The Earth is warming, most likely due to human activities. Even if you don’t believe in the latter, the science for the former is unequivocal. Birds are migrating earlier, the polar ice packs are melting, and sea water is warmer than the Navy measured just a few decades ago. (See: http://blogs.nicholas.duke.edu/citizenscientist/warming-the-gulf-of-maine/ )
Warmer days are best for drying clothes outdoors and for enhancing evaporation from any surface. So, it is reasonable to expect that warmer days will dry out the land surface—agricultural soils, forests, and suburban yards. Already, evaporation removes about 60% of the rain that falls on land, and about 60% of that is returned to the atmosphere by the loss of water from plants through pores in the leaf surface called stomates—a process known as transpiration.
Transpiration keeps the leaves cool and inadvertently provides regional cooling, because each gallon of water lost as vapor to the atmosphere carries away heat—known as latent heat—that would otherwise remain to warm the environment. One doesn’t need much experience to recognize that suburban parking lots are warmer than shady lawns on a summer day—that is transpiration in action. The cooling effect of trees counteracts the greater reflectivity of bare surfaces. We like trees in our yards because their transpiration of water cools the atmosphere on hot summer days.
Given that the oceans’ waters are likely to warm up more slowly than the land surface, an increasing evaporation from the ocean surface—the source of most rainfall—is likely to lag behind the increase evaporation of water from land, leading to widespread drought.
In the desert Southwest, where water is in short supply, homeowners are encouraged to use xeriscaping—planting of cactus and other plants which use and lose water at low rates. A lush, green lawn is not a welcome sight in Phoenix.
A similar philosophy can apply in regions with traditionally moist climates. Depending on their size and species, urban trees can use between 1 and 60 gallons of water each day. Typically, pine trees have greater overall water use than hardwoods, but among hardwoods, water use varies greatly between profligate species, like sycamore, and misers, like oaks.
To a large extent one can predict water-use by trees based on where they occur in nature. Sycamores are found on floodplains, where water is likely to be abundant; many oaks live on rocky and sandy soils, which are likely to dry out rapidly. The species are closely adapted to their native habitat, so vulnerability of all trees to drought, in suburbs or in nature, is likely to increase in the drier soils of the future.
Farmers are well acquainted with differential water-use by crop species, which determines what can grow where and thus, what might do best in the hotter conditions expected of the future. Crop physiologists have known for years that a plant hormone, abscisic acid, controls the water loss by plants by regulating the size of the stomatal pores. One promising development is the possibility that genetically-modified plants might be produced that would respond to existing agricultural chemicals in a way that mimics the effect of abscisic acid, reducing their loss of water in transpiration.
Future drought is likely to have major effects on natural landscapes and the plants that inhabit them. Some of these effects will be driven by the physical environment, such as greater evaporation under warmer temperatures. These are likely to spur increasing rates of forest fire and drought-related forest death. Other effects will depend upon biological processes, including what trees and crops we decide to plant and whether or not we must irrigate them. With hope, we can look to improvements in agriculture, which may lower its consumption of water in a future, dry world.
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