Nature's thermometers say spring is springing earlier by 3 days per decade

In a welcome sign of spring, Washington DC's famous cherry trees are beginning to burst into bloom, with the peak bloom predicted to come around April 5. This is two weeks behind last year's peak bloom date of March 20, which was the third earliest on record, according the Capital Weather Gang at the Washington Post. The only earlier bloom dates in the National Park Service's 92-year record dating back to 1921 were March 15, 1990 and March 17, 2000. The average bloom date is April 4, and the latest bloom date on record was April 18, 1958.


Figure 1. Cherry blossoms on March 21, 2012, in Washington D.C. Image credit: wunderphotographer KEM.

Nature's thermometers are reacting to global warming
Flowering cherry trees are excellent "natural thermometers" that give evidence that the climate is warming. Their flowering time is highly sensitive to temperatures in February and March. Last year's early bloom was triggered by the record-warm "Summer in March" temperatures that gave DC its warmest March on record. February and March temperatures in Washington D.C. have warmed by nearly 3°F over the past century, causing the city's famous cherry trees to bloom, on average, five days earlier than they did in 1921. The earlier blooming times of D.C.'s cherry trees are part of a global trend towards earlier spring blooms. A 2007 study by Parmesan et al. found that Northern Hemisphere spring events such as flowering times, bird and butterfly migrations, and frog breeding times have been occurring an average of 2.8 days per decade earlier in spring since the 1950s, averaged over all species.


Figure 2. Average temperatures in Northeast Virginia, including the northwestern suburbs of Washington D.C., have warmed by nearly 3°F over the past century. Temperatures in 2012 were the warmest on record. Image credit: NOAA/NCDC.


Figure 3. Comparison of the 1990 and 2012 USDA Plant Hardiness Zone Maps. Wintertime minimum temperatures in the U.S. have risen so much in recent decades that the United States Department of Agriculture decided in 2012 to update their Plant Hardiness Zone Map for gardeners for the first time since 1990. The Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10°F zones. Compared to the 1990 version, zone boundaries in the new 2012 edition of the map have generally shifted one 5°F half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period. The old 1990 map was based on temperature data from only a 13-year period of 1974-1986, while the new map uses data from the 30-year period 1976-2005. Image credit: USDA and Arbor Day Foundation

The forecast: DC's cherry trees blooming in early March by 2080?
Using a computer model of spring bloom driven by the expected climate of this century from climate models, a 2011 paper by Chung et al. predicted that peak bloom of Washington D.C.'s cherry trees will come five days earlier by 2050, and ten days earlier by 2080, assuming a middle-of-the-road climate change scenario called A1B. However, carbon dioxide emissions are at record high levels, and humankind is currently on a path likely to cause much greater warming. Under a higher emission scenario, the authors predict that by 2080, the cherry trees will bloom in early March, nearly a full month earlier (29 days) than at present. One major concern the authors point out: shorter and warmer winters can reduce the cold hardening of trees, leaving them vulnerable to frost injury.


Figure 4. Past and projected peak bloom dates of the Yoshino cherry trees in Washington, DC and surrounding areas. The future projections were made under the IPCC middle-of-the-road emission scenario A1B (middle panel) and high emissions scenario A2 (right panel.) Humanity is currently burning oil, coal, and natural gas at a rate that puts us closer to the high emissions scenario. Image credit: Chung et al., 2011, "Predicting the Timing of Cherry Blossoms in Washington, DC and Mid-Atlantic States in Response to Climate Change", PLoS ONE 6(11): e27439. doi:10.1371/journal.pone.0027439

Unusual winter jet stream patterns tied to Arctic sea ice loss may slow down cherry tree blooming changes
However, the authors' cherry tree bloom model did not take into account the fact that unusual jet stream contortions in winter have become increasingly common in recent years, increasing the odds of cold winters over the Eastern U.S. According to a March 2013 paper by Tang et al., "Cold winter extremes in northern continents linked to Arctic sea ice loss", wintertime Arctic sea ice loss can cause an increase in unusual jet stream patterns capable of bringing cold, snowy weather to the Eastern U.S., Western Europe, and East Asia. They theorized that sea ice loss in the Arctic promotes more evaporation, resulting in earlier snowfall in Siberia and other Arctic lands. The earlier snow insulates the soil, allowing the land to cool more rapidly. This results in a southwards shift of the jet stream and builds higher atmospheric pressures farther to the south, which increases the odds of cold spells and blocking high pressure systems that can cause extended periods of unusually cold and snowy weather in the Eastern U.S., Western Europe, and East Asia. Climate scientist Stefan Rahmstorf has an intriguing blog post (translated from the German by Eli Rabbett) that shows a dramatic agreement between temperature and pressure patterns during March 2013, and those expected to occur as a result of Arctic sea ice loss. He concludes, "In my view, the above studies provide strong evidence for a link between Arctic ice loss due to global warming, more frequent winter high pressure air masses, especially over the Atlantic-European part of the Arctic, and an associated influx of cold air to Europe." With Arctic sea ice expected to steadily dwindle in the coming decades, there may be an increase in cold February and March temperatures in the Eastern U.S. that will slow down the shift in cherry tree bloom times.

References
Chung et al., 2011, "Predicting the Timing of Cherry Blossoms in Washington, DC and Mid-Atlantic States in Response to Climate Change", PLoS ONE 6(11): e27439. doi:10.1371/journal.pone.0027439

Parmesan, 2007, “Influences of species, latitudes and methodologies on estimates of phenological response to global warming”, Glob. Change Biol. 13, 1860–1872

Root, T. L. et al., 2005, "Human-modified temperatures induce species changes: Joint attribution", Proc. Natl. Acad. Sci. USA 102, 7465–7469.

Related posts
From Heat Wave to Snowstorms, March Goes to Extremes by Andrew Freedman of Climate Central
Extreme jet stream causing record warmth in the east, record cold in the west (January 2013)
Arctic sea ice loss tied to unusual jet stream patterns (April 2012)
Our extreme weather: Arctic changes to blame? (December 2011)
New USDA Plant Hardiness Zone Map for gardeners shows a warming climate (February 2012)

Have a great weekend, everyone, and I'll be back with a new post on April Fool's Day, appropriate for the occasion.

Jeff Masters