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From Gas-guzzlers to Grass Grazers: What Is the Greenhouse Effect and How Is Human Activity Altering It?

May 1998

Although greenhouse gases together make up less than one-tenth of 1 percent of our atmosphere, they act as a kind of thermal blanket around the whole earth. They prevent a significant amount of incoming solar energy from being radiated back out into space. This makes our planet about 30 Celcius degrees warmer than if those gases weren’t present—warm enough to support life as we know it.

What worries many scientists is that this blanket is getting thicker as the proportion of greenhouse gases increases because of human influences. This, they argue, may be causing a dangerous rise in the average temperature of our planet’s atmosphere. It is estimated that the global temperature would increase by between 1 and 3.5 degrees Celsius if CO2 concentrations were to double. At our current rate of ejecting carbon into the atmosphere, this would happen before the end of the 21st century.

A higher global temperature threatens to raise sea levels and alter regional climate patterns. Such changes could trigger major disruptions around the world in the coming century. Food production patterns could shift as agriculture becomes more difficult in some areas and easier in others. Large numbers of plant and animal species, unable to adapt rapidly enough to environmental change, could become extinct. Forests, water supplies, and shorelines could be threatened. Economic growth in many areas could be severely stunted, especially in developing countries, resulting in greater poverty.

How do Plants Capture and Store Carbon Dioxide?

Photosynthesis is the production of organic molecules from inorganic molecules by plants as well as certain bacteria and algae. In plants, cell pigments called chlorophylls trap light from the sun. The photochemical reactions in this first phase of photosynthesis produce energy-rich compounds and release oxygen. In the second phase, enzymes in the plant use these compounds to "fix" carbon dioxide. That is, they combine atmospheric CO2 with these other compounds to form organic compounds for plant nutrition and growth. Much of this locked-up carbon is recycled into the soil as plant matter such as leaves die and decay, and as worms and microorganisms like bacteria break down the organic matter.

Why are Plants’ Ability to Store Carbon Crucial to Protecting the Earth’s Atmosphere?

Through photosynthesis, the earth’s vegetative cover is a key component in the global carbon cycle. Forests and grasslands (as well as the oceans) serve as vast carbon reservoirs, or "sinks," because of their ability to absorb CO2 from the atmosphere. By reducing our carbon emissions and increasing the extent of the earth’s plant cover—for example, by protecting forests—human beings can slow down or even reverse the buildup of atmospheric CO2.

What is the Significance of Deep-rooted African Grasses in Storing Carbon?

In parts of South America, particularly Brazil, many cattle producers in recent years have been planting deep-rooted grasses, originally from Africa, as forage for their herds. More productive than native savanna grasses, these introduced grasses are often mixed with pasture legumes to improve animal nutrition and soil fertility. Research by the International Center for Tropical Agriculture (CIAT) in Colombia has shown that the African grasses also have the ability to store large amounts of carbon deep in the soil—much more than previously estimated. What’s more, the Center’s work indicates that grass-legume combinations result in greater carbon storage than that seen with native savanna grasses or with African grasses alone. Because of the vast surface area of savannas in South America and their wide use in raising livestock, the research suggests that African grasses, as natural carbon "sinks" or reservoirs, have enormous potential for slowing CO2-related global warming while improving agricultural productivity.

Why was the Carbon Storage Potential of Savanna Grasses Previously Underestimated?

According to Myles Fisher, a plant physiologist with CIAT, it was largely because of the way scientists measured grassland productivity in past decades. They were then mainly interested in the above-ground production of biomass by grasses—the forage available for grazing animals. Estimates typically did not include root production and decaying plant debris in the soil. Both literally and figuratively, scientists weren’t digging deep enough to get to the root of the carbon-storage issue.

In the late 1980s and early 1990s, other researchers made a major contribution to understanding soil organic matter by looking deeper into the soil, down to 15 centimeters (about 6 inches). Their estimates of grassland productivity shot up, to as much as five times the value of the earlier figures.

For many years CIAT has emphasized plants for cattle fodder that tolerate dry spells and can thrive in infertile soils like the acid savannas of South America, which are low in phosphorus and calcium. So its scientists were naturally interested in grasses with robust roots that can grow deep in the soil to pick up nutrients and water. This led them to look even deeper into the soil, down a meter or so, to observe the dynamics of pastures, especially the buildup of soil organic matter. Their data suggest that a single hectare of pasture planted to deep-rooted grasses of African origin can store as much as 15 tons of carbon per hectare each year, though it is not yet known how this rate changes over the longer term.

What are the UN Framework Convention and Kyoto Protocol?

In centuries gone by, "the global climate changed human beings. Now human beings seem to be changing the global climate." This disturbing reversal of roles is the motive behind the 1992 UN Framework Convention on Climate Change. This international treaty was signed by 165 states and took effect in March 1994. Its main aim is to stabilize "greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic (human-induced) interference with the climate system." The Convention didn’t lay out a specific blueprint for tackling the problems of climate change. Rather, it established a process whereby countries could agree on specific actions for the future.

The future has arrived. In December 1997, an international agreement was struck in Kyoto, Japan, on reduction of emissions of six greenhouse gases. The Kyoto Protocol contains legally binding targets for emissions of industrialized countries. They must reduce their combined emissions by at least 5 percent by the period 2008-2012, calculated as an average over these five years. According to the United Nations Environment Programme, "the Protocol will result in 2010 emissions levels that are approximately 29 percent below what they would have been in the absence of the Protocol."

While the UN Framework Convention set the stage for monitoring and cutting greenhouse gas emissions through protocols like the one negotiated in Kyoto, it also encourages other actions. It specifically calls for more scientific research on climate change and for countries to prepare inventories of sinks for greenhouse gases such as grasslands and forests.

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