Until recently, efforts throughout North America to increase the habitat of the monarch butterfly by planting milkweed, had been successful in increasing their population. However, something changed in 2019, and the monarch population started decreasing once again.

Chip Taylor, the director of Monarch Watch and professor emeritus of ecology and evolutionary biology at the University of Kansas, has been trying to determine what factor was responsible for the recent monarch population decline. The answer appears to lie in migration patterns and how these patterns are impacted by climate change. North American monarchs winter in Mexico. When the weather warms, these monarchs migrate to the southern United States. After laying their eggs, the parent generation dies, and the next generation proceeds on the journey northward. This cycle repeats itself until the monarchs reach Canada.

The timing of the northward journey for each successive generation provides a clue to predict monarch populations. If the temperature in Texas is higher than normal in March, the monarchs arrive too soon. When the next generation hatches and moves north, it encounters cooler than normal temperatures which slows the development of the next generation. The accumulated slowdown along the northern migration was about two weeks in 2019. In addition to this delay, droughts in late summer of 2019 reduced nectar supply, further stressing the monarch population as it made its return trip to Mexico.

Since high temperatures and droughts are a result of climate change, Taylor predicts that "The consequences aren't going to be good for the monarchs. We're going to lose this migration unless we can slow down these greenhouse gases."

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