Can Earth's Plants Keep up with Us?

by Stephanie Renfrow • design by Michon Scott • September 26, 2007

Our lives depend on plants. Plants turn the energy of the Sun into our most basic needs: lumber for houses, fuel for cooking, fiber for clothing, feed for livestock, and food for our own growing bodies. But as global population and incomes rise, will plants be able to keep up with the human appetite? And if they cannot, which regions will be short on food and other plant-based resources, and what will that mean for nations as they try to assure food security for their citizens?

Marc Imhoff, a biophysical scientist with NASA, has been exploring these questions with colleagues from the University of Maryland’s Earth System Science Interdisciplinary Center, the World Wildlife Fund, and the International Food Policy Research Institute for six years. He said, “Our primary motivation has been to find out where we stand relative to our survival on the planet, and what our needs are compared to the capability of the biosphere to sustain them. In fact, it goes beyond just need; it includes our different lifestyles—our appetites.” To build some answers, Imhoff set about measuring global plant productivity, calculating human consumption levels on a cultural level, and then comparing what he learned. His findings remind us that we all rely on the same finite Earth.

The Green Supply

Net primary production is a measure of plant productivity, the amount of plant material left over after respiration. Imhoff put it this way: “Net primary production is the plant material that we see above ground, as well as what is below ground, like root systems. All of our food, much of our fiber, and for many people in developing countries fuel for cooking, is derived from plant material.”


Adapted from an article published by Earth System Science Data and Services.

  Global net primary productivity

To measure net primary production, Imhoff used an index, or scale, of vegetation based on satellite data from the Advanced Very High Resolution Radiometer (AVHRR) instrument. The satellite observations spanned every sixteen-day period from 1982 to 1998, allowing Imhoff to compute an average maximum vegetation amount for each month of the year. He combined the monthly vegetation data with temperature, humidity, rainfall, and land cover type in a model that simulates plant growth. The model provided Imhoff and his colleagues an estimate of the planet’s net primary production. Imhoff said, “This information gave us the planetary supply of plant production on land that is available to humans in an average year.”

The Human Appetite

Imhoff’s next step was to measure the amount of net primary production that humans use worldwide in an average year, and then tie it to cultural consumption habits. To do that, he turned to statistics from the United Nations Food and Agricultural Organization (FAO) on food and fiber consumption by country, taking the data from 1995 as a typical year that matched the satellite timeline. He said, “We divided the consumption statistics into food, both plant- and animal-derived; and fiber, including wood, wood-based fuel, and paper. Then, we backed out what you would need to see in the field to get those products,” he said. “This way, we could double-check what the AVHRR data would have shown in the field with what the consumption statistics indicated was actually used.”


The first step in measuring plants’ ability to sustain the human population is to measure Earth’s annual net primary production, the amount of plant matter an ecosystem produces. This image, based on observations of vegetation from satellites, shows productivity on a scale from 0 to 2 trillion grams. Dark green areas indicate the most productive areas, and beige indicates the least productive. (Image by Jesse Allen, based on data from NASA’s Socioeconomic Data Center.)
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  Human appropriated net primary productivity (millions of grams)

Next, Imhoff requested data on global population numbers and density from NASA’s Socioeconomic Data and Applications Center. “We overlaid the consumption data on the population map and ended up with a gridded surface map showing the amount of net primary production required to support the consumption habits of different human populations all over the world.” This map gave Imhoff the information he needed to compare nature’s supply of plant production with human demand.


People everywhere depend on Earth's net primary production (NPP) for everything from food to clothing to fuel. This figure shows how much (millions of grams) of the Earth's NPP people in different parts of the world consume. High national consumption can be due to large populations with low per-person consumption levels (e.g., India), or to small populations (United States, European countries) with high per-person consumption levels. (Image by Jesse Allen, based on data from NASA’s Socioeconomic Data Center.)
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Keeping up with Demand


When he compared the global supply of net primary production to the human appetite, Imhoff confirmed some ideas that did not surprise him. “Some things were a no-brainer,” he said. “For example, urban populations with a high density consume way more primary production than local ecosystems can produce.” One sharp example of this was New York City, which consumed 30,000 percent more primary production than it created. “That says a lot about the dependence of urban areas on our transportation networks and agricultural infrastructure,” he said. The ratio of consumption to regional net primary productivity might prove to be a useful indicator of potential trouble spots should natural disasters, economic insecurity, or other problems undermine networks or infrastructure.

  Map of Human-appropriated net primary productivity.

Having enough food may seem like a concern only for developing countries, but industrialized countries also have concerns about food security, which is defined simply as always having enough food for an active, healthy life. Developed countries may have dense urban populations, import more food, and be accustomed to high levels of consumption—all of which make these countries susceptible to transitory food supply disruptions. In addition, developed countries may have poor populations that are vulnerable to rising food prices in spite of typical governmental support services. Imhoff said, “Worldwide, we have some very vulnerable populations that could never survive just on the productivity of the land on which they live—with some important implications for national and regional food security.”

Closely tied to the question of having enough food for survival is the idea of having enough fuel, clothing, and building materials for survival. The availability of everything from firewood to winter coats begins with plants. Consumption of material goods is an important factor in economic stability and security, as well as in maintaining or improving lifestyle levels. The more a population consumes, the more effort it takes to maintain that standard of consumption. Imhoff found that there were two big factors that lead to high consumption levels. The first is high per-capita consumption rates, as seen in much of the developed world; the second is large populations. Even a low per-capita consumption rate can result in a huge overall level of total consumption if multiplied over a large number of people.


In many areas, human consumption far exceeds what the land in the immediate vicinity can provide. Densely populated countries, such as India and China, often consume more than 100 percent of locally available net primary production (NPP), but so do smaller countries in areas with naturally low NPP, such as Saudi Arabia. Even in productive areas like the eastern United States and Europe, demand exceeds local supply due to high levels of per-person consumption. (Image by Jesse Allen, based on data from NASA’s Socioeconomic Data Center.)
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  Landsat image of Riyadh

To Imhoff, a more surprising finding was the importance of technology in helping balance the equation between supply and consumption. “We found that using improved technology—especially in harvesting and storage techniques—can actually halve the amount of waste in agricultural production,” he said. “Take logging. Without the benefits of improved harvesting technology, you might literally lose a tree for every one that you use.”

The interplay between population, consumption rates, affluence, and technology leads to some thought-provoking realizations. “For example, Asia’s per-capita consumption is on the rise,” he said. “If consumption begins to match Western levels, there will be a significant increase in demand for food and fiber products. If technology improvements do not come with that growth, then you’ll see populations that are outstripping their regional food production capacity. They’ll be more dependent on resources elsewhere, and will have to compete for them.” Although citizens in industrialized countries may not find the rising population in developing nations of immediate concern, poverty has been connected to terrorism, war, underemployment, border pressures, disease, and political unrest.

“It’s a question of how much we are willing to pay to keep getting the level of production that we want, and to transport it from one place to another,” Imhoff said.


In marginally productive areas, such as the desert landscapes of the Arabian Peninsula where Saudi Arabia's capital, Riyadh, is located, it is easy for human populations to consume far more primary production than the local landscape can supply. Landsat 7 captured this image of Riyadh on December 16, 2000. The urban grid appears gray, isolated patches of vegetation are dark green, and the bare ground surrounding the city is pinkish-tan. Riyadh has a population of more than 4 million residents. (NASA image created by Jesse Allen, using Landsat data provided by the University of Maryland’s Global Land Cover Facility.)


Can We Afford It?


Stanley Wood, of the International Food Policy Research Institute (IFPRI), agreed with Imhoff’s emphasis on the question of costs. “The bottom line is food availability and food affordability. How stretched will our incomes be to meet our food requirements?” Wood works with a team from the IFPRI that collaborated with Imhoff on the net primary production work and which has joined with him on a new proposal that builds on the initial work.

Imhoff and the IFPRI team hope to improve their understanding of the flow of net primary production between countries. Wood said, “For example, let’s say that energy security becomes an increasing concern and the U.S. turns to biofuels. The global price of maize could rise steeply because of competition between maize for food and feed, and maize for biofuels. This would create a double-edged sword for poor countries: the increased prices would generate more income for developing country farmers, but would be bad news for poor consumers.”

Widening the global picture is also something that Imhoff looks to do. “We have begun projecting what would happen to plant production with climate change, and you don’t have to look very far to see that the geopolitics of food production could change significantly, with some countries winning and others losing. Even without climate change, we are already rubbing up against some limits in our planet’s ability to supply us,” he said.

  Bwindi Forest Reserve and adjacent farmland

Both Wood and Imhoff hope their data set on human use of net primary productivity, which is now available through SEDAC, will be useful to policy and decision makers, both in governmental and nongovernmental agencies. “We hope to have more one-on-one conversations with users in the future,” Imhoff said. “With the unprecedented population levels that we have now, surprises can develop very quickly. We need to be ready.”

However, even with a growing global population, increasing consumption levels, and other global changes bearing down on us, Imhoff emphasized the positive. “We have the technology to get out ahead of this. The data isn’t just showing us the bad news; it is also giving us the power to study the changes ahead and understand them,” he said. “We are far from being helpless. Our ability to assess our environment and our situation should give us a sense of empowerment.”


In southwestern Uganda, demand for farmland has brought people to the very edge of Bwindi National Park, one of the last remaining fragments of habitat for the critically endangered mountain gorilla. Despite national intentions to preserve the remaining gorilla habitat in Uganda, Democratic Republic of Congo, Rwanda, and Burundi, the area is one of Africa’s most densely populated. Human pressure on the forest reserves is intense; illegal logging, poaching, and farming in protected areas are constant threats. (Photograph courtesy Nadine Laporte.)

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