Future climate scenarios

To simulate the response to a particular climate scenario, GCMs are run with specified amounts of CO₂ and other greenhouse gases that effectively influence the radiative characteristics of the atmosphere. An RCP 8.5 scenario means that by 2100 the model will be influenced by greenhouses gases that yield 8.5 W/m² of longwave radiation back to Earth. There are currently six RCPs: 1.9, 2.6, 3.4, 4.5, 6.0 and 8.5. But how the atmosphere gets to any level of radiative forcing by the end of the century also depends on what happens over the course of the century in terms of economy, policy, population growth, etc.

Thus, future human behavior will play an important role in determining future CO₂ levels. To account for this important dimension, a set of scenarios called Shared Socioeconomic Pathways (SSPs) were developed as a climate community effort (Riahi et al. 2017) to capture the range of socioeconomic outcomes. There are five SSPs (1-5) and each differs in many ways such as the speed of technological progress, future policy decisions, population change, and the use of fossil fuels and renewables.

A completely specified climate scenario includes an SSP and an RCP component. Certain pairings of SSPs and RCPs are more appropriate than others. For example, SSP5 which represents a future heavily reliant on fossil fuels can be paired with RCP8.5, the RCP with the largest radiative forcing. The Verisk Climate Change Projections for U.S. Wildfire consist of four Tier 1 climate scenarios, namely: SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. A description of each of these four SSP-RCPs is provided in SSP-RCP scenarios upon which the Verisk U.S. Wildfire Climate Change Projections are based.

Table 1. SSP-RCP scenarios upon which the Verisk U.S. Wildfire Climate Change Projections are based
Scenario	Description
SSP1-2.6	A future focused on sustainability with emphasis on low material growth, low population growth, and lower resource and energy use (O’Neill et al., 2016). Net zero CO₂ emissions are reached by 2075. This is a low forcing pathway.
SSP2-4.5	A ‘middle-of-the-road’ pathway with historical trends continuing as usual. Annual CO₂ emissions peak between 2030 and 2040. This is a medium forcing pathway.
SSP3-7.0	A future of sustained, increasing annual emissions of CO₂ with fast population growth, little investment in global education and health, and increasing inequalities due to countries prioritizing regional security. This is categorized as a medium to high forcing pathway.
SSP5-8.5	A future heavily reliant on fossil fuel use with high material growth and population growth, but like SSP1-2.6 there is significant investment in global education and health. Annual CO₂ emissions increase and peak in 2090. This is considered a high forcing pathway.