Climate Projections: Choose your own adventure

Bio: Senne Van Loon got his PhD in physics at the University of Antwerp in 2020. Currently, he is a climate scientist at Colorado State University investigating the flows of energy through the Earth system.

The future is uncertain. This is a simple and timeless fact. Yet, for some reason, we humans continue trying to predict the future. Here at Seasoned Chaos, we often discuss weather and climate predictions – be that forecasting tornado activity the following week or creating the seasonal outlook for U.S. temperature. Scientists around the world are working on advanced warning of weather risks, as earlier, more accurate forecasts allow people to allocate resources, plan evacuations, and save infrastructure and lives. These are challenging feats as chaos in the atmosphere, errors in forecast models, and complexities in the Earth system all pose challenges for getting that perfect weather forecast on your phone. But beyond the weekly, seasonal, and even interannual timescales, can we predict 10, 20, or even 100 years into the future?

Why run climate projections?

You may be thinking, if we struggle with predicting next season’s weather, why even venture into predicting future climates? It turns out it’s quite difficult to predict decades into the future. Fret not! This is where climate projections come in. A climate projection is a model simulation that considers evolving greenhouse gas emissions in addition to the physical evolution of the Earth system. Widespread evidence^a,b points to the link between greenhouse gas emissions and rising temperatures around the globe. Given this information, questions emerge such as how exactly will the climate system evolve? How do we know what actions to take to prepare for a changing climate?

We realize these questions can feel a bit daunting. Instead, imagine you’re standing at the top of a slide ride at an amusement park. You need to choose which slide to go down, but you can’t tell the difference between them (see graphic below). Which one should you pick when you don’t know what twists and turns each could entail? Now, imagine that you do have an idea of what the outcome of each slide could be. You could choose the slide on the left and have a pleasant ride down ending in an oasis with trees and sunshine. Or you could choose the slide on the right, with many turns ending in fires and storms. Your knowledge of the slide’s trajectory and the final outcome would (probably) impact your current actions based on your understanding of the consequences. In other words, if you could see where the slide ended up, you’d hopefully make some different choices at the beginning!


_{Very dramatic visualization of the choices we as a society have to make, where the slides represent the climate projection scenarios and the bottom of the slide represents aspects of the future that may await us.}

Climate projections help us to make choices today that will influence our future. The model simulations can include a range of carbon emissions based on human activity, natural processes such as volcanoes, and even some climate intervention strategies. Climate projections use various complex global circulation models to produce a range of outcomes that include temperature, precipitation, sea level, and other variables. An important distinction here is projection vs. prediction. Climate predictions refer to relatively short-scale forecasting (weeks to years) of a specific variable or phenomenon (weekly atmospheric pressure system/jet stream waviness, seasonal El Niño likelihood). Put simply, predictions tell us what the Earth will look like, projections tell us what the Earth could look like (footnote 1).

Projections are used by policymakers, urban planners, emergency risk managers, and researchers (to name a few) to guide future policy, decisions, and planning by understanding what the world could look like. Decision-making using projections boils down to two key factors: mitigation and adaptation. Mitigation refers to anthropogenic intervention to reduce the sources or enhance the sinks of greenhouse gases. Adaptation refers to adjustment in natural or human systems in response to actual or expected climatic events, which moderates harm or seizes beneficial opportunities.

How are climate projections made?

The scenario-based modeling for climate projections provides crucial information for anticipating the potential consequences of our collective action. Often, climate projections are discussed in the context of the Intergovernmental Panel on Climate Change (IPCC) report. Standardized climate projections gained widespread attention in 1992, when the IPCC released a “pathbreaking” set of emission scenarios that were incorporated into global circulation models to produce so-called climate change scenarios. With the ever-growing need for more information of plausible future scenarios, the “Representative Concentration Pathways (RCPs)” were developed. These RCPs are possibilities of future emissions, such as carbon dioxide, CO₂, or aerosols. Different scenarios get assigned a number based on the approximate excess energy (radiative forcing) in the Earth system in 2100 (in Watts per square meter, e.g. 2.6 W/m²). These values are indicative of how much warming will occur at the end of the century. However, RCPs don’t take into account the two-way interaction between societal changes and evolving greenhouse gas emissions.

Thus, came the development of the Shared Socioeconomic Pathways (SSPs). These were developed to include a socioeconomic narrative for depicting how the future could evolve, accounting for population and technology changes that scale with economic growth. They include 5 families: a world of sustainability-focused growth and equality (SSP1); a middle-of-the-road world where economic trends broadly follow their historical patterns (SSP2); a competitive world with national security concerns restricting global cooperation (SSP3); a world of ever-increasing inequality with moderate economic growth (SSP4); and a world of rapid and unconstrained growth in economic output and energy use (SSP5) (see figure below).


_{Source: Climate Data Canada}

To combine socio-economic, geopolitical, and emissions information, a global coordinated effort consisting of approximately 25 different institutions run their own global climate models to simulate how the Earth will warm under different future scenarios.

A breakdown:

• Scenarios: Integrated Assessment Models (IAMs) take each SSP narrative and calculate what emissions pathway would be needed to achieve a specific excess energy (radiative forcing) target (from the RCPs) by 2100 — for example, determining what mitigation policies would get SSP1 to the 2.6 W/m² climate policy goal.
• Inputs: The IAMs produce emissions and land-use information that are then used in the climate model simulations, creating combined scenarios like SSP1-2.6 or SSP5-8.5.
• Running simulations: The various modeling centers run these scenarios to produce the full range of projections that we see in the figure below. These projections range from the most optimistic (SSP1-1.9, global CO₂ emissions are cut to net zero by ~2050) to the avoid-at-all-costs scenario (SSP5-8.5, very high greenhouse gas emissions by 2100).
• Capturing unknowns: The spread in projected temperature accounts for both differences between models and variations inherent to the climate system. While no one model is perfect, running many model simulations gives an educated guess for a plausible range that we expect the truth (whatever that may be) to lie within.

_{Adapted from IPCC AR6: The terminology SSPx-y is used, where ‘SSPx’ refers to the Shared Socio-economic Pathway or ‘SSP’ describing the socio-economic trends underlying the scenario, and ‘y’ refers to the approximate level of radiative forcing (in watts per square metre, or W m–2) resulting from the scenario in the year 2100. Color shading shows range for SSP1-2.6 and SSP3-7.0. Source: UCAR SciEd}

Choosing our slide of fate

We can use these scenarios to ask questions like “what happens after the planet warms 1.5°C?” versus “what will the climate look like in 2100?”. The IPCC often uses temperature thresholds, or “global warming levels”, as a framing of climate change impacts (footnote 2). The idea is that crossing the 1.5°C threshold has similar impacts independent of when it actually happens. We might cross 1.5°C next year, in 20 years, or in 50 years, but the results of a 1.5°C-warmed planet will be the same: more intense heat waves and floods, loss of biodiversity, food and water shortages, and so on.

Understanding impacts to our environment and society in these different scenarios is an active area of research. One such example is recent work that dives into how the climate could evolve after crossing various temperature thresholds and how this could impact heat stress for cattle in America’s breadbasket. It is important to realize that even in the SSP1-1.9 scenario – aggressive mitigation with dramatic policy changes and reduction in greenhouse gases – the temperature still increases for 20-30 years due to inertia in the climate system. In other words, we as a planet have already committed ourselves to about 1°C of warming above pre-Industrial Revolution levels based on our collective actions over the past few decades.

The actions we choose today truly decide our slide of fate. Which of the 3 slides in the graphic will we take? The pleasant slide ending in environmental sustainability and justice? The yellow twisty slide ending in consumerism and pollution? The purple slide that falls somewhere in between? By knowing the extent of damaging impacts from increased greenhouse gas emissions, it encourages and necessitates mitigating these emissions – reducing greenhouse gases immediately. It also helps us adapt and prepare for future scenarios, including new temperature and precipitation normals or sea level rise. Will we invest in resilient infrastructure that can withstand more intense climate hazards? Will we slash global greenhouse gas emissions to reduce those hazards? What will our world become in 2050, 2070, 2100? The answer is up to us.

Footnotes:

1. Let’s bring this back to our favorite butterfly- our Lorenz butterfly for understanding chaos. Weather and climate forecasting focuses on predicting where on the butterfly’s wings we will be at some time in the future. Climate projections on the other hand estimate, or show potential outcomes, of how the butterfly’s wings themselves will reshape in the future. In more technical terms, the distinction between them is an initial condition vs. boundary condition problem. I spy another Seasoned Chaos post to slide more into this in the future!
   
2. The Special Report on 1.5 °C highlighted the major environmental and societal risks associated with crossing both the 1.5 °C and 2°C thresholds, including impacts on biodiversity, water and food availability, and security risks. The impacts are assessed independent of the specific warming scenario, not on when the temperature threshold is crossed. It replaces the dimension of time with the dimension of temperature.
   
   References:
   a. IPCC 6th Assessment Report, The Physical Science Basis, 2021. Climate Change in Data. https://www.ipcc.ch/report/ar6/wg1/resources/climate-change-in-data/
   b. NASA,The Causes of Climate Change. https://science.nasa.gov/climate-change/causes/