Greenhouse gases enter the atmosphere mainly as a result of human activity, and they create global warming and other worldwide climatic effects. An IPCC report (2007) summarizes the current scientific knowledge regarding climate change. Emissions of gases such as carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons have increased drastically since the start of the industrial revolution, and once in the atmosphere, those gases trap heat radiated from the earth in what is called the greenhouse effect, resulting in increased average temperatures at the earth’s surface. This warming results in varied and somewhat unpredictable effects on the earth’s climate system, such as altered patterns of precipitation, higher frequency of extreme weather events, and sea level rise from melting ice caps.
Greenhouse gas emissions inevitably accompany the modern process of industrialization. The energy required to maintain a consumer society comes mainly from the burning of fossil fuels, which releases carbon dioxide, the largest contributor to global warming, into the atmosphere. Plants that burn coal and natural gas provide most of the world’s electricity, and petroleum-burning engines power most of the world’s transportation. Kahn (2009) notes that the trend of urbanization increases the demand for energy, and this demand will continue to rise as an increasing portion of the world’s population moves into cities. About 30% of people lived in urban areas in 1950, but 60% are expected to live in urban areas by 2030.
The intensive agriculture required to support a growing urban population has further consequences greenhouse gas emissions. Van Beek et al. (2010) predict that agricultural emissions will increase rapidly until 2040, when they may start to level off. While emissions from urbanization are mainly carbon dioxide, agricultural emissions are mainly nitrous oxide and methane, which are produced through fertilizer use and livestock production. These gases absorb more heat than carbon dioxide, yet they are emitted in lower quantities. Agriculture is also responsible for carbon dioxide emissions from deforestation and other land-use changes as well as emissions from fossil fuel energy used to create fertilizer and other products.
Because greenhouse gases come from so many different sources, the strategies to curb emissions are varied as well. These strategies generally fall into the categories of changes in energy generation and changes in land-use practices. For more about reducing emissions, see Sustainable Management: Greenhouse Gases.