Planet of the Plants
Humanity is on the threshold of a century of extraordinary bounty, courtesy of global climate change. That's the opinion of Robert Balling, former scientific adviser to the Greening Earth Society, a lobbying arm of the power industry founded by the Western Fuels Association. In a world where atmospheric carbon dioxide levels soar from the burning of fossil fuels, he says, "crops will grow faster, larger, more water-use efficient, and more resistant to stress." Quoting study after study, he invokes visions of massive melon yields, heftier potatoes, and "pumped-up pastureland." Bumper crops of wheat and rice, he says, will benefit the world's farmers and the hungry.
Balling's assertions are backed by solid science: Gaseous CO2 fertilization does cause remarkable growth spurts in many plants, and could create a greener planet with beefier tomatoes and faster-growing, bigger trees. But there's a catch: The insects, mammals, and impoverished people in developing countries who feed on this bounty may end up malnourished, or even starving.
A small but growing body of research is finding that elevated levels of atmospheric carbon dioxide, while increasing crop yield, decrease the nutritional value of plants. More than a hundred studies, for example, have found that when CO2 from fossil-fuel burning builds up in plant tissues, nitrogen (essential for making protein) declines. A smaller number of studies hint at another troubling impact: As atmospheric CO2 levels go up, trace elements in plants (such as zinc and iron, which are vital to animal and human life) go down, potentially malnourishing all those that subsist on the plants. This preliminary research has given scientists reason to worry about bigger unknowns: Virtually no studies have been done on the effects of elevated CO2 on other essential trace elements, such as selenium, an important antioxidant, or chromium, which is believed to regulate blood-sugar levels.
The less-nutritious plants of a CO2-enriched world will likely not be a problem for rich nations, where "super-sized" meals and vitamin supplements are a dietary mainstay. But things could be very different in the developing world, where millions already live on the edge of starvation, and where the micronutrient deficit, known as "hidden hunger," is already considered one of the world's leading health problems by the United Nations.
The problem of hidden hunger grew out of the 1960s "green revolution." That boom in agriculture relied on new varieties of high-yield crops and chemical fertilizers to staunch world hunger by upping caloric intake in the developing world. Unfortunately, those high-yield crops are typically low in micronutrients, and eating them has resulted in an epidemic of hidden hunger. At least a third of the world is already lacking in some chemical element, according to the U.N., and the problem is due in part to a steady diet of micronutrient-deficient green-revolution plants. Iron deficiency alone, which can cause cognitive impairment in children and increase the rate of stillbirths, affects some 4.5 billion people. Lack of iodine, another micronutrient, can result in brain damage and is a serious problem in 130 countries. According to the World Bank, hidden hunger is one of the most important causes of slowed economic development in the Third World.
Enter rising CO2 levels, which could exacerbate hidden hunger in this century. Current concentrations of atmospheric CO2 now exceed anything seen in the last 420,000 years -- and likely in the last 20 million years, according to the U.N. Intergovernmental Panel on Climate Change. And forecasts call for CO2 levels to rise dramatically, from today's 378 parts per million to 560 parts per million or more by as early as 2050. The micronutrient decline brought by these ballooning CO2 levels could collide dangerously with the developing world's nutrient-poor green-revolution crops and its exploding population. Scientists also worry about how plant nutrient deficiencies might destabilize the world's wild ecosystems in unexpected ways.
"This is one of those slow-motion effects that does not hit us like a hammer, so we don't notice it," says Irakli Loladze, an assistant professor at the University of Nebraska. But, he says, failing to notice the hidden hunger fueled by changing CO2 levels does not lessen its potential impact: "The structure of the whole food web could change."
Diet for a Nitrogen-Deprived Planet
Early carbon-dioxide enrichment experiments were relatively simple: All kinds of wild and cultivated plants were exposed in field or lab to current, doubled, and tripled levels of CO2, and scientists watched what happened. In more than 2,700 studies, plant growth typically exploded. Doubled CO2 levels resulted in an average increase in agricultural yield of over 40 percent.
But after about 1993, some scientists began to question this approach. While the early studies looked at overall growth, they ignored the nutritional quality of the bigger, faster-growing plants, according to Loladze. When researchers began measuring the nutritive value of CO2-enriched plants and feeding the vegetation to insects and livestock, they started getting discomforting data.
Those data reveal a clear pattern for the macronutrient nitrogen, the only dietary chemical element that has been extensively studied to date. Peter Curtis, a professor of plant ecology at Ohio State University, gathered 159 papers addressing the nitrogen-depletion problem and found a "reduction of nitrogen in seeds in both wild and crop species," he says. Some species, like soybeans, showed no change, while barley and wheat showed a 20 percent reduction.
Though Curtis doesn't see this nitrogen shortage as a crisis for industrial agriculture, where chemical fertilizers can make up nutritional shortfalls, he wonders how protein declines might affect "wildlife that rely on plant seeds -- insects, seed-eating birds, or mammals, for example. For them, the nitrogen levels are really quite important."
CO2-induced nitrogen deficiency in plants has already been shown to affect herbivorous insects and the carnivores that eat them. To make up for the plunge in plant protein, some plant-eating insects must dramatically increase their intake of vegetation. But unable to keep up with the need to eat enough food, some bugs suffer increased malnutrition, starvation, predation, and mortality, writes evolutionary biologist David Seaborg in a recent issue of Earth Island Journal.
When Western Michigan University entomologist David Karowe fed cabbage white butterfly caterpillars leaves grown in an atmosphere with double the earth's current CO2 levels, the insects ate about 40 percent more plant matter than under current atmospheric conditions. But they still couldn't meet their dietary needs. Their growth rate slowed by about 10 percent and their adult size was smaller. Peter Stiling at the University of South Florida made similar findings for leaf miners, insects that eat out tiny caverns in leaves where they live. When they took up housekeeping in CO2-enriched leaves, the insects had to eat out 20 percent larger leaf homes. But the bugs were still twice as likely to die of starvation as insects living at today's CO2 levels.
As serious as these results seem, no one should jump to conclusions, says William Mattson, chief insect ecologist with the U.S. Forest Service in Rhinelander, Wis. He has spent the past five years monitoring 10 insect species and found they react differently to raised CO2 levels and lowered nitrogen levels, with some showing no change and others harmed, and no clear pattern yet in sight. He worries, though, that CO2 fertilization and nitrogen depletion could combine to alter insect balances in unexpected ways. For example, the leaf miners described above were also four times more likely to be killed by parasitic wasps -- bad news for the miners but good news for the wasps. In another study, aphids reproduced 10 to 15 percent faster in enriched CO2 atmospheres -- good for the aphids, but bad for the crops they infest.
Sorting out CO2 winners and losers ultimately depends on your point of view. To most people, "good insects" pollinate our crops, provide food for fish and birds, and regulate wild and domestic plant growth, and their decline would be problematic. However, farmers would likely herald a population crash in "bad bugs" -- that is, crop-eating pests. Unfortunately, no one can guess what CO2-altered natural and cultivated systems might look like.
The problem gets more complex with bigger animals. Clenton Owensby of Kansas State University has conducted one of the most extensive CO2 experiments involving mammals -- specifically, sheep. "We got around a 22 percent increase in yield of forage grasses over an eight-year period in an enriched CO2 environment," Owensby says -- but, "over that same time period, we also saw an 8 to 12 percent reduction in nitrogen concentration in the grasses, with a 5 to 10 percent reduction in ruminant animal productivity." That, he says, could translate into longer times spent raising sheep and cattle in the future, shaving already thin profit margins from financially strapped ranches. The problem, Owensby says, is that sheep and cattle cannot digest forage directly; they rely on microbes in their guts to break down cellulose. But reduced nitrogen decreases the microbial population, which slows the rate at which the forage can be digested, which in turn slows the rate at which forage can be eaten, and ultimately the rate at which the animals grow.
Owensby assumes it will be easy for industrialized nations to compensate. They can add nitrogen supplements to livestock diets, though that will still add some cost to meat production. But this would not be so easy in the developing world, where livestock productivity is often already marginal. And it would be nearly impossible with wild ruminants, such as browsing deer, elk, and gazelles, among which nitrogen deficiency remains unstudied.
Oddly, air pollution from fossil fuels may help offset the negative impacts of increased CO2 in plants. Auto exhaust and coal-burning emissions have increased nitrogen deposits in soils in the farm country of industrial nations by up to 50 times natural levels, according to Christian Korner of the Institute of Botany at the University of Basel, Switzerland. While this brings with it other serious problems such as acid rain, it could help ease or even solve nitrogen and protein deficiencies. But not without other repercussions, says Curtis: "The bottom line is that the combination of high CO2 and high nitrogen favors typical human-camp followers, mostly weedy species," such as Canadian thistle, spotted knapweed, leafy spurge, and kudzu, all of which seriously damage croplands and ecosystems and compete with native plants. "That could lead to an acceleration in the decline of biodiversity," he says.
Elementary, My Dear
What about the other 24 elements known to be vital to the human diet? Precious few studies have been conducted on these micronutrients, but the University of Nebraska's Loladze surveyed the entire available scientific literature. He found that an overwhelming number of the three-dozen-plus experiments conducted to date showed that CO2 enrichment caused a significant decline in one or more micronutrients, which include zinc and magnesium.
"It is obviously known that carbon dioxide boosts plant growth; it is after all a 'greenhouse' gas," says Loladze. "Even a high-school student in New Zealand growing plants with high amounts of CO2 was able to grow huge tomatoes. But when she investigated their quality, it turned out that the tomatoes had lower levels of micronutrients, and less nutrition in them."
Loladze, to his dismay, found just two studies on rice, the world's most important crop, and four on wheat, the second most important. One rice study found that four out of five elements decreased when grown in CO2-enriched air, with nitrogen dropping 14 percent, phosphorus 5 percent, iron 17 percent, and zinc 28 percent. Only calcium showed an increase, of 32 percent. The other rice study showed no significant change in micronutrient levels. In wheat, on average, every measured element except potassium declined in three studies. A just-published study by Chinese researchers led by Dong-Xiu Wu found that while high CO2 levels significantly increased grain yield, they severely decreased nutrient quality: nitrogen concentrations fell by 15 percent, phosphorus by 36 percent, potassium by 23 percent, and zinc by 32 percent.
Mattson points to still another problem with CO2. "Something else that may exacerbate micronutrient deficiency is that added CO2 tends to drive up [the production of] many plant non-nutrients" -- poisons that enhance plant defenses against their would-be consumers. "The sum total of lowered nitrogen, lowered essential micronutrients, and heightened [plant poisons such as] tannins and other phenolics could be the worst kind of soup," he says. What we're doing, he believes, is running an unregulated and probably irrevocable chemical experiment on earth's ecosystems.
Dude, Where's My Carbon?
Now that researchers have detected CO2-induced nutrient deficiencies, they are seeking to understand why they happen. And they think they have found some relatively simple underlying causes -- simple to scientists, that is, although perhaps not to those of us who glazed over in high-school biology.
We live in a carbon world, scientists explain: All life on earth, from oranges to orangutans, is carbon-based. Most of this carbon comes from our atmosphere, which is absorbed by plants, which pass it on to grazing animals, which in turn pass it on to their predators. Change the levels of atmospheric carbon, and all plants and animals along the chain may be affected.
Here's how: Plants create much of their biomass out of thin air, from a steady diet of CO2 sucked through small leaf openings called stomata. Then, via the miraculous sleight-of-hand known as photosynthesis, the plants combine CO2 and water in the presence of chlorophyll and sunlight to make carbohydrates, simple sugars, and complex starches, which provide energy for plant growth. Much of the remainder of what plants need -- nitrogen and trace elements -- doesn't come from the air, but is pulled up through the root system from the soil.
Scientists have isolated two mechanisms that potentially explain how elevated CO2 levels reduce plant nutrients. The first is a "biomass dilution" effect. As plants absorb more airborne carbon, they produce higher-than-normal levels of carbohydrates but are unable to boost their relative intake of soil nutrients. The result of this dilution effect is increased yields of carbohydrate-rich fruits, vegetables, and grains that contain lower levels of macro- and micronutrients. Put simply, a bite of bread in our current CO2 atmosphere ends up being more nutritious than one in the CO2-enriched atmosphere of the future.
A second problem: Plants exposed to increased CO2 levels start to narrow the stomata through which they inhale CO2 and exhale water vapor via transpiration. This benefits plants by making them more drought resistant, but it also means that fewer waterborne nutrients flow into the roots. According to Loladze, if carbon-dioxide levels are doubled, transpiration decreases by about 23 percent.
A particularly disturbing study suggests that the mechanisms of CO2 nutrient depletion may already be causing a decline in the quality of our food supply. Josep Penuelas of the Center for Ecological Research and Forestry Applications in Barcelona, Spain, compared historical plant samples grown at preindustrial levels of atmospheric CO2 with modern equivalents. He found that today's plants had the lowest levels of calcium, copper, iron, potassium, magnesium, sodium, sulfur, and zinc than at any time in the last three centuries.
Research for Tomorrow
The obvious way to reduce the risk of declining food quality is to cut fossil-fuel emissions, thereby reducing atmospheric CO2 concentrations. But political resistance in the U.S. and the global failure to effectively curtail emissions means that CO2 levels will rise far higher in coming decades. Therefore, scientists say, we need to quickly embark on a crash program to research the biochemical impacts of CO2 and prepare for the potential nutritional harm.
"Nobody really knows how serious the changing chemical composition of plants caused by heightened CO2 will be," warns Mattson. "We are just scratching the surface here. ... It is a wide-open question about what impact this will have on the nutritional physiology or reproductive success of animals."
Loladze agrees that three dozen studies, or even 200, prove nothing conclusively. Curtis suggests a novel fast-track strategy for quickly expanding that database: He says that data may not need to come from new experiments, but may already exist "as archived seeds" and other stored vegetative matter left over from the 2,700 CO2 plant experiments already completed. Korner, however, calls for an aggressive new round of nutrient experiments conducted on a global scale.
Such massive research would require major funding, something the Bush administration seems unlikely to provide. Still, throw more money at the problem, agrees Mattson, and, "you'll get more people working, and you'll accrue the knowledge faster. Whether it can influence policy, that's difficult to say. We have an administration that has its mind set on what the policy should be. And it's always possible for them to say we just don't know enough yet to act. It's a [faulty] defense anyone can employ: to say, 'there is so much unknown; let's not do anything.'"
At some point, though, there will be a tipping point, which is what most worries scientists like Mattson. He looks at the vast array of harm caused by increased greenhouse-gas levels -- melting ice caps, extreme weather, the altering of wildlife habitat, and the biochemical impacts of rising CO2 levels -- and concludes, "You push something a little bit every year over the long term, and you see little or nothing changing. And all of a sudden ... one of those nonlinear changes occurs, where you push everything just far enough, and you're over a cliff."