Smithsonian.com

High temperatures and high levels of humidity reduce the human body’s ability to do work. Image via Flickr user zoonabar

If you feel sluggish and have difficulty getting physical work done on very hot, humid days, it’s not your imagination. Our bodies are equipped with an adaptation to handle high temperatures—perspiration—but sweating becomes ineffective at cooling us down when the air around us is extremely humid.

Add in the fact that climate change is projected to increase the average humidity of Earth as well as its temperature, and you could have a recipe for a rather unexpected consequence of greenhouse gas emissions: a reduced overall ability to get work done. According to a study published yesterday in Nature Climate Change, increased heat and humidity has already reduced our species’ work capacity by 10% in the warmest months, and that figure could rise to 20% by 2050 and 60% by the year 2200, given current projections.

The Princeton research team behind the study, led by John Dunne, came to the finding by combining the latest data on global temperature and humidity over the past few decades with American military and industrial guidelines for how much work a person can safely do under environmental heat stress. For their projections, they used two sets of climate regimes: a pessimistic scenario, in which greenhouse gas emissions rise unchecked through 2200, and an optimistic one, in which they begin to stabilize after 2060.

The team also considered a range of possible activities we might consider work: heavy labor (such as heavy lifting or digging) that burns 350-500 Calories per hour, moderate labor (such as continuous walking) that burns 200-350 Calories per hour and light labor (such as standing in place) that burns less than 200. For each of these levels of activity, there is a cut-off point of temperature and humidity past which the human body cannot safely work at full capacity.

Much of the reduced work capacity, the researchers say, will occur in tropical latitudes. In the map from the study below, shaded areas correspond to places where, over the course of a year, there are more than 30 days during which heat and humidity stresses reduce work capacity. Purple and blue cover areas for which this is only true for mostly heavy labor, while green and yellow indicate regions where even moderate labor is impacted:

Image via Nature Climate Change/Dunne et. al.

Under the pessimistic emissions scenario, in 2100, the area of the globe for which humidity curtails work will expand dramatically, covering much of the U.S., and reducing total human work capacity by 37% overall worldwide during the hottest months. Red covers areas for which capacity for even light labor is reduced due to climate for more than 30 days per year:

Image via Nature Climate Change/Dunne et. al.

The effect, they note, is that “heat stress in Washington DC becomes higher than present-day New Orleans, and New Orleans exceeds present-day Bahrain.” This doesn’t include other types of dynamics which could accelerate the consequences of climate change in highly populated areas, such as the urban heat island effect—it’s just a basic calculation given what we project will happen to the climate and what we know about how the human body works.

Looking at the map and thinking about how the study defines “work” can lead to a troubling conclusion: in 2100, throughout much of the U.S., simply taking an extended walk outdoors might not be possible for many people. The economic impacts—in terms of construction and other fields that rely upon heavy manual labor—are another issue entirely. Climate change is certain to bring a wide range of unpleasantconsequences, butthe effect of humidity on a person’s ability to work could be the one that impacts daily life the most.