The Impact of Climate Change on the Western Monarch Butterfly | Cal Parks
Published: November 17, 2023

By Emily Doyle, PhD | Program Manager, Climate Resilience 

The monarch butterfly is one of North America’s most recognizable and charismatic butterflies, known for its vibrant orange and black wings and extraordinary annual migration. Every year, western monarch butterflies (the subspecies found west of the Rocky Mountains) travel thousands of miles — from Baja California to the Canadian border and back again — to spend the winter at overwintering sites along the California coast. Many of these sites are found in California state parks, including Natural Bridges State Beach, Pismo State Beach, and Lighthouse Field State Beach.  

Millions of western monarchs used to return to California every year, but the population has plummeted in recent decades, and fewer than 2,000 butterflies were counted in California in 2020. Numerous factors have contributed to this decline, including habitat loss, pesticides, and disease. Fortunately, the western monarch population has rebounded in the past couple of years, with almost 250,000 butterflies counted in 2021–2022. However, this gain is increasingly threatened by climate change, including prolonged drought and intensifying storms as shown by the unprecedented winter storms this past year. As the single largest land manager of overwintering groves for western monarchs, California State Parks can play a key role in protecting this iconic species. 

Lifecycle and migration of the western monarch butterfly

Western monarchs’ life cycle is explicitly linked to their annual migration. In the spring and summer, monarchs continue to breed as they fly north, following warmer temperatures and plentiful milkweed plants to lay their eggs. As fall approaches, changes in day length, temperature, and a decline in milkweed quality prompt the last generation of monarchs to return south to coastal overwintering groves on the California coast. These locations provide the specific microclimate that monarchs need to survive the winter, including protection from the cold and wind, dappled sunlight, and a suitable water source. The overwintering monarchs enter a suspended developmental state called diapause, during which activities are limited to sunning, feeding on nectar, and rehydrating. This period allows monarchs to conserve the necessary energy to survive the winter before dispersing in the late winter or early spring to begin their northward migration. 

How is climate change affecting western monarchs? 

Climate change poses an increasing threat to western monarch butterflies and is impacting habitat health, resource availability, and even migration patterns. Droughts are becoming longer, more severe, and more frequent, reducing water availability and threatening habitat. Rising temperatures also decrease the amount of fog and dew at the coast, which are significant water sources for overwintering monarchs.  

Worsening droughts also impact the health and structure of overwintering groves, which are critical for western monarchs’ survival. Overwintering sites typically include roost trees surrounded by a larger grove of trees and shrubs that protect clustering monarchs from the wind. Many of these sites are already deteriorating because of age, human development, and other factors that alter the microclimate of the grove and make it less suitable for monarchs. Climate change is now exacerbating this crisis. Droughts are increasing tree die-off, limb loss, and susceptibility to disease, potentially killing important roosting trees or critical windbreaks. This also creates more fuel for a potential fire — a particularly frightening consequence because rising temperatures will increase the risk of a winter fire that could wipe out large populations of overwintering monarchs. 

Warming temperatures also impact the range, productivity, and phenology — when flowers bloom every year — of the nectar plants and milkweed that monarchs depend on. As temperatures continue to rise, nectar plants may become less abundant and bloom earlier in the season. This could potentially lead to insufficient resources for overwintering monarchs, particularly later in the season. As winters get warmer, western monarchs will also spend less time in diapause and have a greater need for nectar sources, further exacerbating this potential nectar shortage. Milkweed on migration routes could become less abundant and shift further north as temperatures rise, leading to longer migration distances, and potentially increasing monarch mortality during migration. 

Severe 2022 winter storms forecast greater vulnerability for monarchs

The winter storms of 2022–2023 caused extreme damage to coastal California, including an estimated $200,000 of damage to state parks. From late-December to mid-January, California was hit by nine back-to-back atmospheric rivers that dumped 32 trillion gallons of water across the state in a three-week period — almost equal to the volume of Lake Tahoe. The National Oceanic and Atmospheric Administration describes atmospheric rivers as “naturally occurring air currents” that can create extreme flooding and precipitation events. Last winter, these atmospheric rivers were accompanied by a bomb cyclone to create an unprecedented winter storm season. 

These winter storms had a significant impact on the western monarchs clustered in overwintering groves up and down the coast. Every year, the Western Monarch Count records how many monarchs are overwintering in California around Thanksgiving and the New Year. In a typical year, the number of monarchs decreases 35–49% over this period due to weather, predators, and other factors. The 2022–2023 counts, however, saw a 58% drop in the monarch population from around 275,000 to less than 117,000. Furthermore, the New Year’s count ran from December 24, 2022 to January 8, 2023, so it does not reflect monarchs that died in the continuing storms that winter. The 2023 Thanksgiving count is currently underway and may show a fuller picture.  

The 2022–2023 winter storms were only a glimpse of what western monarchs will face in the future. Climate change is increasing the chances of heavy precipitation events and will make winter storms more intense as temperatures rise, and the atmosphere can hold more moisture. The Fourth National Climate Assessment also concluded that atmospheric rivers on the West Coast will become more frequent and more severe. Protecting western monarchs from extreme weather events is critical to ensure the species’ long-term survival. 

Protecting monarchs

Restoring and preserving overwintering groves is critical to monarch conservation. Healthy, well-managed groves are more resilient to climate change and can better protect monarchs from extreme weather. Restoration work can include planting trees as windbreaks and planning how to mitigate flooding. Overwintering grove management plans can also be developed to lay out an adaptive strategy to restore and preserve these critical sites.

California State Parks Foundation has supported the development of grove management plans at several state parks, including Natural Bridges State Beach, Lighthouse Field State Beach, and San Simeon State Park. We also work with partners like Groundswell Coastal Ecology to host volunteer events where participants plant native nectar plants, remove invasive plants, and improve signage to educate the public about western monarch butterflies. By working together, we can help protect this beloved species for future generations. 

Join us this #GivingTuesday to help protect the western monarch, critical overwintering habitat in California state parks, and all wildlife by giving to our parks today!

All monarch population data sourced from the Western Monarch Count