3. Understand Exposure

3.1 Introduction to Exposure

Most frameworks include a step to understand how exposure to climate hazards may change over time. At a very basic level, this exposure assessment helps airports understand:

• The extent to which the intensity and frequency of extreme temperatures, flooding, and storms may increase.
• How precipitation patterns may change—such as more rain falling during summer months rather than winter months.
• The extent of sea level rise projections and associated storm surge.
• Changes in other weather and climate patterns that could affect successful operation of airports.

This section describes how to define climate hazards, where to obtain data, how to select future emissions scenarios, how to choose climate models, and how to select appropriate timeframes.

3.2 Defining Climate Hazards

Exposure assessments usually consider changes in temperature, precipitation, riverine flooding, sea level rise, and other related hazards. Before beginning an assessment, your airport should consider how those hazards are defined. It is typically more useful to consider shorter events such as the number of days above 95 degrees Fahrenheit each year rather than, for example, average annual temperature. These shorter events are usually associated with impacts more so than annual averages.

At the start of the exposure assessment, reach out to individuals responsible for designing, operating, maintaining, and repairing infrastructure, as well as individuals involved in staffing and operations. They will likely have insights into the types of heat, precipitation, storm, and flooding events that are problematic, and for which your airport may wish to understand the projected increase in intensity or frequency.

3.3 Locating Exposure Data

Several resources are available to help you understand your airport’s exposure to future climate hazards. Appendix C includes a list of climate data resources that airports can use. Your state may also have more localized projections available. Airports can reach out to local climatologists to assess what resources are available before defaulting to the resources provided in Appendix C.

Using these or similar resources, airports can estimate how the hazards of concern may change in the future. This information, paired with the trends highlighted in the National Climate Assessment for your region, provides the basis for estimating how the climate will change in your region.

When obtaining datasets, you may need to make decisions about future emissions scenarios, climate models, and timeframes—which are discussed in more detail below.

Some extreme weather events are difficult to model, such as hurricanes and ice storms. Airports can look at a combination of historical events and climate projections to develop scenarios that portray potential future high-impact weather events. These event scenarios may help identify where a broader set of potential resilience investments may be needed.

3.4 Selecting Future Emissions Scenarios

Airports can select future emissions scenarios from climate models, which reflect different assumptions about how quickly society will reduce GHG emissions. Climate models are complex numerical models that estimate future climate based on a range of human and natural systems, each with their own assumptions and uncertainties.

The most up-to-date emissions scenarios are the Shared Socio-economic Pathways (SSPs) from the World Climate Research Programme’s Coupled Model Intercomparison Project (CMIP) Phase 6 dataset. The SSPs are a series of projections representing a range of future climate change scenarios and development pathways that encompass different trajectories of global GHG emissions.

Some frameworks may reference Relative Concentration Pathways (RCPs), which are from the earlier CMIP Phase 5. RCPs explore the effects of different emissions trajectories, but do not have standardized socioeconomic characteristics like those that define the SSPs. Although not as up-to-date, RCPs are still acceptable to use in vulnerability assessments.

The current best practice is to use the moderate and high emissions scenarios, SSP 2-4.5 and SSP 5-8.5 (or the previous RCP 4.5 and RCP 8.5) to provide lower and upper bounds for the range of possible future GHG emissions. SSP 2-4.5 represents a world that follows a path in which social, economic, and technological trends do not shift significantly from historical patterns. SSP 5-8.5 represents a world with intensified fossil-fuel development and is the upper boundary of the range of SSP scenarios.

Projections across scenarios should not be averaged but should be viewed as distinct future scenarios.

Capturing a range of emissions scenarios is important for addressing the underlying uncertainty associated with climate models, which arises from the complexity of the climate system and the abilities of models to represent it, as well as the inability to predict the decisions that society will make that impact GHG emissions. While uncertainty may indicate that the degree of change is unknown, the direction of change is clear and there is ample information to make informed decisions.

Although airports may consider more than two emissions scenarios, it may not be useful to do so. More information is not necessarily better, and can make communication about results more difficult. Moreover, when given information about an odd number of scenarios, people tend to focus on the middle option. Therefore, this primer recommends using just two scenarios to represent a reasonable upper and lower bound.

3.5 Choosing Climate Models

A variety of climate models can be used to make projections of future climate change under each scenario. Airports can reach out to climate science experts for recommendations on a targeted set of climate models to use. If no expert guidance is available, you can consider the full range of climate models available for each scenario, weighted equally.

3.6 Selecting Timeframes

Common planning timeframes are mid-century (2050) and late-century (2080). Future timeframes are sometimes referred to as 2050 or 2080; however, this is not a completely accurate description of the timeframe. These timeframes actually usually represent the 20- or 30-year period in which 2050 and 2080 are the midpoints. For example, mid-century may actually refer to an average of the model outputs across each year between 2035 and 2064. Because there are natural variations in weather patterns across years, it is important to look at a multidecadal average (rather than a single year), to understand how climate is changing over time.

It is common to look at two timeframes when looking at exposure assessments, but they have different applicability to different planning decisions. Late-century planning timeframes can be useful if you have long-lived assets and want to plan for potential impacts in the far future. The projected changes in climate often are more significant farther out in time, so late century can also be useful in communicating how much climate could potentially change within the conceivable lifetime of some people living today.

Alternatively, projections used for mid-century planning timeframes may not always have changes as dramatic as those in late-century projections, but they are usually more relevant to present-day decision making.