Tuesday, March 20, 2012

So, When Is Spring?

Many use the calendar to determine when spring arrives while others use a rodent to tell them when spring will arrive.  The first uses an artifact of astronomy and the latter superstition.  Meteorologists use a different calendar for spring.  Records are not kept on weather and climate for periods of say March 20 to June 21 such as the calendar.  The records are neatly arranged as daily, weekly, monthly, seasonally, and annually.  Meteorologist arrange the seasons in three-month increments, i.e. spring is March 1 - May 31.

Incidently, the start of spring in astronomy is not always the same date and neither is the end date.  It depends on when the vertical rays of the sun are over the equator for the start.  This would greatly complicate record-keeping.

These are all artifacts of man and Mother Nature has her own ideas.  An index for the onset of spring was developed by Mark D. Schwartz (University of Wisconsin-Milwaukee) and USA National Phenology Network colleagues.  This index, based on temperature variables measured at individual weather stations, estimates the first day that leaves appear on plants in a given state.

Image Credit: Climate Central.

Over the past several decades, with the exception of the Southeast, spring weather has been arriving earlier in most parts of the United States. This shift affects all sorts of biological processes that are triggered by warmer temperatures — not just flowering, but animal migration and giving birth and the shedding of winter coats and the emergence from cocoons.  Signs of spring, such as leaves unfolding on trees, first flowers, bird migrations, sounds of amphibians, and the appearance of butterflies, are occurring about 2 to 5 days per decade earlier in the last 30 years in response to recent climate warming.

The calendar above is a U.S. estimate.  To come up with the estimate, the average change was taken across 716 weather stations spread across the lower 48 states.

Click here for an interactive version of the graphic above.  Image Credit: Climate Central.

In South Carolina, the date of “first leaf” has not shifted, a trend that does not hold true for the lower 48 as a whole (see calendar above). On average, the lower 48 states show a shift of “first leaf” from March 20th (1951-1980 average) to March 17th now (1981-2010 average) – approximately 3 days earlier.

In the graph above, you can see how South Carolina's shift looks against the backdrop of year-to-year changes, which are even more variable than decadal swings (no surprise to meteorologists). Click on the link to the image for an interactive version where you can show or hide the linear trend, exponentially weighted moving average, and annual variations by selecting on the appropriate item in the legend,

There is a USA National Phenology Network that monitors the impacts of climate change on plant and animal species across the country. The network is comprised of government, education and non-porfit organizations along with hundreds of volunteer citizen scientists that observe phenological events, such as first leaf out, flowering, migrations and growing season, in their local areas. By harnessing the power of people, the network provides researchers with far more data than they could collect alone. Want to join? Visit the participate section of the USA-NPN website for more details.