Emission Scenarios

The greatest source of uncertainty for anyone trying to predict how the climate will change is us. How will we behave in the future? Will we take the issue seriously and rapidly cut our emissions of greenhouse gases? Or will it be business as usual? There are a number of Emission Scenarios, which have been modelled to try and understand this. This section has been updated to include the new AR5 scenarios.

The Intergovernmental Panel on Climate Change (IPCC) made projections for our future emissions of greenhouse gases in the Special Report on Emission Scenarios (SRES). This report considers 40 scenarios, which are grouped into 4 scenario families. Each scenario needs to consider a lot of things, including:

• How will the population grow: how rapidly and when does it peak?
• What is economic growth going to be like?
• What are environmental policies going to be like: are we going to keep burning fossil fuels, or are we going to switch to renewable forms of energy?

The following figure gives an example of some of the results of the scenarios. There is one graph from each of the four families. You can see from these that there are a wide range of possible futures in these from High emissions scenarios like A1FI to low scenarios like B1.

Each scenario has a narrative like the one in the following box. A number of different models have been used calculate the amounts of each greenhouse gas would be emitted by each scenario (hence the uncertainty bands in the figure above). The full report, including data tables, can be found here.

"The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B) (where balanced is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and end-use technologies)." quoted from the SRES report

New emission scenarios for the IPCC 5th Assessment Report

The latest IPCC report uses Representative Concentration Pathways (RCPs), rather than the SRES scenarios. The Skeptical Science website has a useful introduction to them in their The Beginner's Guide to Representative Concentration Pathways.

There are four emissions pathways: RCP8.5, RCP6, RCP4.5 and RCP2.6. The numbers indicate the increase in radiative forcing (i.e. warming, [Watts per square metre]) associated with each pathway. You can see in the following how they compare to SRES. Notice that:

• RCP8.5 is similar to one of the highest emissions (A2) scenarios. Whilst RCP4.5 is close to the Low emissions (B1) scenario.
• RCP2.6 is a new, very low emissions scenario, which is designed to be likely (i.e. 67% chance) of limiting global warming to 2ºC. This pathway will require a phenomenal effort in global emissions reduction: we will need to reduce emissions by between 40% and 70% by the year 2050; by the end of the century we will need to drop greenhouse gas emissions below zero (e.g. find a way of stripping carbon dioxide out of the atmosphere)!

Notice that the RCP database is available online, with downloadable data and the ability to generate simple graphs and maps of the different pathways.