Greenhouse gases absorb the heat of the sun and cause a rise in atmospheric temperature. In Japan, the Law Concerning the Promotion of Measures to Cope with Global Warming recognizes six types of greenhouse gas: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride.

The percentage of greenhouse gases in the atmosphere is small, but their effect is large: without them, the mean global temperature would drop from 15℃ to -18℃. In discussions about global warming, there is a tendency to focus only on carbon dioxide. There are two reasons for this focus:

• Carbon dioxide emissions are by far the greatest in magnitude.
• Individuals can have a large effect on carbon dioxide reductions, which is not true of the other gases.

The following chart indicates total amount of carbon dioxide on the planet and the yearly increase in that total.

Unit: Billion metric tons

Source: IPCC Fourth Assessment Report

As the chart shows, atmospheric carbon dioxide represents only 11% of the total amount on the planet, a percentage indicating the leverage this gas exerts on the ecosystem. In addition, since the effects of global warming first appear in the atmosphere, increases in greenhouse gas emissions have a direct effect on our lives.

There are 760 billion metric tons of carbon dioxide in the atmosphere with 3.2 billion added every year. At this rate, doubling the atmospheric amount would require 237.5 years, an event likely to occur in a much shorter period of time under present circumstances. In either case, the result would be catastrophic. The causes of increased emissions are many, but, as the graph below suggests, the growth in global human population and the rise of industry and consumption in developing countries are chief among them.

The following graph indicates greenhouse gas emissions by country, with the populous countries of China, India, the United States, and Indonesia at the top. In addition, China now supports so much industry that it is known as the "factory of the world." Apportioning responsibility for global emissions is no simple task, however, since Japan, for example, has developed energy conservation and environmental technologies by which to allay its otherwise greater emissions. The data may surprise or seem obvious depending on one's individual perspective; what cannot be disputed, however, is the necessity of reductions by high-emissions countries to effect a global change for the better.

The graph "CO₂ emissions from fuel combustion by country" below reveals another problem. The combined emissions of countries that have made reduction commitments under the Kyoto Protocol account for only 1/3 of the global total, yet they are less than the combined emissions of the United States, which has not ratified the Protocol, and China, of which no reduction commitments have been required. For this reason, it seems that additional steps in the development of the Kyoto Protocol are required.

The graph below is based on data from the World Data Centre for Greenhouse Gases (WDCGG) of the Japan Meteorological Agency (JMA) showing the change over time of the average monthly concentration of atmospheric carbon dioxide average by latitude.

From this graph, the following may be understood:

• 1. The atmospheric concentration of carbon dioxide rises as one approaches the poles.
• 2. Although there are variations within a year, there is a straight-line trend of increasing carbon dioxide concentrations over time.
• 3. Seasonal variations increase in magnitude as one approaches the North Pole.

The greater land area and human population in the Northern Hemisphere helps explain these trends:

• 1. A greater number of plants on the more abundant land in the Northern Hemisphere grow and absorb carbon dioxide during the spring and summer.
• 2. A greater number of people and a greater amount of industry in the Northern Hemisphere produces a higher concentration of carbon dioxide throughout the year.

Japan produces 4.7% of the world's greenhouse gas emissions, which may be further broken down per the chart below. Although Japan has made a commitment to reduce greenhouse gas emissions to 94% of 1990 levels by the year 2012, it had instead increased these emissions by 6.2% by the year 2006. A breakdown by greenhouse gas type indicates that only carbon dioxide emissions have increased over the time period, a disturbing trend inasmuch as every individual has the power to contribute to reductions. Japan must face these facts squarely and take action.

Source: "Annual Report on the Environment and the Sound Material-Cycle Society in Japan 2008," Ministry of the Environment.