Greenhouse gases: increases followed by (some) decreases

The International Panel on Climate Change has created six scenarios — called A1 (with three different versions), A2, B1, and B2 — to portray images of the future. These scenarios provide a glimpse of what the future climate could be like given certain conditions, but they are not predictions or forecasts. (To see more about how climate modeling works, visit the NOAA website.)

Each scenario compiles possibilities for several conditions that influence the global production of greenhouse gases to estimate future greenhouse gas emissions. The graph below, produced from data given by the IPCC, shows how yearly greenhouse gas emissions and cumulative emissions could change over this century in each scenario.

Although the primary greenhouse gas is carbon dioxide (CO₂), emissions also include methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphurhexafluoride (SF₆).

The IPCC generally focuses on CO₂ emissions, and this is reflected in the below chart that includes only carbon dioxide equivalents emissions, excluding all other greenhouse gases.

Several scenarios show a peak in emissions after several decades; this is attributed to decreasing forest area. However, all scenarios but one (A1FI) show a later decrease in emissions due to slowed population growth, changing technologies, and regional interaction.

Temperatures increase in all scenarios

Each change in yearly greenhouse gas emissions carries with it a corresponding temperature change. This emissions-temperature relationship is not a simple correlation, but instead involves the compilation of predictions of various models of how the Earth will react to increased levels of greenhouse gases.

The IPCC has synthesized 40 models in order to estimate how each change in emissions will affect the global temperature. The following table summarizes the IPCC's best estimates for temperature change given each scenarios level of greenhouse gases:

Our way of life affects the future

The IPCC created each scenario using "harmonious" elements; that is, factors are not randomly chosen but instead work together in a logical scheme.

The contributing variables of demographic change, social and economic development, and pace and direction of technological change are broken down as follows:

• Population growth — population will grow but eventually slow in most scenarios
• Affluence — gross world production will increase to 10 to 26 times today's production by 2100
• Income differences among world regions — most scenarios assume a closing in income gap
• Technolgy — how energy and land are used
• Energy structures — examines are use of fossil or alternative fuels
• Forests — changing food demands due to shifts in demographics and dietary habits will initially deplete forests, but most scenarios predict an eventual reversal of this trend

The chart in the above section shows how world population and wealth are expected to grow over the next century under each scenario, and how these factors will indirectly influence temperature change through their impact on greenhouse gas emissions.

Scenario Neutrality

The IPCC reiterates that no scenario is considered more likely than another. Instead, scenarios are meant to encompass the range of possibilities included in climate change literature.

It is important to note that "disaster", "surprise", and outlying scenarios have been excluded from this analysis. Although much of the literature does discuss a catastrophic scenario resulting in global breakdown or economic collapse, the IPCC contends that the problems caused in these situations would shift focus from climate change to the more immediate problems, rendering emissions irrelevant.

In addition, there is no "business as usual" scenario; each one assumes that the world will change in significant ways in the next century.