Ocean waters off California’s coast are warming, particularly in Southern California. An unprecedented marine heatwave affected the West Coast of the United States from 2014 to 2016.

Oceans have absorbed about 90 percent of the Earth’s increased heat energy over the last 50 years. Globally, ocean waters have warmed to depths of 3,000 meters over the past several decades. In recent years, prolonged periods of unusually high ocean temperatures, known as marine heatwaves, have occurred across the globe, creating devastating effects on the marine ecosystem. For more information, download the Coastal Ocean Temperature chapter.

The graph shows annual average sea temperatures in degrees Fahrenheit (°F) at Trinidad Bay (1973-2020), Pacific Grove (1919-2020), and La Jolla (1916-2020) shore stations. These locations are shown on the map below. 

The three locations studied are shown in the map below. 

• Long-term measurements taken at shore stations along the California coast show warming sea surface temperatures. Warming has occurred fastest at La Jolla, especially in the last 50 years. From 1971 to 2020, sea surface temperatures at La Jolla have risen at a rate about two to four times faster than at the two other stations.

• Satellite-based data show that waters offshore have largely warmed. Along the continental shelf, however, some locations have warmed while others have cooled. Cooler temperatures reflect the influence of a seasonal wind-driven process called upwelling.

• A marine heat wave that occurred from 2014 to 2016, known as “the Blob”, was the largest and most intense recorded off California since 1980.

• Warming ocean temperatures directly impact marine ecosystems. The 2014-2016 marine heat wave clearly illustrated the devastating effects of high temperatures. Impacts observed included a large number of deaths among marine mammals and sea birds; the loss of kelp forests; changes in the distribution of commercially important fish and many other marine animals (such as sea slugs and certain zooplankton); and a prolonged toxic algal bloom that poisoned marine mammals and led to the closure of the Dungeness crab fishery.

• Warmer ocean temperatures contribute to global sea level rise; warming water not only expands but also accelerates the melting of land-based ice sheets.

• Surface ocean water temperature affects weather. Warmer waters play a role in extreme weather events by increasing the energy and moisture of the atmosphere.

An alarming rise in whale entanglements with crab fishing gear coincided with the 2014-2016 marine heat wave. This record increase was linked to a combination of factors, including changes in prey abundance and distribution (along a narrow band closer to shore) and the delayed crab season. Photo shows wrapped line and buoy around a humpback whale.

Credit: NOAA Fisheries, MMHSRP Permit #18786-06

• Warming ocean temperatures are a direct result of climate change. As the oceans absorb increased heat energy from the atmosphere, sea water temperatures rise.

• Ocean currents, winds, climate modes like El Niño, and other oceanic and atmospheric phenomena can affect ocean water temperature.

• Near-surface ocean water temperatures along the California coast are influenced by seasonal upwelling, a wind-driven process that brings deep, colder, nutrient-rich waters to the surface. Considerably less effective upwelling occurs along the Southern California coast, where waters have warmed faster.

• National Oceanic and Atmospheric Administration (NOAA), Coastal Water Temperature Guide and The California Current Marine Heatwave Tracker - Blobtracker

• UC San Diego, Scripps Institution of Oceanography, Shore Stations Program