I was nearing the end of my trip to my parents’ house in western Massachusetts for Thanksgiving when my phone lighted up with an alert that was both familiar and shocking: “ONGOING WILDFIRE; VISIBILITY AND AIR QUALITY IN AREA DUE TO SMOKE MAY BE REDUCED.” During my childhood in rural New England, wildfires were virtually unheard of, mostly because the region receives more than 3 feet of precipitation in an average year, with autumn typically being one of the wettest periods. This year, however, September and October each saw less than 2 inches of rain, making this the driest fall in at least two decades.
The same was true for most of the country. On Nov. 5, the U.S. Drought Monitor reported that more than 85% of the continental U.S. was experiencing “abnormally dry” conditions (or worse) — the highest proportion since the organization — a partnership between the University of Nebraska-Lincoln, the U.S. Department of Agriculture and the National Oceanic and Atmospheric Administration — began keeping records in 2000.
Amazingly, this included places like Asheville, N.C., that had been devastated by flooding during Hurricane Helene in late September. After being deluged with 14 inches of rain over three days, which was far more than the soil could absorb, the city saw just 0.03 inch in the entire month of October. The ground has dried out enough that almost 90% of the state of North Carolina is now in a drought, less than three months after the deadliest flooding in the state’s history.
Closer to home, while reservoir levels in Northern California are healthy after recent atmospheric rivers brought historic rainfall, conditions in the lower Colorado River Basin have worsened in recent months. The area is currently classified by the U.S. Drought Monitor as experiencing “extreme drought,” with Lake Powell and Lake Mead both barely one-third full.
This could be a problem for water managers in Los Angeles and San Diego, especially since those cities have received less than 10% of the typical rainfall amounts since the beginning of October. That in turn has contributed to explosive wildfires like the Franklin fire in Malibu, as dangerous winter wind patterns are beginning to occur at a moment when, in a more normal year, the fire season would already have been ended by heavier rain.
These patterns of boom and bust, where unusually heavy rainfall is interspersed among periods of acute drought, are a notable effect of climate change. Hotter air is also “thirstier” — it can hold more moisture and it causes water to evaporate more quickly. As a result, climate change is causing droughts to set in faster during periods of low rain as more groundwater is lost into the atmosphere. Once this extra water vapor is in the atmosphere, there is an increased potential for extreme precipitation. But more intense rain does not necessarily lead to more replenishment of groundwater — instead it mostly means more runoff since the top layers of dirt can easily receive more rain than they can absorb.
Counterintuitively, this is particularly true after a long drought — soil that is well-hydrated (but not saturated) is able to quickly conduct water downward because of surface tension, while the same process can take more than 100 times longer in parched soil. Moreover, without proper stormwater management, every gallon of water that escapes as runoff is a gallon that is unavailable for use during subsequent droughts. (A 2021 report from the American Society of Civil Engineers gave a “D” grade to the country’s stormwater infrastructure.)
The result of all this is that, according to a recent U.N. report, more than 75% of land worldwide experienced higher aridity during the last 30 years (as compared with the previous three decades) even as global precipitation increased by almost a tenth of an inch over the same period.
One additional complication comes from vegetation. Land plants not only depend on rain to survive, but they also play an important role in returning moisture to the atmosphere to produce more rain. In fact, the dominant mode of terrestrial groundwater loss is a process called “transpiration,” which occurs when water is pulled out of the soil by a plant’s roots and then evaporates from its leaves during photosynthesis.
As the air becomes hotter, it becomes easier for plants to lose water to transpiration, especially because photosynthesis occurs during daylight hours when temperatures are highest. If plants cannot get enough water through their roots to meet this demand, they will develop air bubbles in their vascular system and die.
This can lead to a dangerous feedback cycle in parts of the world where transpiration is a major source of water vapor that becomes precipitation, such as the Amazon rainforest: as drought conditions become more common, parts of the rainforest wither and die (or are burned in wildfires), leading to decreased transpiration, which causes less rain to fall, which kills more trees, and so on. A recent study suggested that if deforestation and climate change continue unabated, between 10% and 47% of the Amazon could transition from lush rainforest to arid savanna over the next 25 years.
There is one factor working in the other direction: carbon fertilization. A major reason that plants lose so much water through their leaves is that they need to open microscopic pores called stomata to absorb carbon dioxide, which (along with water and sunlight) is a key ingredient in photosynthesis. As carbon dioxide levels in the atmosphere increase, plants may need to open their stomata less frequently, leading to decreased transpiration and preserving more groundwater.
There is some disagreement in the scientific community over the question of how climate change will affect plant health and groundwater availability across a variety of biomes, with competing models and approaches providing different answers.
Ultimately, this uncertainty should prompt increased preparation, not less. This means implementing measures that will alleviate catastrophic flooding, such as floodplain management and replacing pavement with living “green” infrastructure rather, as well as improvements that will conserve water during droughts, including more efficient irrigation systems and replacing thirsty lawns with xeriscaping. Otherwise, communities will see only more death and economic disruption as the drought-deluge cycle continues to intensify.
Ned Kleiner is a scientist and catastrophe modeler at Verisk. He has a doctorate in atmospheric science from Harvard University.
