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Climate Projection FAQ

Definition List for all Variables Used in Our Climate Projections

What are Global Climate Models?

In an effort to see what a future climate looks like on Earth, scientists use Global Climate Models (GCMs). GCMs are computer-driven models that simulate the physics and chemistry interactions between the atmosphere, land, and the oceans. To simulate the global climate by the end of the century, climate models produce information on a 3-dimensional grid covering the globe and the distance between each grid point is on average 100 km (~60 miles).

What is downscaling?

Since GCMs have one grid-point every 100 km (60 miles), the information provided is too broad to pick up details at the local and regional scales. Downscaling is used to translate the global “big picture” changes from GCMs to local and regional scales, usually with grid points from 4-50 km (~2-30 miles). For the projections shown on this site, the grid points are spaced 10 km (~6 miles) apart.

What are Climate Projections? An ensemble mean?

Climate projections come from the GCMs and downscaled output. They give us information on potential future changes in temperature, precipitation, and other variables. It’s important to note that one downscaling method, GCM, or scenario provides us with a single projection of the future climate. To get a better idea of how the climate could be by the end of the century, scientists use multiple downscaling methods, GCMs, and scenarios. Using multiple GCMs and downscaling techniques can create what scientists describe as an ensemble mean, which is an average of all the climate projections. Each climate projection is referred to as an ensemble member, which is an average of all the projections. While we recommend examining the range of changes shown by the projections, the ensemble mean does provide a simple way to quickly examine the projected changes for a given variable.

What should users keep in mind when exploring climate projections?

Climate modeling and downscaling are powerful tools for impact assessments and decision making, but it’s important to understand the limitations of these projections:

1 – Climate projections are based in part on assumptions about future human emissions of greenhouse gases.

2 – Though some gaps in scientific understanding still remain, scientists put their best understanding of the climate system into these models.

3 – The location and timing of future weather events (such as floods or droughts) cannot be determined from climate projections. However, the projections can be used to assess the changing climatology that influence events such as the average number of days over 95ºF and under 32ºF.

Why are there separate future periods of 30 years?

Just like the present, weather events in the future will continue to change daily, monthly and yearly. This is why scientists require a longer time period to measure the average weather, or climate, that’s used in GCMs to produce climate projections of future climate conditions. For our climate projections, scientists use 30 year time periods to average all the weather events and create a climatology that represents what the future climate could be.

What are RCPs and what do the different numbers mean? Which RCP Should I use?

The Representative Concentration Pathways (RCPs) represent different storylines about the amount of heat-trapping greenhouse gasses in the atmosphere through 2100. In the Fifth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC), the authors defined four scenarios to serve as a consistent set of conditions to use with climate models and downscaling to create climate projections. The four scenarios are RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5. The greater the number, the more heat-trapping gasses in the atmosphere. As for which one to use, RCPs are not considered to be forecasts, nor are they considered definite pathways. While they represent plausible scenarios for the future, none of the RCPs are intended to be used more than the others.

Why are there so many projections to work with?

An ensemble mean can be a helpful way to get a sense of the projected change, but for decision making, vulnerability assessments, and impact assessments, the South Central CASC strongly recommends considering the range of potential changes from the climate projections to account for these sources of uncertainty. Therefore, several reasons exist to account for these sources of uncertainty and why multiple climate projections are created:

1 – The use of multiple RCPs help to guide a range of plausible options for individual and societal actions.

2 – While each GCM has the same fundamental physical equations, some processes are still being actively researched. With each GCM run, the information from each RCP are provided to downscaling.

3 – The best methods for downscaling are still actively being researched by climate modelers. As a result, multiple downscaling methods can be applied with the output from GCMs.