John Abatzoglou and his collaborators at Oregon State have analyzed the climate data for the Pacific Northwest by season over the last century. For the most part, their results are likely not a surprise. Annual temperatures warmed, as did summer, fall, and winter, and the rate of warming increased over time. But they also found one major exception, in spring. Temperatures in spring have cooled slightly over the past three decades, most notably since the early 1990s. Cooler springs during the last few decades affect cropping decisions, timing of field operations like planting or spraying, and pest cycles.
Why is this? There are lots of influences on the climate, including volcanoes, solar output (the total radiation coming from the sun, which can vary based on sunspots and solar flares), known large-scale climate cycles (such as El Niño and the lesser known Pacific North American pattern), and human caused greenhouse forcing (the factor most prominently associated with global climate change).
Their analysis (see a factsheet and presentation summarizing the results here) suggests that the cooling is largely the result of natural large-scale climate cycles. In the absence of these cycles, spring warming would have likely occurred, at about the same rate as predicted by climate change models that consider greenhouse gas emissions. You can see these cycles in spring temperature clearly in the graphic below, showing mean spring temperatures in Lewiston, Idaho. The mean springtime temperature for the entire time period was 52°, and you can see departures from that mean for each year (red bars for years when the mean temperature exceeded 52°, blue bars for years when it was below 52°). The black line denotes the 11 year running mean and makes the cycles evident, with periods during which there are cooler-than-average spring temperatures followed by periods when there are warmer-than-average temperatures. (Similar data for other locations in the Northwest is available here).
Better understanding these climate cycles, and the combined impact of multiple drivers on seasonal climate, is of critical importance to agricultural producers, particularly in light of a changing climate. The natural factors that have resulted in cooler springs are not likely to continue indefinitely. Instead, it is likely that when these processes reverse, and large-scale natural factors and human caused greenhouse forcing are acting in the same direction, we will see significant seasonal warming.