Garrett Marino and his colleagues were motivated to look at the frequency of false springs after this phenomenon caused an estimated $2 bn worth of agricultural losses in the south-central and south-eastern US in 2007.

To look for a trend, the researchers first had to define when a false spring had occurred. "We derived a false-spring index that examined the relative timing of the start of the growing season (SGS), or leaf emergence, to the timing of a potentially damaging last hard freeze (minimum temperature ≤ –2.2 °C)," Marino told environmentalresearchweb. "Determining historical timing of last freezes was straightforward from analysis of surface temperature observations, but calculating the SGS required a model approach due to the scarcity of ground-based observations of phenological events."

The team developed an SGS model that combined Enhanced Vegetation Index (EVI) data derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations with a ground-based temperature 'degree-day' approach reflecting the rate of springtime warming. Growing degree-days were calculated by looking at days after 1 March on which the average temperature exceeded 10 °C, which was set as the base temperature. If the temperature topped 10 °C, then the difference between the actual and base temperatures was taken as the degree-day contribution for that day. Degree-days were accumulated from 1 March to the satellite-derived SGS date. These derived degree-days values necessary for SGS to commence were then applied to temperature observations going back much further than the satellite era to estimate historical SGS.

The researchers did not find a significant area-wide, long-term trend in SGS. However, they did find that in the south-eastern region of the study area, stretching from the Mississippi eastward to the Carolinas, the timing of the last hard freeze occurred significantly later – by more than one day per decade – along with an increased frequency of false springs. Earlier last hard freeze dates and decreased frequency of false springs were found over much of the north-western part of the study region, including Arkansas and southern Missouri.

"Our results show that climate change can cause regional impacts that do not fit the global trend," said Marino. "A changing climate can result in an increase in non-intuitive events, such as false springs in some regions."

Marino and colleagues published their study in Environmental Research Letters (ERL).