatest degree possible, to protect food production from the heat and instability of this brave new world.

And, since we know that the temperature will keep rising, we should be planning for extreme heat waves, even if we do manage to literally green our cities. That means accounting for the fact that the transpiration that works so well to lower the temperature also works to increase humidity. Even if all the plant life lowers a city’s temperature by ten degrees, that won’t make it safe outside if the humidity creates wet-bulb conditions, in which we lose our ability to cool ourselves by sweating. The one advantage that cities have in this regard is that they tend to be drier than their surroundings, and bringing in more plants could make the heat deadly at lower temperatures:

A new study, led by Yale School of the Environment scientists and published in Nature, investigated the combined effect of temperature and humidity on urban heat stress using observational data and an urban climate model calculation. Researchers found that the heat stress burden is dependent on local climate and a humidifying effect can erase the cooling benefits that would come from trees and vegetation.

“A widely held view is that urban residents suffer more heat burden than the general population owing to the urban heat island phenomenon. This view is incomplete because it omits another ubiquitous urban microclimate phenomenon called the urban dry island — that urban land tends to be less humid than the surrounding rural land,” says Xuhui Lee, Sara Shallenberger Brown Professor of Meteorology, who directed the study.  “In dry, temperate, and boreal climates, urban residents are actually less heat-stressed than rural residents. But in the humid Global South, the urban heat island is dominant over the urban dry island, resulting in two to six extra dangerous heat stress days per summer.”

Lee and YSE doctoral student Keer Zhang, lead author of the study, say they were motivated to investigate the issue for several reasons: a large percentage of the global population lives in urban areas; many people in informal urban settlements do not have access to air conditioning; and the problem is going to get worse as temperatures rise and more people move to cities. About 4.3 billion people, or 55% of the world’s population, live in urban settings, and the number is expected to rise to 80% by 2050, according to the World Economic Forum.

The researchers developed a theoretical framework on how urban land modifies both air temperature and air humidity and showed that these two effects have equal weight in heat stress as measured by the wet-bulb temperature, in contrary to other heat indexes, which weigh temperature more heavily than humidity. Wet-bulb temperature combines dry air temperature with humidity to measure humid heat. The results of the study, the authors note, raise important questions.

Green vegetation can lower air temperature via water evaporation, but it can also increase heat burden because of air humidity. The question then is to what extent this humidifying effect erases the cooling benefit arising from temperature reduction. We hope to answer this question in a follow-up study, where we are comparing observations of the wet-bulb temperature in urban greenspaces (with dense tree cover) and those in built-up neighborhoods,” Lee says.

I’ve made the same assumption they’re calling out. This doesn’t negate the various benefits I mentioned at the top, which is why I still like the “green cities” idea, but it underscores the importance of guaranteeing access to artificial cooling. I’ve said before that we’re pretty close to a world in which spending time outside will be lethal in a growing portion of the the population, for a growing portion of the time. We know how to deal with lethally cold temperatures – the fact that we generate heat just by living, means that we can insulate ourselves against the cold, at least for a time. That’s not an option when it comes to heat. I suppose we could try to give everyone a version the liquid cooling garments that astronauts wear, but to me, it seems more practical to start rebuilding cities so that, in addition to the goals of the 15 minute city concept, it’s possible for most of the population live their day to day lives without having to go outside at all. This would require a pretty radical rebuilding of most cities, but in the face of the coming heat, we need to do that anyway.

I will probably keep being nervous about the recent unexplained spike in sea surface temperatures going forward. Even the best-case scenario, going forward, is a terrifying reminder that the really bad times the scientists have been warning us about are a lot closer than most people realize. Having plants around is a good thing, but the rules are changing as the temperature rises, and we have to change with them if we want to survive.

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