During the preparation of an adaptation plan, it is likely that you will need to engage with internal and external stakeholders about the climate risks faced in your area of interest or organisation. This engagement might include undertaking risk assessments, presenting a case for undertaking adaptation planning and developing appropriate and relevant climate adaptation strategies and actions. Engagement will require you to provide information on future climate, which includes selecting, justifying and presenting scenarios based on climate models called General Circulation Models (GCMs).

CoastAdapt contains advice on Understanding climate scenarios, Accessing climate scenarios and Using climate scenarios. Given that GCMs are technical and complex and judgement is required to select and construct scenarios from model information, many adaptation planners report that they find models and scenarios difficult to explain to their stakeholders. Here we outline some tips to consider when you need to communicate climate scenarios.

Communicating about climate scenarios and models

Scenarios are commonly used by decision-makers and planners across a wide range of issues that have uncertain outcomes in the future, such as defence, architecture and healthcare, as well as climate.

Audiences for climate scenarios want to know:

• What is a climate scenario (and how does it relate to a climate model)?
• Why are there different climate scenarios?
• Why should we use more than one climate scenario?
• Given this choice, which climate scenarios should we use?
• Given that there are uncertainties in climate models, are the models reliable?

In the following, we address these five questions.

What is a climate scenario?

GCMs tell us how different climate variables interact based on our mathematical understanding of how the Earth-Atmosphere-Ocean system works. These models can include the biological, chemical and physical processes and interactions, for example, the exchange of heat and CO₂ between the ocean and atmosphere and the exchange of water vapour and CO₂ between a forest canopy and the atmosphere.

The purpose of using climate models is to build scenarios to help understand how the climate is likely to behave under future changes, especially the changes in atmospheric concentrations of greenhouse gases such as CO₂.

GCMs generate data on a three-dimensional grid of points throughout the model domain, which covers the atmosphere, the ocean and the land surface in most GCMs. These data are available for the duration of the model simulation, which is typically from the present day through to the end of the century. Typically, the GCM is ‘forced’ by increasing the concentrations of greenhouse gases in the model atmosphere throughout the simulation.

The data generated by GCMs, or the output, are used to build climate scenarios such as a representation of how temperatures will increase and how rainfall will change. Because GCMs cover the whole world, and because there are limitations imposed by the speed and power of computers, the model grid can be quite coarse: there can be 100 km or more between grid points across the Earth’s surface. People who want to look at the impacts of climate change at the regional or local scale can find this grid resolution too coarse. The GCM output can be ‘downscaled’ to provide greater spatial detail. Downscaling can be done using statistical techniques or by using a regional climate model (RCM).

Scenarios constructed from GCM and RCM output help us to consider how a natural or human ecosystem might be sensitive or vulnerable to various future changes in climate. This means you can start to consider potential adaptation options according to your priorities and risk. Some resources for explaining climate models are (links accessed 15 June 2016):

• https://theconversation.com/explainer-the-models-that-help-us-predict-climate-change-39568
• https://www.skepticalscience.com/rcp.php

Why are there different climate scenarios?

There are many different modelling groups carrying out GCM simulations of future climate changes, in all the major countries. Depending on a number of factors – including how sensitive a model is to increasing CO₂ concentrations and the observations used to start the model simulation off at the present day – different models will produce a different outcome for the same changes in atmospheric concentrations of greenhouse gases. It isn’t possible to say that one is the right answer, and one is the wrong answer; they are just different answers. For this reason, it is usually important to include output from several different GCMs when building a climate scenario.

It is important to understand that scenarios are not predictions. The model runs provide plausible futures of what might happen, not what will happen.

Why should we use more than one climate scenario?

One model is not enough for scenario planning. Because different models may produce different results, scenario planning generally considers a range of model runs to give a representative sample.

Sometime GCMs results are combined to build a single, composite, scenario, sometimes each model is used to produce an individual scenario and the results are compared.  Only looking at a composite is not a good idea - you may be suppressing important information about what some of the models are telling you about what might happen at the extremes.  Looking at a number of scenarios side-by-side will give a picture of the likely range of futures (see Using climate scenarios for a further discussion of this). 

Given this choice, which climate scenarios should we use?

In practice, you are unlikely to build your own climate scenario. There are groups that produce climate scenarios from climate model output (both GCMs and RCMs). A good place to start is the Climate Change in Australia website. This site gives extensive guidance on selecting climate models as a basis for scenarios and provides regional climate scenarios for Australia. Nevertheless, even to use these websites, you will need to balance factors such as planning purpose and planning time horizon in making decisions about your scenarios.

For example, you will need to make a choice about future greenhouse gas concentrations. These are encapsulated in the Representative Concentration Pathways (RCPs) used in GCM simulations. You must select the RCP most relevant to your concerns and be able to explain to your stakeholders the rationale underlying your choice. The RCP you select will affect the information in the scenario and hence the decisions that you make on what you need to adapt to.

The infographic What are the RCPs? may be useful. This explains the difference between the four RCPs. There is a wealth of technical information available on how and why the RCPs were developed. Useful resources include (links accessed 15 June 2016):

• http://www.skepticalscience.com/docs/RCP_Guide.pdf. This is a very detailed resource with relatively simple explanations of technical information.

• https://theconversation.com/explainer-how-scientists-know-climate-change-is-happening-51421. This  includes a short section on RCPs within a broader discussion of climate change.
• http://link.springer.com/article/10.1007/s10584-011-0148-z. This is an open access scientific article that explains the background and rationale of RCPs.

It is unlikely that many stakeholders will be interested in this level of detail but, in engaging with stakeholders, you could include some links to this information in a handout that can be read in more detail.

Another choice you will need to consider in building your climate change scenario is the time period: are you interested in the far future (i.e., towards the end of the current century) or the nearer term mid-century?

CoastAdapt provides advice on how to select scenarios for use in adaptation planning (Understanding climate scenarios). In communicating about the scenarios, it is important to be clear on the rationale for your choice.

Given that there are uncertainties in climate models, are the models reliable?

Uncertainty is inherent in climate change science, and the known unknowns are often discussed. There are three types of scientific uncertainties, and ways to deal with these are outlined in Understanding climate scenarios. Important key messages about uncertainty include the following:

• Just because there are uncertainties in climate models doesn’t mean that the science is not well understood; the uncertainties mean that we are dealing with plausible futures rather than predictions, not a lack of knowledge or understanding that climate change is occurring. Everyday life is far more uncertain, yet we still make strategic decisions about the future.
• Uncertainties in climate modelling are not a reason to delay action; we have sufficient knowledge to confirm that action is needed to respond to the risks of climate-related impacts and to map out appropriate actions.
• Because some climate processes are better simulated by climate models than others, there are different levels of uncertainty for different climate impacts. For instance, there is higher certainty in projections of sea-level rise (a large-scale process) than there is in local rainfall (a small-scale process, which is harder for GCMs to simulate).

Useful resources on uncertainty include (all links accessed 15 June 2016):

https://theconversation.com/why-climate-uncertainty-is-no-excuse-for-doing-nothing-32924  - a relatively simple explanation of scientific uncertainties.

• https://theconversation.com/lost-in-translation-confidence-and-certainty-in-climate-science-17181 - an explanation of confidence terminology.

Using climate projections as a participatory process

Selecting and using different future climate scenarios can be a useful opportunity to engage with others in your organisation or community around adaptation issues. For example, projections of different climate variables (such as temperature and rainfall) and sea level under different RCPs can underpin a participatory approach that engages a range of stakeholders to discuss regional and local impacts.

Communities can consider selection of the most appropriate RCPs and situate this within local knowledge of influencers or drivers, gaps and overlaps between scientific and local knowledge, community values and adaptation options. Participatory processes can range from simple through to elaborate approaches and typically include consideration of scale and spatial data, social and economic data, alternative scenarios for land-use options, visualising data and scenarios, or a form of assessment such as Multi-Criteria Analysis (MCA) to define different local priorities.

For more information on participatory processes, consult Information Manual 9: Community Engagement.  CoastAdapt contains case studies looking at the use of scenarios in community engagement (The SCCG experience and Sydney's adaptation strategy deliberation), and the use of MCA in engagement around adaptation (Kakadu's vulnerability and The SCCG experience).