Warmer Springs Mean Less Snow Cover, Disruptions for Plants and Animals, and More Allergies, Scientists Say
WASHINGTON (March 19, 2013) – Warmer springs are leading to declines in snow cover, changes to species’ biological clockwork, and longer and more intense allergy seasons, according to scientists who spoke at a telephone press conference convened by the Union of Concerned Scientists (UCS) earlier today.
Warmer, earlier springs are a clear signal of a changing climate. March temperatures have grown 2.1 degrees (F) hotter, on average, in the United States since reliable record-keeping began in 1880s. Similarly, the first leaves have started appearing on plants several days earlier than they used to across the country.
Shrinking Snow Cover and Snow Pack in a Warmer Spring
Warmer temperatures are leading to a decline in snow cover, a measure of how much land is covered by snow, and snow pack, which builds over the winter, begins melting in the spring, and is a major water source, especially in the drier summer months.
“When it comes to snow, 32 degrees is the magic number,” said David Robinson, the director of the Global Snow Lab at Rutgers University. “A few degrees of climate change is a world of difference for precipitation falling as snow or rain.”
The snow lab’s data show that March and April snow cover in the United States and Southern Canada has been lower over the past decade than at any time since reliable record-keeping began in the 1920s. Last May, the Northern Hemisphere experienced its second lowest snow cover extent. Historically, spring snow cover has remained at relatively low levels while snow cover in June has been lower over the past five years than at any time since satellite observations began in 1967.
Snow cover tends to insulate crops against cold weather, Robinson said, while also keeping soil moist. Earlier loss of snow cover can therefore throw off timing for farmers and increase irrigation costs.
Water managers have been warily eyeing snow pack in California and Colorado this year. Most sites where scientists have collected snowpack data have shown declines over the past 50 years. According to the draft National Climate Assessment (NCA), climate change is expected to lead to significant declines in snow pack. Under a scenario in which emissions continue to rise, the NCA projects a 13 percent decline in Colorado snowpack by mid-century and a 34 percent decline in California, compared to historic averages.
Species Falling out of Sync with the Season
Warmer springs can also affect the range for migratory species and lead to mismatches in annual cycles for plants and animals that interact with one another.
“A lot of plants and animals are trying to outrun climate change,” said Jake Weltzin, an ecologist with the United States Geological Survey and executive director of the USA National Phenology Network (USA-NPN). “We’re seeing plants and animals whose biological clocks are springing forward by days and weeks in the spring.”
USA-NPN data has contributed to several new pieces of research over the past year and the organization continues to collect data through a network of scientists and citizen groups. For instance, historical data being preserved by USA-NPN has revealed that ruby-throated hummingbirds are arriving in the eastern United States 11.4 to 18.2 days earlier than they used to, depending on latitude.
A Geophysical Research Letters paper used USA-NPN data to determine that the “green-wave,” a measure of how quickly trees grow leaves in the spring, will shift dramatically under a changing climate. Historically, the green-wave takes 74.7 days to travel from Miami to Maine as spring arrives. By the end of the century, it is projected to take only 59.1 days, an indicator that forest areas are likely to become more similar to one another along the U.S. East Coast.
Other research relying on independent data found that 10 species of butterfly in Massachusetts have been shown to take off 3.6 days earlier for every 1.8 F degree temperature increase. Also, a review of historical data collected by Henry David Thoreau found that 32 plants in Concord, Massachusetts are flowering 11 days earlier than they used to. Similarly, data collected by scientist Aldo Leopold helped researchers find that flowering dates for 23 plants have advanced 7 days in south-central Wisconsin.
Finally, a study in the journal Ecology found that Broad-tailed hummingbirds, which migrate in the spring northward from Central America, are becoming decoupled from their nectar resources the further north they migrate. Broad-tails arrive across the border into Arizona right when their nectar resources are present, but by the time they reach the central Rocky Mountains, even though they’ve sped up their migration by 5.5 days over the last 37 years, their nectar resources are advancing 3 times faster, by 17 days over the same time period. Over time, the researchers say, warmer springs that disproportionately accelerate flowering and nectar production in the north could squeeze hummingbirds out of the northern ranges of their habitat.
Spring and Summer Allergy Seasons Growing Longer, More Intense
Multiple lines of research indicate that higher temperatures and atmospheric carbon dioxide levels increase the length of allergy season and the potency of many allergens.
“Warmer temperatures and carbon dioxide are like fertilizer for many plants that produce allergens,” said Lew Ziska, a research plant physiologist at the United States Department of Agriculture. “Given how many people already have respiratory problems like asthma, a longer, more intense allergy season can be a real public health concern.”
Ziska coauthored a review of human-induced climate change and allergen exposure in the Journal of Allergy and Clinical Immunology. Research has found that warmer temperatures and higher carbon dioxide levels are linked to longer and more intense allergen production from a number of tree species that flower in the spring, including, oak, birch, olive and loblolly pine, though these findings can vary greatly by subspecies and location. Researchers have found, for instance, that some species of oak are producing pollen 4 weeks earlier than they used to and experiments at Duke University have found that higher carbon dioxide levels can lead to greater allergen production from loblolly pines. At the same time, warmer winters can lead some tree species to produce less pollen when they flower in the spring.
Summer brings allergens from weeds and grasses, which can produce more allergens when exposed to higher temperatures and carbon dioxide levels. Similarly, fall ragweed season has grown 13 to 27 days longer, depending on latitude, as late fall temperatures have increased. Further, Ziska and his colleagues have found that ragweed pollen production has increased 60 to 90 percent as the climate has warmed.
Finally, urban locations tend to produce more allergens than rural ones due to their higher temperatures and higher local concentrations of carbon dioxide.