Is the climate changing over time? You probably have heard many different opinions about climate change. In the past few years, most scientists have agreed on at least one thing about climate change. They have agreed that measured and recorded changes in Earth’s climate over the past 100 or more years point to a warming of Earth’s surface greater than they would have expected from normal cycles.

Earth’s average temperature depends on how much of the sun’s energy comes through the atmosphere to Earth’s surface, and how much escapes back into space. About 90 percent of the sun’s energy is trapped by gases in the atmosphere, including carbon dioxide, methane, and nitrous oxide. This trapped energy is sent back to Earth in all directions, warming the planet. This warming is called the greenhouse effect, and the gases are called greenhouse gases. Without these gases, humans and other forms of life would not be able to survive on our planet.

During the past 150 years, however, the amount of greenhouse gases in the atmosphere has risen sharply (Figure 1). This increase has been happening since the beginning of the Industrial Revolution in the mid-1700s. The amount has been increasing even more over the past 100 years. Scientists believe the sharp rise in greenhouse gases is caused by an increase in the burning of fossil fuels, such as oil, coal, and natural gas. These higher levels of greenhouse gases in the atmosphere trap more of the sun’s heat that is reflected off of Earth’s surface. This additional trapped heat leads to increasing temperatures on Earth.

Carbon dioxide is one of the greenhouse gases. Carbon dioxide naturally occurs in our atmosphere and a certain amount is necessary to keep Earth warm enough for life to continue. When too much carbon dioxide is emitted into the atmosphere, however, Earth’s surface continues to get warmer. As Earth’s surface warms beyond its normal temperatures, plants and animals are affected.

Carbon is present in rocks, oceans, and the air. Every living thing and once-living thing contains carbon, including all plants and animals. Carbon is always moving around. You can see how it moves in what is called the carbon cycle (Figure 2).

Since a tree is a living thing, it also contains carbon. As a tree grows, it absorbs carbon from the air and stores it in its wood (Figure 3). The world’s forests are related to Earth’s climate because they hold large amounts of carbon on Earth. The carbon stored in the world’s forests does not get emitted into our atmosphere unless the trees are felled and burned or when they die and decay. As you can see, the world’s forests play a role in helping to reduce the effects of global climate change.

FAO and the National Correspondents wanted to know how much carbon is held on Earth by the world’s forests. They also wanted to know whether this number is staying the same, increasing, or decreasing over time.

The amount of carbon in a tree is closely related to the weight of its living material, minus any water. Some scientists estimate that half of a tree’s dry weight is carbon. Others estimate that about 45 percent of a tree’s dry weight is carbon. FAO, used an estimate of 47 percent to calculate the amount of carbon being held by the world’s forests.

The National Correspondents and FAO used three measures of carbon storage. First, they estimated the amount of carbon being held in trees growing in the forests. Recall that carbon is also present in once-living things.

FAO and the correspondents, therefore, also estimated the amount of carbon in the litter and deadwood lying on the forest floor. Litter includes leaves that have fallen from the trees (Figure 4).

Deadwood is the remains of fallen branches and trees that have died (Figure 5). The third measure of carbon was an estimate of the amount of carbon held in the world’s forests soils. Because soil contains living beings such as microbes and once-living material, it also contains carbon. Trees also release carbon into the soil (See Figure 3).

The amount of carbon being held in the world’s forests’ trees was estimated to be 289 Gigatonnes, or Gt (Figures 6 and 8). One Gt is equal to 1 billion tonnes. One tonne is equal to 1 000 kilograms, or 2 205 pounds.

The amount of carbon in the forests’ litter and deadwood was estimated to be 72 Gt, or 17.8 tonnes per hectare. Compare this with the amount of carbon held in the soil, which was estimated at 292 Gt or 72.3 tonnes per hectare. One of the questions FAO wanted to answer is what is happening over time to the amount of carbon being held by the world’s forests.

When FAO estimated the amount of carbon being held by the world’s forests’ trees per hectare, it discovered something surprising (Table 1).

Over the past 10 years, the average amount of carbon held by a hectare of forest has changed very little. The decrease in carbon held by the world’s forests over the past 20 years (Figure 9) is due to the decrease in forest area worldwide (See Inquiry 1).

The United Nations Intergovernmental Panel on Climate Change, or IPCC, is interested in better understanding carbon and its role in climate change. In 2007, the IPCC reported that 17.4 percent of the world’s greenhouse gases come from the forest sector. Most of these emissions come from the loss of forests in developing countries (Figure 10).

The United Nations has recognized the importance of taking action to reduce climate change. It developed an effort to reduce the amount of greenhouse gas emissions from deforestation and degradation in developing countries. This effort is known as REDD (Reduce emissions from deforestation and forest degradation). In December 2010, the United Nations took another step forward. The new effort, known as REDD+, includes the following actions.

REDD+ Reduce emissions from deforestation Reduce emissions from forest degradation Manage forests sustainably Conserve the carbon contained in forests Enhance the carbon contained in forests.

Under REDD+, developing countries will receive money if they successfully complete one or more of these actions. This money can be used to cover some of their costs. The United Nations views REDD+ as an investment that will help countries to reduce the impact of climate change.

Recognizing that the carbon held on Earth within forests helps to address climate change has created a number of opportunities and challenges. People now recognize carbon sequestration as a benefit of having forests. This is in addition to other forest benefits that we explored in Inquiry 3. When thought of this way, you can see that by holding carbon, forests provide a service that has value. A government or a business may decide that maintaining forests for the carbon they hold is worth more than felling forests for fuelwood, agriculture, or for other reasons.

The United Nations also recognized that when forest managers work sustainably, they help people and forests worldwide adapt to a changing climate. As the climate changes, forest management will play an important role in maintaining the health and wellbeing of the world’s citizens.