What if your city's "normal" temperature is already gone? A decade of California climate data says it is.

CMIP6 · CESM2 SSP585 Climate Projections · 2015–2025

A Decade of Heat:
California's New Normal

Ten years of model data reveal a state permanently shifted toward a warmer baseline — and what that means for 39 million people.

Gabriel Borjas-Perez · Jared Parra · Dylan Craver · Christian Ramirez — UCSD

California has always been a land of extremes — droughts, fires, floods. Its climate swings are part of its identity. Scientists have tracked these patterns for over a century.

Using the CESM2 global climate model under the high-emissions SSP5-8.5 scenario, we mapped yearly surface temperatures across California from 2015 through 2025. The model provides gridded data at roughly 1.25° resolution — enough to see regional differences between the coast, the Central Valley, and the Sierra Nevada.

But something has shifted. The data no longer look like natural variation around a stable average. They look like a staircase — each warm year becoming the new floor.

When we set 2015 as a baseline — itself already a record-warm drought year — the subsequent decade shows a pattern hard to dismiss as noise. Inland regions warm faster than the coast. The hottest years cluster at the end of the decade. The consequences are already visible: record wildfires, historic heat domes, and a water system strained beyond its design limits.

+1.3°C
Peak Avg Anomaly
Average warming across all CA grid points peaked above +1.3°C in 2021 vs. the 2015 baseline.
4.2M
Acres Burned, 2020
California's record wildfire season coincided with one of the largest negative anomalies — extreme dryness locked into the land.
7 of 10
Years Above Baseline
Seven of the ten years in the dataset recorded a positive average temperature anomaly vs. 2015.

🗺️ Interactive Map

Where Is California Warming?

Each square cell represents the average temperature for an area of 85 x 85 miles. Red = hotter regions  ·  Yellow = warm  ·  Light = cooler. Colors are fixed across all years (9°C–26°C) so you can compare directly. Drag the slider, hover for exact temps.

2015
2015
Baseline Year
Our Starting Point
2015 was already California's warmest year on record at the time, in the middle of a severe drought. All subsequent years are measured against this already-elevated baseline.
2016
Heat Event
El Niño Fades, Heat Stays
The strong 2015–16 El Niño brought some winter rain but record warmth. Inland valleys began showing the first consistent warming signals above the 2015 baseline.
2017
Water
A Wet Year — Warming Masked
Above-average rain briefly cooled the anomaly. But inland California kept logging higher temperatures, straining agriculture and deepening groundwater deficits built up over years of drought.
2018
Wildfire
Camp Fire Destroys Paradise
November 2018. The Camp Fire kills 85 people and erases an entire town. Northern California shows a strong cooling anomaly — but extreme dryness, not average temperature, drove the disaster.
2019
Drought
Anomalies Push Positive
Southern California and the Central Valley record warming anomalies approaching +1°C above 2015. The alternating wet–dry whiplash pattern that climate models predict begins accelerating.
2020
Record Fire Season
4.2 Million Acres
California shatters its wildfire record. Five of the six largest fires in state history occur this year. The cooling anomaly reflects extreme dryness consuming moisture — heat locked into the landscape.
2021
Extreme Heat
Death Valley, 130°F
Death Valley hits 130°F — one of the highest temperatures ever reliably recorded on Earth. A June heat dome kills hundreds across the West. The map shows the strongest inland warming of the entire decade.
2022
Drought
Lake Mead Hits Record Low
Three consecutive dry years drain lakes Mead and Powell to historic lows. The Colorado River, which supplies Southern California, enters a water-sharing crisis. Inland temps remain well above baseline.
2023
Atmospheric Rivers
Flood After Fire
A succession of atmospheric rivers brings record flooding. Burn scars become mudslide zones. The extreme swing from drought to deluge — exactly what models project — causes billions in damage.
2024
Sustained Warming
No Return to Normal
The interior continues above baseline. Coastal moderation offers diminishing relief as Pacific sea surface temperatures also rise. The warm anomaly is no longer a spike — it is the baseline shifting upward.
2025
Present Day
A Decade Later
Ten years of data show that California's 2015 "record" warmth is now an ordinary year. The gap between the old baseline and current conditions continues to widen — and the trajectory points only one direction.
Data: CESM2 SSP5-8.5 yearly climate projections, 2015–2025. Each grid cell (~1.25°) shows the temperature anomaly in °C vs. the 2015 baseline. Red = warmer; Blue = cooler. Hover any point for exact coordinates and anomaly value. Scroll to zoom, drag to pan.

📈 Ten-Year Trend

The Decade at a Glance

Average absolute temperature across all California grid points, 2015–2025. The slider above moves the marker on this chart — see exactly where each year sits. Dot color matches the map scale: darker orange/red = hotter years. Key events annotated.

Note: Average computed across all ~110 grid points covering California's lat/lon bounding box. Inland areas run hotter than coastal cells — the line reflects their combined average. Dot colors use the same YlOrRd scale as the map.

🌊 Therefore — Why It Matters

Three Ways This Reshapes California

Temperature is not just a number. These shifts compound across every system Californians depend on.

🔥
Wildfire Risk
Hotter, drier conditions extend fire season to nearly year-round. The eight largest California wildfires in recorded history have all occurred since 2017 — the same period shown in this dataset.
💧
Water Security
California's water system was engineered around snowpack as a natural reservoir. As temperatures rise, precipitation runs off as rain immediately rather than storing as snow — threatening farms that feed a third of the nation.
🏥
Public Health
Heat waves kill more Californians than any other weather event. The burden falls hardest on outdoor workers, the elderly, and low-income communities. Longer hot seasons also extend wildfire smoke exposure linked to heart disease and preterm birth.

☀️ The One Thing to Remember

2015 Is No Longer "Hot" — It's Average

7 of 10
years warmer than 2015

In 2015, California broke its all-time temperature record. That year is now our baseline — the coolest point in our decade of data. Seven of the ten subsequent years exceeded it.

This visualization succeeds at showing something a table of numbers cannot: the geography of warming. Inland California bakes while the coast lags behind. The Central Valley — where most of California's food is grown — shows the strongest and most consistent positive anomalies. That spatial pattern matters as much as the trend itself.

📋 Project Notes

About This Project

We built this visualization using CESM2 SSP5-8.5 climate model output, with 2015 as a baseline year. Temperature anomalies are computed by subtracting each grid cell's 2015 temperature from the corresponding value in each subsequent year. The map uses a D3 Mercator projection hand-tuned to fill the viewport with California, and the trend chart computes the mean anomaly across all ~110 grid cells per year.

We chose California because it offers a regionally compelling story: a state large enough to span multiple climate zones — coast, mountain, desert, farmland — all responding differently to the same warming signal. The side-by-side map and trend chart allow the viewer to connect spatial pattern with temporal trend simultaneously.

Future extensions could use higher-resolution regional data, extend projections beyond 2025, and overlay socioeconomic vulnerability data to show which communities face the greatest compounded risk.