Every spring, the Earth blooms. Scientists call it greenup: New buds sprout, fresh leaves unfurl, and continents turns green.

Exactly when greenup takes place each year is the result of an algorithm embedded in the DNA of plants. They track the rising temperature in spring, gauge the rainfall, measure the lengthening days, and then calculate when to start growing.

People have been trying to figure out the greenup algorithm for millennia. Farmers needed to know when to plant their crops; herders needed to know when pastures would be ready for their herds. In China and Japan, greenup records reach back 5,000 years.

In 18th-century Europe, greenup became a hobby. A wealthy British landowner named Robert Marsham noted the days on which oaks began to bloom. His children and their children continued to record the coming of spring for 211 years. They were joined by other citizen scientists, who work together today through online networks such as Project BudBurst.

Professional scientists are managing networks of their own. Mark Schwartz at the University of Wisconsin oversees 1,100 sites in the United States where researchers track the greenup of lilacs.

Among the many things these networks have revealed is that the greenup algorithm isn't a simple calendar. Plants do not bloom on the same day every year. In a cold spring, greenup may come late; in a warm one it may come early. But over the past few decades, the greenup records have revealed something new. On average, greenup has been arriving steadily earlier and earlier in the year. Spring, in other words, is on the march.

There was a good reason to expect this to happen: The planet has gotten warmer. Today Earth's average temperature is 1.2 degrees Fahrenheit higher than it was a century ago. The consensus (.pdf) of the world's climate scientists is that humans are mainly responsible for that rise by releasing heat-trapping gases like carbon dioxide. In a rapidly warming world, you might well expect plants to respond with a faster greenup.

But nature does not play by simple rules. Xiaoyang Zhang and his colleagues at the National Oceanic and Atmospheric Administration analyzed satellite observations of greenup from 1982 to 2005. In much of North America greenup is coming earlier – in some places more than three weeks earlier than in 1982.

But in 30 percent of North America, the greenup comes later – in some cases more than three weeks later. In the past, scientists have noticed delayed greenups in several places. Zhang's new map makes clear that it's not just happening in a couple of isolated valleys.

So why is it happening? It has to do with something called a chill unit.

If you grow peaches for a living, you know all about chill units. They measure how much cold a plant experiences during a winter. And chill units are essential to a good crop of peaches. When plants go dormant at the end of summer, many of them have to experience a certain amount of time in cold weather before they can grow on schedule in the spring. If a peach tree doesn’t get enough chill units, it can’t respond promptly to the warmth of spring. It is still in a zombie-like dormancy, and it can only rouse itself after the spring gets even warmer.

Zhang and his colleagues discovered that global warming has been affecting chill units. North of about 35 degrees latitude – a line that cuts through North Carolina – plants still get enough chilling in the winter to be ready for the spring. When the warm temperatures come early, they start growing. That’s why much of the northern part of the continent is experiencing early greenup. But south of that line, plants have lost as many as 11 chilling days. Now they come into spring still dormant, and only after spring has gotten warmer can they finally start to come to life.

There’s probably more to the new greenup map than chill units. The NOAA team did not take water into account, for instance. Rainfall has been changing across North America over the past 23 years, and it may have put a thumb on the greenup scales. Nevertheless, warmer winters have had a big effect on greenup, and they will have an even bigger one if, as climate scientists project, temperatures rise several degrees over the next century.

The NOAA scientists predict that in Canada and the northern part of the United States, greenup will start even earlier. In the far south, it will start even later. And in a band stretching from North Carolina all the way up to New Jersey, spring will make an about-face. In this band, spring is now coming earlier than before, but in a matter of decades it will lose so many chill units that spring will start coming later.

These shifts will force peach farmers – and other farmers, too – to change how they raise crops. Insects, birds and other animals will also get a rude surprise. They time their own life cycles to match plant cycles, but have different genetic algorithms (to factor temperature, rainfall, length of days, etc.) that set when they hatch or migrate. The plants they depend on for food may come too early or too late for them.

What makes the greenup map especially important is that it shows that global warming is not some simple, all-or-nothing thing. It’s always tempting to use an easy shorthand and say that the Earth is ablaze, as if global warming were a switch that was turned on or off.

Climate change denialists sometimes try to exploit this yes-or-no way of thinking by finding one bit of data that defies the trend. A single cold year, or a single place where the temperature has not risen, is not proof that global warming is a hoax.

The fact that spring is coming later and earlier in large parts of North America does not either. In fact, a close look at how plants really work shows that this greenup map is just the sort of thing you’d expect global warming to do. Like everything in nature, global warming won’t submit to simple overgeneralizations. But its complexity offers no comfort, either. Spring is changing fast – in ways we are only beginning to understand.

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Freelance writer Carl Zimmer won the 2007 National Academies Communications Award. His next book, Microcosm: E. Coli and the New Science of Life, will be published in May by Pantheon.