Important for the world temperature level and climate is the amount of greenhouse gases (GHGs) in the atmosphere. These gases allow sunlight to pass through the atmosphere while they restrict the out-radiation of heat. The more GHGs, the higher the average world surface temperature will become. The current amount of GHGs helps maintain a world average temperature of 14°C, which is about 33°C higher than what the temperature would have been without GHGs. Since the atmosphere "delivers climate services" without being depleted, GHGs can be thought of as a renewable resource. The most important GHG is CO2, and the following task is about controlling the amount of CO2 in the atmosphere.

The Intergovernmental Panel on Climate Change (IPCC), a scientific panel organized by the United Nations, has concluded that carbon dioxide (CO₂) and other greenhouse gas emissions contribute to global warming. The panel stated that: “most of the warming observed over the last 50 years is attributable to human activities.”

The amount of CO₂ in the atmosphere is affected by natural processes and by human activity. Anthropogenic CO₂ emissions result from human activities such as combustion of fossil fuels, changes in land use, and deforestation. Emissions have been growing since the start of the industrial revolution, reaching about 9.6 GtC/year in 2010. GtC denotes gigatons of carbon, meaning billion tons in American English and milliard tons in British English (to avoid confusion, we use gigatons of carbon, which is the same in both languages). Here, GtC refers to carbon only and does not include the weight of the oxygen in CO_2. Natural processes gradually remove CO₂ from the atmosphere. For example, CO₂ is used by plants and it dissolves in oceans. In 2010, the net removal of anthropogenic CO₂ by natural processes was about half of the anthropogenic CO₂ emissions. Over time, the amount of anthropogenic CO₂ in the atmosphere has increased from about 20 GtC in 1900 to about 240 GtC in 2010.

Assume that one president governs the entire world. She wants to stabilize the amount of CO₂ in the atmosphere, at the level achieved by 2020. She has asked you to give advice on how world emissions of CO₂ must develop to reach that goal. To simplify the task, give your answer in terms of one number for yearly global emissions for the period 2020 to 2050 in GtC/year.

The task is shown in the below interactive learning environment (ILE). If the text or the illustrations are too small, you may open the ILEs in separate windows. If so, click new window.