Climate Change Is Disrupting Nature’s Calendar. Thoreau’s Notebooks Are Helping Show Us How.

, Deputy director, Climate & Energy | May 10, 2013, 10:10 am EDT
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Spring is not what it used to be. The seasonal cycles that generations of naturalists, including Henry David Thoreau and Aldo Leopold, documented so meticulously in their field notes are being thrown badly off kilter by climate change.

Of course, spring is still the time to relax in your yard listening to warblers sing and breathing in the gorgeous perfume of lilacs. But the timing is off – increasingly so – and this is likely to be bad news for many species.

Lilac. Photo: Adam Markham

Lilac. Photo: Adam Markham

Just like us, Thoreau smelled the “vivacious lilac…unfolding its sweet-scented flowers each spring” when he lived in Concord, Massachusetts from the 1840s, but he undoubtedly smelled it later in the year than we do now. Cornell biology professor David Wolfe’s analysis of long-term records of flowering times of lilacs in the Northeast shows that they now bloom an average of 4 to 8 days earlier than they did 50 years ago. The Arnold Arboretum has permanently moved its famous Lilac Sunday two weeks forward so as not to miss the blooms.

Thoreau’s pioneering analysis

In recent years there has been a surge of new scientific studies analyzing the links between climate change and alterations in the timing of natural phenomena such as leaf-out, flowering, insect emergence, and egg-laying by birds.

The scientists who conduct these studies are known as phenologists and one such is Richard Primack at Boston University (BU). Primack’s team used their own field surveys, Thoreau’s original records of his annual field observations, and those of other local naturalists to demonstrate that by the time of the record hot spring of 2012, flowers in Concord were flowering up to 20 days earlier than they did in Thoreau’s day. In fact, in 2012, Concord’s high bush blueberries flowered a remarkable 6 weeks earlier than Thoreau ever would have seen.

Primack’s work forms the core of a fascinating new exhibition that has just opened at the Concord Museum and will run until September 15, 2013: Early Spring, Henry Thoreau and Climate Change. It’s truly a thrill see some of Thoreau’s original seasonal charts and field notes displayed there on loan from the Morgan Library.

The BU team also worked with colleagues from the University of Wisconsin, the Aldo Leopold Foundation, and Harvard to extend their analysis to data Leopold collected on flowers in southern Wisconsin. Lead author Elizabeth Ellwood says “our data shows that plants keep shifting their flowering times ever earlier as the climate continues to warm.”

Many signs of early spring

But climate-driven changes in phenology are by no means restricted to plants. According to the USA National Phenology Network there are many examples of change: Native bees are emerging 10 days earlier in the Northeast, numerous butterfly species are flying earlier in California’s central valley, American pipits are laying their eggs earlier in the mountains of Wyoming, and yellow-bellied marmots are emerging earlier from hibernation in Colorado. Meanwhile, northern flickers are arriving earlier in their breeding grounds in the Pacific Northwest and yellow-rumped warblers have shifted the timing of their migration to Minnesota northern woods.

Many migratory birds are arriving in their breeding areas sooner in Europe too. One of Britain’s most famous amateur naturalists, Gilbert White, assiduously noted in his diary when the first swallows were seen in Selborne, the village where he lived. On April 29, 1793 he recorded “I have seen no hirundo yet myself” — meaning he’d seen no swallows or martins. These days, March 28, a full month earlier, is a much more likely date to see the first swallows in the south of England, and a recent study showed that one third of the bird species tracked by a British Trust for Ornithology study now nest over a week earlier than they did in the 1970s.

Pied flycatcher. Photo: Andy Morffew.

Pied flycatcher. Photo: Andy Morffew.

Seasonal cycles disrupted

The problem for many species may turn out to be that variable seasonal responses to climate change will cause mismatches in ecological synchronicity. For example, many birds have evolved their annual spring migrations to arrive in their breeding grounds at the best time to take advantage of peak food supply.

Scientists have found that in the northern parts of the broad-tailed hummingbird’s range in the interior West, food plants are flowering earlier but birds have not changed their migration in response. If this trend continues, the hummingbirds will eventually arrive after the flowers have begun to bloom and miss part of the vital annual nectar flow. In Europe, Pied flycatchers nesting in some Dutch woodlands are now laying eggs after the peak emergence of the caterpillars they feed on, resulting in poor breeding success and a major population decline.

So go ahead and enjoy the spring as it unfolds in all its glory — for as Thoreau said, “We need the tonic of wildness” — but stay alert. Global warming is already driving noticeable changes in the natural cycles in the fields and woods around us, and the harbingers of bigger changes to come are the flowers, insects, and birds in our own backyards.

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  • And Aldo Leopold’s daughter, the late Nina Leopold-Bradley, continued her father’s meticulous work of phenology. We had the pleasure to interview her for our documentary on Leopold (“Green Fire: Aldo Leopold and a Land Ethic for our Time”).
    During our work on the documentary, we also learned of a school in Ohio that had their students observe nature on a regular basis from “sit spots.” (New Albany-Plain Local Schools) And now, my 5 and 7 year olds have their own “sit spots” in our modest, urban backyard. My hopes (and worries) are great for these next generations to come.

    • Adam Markham

      Alana, so nice to hear that your family is stepping into Aldo Leopold’s shoes and that the next generation is involved already.