Human Dimensions of Global Warming

Categories of Proximate Causes


Proximate causes are the human activities that directly cause climate change. There are two overarching categories of proximate causes: land transformation and industrial processes. People transform the land surface in many ways, with some important types being deforestation, agriculture, urbanization, and transportation network building. Industrial processes include energy production, transportation, manufacturing, waste disposal, and many other activities. Note that land transformation and industrial processes are not always distinct. For example, building a road transforms the land but requires powerful industrial equipment, mined materials, and processed chemicals. Once the road is built, vehicles traveling on it are manufactured using energy, petrochemicals, and mined materials, and are run by burning mined and processed petroleum products.

There are many other ways to categorize the proximate causes. The US government, as part of their greenhouse gas emissions inventory process, breaks human activities producing GHGs into six categories: energy production and consumption, non-energy industrial processes, solvent and other chemical use, animal agriculture, waste management, and land use. Because energy and transportation are the biggest contributors to greenhouse gas emissions, the next lesson in this course covers the energy production and consumption category, and the following lesson explores transportation, a crucial subcategory that falls under energy in the US system. The following paragraphs will discuss the remaining five categories (non-energy industrial processes through land use) of human activities generating GHGs. Note that these paragraphs only present a select sample of important GHG-producing activities; there are many, many more proximal causes than those described here.

Although there are numerous industrial processes that produce GHGs, three stand out as being especially important: mineral production, chemical production, and metal production. Mineral production emits prodigious amounts of GHGs through cement making, lime calcining, and limestone and soda ash use. Chemical production emitting significant quantities of GHGs include ammonia, nitric acid, and adipic acid production. Although less well known, adipic acid is essential to nylon production, and making this acid releases large amounts of N20. Many metal production processes release GHGs; iron, steel, and aluminum production, plus magnesium smelting and casting, are the largest sources.

Nitrous Oxide Canisters in back of car
Figure 2.4: Nitrous Oxide Boosters.
Credit: Used by permission Karthick Subramonian. Accessed July 7, 2011 from Reflections From My Life! Working of NItrous Oxide boosters in a car!

Solvent use, such as observed in degreasing, dry cleaning, and the graphic arts, emits a substantial amount of GHGs. The use of N2O emits fugitive N2O and includes such diverse uses as medicine, rockets and car racing, and aerosol propellants.

Animal management is an important contributor to climate change. Animal husbandry, feedlot management, and animal manure management together produce considerable quantities of CH4.

many cows eating in a feed lot
Cattle Feed Lot.
Credit: Imperial Valley Feed Lot Jan 2011 #1 / Chris Austin (The link,, is no longer active.) / CC BY-NC-ND 2.0

Waste disposal is responsible for large sums of CH4 and CO2. Solid waste disposal generates so much CH4 through the decomposition of garbage that many municipalities around the world collect it to generate electricity or to power other activities. Most sewage-handling facilities also produce considerable amounts of CH4, although some advanced sites have eliminated this source of GHGs. Waste incineration produces vast quantities of CO2.

people sifting through garbage at a Sudanese landfill
Waste pickers at the main Khartoum landfill site.

Land use generates some of the biggest GHG totals. Forestry produces CO2 through the use of heavy machinery for logging, but far more important is the double whammy that results from cutting down trees: when a tree dies, it no longer pulls CO2 from the air; the burning or decomposition of the wood releases the stored CO2 to the atmosphere. Agricultural soil management (crop, grassland, and pasture management) releases large quantities of CO2 through the breakdown of soil humic acids and N2O by the misuse of nitrogenous fertilizers. Wetland and rice paddy management produce huge sums of CH4. Construction for settlements breaks up the soil, releasing CO2 to the atmosphere, and removes vegetation, also releasing CO2 and eliminating its ability to remove CO2 from the air.