The last thing a seasoned gardener would expect from global warming is for leaves to appear later in spring, but exactly that is happening across the southern US.
鈥淚t鈥檚 really surprising,鈥� says Xiaoyang Zhang of Earth Resources Technology in Camp Springs, Maryland, because studies usually show plants greening earlier. 鈥淣obody had noticed how warming temperatures can delay the green-up.鈥�
Zhang spotted the anomaly when he examined satellite images showing seasonal changes in vegetation colour across the US from 1982 to 2005. In latitudes above 40掳 north, plants came into spring bloom an average of 0.32 days per year earlier over the period. But below 31掳 north, plants bloomed an average of 0.15 days later. The tipping point, where climate change had no effect, was at 35掳 north (Geophysical Research Letters, ). A similar pattern emerged when Zhang examined records of when lilacs flower each year.
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Zhang thinks that some plants need to be exposed to a short cold snap to sprout. Plants at northern latitudes still get this, but those below 35掳 north do not, he says, causing them to sprout later as the climate warms.
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