Updated: Aug 12, 2019
Years ago, I moved from Florida to Colorado. “You know it snows there?” my friends said, as though I was going to be in for a rude awakening! But it wasn’t the snow that tested my ability to adapt. It was the change in humidity – especially in the summer. While I was accustomed to the combination of heat and moist air, basically a “steam room”, Colorado summers mixed heat and dry air, your typical “sauna” condition. It was a shock to my system at first, but I learned to regulate my heat by increasing my water intake and avoiding activity during the afternoons. Likewise, the plants in the tallgrass prairie field site where I worked managed water differently than the grasses in the marshes and flatwoods of Florida.
Summer heat and humidity in the Midwest has generally fallen somewhere between these two extremes. The combination of heat and humidity can often shift during the same season in the Chicago region, especially once an early rainy period dries up. I’m sure those of us attending the CW Prairies Adaptation workshop at Sagawau last September recall that our tour outlasted our comfort level outside!
As the climate warms, the energy available to evaporate water into vapor increases and results in more water vapor. In addition, warmer air is capable of holding more water – so these two things result in more water vapor in the atmosphere. There is a limit to how much vapor the air can hold, known as saturation, beyond which liquid water condenses and precipitation falls out. But before it reaches that saturation point, the air contains some amount of water vapor and this is known as humidity. Specifically, relative humidity is the fraction of actual water vapor in the air to the total water vapor the air could hold. So a relative humidity value of 20% is pretty dry, while a value of 80-90% is your typical muggy summer day in Tampa.
Since we cool ourselves by evaporating sweat from our skin, drier air makes this easier than moist air. So shouldn’t that have been more comfortable for me in Colorado? We can grow accustomed to regulating our water intake and loss to match the environment we live in. In my case, the dry air in Colorado dehydrated me faster as my sweat evaporated more efficiently!
Plants regulate water in the environment they live in too, but by using a different process from humans. They use water to photosynthesize, bringing it up through their roots and stems, and transpiring the water vapor through stomata in their leaves. This time it’s pressure that provides the force to move water from the plant to the air. All gasses in the air exert a pressure which is temperature dependent. As long as there is capacity for more water vapor, the atmosphere exerts a force on the water vapor coming from plants leaves. This is vapor pressure deficit (VPD). The greater the VPD, the more water demand on the plants. As you can see from this helpful animation from the Fourth National Climate Assessment (Fig. 21.3, Midwest Chapter), VPD increases with warmer temperatures and it is independent of relative humidity:
The implications for prairies is that we should expect to see more water demands on grasses and other plants as the climate warms. What will this mean? A study by scientists at Stanford University and Columbia University published in 2017 was not optimistic. They found that grasslands in the U.S. are more than three times more sensitive to VPD than they are to changes in precipitation. While this study included all grasslands together – from our Midwest tallgrass prairies to southern California coastal grasslands – it’s worth noting that there is a significant sensitivity to VPD that translates into lost productivity. Other research has shown that some grassland plant species can adjust to changes in precipitation over time, but this could be limited to trends that happen slowly and they did not address VPD. Severe events or changes that occur too quickly for plant species to adapt to could spell real trouble.
One take away from this research is to be aware that changing temperatures and precipitation may not be the primary climate factors affecting prairie habitats.
Similar to the “It’s not just the heat, it’s the humidity” advice you’d give a new resident moving from Florida to Colorado, perhaps we should heed a new refrain: “it’s not the humidity, it’s the vapor pressure deficit!”