Storylines of Regional and Seasonal Climate Change in the United States in the 21st Century

Forecasts of regional climate change, particularly those linked to changes in the
atmospheric circulation, have considerable uncertainty. Large ensembles of simulations with
individual climate models have revealed that the mean of model ensemble members (commonly
used to communicate climate change impacts) is a poor measure of the many plausible pathways
of climate variability in the 21st century. Additionally, the 21st century projections from different
climate models vary greatly due to differences in their climate sensitivities, patterns of sea
surface temperature changes, and Arctic sea ice loss, among other factors. Given these
uncertainties in regional climate projections, a “storyline” approach may be a more accurate
method to convey future climate change to policy makers and other stakeholders. The storyline
approach suggests that climate change impacts in a particular region should be viewed as a set of
“storylines” (i.e., hot and dry vs. warm and wet), each of which are plausibly possible given the
range of climate model projections for the 21st century. While more models or model ensemble
members may suggest one storyline over another, this does not rule out the viability of
alternative storylines conveyed by a smaller number of models or model ensemble members.

The proposed project applies this “storyline” approach to 21st century climate projections
from global climate models from phase 6 of the Coupled Model Intercomparison Project
(CMIP6) for each National Climate Assessment (NCA) region of the United States. For each
NCA region for all four seasons, 21st century trends in societally relevant climate variables (such
as means and extremes in temperature, precipitation, and wind speed) will be decomposed (via
multiple linear regression) into components associated with the warming global-mean surface
temperature and four indices of atmospheric circulation change. The analysis will be repeated in
CMIP5 models to assess the robustness of the results across model generations. A group of
meaningful storylines of combined temperature and circulation changes will then be constructed
to illustrate low impact and high impact scenarios of 21st century climate change in each NCA
region. For example, models that indicate the greatest global-mean surface temperature warming
and La Niña-like sea surface temperature changes over the 21st century might project the most
severe drought impacts in the southwestern United States. Conveying the possibility of multiple
scenarios will be an important way for stakeholders to prepare for a range of impacts from
climate change due to uncertain atmospheric circulation changes in CMIP models.

One of the objectives of this competition is to capture the “combined effect of a variety of
changes in the Earth’s climate system and also adequately characterizing associated
uncertainties.” The storyline approach of the proposed project combines multiple climate factors
(temperature and various atmospheric circulation changes) into single storylines that are
plausibly possible for the 21st century United States given the latest climate model projections.
Our findings will help further the mission of the NOAA MAPP program “to enhance the
Nation’s capability to predict variability and change in Earth’s climate system.” Specifically, the
project targets aspects of both Priority Areas A and B of the proposal competition. For Priority
Area A, the proposal will address the combined effects of temperature and atmospheric
circulation changes on each of the NCA regions. For Priority Area B, the proposal will identify
the range of uncertainty in societally relevant climate variables in each region and quantify the
component of that uncertainty associated with global temperature warming and various
atmospheric circulation changes unrelated to the global-mean surface temperature.