At ground level, tulips and daffodils mark the arrival of spring. But from a satellite's vantage point, the wash of green that appears across the forests of the eastern United States is one of the most noticeable signs of winter's passing. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Aqua satellite captured this view of spring greening on April 7, 2012, an unusually cloud free day.
The deepest greens of lush foliage are most visible throughout the Piedmont, a forested plateau between the Appalachians and the lower elevation plains along the Atlantic coast. The Appalachians appear brown because cooler temperatures at higher elevations cause a lag in the greening. In this case, the trees at the higher elevations were likely still in bloom and hadn't started to produce leaves. The speckles of tan throughout the coastal plain are farmlands, where fields often stay bare or filled with dry crop stubble until late spring planting.
There are a number of ways to measure the onset of spring, such as the blooming dates of particular plant species or the arrival of migrating birds. Researchers have shown, for example, that red maple trees in Washington, DC, now bloom five days earlier than they did in 1970; yarrow plants bloom 17 days earlier.
But only satellites can provide a broad view of greening, and the appearance of spring foliage has become an important tool for determining how the start of spring has changed over time. Scientists such as Mark D. Schwartz, a geographer at the University of Wisconsin-Milwaukee, have worked to integrate satellite measurements with detailed, ground-based studies of plant species. Schwartz’s research suggests that the onset of spring has moved up by 1.1 days per decade in North America since 1960. Change has been particularly fast in the west, where it has advanced by 1.5 days per decade.
Schwartz described the timing of this year’s greening as “exceptionally early” in comparison to other years—something that is not surprising given the sweltering temperatures North America has experienced this spring. The earlier arrival of spring also fits with what we know about global warming.
However, research has also shown that the effect of increasing temperatures is not always straightforward. Many woody species require a certain amount of exposure to cold in the winter in order to grow properly in the spring. A study of satellite observations from 1982 to 2005 found that about 30 percent of North America—particularly areas south of 35 degrees latitude—has actually been greening later due to the lack of cool winters.