Specifying the scenarios

You can choose the time period to be visualised by setting the Start year for the visualisation and the number of Years which it should cover

There are two types of analysis you can visualise: component or collection.

The collection analysis visualises only one scenario. The visualisation compares the emissions from all the sources in the group, but does not show the total emissions.

An emitter group could for example represent a set of design options for a construction project, with each source representing the whole of life carbon emissions from a single option, or it could represent the whole world, with separate sources for different countries or groups of countries.

There are several ways in which you can use a component analysis, including:

You can choose the number of simulations that the visualisation should perform. The higher the number of simulations, the longer it will take to run and analyse the results, but the more robust the results are likely to be.

It is sometimes useful to use only a small number of simulations (say 100) as you first set things up and experiment with your inputs, and then when you are confident that you have got it right use a larger number (say 1000) to produce the final results.

The component analysis treats the sources in an emitter group as components of a single overall emitter. The visualisation shows either the total emissions from all sources, or the emissions from a single source, as specified on the results page. With a component analysis you can compare the emissions from the emitter group in two scenarios.

An emitter group could for example represent a whole of life carbon assessment for a building, with each source representing a specific construction stage, maintenance operation or usage element, or it could represent the whole world, with separate sources for different countries or groups of countries.

There are several ways in which you can use a component analysis, including:

You need to specify five types of information for each scenario:

A carbon pricing consists of a set of yearly carbon prices.

The ACC Carbon Tool includes several predefined carbon pricings available to all users which you cannot overwrite, delete, or change. You can also add your own carbon pricings, which will be available only to you.

You can see the definition of any carbon pricing by selecting it on the Carbon pricings page. The Carbon pricings help page describes carbon pricings in more detail.

An emitter group consists of a set of carbon sources which emit carbon (or other greenhouse gases) into the atmosphere each year. Absorption from the atmosphere is represented as a negative emission.

The ACC Carbon Tool includes several predefined emitter groups available to all users which you cannot overwrite, delete, or change. You can also add your own emitter groups, which will be available only to you.

You can see the definition of any emitter group by selecting it on the Emitter groups page. The Emitter groups help page describes emitter groups in more detail.

The discount rate is used to calculate the present value of carbon costs. It should be consistent with type of pricing you are using — if your prices are nominal, the discount rate should include inflation, for example.

You can choose to run one scenario with either a component or or collection analysis, or two scenarios with a component analysis. Comparing the results from two scenarios can be useful in several ways:

There are six NGFS scenarios, which are described in some detail on the NFGS Scenarios Portal. They are:

The ACC Carbon Tool visualises the emissions in Mt CO2-equiv/yr, Mt CH4/yr or Mt CO/yr, depending on which emissions type you choose. The NGFS Scenario Explorer gives values for these emissions calculated by three different models: GCAM5.3_NGFS, MESSAGEix-GLOBIOM 1.1, and REMIND-MAgPIE 2.1-4.2. The ACC Carbon tool uses the results from the three models to come up with an estimated probability distribution of emissions.

Each of the three models produces estimated emissions for different future years. The ACC Carbon Tool uses only those results for years for which all three models produce results, and performs linear interpolation for the intervening years. Typically, this means we use estimates at five yearly intervals until 2050 and at ten yearly intervals thereafter.

The data for the NGFS scenarios is taken from the NGFS Scenario Explorer hosted by IIASA. We are grateful to IIASA and the NGFS for making such an extensive data source publicly available.

You need to specify six pieces of information for your scenario:

The NGFS base scenario, emissions type, and probability distribution are used to construct the emitter group.

The NGFS base scenario

is used to construct the carbon pricing.

There are many, many output variables available for the NGFS scenarios. The ACC Carbon Tool focuses on emissions, and provides visualisations of those listed below. Note that their units of measurement differ, so comparing absolute values is not necessarily informative, though comparing overall patterns can provide interesting insights.

The ACC Carbon Tool visualises the emissions in Mt CO2-equiv/yr, Mt CH4/yr or Mt CO/yr, depending on which emissions type you choose.

The NGFS Scenario Explorer gives values for carbon prices by region and for the whole world. The ACC Carbon Tool uses the whole world values, whose units are US$2010/t CO2. The tool simply multiplies the emissions by the price to calculate the cost. Note that the pricing is not fully consistent with the units used for the emissions data, which are Mt C02/yr, Mt CO2-equiv/yr, Mt CH4/yr or Mt CO/yr, depending on which emissions type you choose. This makes comparisons of costs between different types of emissions invalid in some cases.

The NGFS Scenario Explorer gives values for carbon prices calculated by three different models: GCAM5.3_NGFS, MESSAGEix-GLOBIOM 1.1, and REMIND-MAgPIE 2.1-4.2. The ACC Carbon tool uses the results from the three models to come up with an estimated probability distribution of carbon prices. We assume a normal distribution, using the mean and standard deviation of the three model results. The distribution is truncated below at zero — we assume that carbon prices can never be negative.

Each of the three models produces estimated prices for different years. The ACC Carbon Tool uses only those results for years for which all three models produce results, and performs linear interpolation for the intervening years. Typically, this means we use estimates at five yearly intervals until 2050 and at ten yearly intervals thereafter.

The emitter groups represent the current estimates of future emissions from specific sources. One of the features of the ACC Carbon Tool is its ability to analyse the impacts of overall emissions changes through changes in the probability distributions — shifting the emission levels and distribution spreads. For example, emissions changes can be used to analyse the trends resulting from the overall decarbonisation of the economy or the electricity supply, a short term shock, resulting from a disorderly transition to a low carbon economy, or a boost to allow for omissions in the source emissions, such as emissions from demolition.

There are three types of change, each of which apply to the level or spread or both.

Change type	Effect on level and spread
Trend	A constant increase or decrease at the specified rates
Shock	A total increase or decrease of the specified proportions, phased over the specified years
Boost	A temporary increase or decrease of the specified proportions, occurring in the specified years

You can specify up to three emissions changes in each scenario by checking the relevant Apply this change checkboxes. The three changes may of the same or different types, and may occur over overlapping or disjoint periods as specified by the start and stop years. If more than one change occurs in a particular year, they are applied cumulatively.

Emissions changes in the distributions start to occur, and stop occurring, in the specified calendar years.

The changes in the level of the emissions distributions that occur over the specified period, as a number between -1 and +1. For example, 0.5 represents a 50% increase, and -0.1 a 10% decrease.

The level for each type of distribution is determined according to the following table.

Distribution	Level
none	Amount (deterministic)
uniform	Median — the average of the minimum and maximum
normal	Mean
lognormal	Mean
triangle	Mode
trapezoidal	Average of the two bending points

The changes in the spread of the emissions distributions that occur over the specified period, as a number between -1 and +1. For example, 0.5 represents a 50% increase, and -0.1 a 10% decrease.

The spread for each type of distribution is determined according to the following table.

Distribution	Spread
none	None (deterministic)
uniform	Range — the difference between the maximum and minimum
normal	Standard deviation
lognormal	Standard deviation
triangle	Range — the difference between the maximum and minimum
trapezoidal	Range — the difference between the maximum and minimum