Emission scenarios are possible pathways that society might take in the the emission of greenhouse gases in the future. It is hard to predict what will occur in society in the next 5 years, let alone to predict the next 100. Society could continue to pollute heavily, or it could switch to minimal emissions in harnessing its energy, or anything in between.
By making assumptions on how society will develop, such as how the world population will grow and how world economies interact, it is possible to estimate the world's future emissions and their effects on the climate. This is known as the Storyline Approach.
Another method is to simply assume emissions at some point in the future, and then see what steps society might need to take to get there. This approach is known as the Representative Concentration Pathways (RCP), and will be discussed alongside the Storyline Approach.
The determining factors for emission scenarios are known as driving forces. The main forces that "drive" society's future emissions are:
Each of the driving forces affect how society emits pollutants, specifically greenhouse gases. If the population grows drastically, GHG emissions are expected to increase with it. If technology develops such that electrical generation becomes more efficient, then GHG emissions may be able to decrease. There are many combinations of these driving forces that can occur, thus leading to various scenarios.
Visit driving forces for emission scenarios for more information.
Below is a summary of the two emission scenario methods taken by the IPCC in past reports. The video at the bottom of the page discusses each in more detail, along with images and results from the different approaches. Effects of the scenarios on the climate are discussed here.
The scenarios developed by the IPCC in the Special Report on Emissions Scenarios (SRES) are a result of four different storylines, titled A1, A2, B1, and B2. Each storyline assumes a distinctly different direction for future developments of driving forces. From these assumptions, each storyline differs in increasingly irreversible ways resulting in drastically different end scenarios.
The four scenarios are as follows:
Representative Concentration Pathways were adopted in the IPCC's 5th assessment report. As mentioned before, RCPs assume end points of emissions giving a clear pathway for emissions to take in the future. How these endpoints are reached is another story, and could depend on a wide variety of driving force combinations. The year that is defined up to is 2100, chosen to give a wide range of possible outcomes.
Four pathways are set, defined by endpoint values for radiative forcing by the year 2100. The lowest emission scenario peaks in emissions by 2050, declining afterwards, therefore called RCP3PD (Peak/Decline). The highest emission scenario on the other hand is called RCP8.5, as it reaches 8.5 W/m2 by the year 2100.
In the video below Professor of Instruction Dr. Harris explains the scenarios resulting from these storylines and representative concentration pathways. Implications of each are discussed along with them, giving a nice summary of the two methods.