Global warming is caused by two types of forcings: natural forcing, which involves solar cycles and the Milankovitch cycles ; and anthropogenic forcing, which is human-based. In this article, we’ll discuss anthropogenic forcing, as it accounts for more than half of the current global warming, according to IPCC’s 5th Assessment Report. The contributions of the two forcings (anthropogenic and natural) to global warming are represented on the graph below.
Anthropogenic forcing is the result of our greenhouse gas emissions. Since the last century, our activities have been releasing unprecedented quantities of greenhouse gases into the atmosphere, worsening the greenhouse effect. Greenhouse gas emissions come primarily from electricity and heat production; agriculture, forestry and other land uses (AFOLU); industrial activities; transport; and buildings. The contributions of each of these economic sectors were studied by the IPCC (graph below).
Fossil fuel combustion is used in nearly all of the economic sectors, being thus the main culprit of global warming. The greenhouse gas emissions from daily human activities, such as driving a car, or heating up a house, eventually sum up to the current level of carbon dioxide in the atmosphere, in the following priority:
Electricity and Heat Production : This sector accounts for 25% of total greenhouse gas (GHG) emissions. According to the EIA (US Energy Information Administration), 67% of the electricity generated in the United States was from burning fossil fuels (coal, natural gas, and petroleum) which release CO2 in the atmosphere.
Agriculture, Forestry, and other Land Uses: Accounts for 24% of total GHG emissions. Deforestation, livestock, rice cultivation, forest fires and peat decay are included in this sector.
Industry: Accounts for 21% of total GHG emissions. The use and production of metals, such as iron and steel, are some of the contributors of GHG emissions in this sector.
Transport: Accounts for 14% of total GHG emissions. This includes the emissions from cars, airplanes, trucks, and ships.
Buildings: Accounts for 6.4% of total GHG emissions. This includes space heating in residential buildings, lighting, cooking, and water-heating.
Other Energy: Accounts for 9.6% of total GHG emissions. This includes petroleum refining and fuel production.
In short, the current global warming is primarily caused by the greenhouse gas emissions of human activities, namely energy production and consumption. Other main contributors of greenhouse gas emissions include agriculture, deforestation, and forest fires. Many countries have engaged in international efforts to reduce our current levels of greenhouse gas emissions, such as the actions delineated in the Kyoto Protocol, but much more is needed to effectively halt global warming.
IPPC, 2014: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
EIA (US Energy Information Administration), 2014. What is U.S. electricity generation by energy source? http://www.eia.gov/tools/faqs/faq.cfm?id=427&t=3