麻豆传媒

IN 1957, geologist Edward Sable photographed the Jago river in Alaska鈥檚 north-east corner. His snap shows a typical Arctic vista, devoid of trees, with low-lying grass growing among glacial debris.

Half a century later, an ecologist by the name of Ken Tape visited the very same site to retake the shot. 鈥淚t was hardly recognisable,鈥� says Tape. Ground-hugging plants had given way to tall bushy shrubs. Even the shape of the land had changed in places, as long-frozen soil had thawed.

This flourishing of vegetation is no isolated phenomenon 鈥� it is happening across vast swathes of the planet. And while it is most pronounced in cold northern regions, it is also happening in more surprising places. From the Middle East to the Australian outback and the African savannah, many of Earth鈥檚 driest regions are getting greener.

A greener planet sounds wonderful, and in some ways it could be. But with climate change, nothing is ever simple 鈥� and more plant growth is not always good. Among other things, it will accelerate global warming. What鈥檚 more, in places the plants that thrive most could be weedy pests, overrunning more desirable plants. So what鈥檚 going on? Why is the planet getting greener, is it going to keep getting greener and what effect will this have?

The first question is fairly easy to answer. Most of the greening has taken place on the land north of 45 degrees latitude, a vast area that includes Canada, northern Europe and Russia (see graphic). The greatest change is in the Arctic tundra and in the coniferous, or boreal, forests, where a whopping 41 per cent of the land surface became greener between the 1980s and late 2000s. 鈥淲hat we鈥檙e seeing in the northern hemisphere is unprecedented in the 20th century and over the last millennium,鈥� says Jonathan Barichivich at the University of East Anglia in Norwich, UK.

In these areas, the cause is clear. Growth is limited by the cold, and the Arctic is warming twice as fast as the rest of the world. , bringing a more gradual transition between seasons. The most notable upshot of this 鈥� clearly visible in satellite imagery 鈥� is that spring comes earlier and growth happens over more of the year. In August, Barichivich reported that, compared with just 30 years ago, the Arctic growing season is now more than six days longer in North America and almost 13 days longer in Eurasia ().

Given all the concern about overgrazing and desertification, a more unexpected finding is that . There are, however, big variations from place to place 鈥� while growth has increased in the Sahel in Africa, the northern Great Plains in North America, Western Australia and parts of the Indian drylands, it has fallen in central Asia, central Australia and Patagonia.

Why there is more growth overall has been much debated, with explanations ranging from increased rainfall to changes in land use. Earlier this year, though, a study led by Randall Donohue of CSIRO, Australia鈥檚 national research agency, suggested that in warm, dry regions, rising carbon dioxide levels are the main cause of the greening.

Plants use the carbon in CO₂ to build the molecules of which life is made, from protein to DNA. So CO₂ is essential for plants, and many experiments over the past century have shown that higher levels promote growth.

The fertiliser effect

Most have been carried out in greenhouses or growth chambers, but in the last 20 years biologists have also released CO₂ into large plots of forest and crops to assess plant responses in a more realistic environment. These 鈥渇ree air CO₂ enrichment鈥� studies show that 鈥� by as much as 40 per cent 鈥� at concentrations of CO₂ ranging from 475 to 600 parts per million. (The typical level today is just under 400 ppm, up from around 280 ppm in 1850.) They also show that some trees exposed to CO₂ levels of around 550 ppm grow faster, and can end up 74 per cent heavier in just six years.

But as any gardener or farmer knows, pouring fertiliser onto plants does not boost growth unless you are meeting all their other needs. If plants are not getting enough light, water, nitrogen and other nutrients, or if conditions are not suitable, they cannot grow.

In many parts of the world growth is already limited by factors other than CO₂. For example, in the boreal forests, nitrogen is often in short supply. 鈥淚n some locations, limiting factors prevent CO₂ fertilisation from being fully expressed,鈥� says Donohue.

In dry areas, lack of water is the main limiting factor. So why should higher CO₂ levels boost growth in these regions?

The reason is that leaves are covered by a waxy, airtight layer to reduce water loss. To get the CO₂ they need, plants have to open pores on their leaves, but as long as the pores remain open, water is lost through them. When CO₂ levels rise, plants do not need to open their leaf pores as wide or for as long, so they lose less water. That means more growth for the same amount of water used.

Donohue鈥檚 team estimated that the 14 per cent rise in atmospheric CO₂ from 1982 to 2010 should produce a 5 to 10 per cent increase in growth in warm, arid regions where the lack of water limits growth. They then analysed satellite imagery 鈥� which does a good job of showing how densely the ground is covered by green leaves 鈥� to see if this was indeed the case. When the effect of varying rainfall was eliminated, they found that foliage increased by 11 per cent over this time, in regions including parts of Australia鈥檚 outback, the Middle East, Africa and North America ().

It has long been suspected that the rise in CO₂ is boosting plant growth. Demonstrating that this is happening has been tough, because where plants are prospering there are so many possible explanations, from warming to regrowth after a fire to fewer herbivores. 鈥淐O₂ fertilisation is really hard to observe,鈥� says Barichivich. Donohue鈥檚 study is the best evidence yet, but it applies only to a small part of the land surface.

What about other areas? In the last 20 years, hundreds of instrument towers have been set up in both temperate and boreal forests across the northern hemisphere to measure CO₂ levels, humidity, wind speed and so on. Trevor Keenan, now at Macquarie University in Sydney, Australia, and his colleagues used data from the most long-standing of these stations to work out how much CO₂ and water trees are using. The team found that the water-use efficiency of the forests 鈥� the ratio of carbon captured to water lost 鈥� is increasing by 3 per cent per year, and attributed this to rising CO₂ levels (). In some places, forests were growing more, while in others they were merely using less water. The numbers are astounding because they mean that water efficiency is increasing six times as fast as CO₂ enrichment experiments suggest.

Astounding numbers

So not everyone is convinced by this study. 鈥淚 don鈥檛 think it鈥檚 plausible,鈥� says Belinda Medlyn, also at Macquarie University, whose team has helped carry out some CO₂ enrichment experiments. 鈥淚 don鈥檛 think we would have missed an effect of the size they鈥檝e detected.鈥� While Medlyn is , she concedes that there is no convincing alternative just yet.

Even if the effect is not as large as Keenan鈥檚 study suggests, it appears that rising CO₂ levels are already affecting plant growth around the world. The big question is, what happens next?

North of the 45th parallel, the greening looks set to continue, at least in the short term. 鈥淭he big picture is that warming will cause the boreal forests to move north, and invade the tundra so that the tundra will shrink,鈥� says Barichivich. The vegetation in parts of the northern hemisphere now resembles that found 4 to 6 degrees farther south as recently as 1982. If this trend continues, vegetation could move as much as 20 degrees north 鈥� about 2200 kilometres 鈥� by the end of the century, relative to where it was.

Fast-forward 50 years into the future and it is easy to imagine that Siberia will look as lush as Europe does now and that the Canadian Arctic will be fully forested. But things might not be that straightforward. After all, we don鈥檛 even fully understand what is happening now. 鈥淪cientists are still trying to figure out what those green pixels [in the satellite images] mean; what is actually happening on the ground,鈥� says Tape, who is at the University of Alaska, Fairbanks.

One big unknown is how plants and animals 鈥� especially us 鈥� will respond and adapt. There will undoubtedly be winners and losers, with some species thriving at the expense of others. In North America, billions of trees have already been killed by swarms of bark beetles, which multiply faster in warmer conditions.

There is also still a lot of uncertainty about the direct effects of CO₂ on plants. 鈥淚t鈥檚 really tough to know if greening will continue globally as we don鈥檛 know whether plants will hit a limit and stop responding to CO₂ fertilisation in the future,鈥� says Donohue.

Then there鈥檚 the climate change due to rising CO₂. While warming should boost growth in cold regions, in hot regions further warming could impair growth by increasing water loss and by its direct effect on plants 鈥� growth falls sharply when temperatures exceed a plant鈥檚 optimum range.

The big wild card is future precipitation, says Jed Kaplan at the Swiss Federal Institute of Technology in Lausanne. The expectation is that dry regions will get drier, but that rainfall will increase overall. However, there is little certainty as to where this extra water will fall.

So we don鈥檛 know exactly how the climate will change, let alone what the effect on vegetation will be. Matters are made even more complicated by the fact that these processes are interrelated. Not only does the climate affect vegetation, but vegetation affects the climate, both locally and globally.

In theory, more growth could help soak up more CO₂ and thus help limit climate change. Indeed, a study published last month claimed that CO₂ levels would already be near 485 ppm were it not for the CO₂ fertilisation effect 鈥� but it was based on a model estimate rather than real-world observations ().

The crucial point is that increased growth will only limit increases in atmospheric levels of CO₂ if the carbon taken up by plants remains locked away in soils or peat bogs or tree trunks. Some studies suggest increased growth could be outweighed by increased carbon loss. One big worry is that tropical forests will die back. This would not only result in the loss of incredibly rich ecosystems like the Amazon rainforest, but also accelerate climate change. For every degree of warming, tropical forests could release about 50 billion tonnes of carbon back into the atmosphere 鈥� equivalent to five years鈥� worth of human emissions.

But a study published in February suggests that CO₂ fertilisation will help prevent dieback, by boosting water efficiency enough that forests survive. 鈥淲e think that the Amazon and other tropical forests generally won鈥檛 suffer catastrophic dieback, but they鈥檙e still likely to suffer,鈥� says Peter Cox of the University of Exeter in the UK, who led the study ().

In the Arctic, meanwhile, continued greening is definitely bad news in climate terms. Land covered in dense vegetation soaks up a lot more heat than barren, snow-covered tundra. 鈥淣o matter which way you look at it, more vegetation will absorb and hold more energy and will lead to further warming,鈥� says Scott Goetz of the Woods Hole Research Center in Massachusetts.

鈥淣o matter which way you look at it, more vegetation will absorb and hold more energy, and will lead to further warming鈥�

Goetz was part of a team that looked at how vegetation will change across the Arctic in the coming years, and how that will affect the climate. The study concluded that the vegetation changes will be far greater than previously thought 鈥� with woody shrub cover increasing by 50 per cent by 2050 鈥� and thus have a much greater warming effect, too ().

One possible outcome of all this warming and drying is that the Arctic鈥檚 permanently frozen ground 鈥� the permafrost 鈥� will thaw, releasing vast amounts of the potent greenhouse gas methane into the atmosphere. Another possibility, which would also release carbon, is that in the tundra, which has been relatively fire-free for the past 11,000 years.

And while talk of a greener planet conjures up an image of Eden, the greening trend is not necessarily good news for wildlife. In grasslands and coral reefs, for instance, adding more nutrients can lower biodiversity by allowing a few vigorous species to take over. In places, CO₂ fertilisation might have a similar effect. For example, there has already been an increase in the growth of vines 鈥� including poison ivy in North America 鈥� at the expense of trees, possibly due to rising CO₂ (麻豆传媒, 5 October, page 42).

What is certain is that we are going to see vast changes in vegetation as CO₂ levels continue to soar and the planet warms. 鈥淭he magnitude of the response that we鈥檙e seeing is huge and the prospect of it continuing is quite frightening,鈥� says Keenan.

This article appeared in print under the headline 鈥淭he great greening鈥�