The Climate Change in the Los Angeles Region Project

As the Intergovernmental Panel on Climate Change (IPCC) reaffirmed in its Fifth Assessment Report, global climate is changing in response to human emissions of greenhouse gases. With the help of global climate models (GCMs), computer models that simulate the climate system, scientists are able to project the future state of the climate given different scenarios of greenhouse-gas emissions. While GCMs can tell us a lot about climate change over large regions, they do not provide enough detail to help us understand likely impacts in our own backyards. That's why our team undertook a comprehensive study of climate change in the Los Angeles region, using a new technique to downscale information from multiple GCMs and produce neighborhood-by-neighborhood projections of future climate. This page summarizes the study's findings on likely changes by the middle and end of this century.

Study Methods

• We downscaled more than 30 global climate models (GCMs) to 2-km resolution using combination of dynamical and statistical techniques.
• We simulated 1981–2000 climate and produced projections for two future time periods (2041–2060 and 2081–2100), under two different greenhouse gas emissions scenarios: Business As Usual and Mitigation.
• Findings include projections we deem most likely (an average of the downscaled GCM results) and the range of possible outcomes (from the GCM showing the least change to the one showing the most change).

Temperature Findings        [skip to Precipitation Findings]

At mid-century (2041–2060):

• Most likely warming at mid-century in Business As Usual scenario is 4.3°F, averaged over the region’s land area.
• The number of days hotter than 95°F increases across the region, but to a greater extent in the interior compared with coastal areas. (See Fig. 1.)
• Temperature changes in the Mitigation scenario are about 70% of those in Business As Usual, meaning significant effects are inevitable.

Fig. 1: A spatial representation of average August temperatures in the baseline period (1981–2000, at left) shows that coastal areas tend to be cooler than inland areas separated from the coast by a mountain range. At right, the corresponding image is down for the mid-century (2041–2060) period under the business-as-usual (RCP8.5) scenario of greenhouse gas concentrations. Temperatures shown range from about 50°F (indigo) to 95°F (dark orange). Values represent the ensemble-mean, or the average of the outcomes from all the downscaled global climate models.

At end-of-century (2081–2100):

• In the Business As Usual scenario, temperatures continue to rise. By end-century, average temperatures across the land region are most likely to be 8.2°F warmer than they were in 1981–2000.
• The number of days hotter than 95°F also continues to rise in the Business As Usual scenario. (See Fig. 2.)
• In the Mitigation scenario, temperatures level off after mid-century. On average, end-century temperatures are about 3°F warmer than in 1981–2000.
• The number of days hotter than 95°F does not increase from mid-century to end-century in the Mitigation scenario.

Fig. 2: The average number of days per year exceeding 95°F in the baseline period (1981–2000) and the two future periods under the business-as-usual (RCP8.5) scenario. Values represent the ensemble-mean, or the average of the outcomes from all the downscaled global climate models.

Precipitation Findings

When we analyzed our mid-century climate projections for total precipitation (rain and snow) during the core of the Los Angeles region's wet season (December through March), we found that not much is likely to change. Some downscaled global climate model projections show a slight overall decrease in annual precipitation, others show a slight overall increase, and still others show no change at all (red dots in Figure 3). In those models that do show a change, the change is small compared with the region's natural variability in precipitation totals from year year to year (black dots in Figure 3). In the current climate, the Los Angeles region experiences some average years, some very wet years, and some much drier years. We expect that this natural year-to-year variability will continue, but that overall averages in local precipitation will remain stable.

Fig. 3: Monthly precipitation accumulations (mm) averaged over CIMIS stations (white dots), WRF–NARR grid points nearest to CIMIS stations (light gray dots), land averaged in the UDel observational dataset (medium gray dots), land averaged in the CPC observational dataset (dark gray dots), and land averaged in the WRF–NARR output (black dots). Larger dots in each case represent monthly climatologies. Also shown are monthly 2041–2060 and 2081–2100 precipitation changes (mm per wet season) relative to the baseline climate period (1981–2000) according to 36 statistically downscaled (red and blue dots, respectively) and interpolated (pink and light blue dots, respectively) CMIP5 GCMs. Larger red/blue and pink/light blue dots represent ensemble-mean monthly changes.

Frequently Asked Questions

What is the significance of these results?
Our results show that significant temperature increases are coming to the Los Angeles region by mid-century, and that they won’t affect all neighborhoods equally. Areas near the coast will experience less warming. Areas in the interior, particularly those separated from the coast by a mountain range, will experience more warming.

By comparing the Business As Usual and Mitigation scenarios, we see that at mid-century, some changes is inevitable and must be adapted to. By end-century, further change can be prevented if the world follows the Mitigation path.

What is the difference between the Business As Usual and Mitigation scenarios?
Business As Usual assumes that the level of greenhouse-gas emissions continues to increase at the same rate it has been increasing in recent years. Mitigation assumes the world comes together to reduce greenhouse-gas emissions in the coming decades. These are standardized scenarios created by the IPCC for use in climate modeling studies.

Why is the research team confident in these findings?
Different GCMs give different results, and downscaling not just one but many GCMs allowed Dr. Hall and his team to account for these differences. In addition, developing the statistical downscaling techniques employed in the study required the team to understand the physics underpinning climate change in the region, enhancing the credibility of their results.

More About the Project

The Climate Change in the Los Angeles Region project was commissioned by the City of Los Angeles in partnership with the Los Angeles Regional Collaborative for Climate Action, with support from the US Department of Energy. Additional funding was provided by the National Science Foundation and the Southwest Climate Science Center.

References

Walton D, F Sun, A Hall, and SC Capps, 2015: A hybrid dynamical–statistical downscaling technique, part I: Development and validation of the technique. Journal of Climate, 28(12): 4597–4617. DOI: 10.1175/JCLI-D-14-00196.1 download

Sun F, D Walton, and A Hall, 2015: A hybrid dynamical–statistical downscaling technique, part II: End-of-century warming projections predict a new climate state in the Los Angeles region. Journal of Climate, 28(12): 4618–4636. DOI: 10.1175/JCLI-D-14-00197.1 download

Berg N, A Hall, F Sun, SC Capps, D Walton, B Langenbrunner, and JD Neelin, 2015: 21st-century precipitation changes over the Los Angeles region. Journal of Climate, 28(2): 401–421. DOI: 10.1175/JCLI-D-14-003161.1 download

Sun F, A Hall, M Schwartz, D Walton, and N Berg, 2015: 21st-century snowfall and snowpack changes in the Southern California mountains. Journal of Climate, submitted.

Schwartz M, A Hall, and F Sun, 2015: Mean surface runoff insensitive to warming in a key Mediterranean-type climate: a case study of the Los Angeles region.Journal of Climate, submitted.

Other Projects by Our Group

For more about our group's work, visit our Research page.