Projections of future climate for U.S. national assessments: past, present, future

Abstract

Climate assessments consolidate our understanding of possible future climate conditions as represented by climate projections, which are largely based on the output of global climate models. Over the past 30 years, the scientific insights gained from climate projections have been refined through model structural improvements, emerging constraints on climate feedbacks, and increased computational efficiency. Within the same period, the process of assessing and evaluating information from climate projections has become more defined and targeted to inform users. As the size and audience of climate assessments has expanded, the framing, relevancy, and accessibility of projections has become increasingly important. This paper reviews the use of climate projections in national climate assessments (NCA) while highlighting challenges and opportunities that have been identified over time. Reflections and lessons learned address the continuous process to understand the broadening assessment audience and evolving user needs. Insights for future NCA development include (1) identifying benchmarks and standards for evaluating downscaled datasets, (2) expanding efforts to gather research gaps and user needs to inform how climate projections are presented in the assessment (3) providing practitioner guidance on the use, interpretation, and reporting of climate projections and uncertainty to better inform decision-making.

Keywords: Assessment; Climate change; Projections; Scenarios.

Fig. 1

The inherent mismatch between IPCC and NCA cycles is depicted by darker-colored bars representing the years in which different IPCC (yellow) and NCA (green) assessment report cycles used the scenarios listed along the y-axis as their primary source for physical climate projections (e.g., greenhouse gas emissions and atmospheric concentrations; temperature, precipitation, sea-level rise, etc.). Lighter-colored bars denote where a subset of IPCC volumes or NCA chapters were still basing findings on sources that rely on scenarios from the prior CMIP cycle. Thus, the lighter bars reflect the fact that the broader scientific literature base, particularly regarding climate impacts and risks, is slower to incorporate shifts in CMIPs across assessment cycles. For example, the IPCC Second Assessment Report (SAR) primarily used IS92 scenarios while Working Group I (WGI) and WGIII of the Third Assessment Report (TAR) and Fourth Assessment Report (AR4) used SRES scenarios; however, WGII of TAR and AR4 also assessed literature that used some of the IS92 scenarios. Similarly, while new SSP-RCP scenarios from CMIP6 were available during the development of NCA5, most of the downstream impacts literature (e.g., climate impacts on ecosystems, health, economics, etc.) utilized the RCP scenarios from CMIP5. Adapted from Fig. 2 in Pedersen et al.

Fig. 2

Figure 2.14 in Marvel et al. (https://nca2023.globalchange.gov/chapter/2#fig-2-14). When (or if) the world reaches 2 °C of global warming depends on future greenhouse gas emissions and the sensitivity of the climate system to those emissions. Shown are the IPCC AR6 assessed warming projections for four future scenarios, with projected years at which the 2 °C (3.6°F) GWL would be reached. For example, under a very high scenario (SSP5-8.5), models project reaching 2 °C between 2033 and 2054, with an average estimate of 2042. Under a low scenario (SSP1-2.6), the 5% CI (confidence interval) range begins in 2041, but the average projection shows that warming would actually stay below 2 °C. Figure credit: Project Drawdown, Stripe Inc., NOAA NCEI, and CISESS NC