The Climate CIRCulator is a monthly newsletter covering climate science and the Northwest written by scientists and communicators.
An early spring onset can yield a more bountiful harvest, but a false spring (when temperatures rise and spring seems to have arrived only to be followed by a hard freeze) can bring damaging, if not devastating, effects.
Predicting when spring has sprung, aka spring onset, and if spring has sprung or is just fooling around, aka false springs, as they relate to climate change in the Northwest is the work of two recent studies.
In their paper published in the Journal of Climate, Toby Ault and colleagues made maps of the U.S. showing the date of spring onset for every year from 1920 to the present. They mapped two types of spring onset, one corresponding to the “first leaf” and the other the “first bloom” of the year. To do this, they relied on an established index of spring onset based on daily fluctuations in temperature and photoperiod (available hours of sunlight) and combined this spring index with a new U.S.-wide dataset of daily minimum and maximum temperatures. Though plant species differ in how they respond to environmental cues to release the first leaves and first blooms of spring, this spring index has proven to work well for many plant species.
The maps Ault and colleagues produced show that overall the Northwest has been trending toward an earlier spring onset — about five days earlier over the last six decades. This is consistent with a study by CIRC authors John Abatzoglou, David Rupp, and Philip Mote that showed warming in all seasons and a reduction in frost days. However, if one starts looking from 1980 forward, the trend reverses, moving toward a later spring onset in most of the Northwest (also consistent with Abatzoglou’s study).
The exception to this is the first leaf along the coast. Ault and his colleagues link this trend reversal to Pacific Ocean temperature patterns, particularly the Pacific Decadal Oscillation, or PDO. When the PDO is in its cool phase, it means the water off the west coast of North America is cooler than normal, slightly tipping the scales in favor of having a cooler-than-average winter and spring in the Northwest, and spring onset tends to come later.
While an early spring onset can lead to more bountiful yields from our vegetable gardens, a hard freeze following an early first leaf or first bloom can be damaging. These false springs are the topic of a recent study by Andrew Allstadt and colleagues published in Environmental Research Letters. The researchers wanted to know if false springs could become more or less common in the future given the generally earlier spring onsets that are projected to occur as global temperatures increase.
The general concern is that with early spring onset occurring earlier in the year, the Northwest could experience more false springs. Like Ault and colleagues, Allstadt and team used the same spring index but mapped changes in false spring frequency over the U.S. to the year 2100.
Allstadt and colleagues show that on average in the Northwest, fewer false springs are expected to occur (some areas show no change, while others show decreases of over 30 percent). Only in some high desert areas of southeastern Oregon are false springs (in this case, associated with a hard freeze after first bloom) expected to become slightly more common. In these areas the future average minimum daily temperatures on the date of first bloom are actually colder in the latter part of the 21st century than in recent history because the spring onset arrives so much earlier.
With temperatures projected to increase over the long-term, the trend over the last few decades toward a later spring onset may reverse again with spring onset arriving earlier and earlier. The above research indicating that false springs will not increase in frequency with the earlier spring onset is good news for our vegetable gardens and our region’s agricultural.