In an astronomical sense, the first day of spring occurs on the vernal equinox, when the hours of the day are almost exactly evenly split between daylight and night. For the past six years, this exact time has fallen on various hours of March 20. Conventionally, this is the date the Gregorian calendar on your refrigerator will call the first day of spring.
But how does a climate scientist define spring? An article published last month in Climate Dynamics, coauthored by Zachary Labe, a graduate student at UC Irvine, Cornell’s Prof. Toby Ault, earth and atmospheric sciences, and Prof. Raul Zurita-Milla, geo-information science at University of Twente, highlighted the importance of this definition in understanding the phenomena of early onset springs. The purpose of the study was to simulate both historical and future spring onset dates to better understand the extent to which natural variability and climate change impact these dates.
Spring is popularly defined by the senses — the bustling and buzzing of renewed insect activity, shrill and gurgling bird song, and the shockingly iridescent greens, purples and yellows that symphonically burst from the landscape.
“It’s one of those things, you know it when you see it, right?” Ault said. “There’s a host of changes that you can really detect, just subjectively, in the middle to end of April, early part of May.”
The study, “Identifying anomalously early spring onsets in the CESM large ensemble project,” is partially unique for its use of what Ault defines as an objective and quantitative metric of what defines the onset of spring.
“What actually was originally a model of lilac bud burst and first blooms but has been repurposed as a more generalized indicator of spring onsets,” Ault said. “What we’ve done is we’ve taken a model that was once tied to plant phenology and adapted it to be more tuned to the atmospheric component so that we can isolate the part that’s driven by the climate system or is driven by the atmosphere.”
Ault and his colleagues used this index in combination with global climate models to create long term predictions of how spring onset dates will change in the coming decades. The results were staggering.
“The average spring in a future… less than half a century in the future looks like the earliest spring on record over the last century,” Ault said.
The earliest spring on record was the spring of 2012. In that year, Ault explained, the indicators of spring onset changed by around two months. In other words, according to the most recent models, within four or five decades one might begin to regularly expect typical April weather beginning around February.
“This is the outcome we get if we continue down this path of burning a lot of fossil fuel and using the atmosphere as a dumping ground for excess CO2 when we make energy,” Ault said. “It’s hard to believe that we’re going to just let this happen to the planet without doing something about it.”
If 2012 is any indicator, these increasingly normal early springs will take a large toll on agriculture. According to the Senate testimony of Joseph Glauber, the chief economist of the United States Department of Agriculture, flowering trees which had an early response to warm temperatures in March were negatively impacted by subfreezing temperatures that continued into April. In particular, tart cherry production was cut nationally by 68 percent, including over 80 percent losses in Michigan.
When spring comes early, the date of the last frost often does not shift along with temperature averages and trends of animal and plant life cycle events or phenological events. As a consequence, the less hardy plants begin to suffer, leaving farmers to either suffer crop losses or find a way around the late freezes.
Despite the gravity of these consequences, Ault points out that the predictions of this study are hardly set in stone.
“It doesn’t have to be that way,” Ault said. “Now is the time to take a different path if we think maybe it would be better not to have all of the increased risks and potential consequences and impacts from rising global temperatures.”