Walden Warming Climate Change Comes to Thoreau's Woods
by Richard B. Primack
University of Chicago Press, 2014
Cloth: 978-0-226-68268-6 | Paper: 978-0-226-27229-0 | Electronic: 978-0-226-06221-1
DOI: 10.7208/chicago/9780226062211.001.0001


In his meticulous notes on the natural history of Concord, Massachusetts, Henry David Thoreau records the first open flowers of highbush blueberry on May 11, 1853. If he were to look for the first blueberry flowers in Concord today, mid-May would be too late. In the 160 years since Thoreau’s writings, warming temperatures have pushed blueberry flowering three weeks earlier, and in 2012, following a winter and spring of record-breaking warmth, blueberries began flowering on April 1—six weeks earlier than in Thoreau’s time. The climate around Thoreau’s beloved Walden Pond is changing, with visible ecological consequences.
In Walden Warming, Richard B. Primack uses Thoreau and Walden, icons of the conservation movement, to track the effects of a warming climate on Concord’s plants and animals. Under the attentive eyes of Primack, the notes that Thoreau made years ago are transformed from charming observations into scientific data sets. Primack finds that many wildflower species that Thoreau observed—including familiar groups such as irises, asters, and lilies—have declined in abundance or have disappeared from Concord. Primack also describes how warming temperatures have altered other aspects of Thoreau’s Concord, from the dates when ice departs from Walden Pond in late winter, to the arrival of birds in the spring, to the populations of fish, salamanders, and butterflies that live in the woodlands, river meadows, and ponds. 
Primack demonstrates that climate change is already here, and it is affecting not just Walden Pond but many other places in Concord and the surrounding region. Although we need to continue pressuring our political leaders to take action, Primack urges us each to heed the advice Thoreau offers in Walden: to “live simply and wisely.” In the process, we can each minimize our own contributions to our warming climate.


Richard B. Primack is professor of biology at Boston University. He is the author of Essentials of Conservation Biology and A Primer of Conservation Biology and coauthor of Tropical Rain Forests: An Ecological and Biogeographical Comparison. He lives in Newton, Massachusetts.


“Thoreau, in Walden, proposed a ‘realometer’ to filter out prejudice and delusion. This eloquent new book fills that role for us, reminding us that global warming is not an abstract future proposition but a very profound current reality.”
— Bill McKibben, author of Oil and Honey: The Making of an Unlikely Activist

“This is an important book that should be required reading for everyone who cares about the future of our planet, and especially for those who remain skeptical about the threats of climate change. What better place to chronicle the effects of global warming than in the cradle of the American environmental movement—Thoreau’s Walden Woods.”
— Don Henley

“Primack’s elegant and eloquent scientific memoir shows how today’s science is advancing thanks to Henry Thoreau’s mid-nineteenth-century observations as recorded in his journal and in his almost completely unknown because unpublished charts containing years and years worth of data on first flowering, bird arrival times, and much else happening in Concord’s natural world. Primack’s book is important in three ways: it is a report on what global warming has already done to a much-loved bit of American space—Walden Pond; it is a detailed warning about what we are now facing; and it is a stirring call to arms, especially to young Americans and students about how they can help. Emerson told Thoreau to keep a journal. Primack is urging people, especially young people, to keep Thoreauvian journals, not for personal reasons, but to advance our knowledge of what happens and when in the natural world we all share. This book is a grand gift, a bracing and appealing take on a difficult and complex problem. I wish I had read it when I was nineteen.”
— Robert J. Richardson Jr., author of Henry Thoreau: A Life of the Mind

“Determined to help the public understand global warming, Primack decided to search for evidence of climate change in Concord, Massachusetts, home of Walden Pond, made famous by pioneering environmentalist Henry David Thoreau. Primack’s plan was to compare field notes of the past with new information about the same plants in the same places, but he despaired of finding reliable old records until he learned about Thoreau’s unpublished, little-known tables precisely documenting the annual flowering dates for more than 300 plant species. Primack also struck gold in the form of invaluable nature journals kept by modern-day citizen scientists. He now tells the deeply instructive story of the challenges he and his dedicated graduate students faced during the past decade as they identified the many plants that have disappeared since Thoreau’s time and those which “are flowering earlier in successive years” as spring temperatures rise. Primack shares striking tales from the field and elucidates from an unnervingly close-to-home perspective the dynamics and impact of climate change on plants, birds, and myriad other species, including us.”
— Donna Seaman, Booklist

“Primack’s book brings the issue of climate change down to earth in a focused approach without hard science; recommended to students of environmental studies as well as to general readers active in the study of the subject.”
— Susan E. Brazer, Library Journal

“Each chapter of this book documents alarming change: the flowering of the pink lady’s slipper orchid has begun three weeks earlier; wild apple blossoms have advanced by two to four weeks; wood sorrel by six weeks. . . . [Walden Warming] show[s] compellingly how a place and its ecosystems can alter dramatically in the face of climate change.”
— Jules Pretty, University of Essex, Times Higher Education Supplement

“The book tells the story of Primack’s struggle to replicate Thoreau and find changes in flowering times, but soon broadens into a hymn to citizen science. Primack finds many others who are not conventional scientists but keep careful records of myriad things, from the times that migratory birds arrive to the date butterflies emerge and ice melts on ponds. It is these extraordinary people who make the book a rich, rewarding read. And there is also the inspiring message that anyone with a keen eye for nature can make a difference, with an afterword on how to become a citizen scientist.”
— Alun Anderson, New Scientist

"This book is more than a clarion testament to the real and present effects of climate change. It is an exhortation to become more engaged in the natural world whether through citizen science or observation, and, in so doing, recognize and limit our own impacts on the earth. A constant presence throughout this book, Thoreau would be pleased to read this volume, which weaves together science, nature, ethics, and human action as part of a single whole."
— Science

"Primack’s story is worth telling, and Primack is a worthy storyteller. . . . Primack clearly demonstrates the value of several non-traditional forms of historical observations for documenting change, including personal journals, butterfly club observations, and fishing lodge records. Perhaps most importantly, Primack clearly demonstrates that our environment is changing rapidly, and this is undoubtedly due to anthropogenic climate change."
— Theresa M. Crimmins, University of Arizona, Ecology



DOI: 10.7208/chicago/9780226062211.003.0001
[climate change, warming climate, Malaysia, Walden Pond, ice-out, flowering times]
The book begins with the author's decision to leave his tropical rainforest studies in Malaysian Borneo and return to the United States to focus more closely on climate change research and find examples closer to home. This chapter in particular emphasizes the dual global-personal nature of climate change and the need for a call to action in the U.S; professional colleagues, for example, were skeptical of the viability of the author's research on gradual warming in Eastern Massachusetts for the reasons that it was a controversial career change for a tropical biologist and would likely earn a cold reception from governmental funding organizations such as the National Science Foundation. This introduction details the early stages of research and the discovery of Henry David Thoreau's plant notes on flowering times and other records as sources of historic climate change data. The chapter finishes with a description of changing ice-out dates, and Walden Pond in particular. Ice-out times at Walden are now earlier than they were in Thoreau's time due to a warming climate. (pages 1 - 15)

DOI: 10.7208/chicago/9780226062211.003.0002
[climate change, flooding, droughts, hurricanes, heat waves, rain, nor'easter, realometer]
After lamenting the politically-charged nature of large-scale discussion on climate change--and offering a few examples from France, Russia, and the United States of catastrophic weather events such as heat waves, droughts, and hurricanes that revealed it as a sobering reality--this chapter examines the effect of climate change on weather patterns and phenomena such as New England nor'easters. The storm that dropped ten inches of rain on the Boston area in mid-March of 2010 is discussed in detail through an anecdote on extreme flooding in the author's neighborhood. The human-created paving and landscaping projects that disrupted runoff and contributed to the downpour's damaging effects are highlighted, as are possible solutions for preventing damage to homes and settlements in the face of increased rainfall due to climate change. The author incorporates Thoreau's ideological invention of an imaginary instrument he calls the "realometer," for penetrating the clamor of "opinion, prejudice and tradition…delusion, and appearance" to measure the truth of human perceptions; such an instrument is needed today, the chapter argues, to break through the lack of understanding and narrow-mindedness surrounding public views on the very real phenomenon of climate change. (pages 16 - 27)

DOI: 10.7208/chicago/9780226062211.003.0003
[flowering times, seasons, field notes, handwriting, rare species, locally extinct, calendar]
This chapter introduces Thoreau as a naturalist and meticulous recorder of seasonal data, not only on flowering plants and the weather but also on birds and every species he could find and track in Concord. Thoreau's lifetime goal of a calendar of the seasons was never completed, but he left behind extensive field notes that forge a connection between his insights on human nature and his scientific observations; the author makes a case for Thoreau as a climate change scientist born ahead of his time. Thoreau's notes on flowering times formed the basis for the author's current study of the same Concord species and their changes through time. Poor handwriting and all, the research team was largely able to compare Thoreau's observations with modern data from the same spots in Eastern Massachusetts. Some species were problematic to decipher in the original records or find in accessible spots, and various research setbacks (such as the difficulty of finding rare specimens in time for first flowering) are discussed; the chapter ends with the somber discovery that, in the intervening hundred and fifty years, many of Concord's plant species have severely declined or are locally extinct in Concord, perhaps due to global warming. (pages 28 - 38)

DOI: 10.7208/chicago/9780226062211.003.0004
[Extinction, rare plants, ovate-leaved violet, pink lady's slipper orchid, Alfred Hosmer, Flora, urban heat island effect, food web]
In this chapter, the author elaborates on the difficulties encountered in finding many of Thoreau's flowering species; one quarter of the species that he observed have in fact vanished from the local landscape, and a further third are rare native plants found in only one or two areas and hovering close to extinction. Individual anecdotes feature plants such as the ovate-leaved violet (which nearly got the author arrested for trespassing on a set of train tracks) and the pink lady slipper orchid. The chapter introduces botanist Alfred Hosmer as a further source of historical plant data and concludes that many species have been flowering incrementally earlier from Thoreau's time to Hosmer's to the present day. Observations from the Blue Hills Meteorological Observatory are included as evidence of the warming climate of Massachusetts, due to both global warming and the urban heat island effect. Finally, the chapter considers the possible dire effects of a warming climate on seasonal food webs and species relationships, such as those between insects which feed on flowering plants and the migrating birds which need to arrive at the right time to nest and feed on spring insects. (pages 39 - 57)

DOI: 10.7208/chicago/9780226062211.003.0005
[missing species, species loss, warming temperatures, habitat destruction, orchid, mountain mint, leafing out, temperature tracking]
Continuing along the line of Concord's missing plants, this chapter aims to explain why so many species observable in Thoreau's and Hosmer's time, and even up to a few decades ago, have now disappeared. Orchids, many aquatic plants, and mountain mints in particular have been severely depleted since the 1850s, visibly due to increasing habitat destruction, succession, pollution, and loss of wetlands. However, the author presents climate change as another prominent factor in species disappearance. One convincing explanation is that the plant species remaining in large numbers from Thoreau's time represent those that are able to withstand great changes in climate; plant families such as orchids and wild mints do not have the flexibility to track seasonal temperatures and adjust their flowering times, and so they are unable to survive in an altered climate. The chapter asserts that plants that are flexible enough to survive probably remain because they are able to adjust their leaf-out times to accommodate for earlier warming temperatures. Certain local botanists are doubtful that so many species from Thoreau's time have completely disappeared, but evidence suggests that more plants have vanished or at least severely declined from Concord in the last forty years than ever before. (pages 58 - 77)

DOI: 10.7208/chicago/9780226062211.003.0006
[invasive species, purple loosestrife, McGrath Farm, species loss, Minot Pratt, assisted colonization, weeds, plant introductions]
The chapter begins with the author's experiences of growing the invasive ornamental plant purple loosestrife, and observing it almost take over his backyard. Part of the success of such invasive species is their ability to flower early in warm years and later in cool years, which might be linked to their ability to grow faster than native species. As a result of invasive plant species, habitat destruction and other threats, native plants in Concord have been declining over the past 150 years, and the percent of non-native species has been increasing. Many rare species in Concord survive in unusual habitats, such as the McGrath Farm, where the farmer seldom weeds and does not use herbicides. It is difficult to restore wildflower populations once they are lost, as shown by the experiences of Minot Pratt in the 19th century; he planted numerous wildflowers in Concord, but almost none persist today. The best strategy to conserve biodiversity and to maintain wildflowers may be to try introducing plant species from further south using a procedure known as assisted colonization. This must be done carefully to avoid having these new species harm species already present, but may be necessary to protect biodiversity from climate change and invasive species. (pages 78 - 94)

DOI: 10.7208/chicago/9780226062211.003.0007
[Birding, birds, Concord, William Brewster, Ludlow Griscom, arrival dates, migration patterns, declining populations]
This chapter describes the extensive tradition of observing birds in Concord, and how these records can be used to detect the impact of climate change on bird migratory patterns. First spring arrival records from the 1850s come directly from Thoreau himself and can be compared with other birding records from Concord, including turn-of-the-century observations of William Brewster, records from the 1930s by Ludlow Griscom, and the 1956-2007 observations of Concord resident Rosita Corey. Results from this large body of evidence were equivocal: while three species--the yellow-rumped warbler, the yellow warbler, and the Baltimore oriole--now arrive in Concord earlier than they did in Thoreau's time, the majority of the 22 species studied have not changed their migration patterns. While migrating birds tend to arrive in Concord earlier in warmer years, the bird population as a whole has not significantly moved up its migration period within the last hundred and fifty years. Part of the problem in interpreting the bird data from Concord is that many bird species have been declining in abundance over the past 160 years, which also affects the date of first arrival in the spring. (pages 95 - 114)

DOI: 10.7208/chicago/9780226062211.003.0008
[Birds, spring arrivals, mist-netting, banding, Manomet, El Niño/ Southern Oscillation, Korean swallows, bird conservation]
The mist-net data gathered by bird banders at the Manomet Center for Conservation Science is used to provide accurate measures of the effects of climate change on bird migration times. The author describes the process of recording information on individual birds during the banding process. Overall, conservation center data from the 1970s to the present shows that eight out of thirty-two birds studied were arriving, on average, earlier in the spring than they had in the past; later first arrival times due to declining populations coupled with earlier first arrival dates attributed to climate change muddled data until researchers examined the average arrival times instead. An in-depth discussion of what drives migration times follows: namely spring temperature and annual climate patterns such as the El Niño/Southern Oscillation Event. The chapter also examines the potential application of later arrival dates to estimate the degree of population decline, using South Korean swallows as an example. The journals of Betty Anderson from her farm in Middleborough demonstrate how one person can provide important information on plants, amphibians, and butterflies. The chapter finishes with a call for increased awareness of the preservation and protection of birds and bird habitat and brings Thoreau's stance on conservation back to the forefront. (pages 115 - 132)

DOI: 10.7208/chicago/9780226062211.003.0009
[Honeybees, solitary bees, butterflies, Massachusetts Butterfly Club, changing distribution, flight times, elfins, hairstreaks]
This chapter describes the author's efforts to locate data on the response of insects to climate change. Even though honeybees are widely used in Massachusetts to produce honey and to pollinate crops, the author was not able to locate any beekeeper that had recorded the start of honeybee flights in the spring. Similarly, solitary bees are locally common in Concord, including in Sleepy Hollow Cemetery, where Thoreau is buried, but no one has recorded when they begin their seasonal behavior. However, there is extensive data available on butterfly flight times recorded by members of the Massachusetts Butterfly Club. Studies of two groups of short-lived butterflies, elfins and hairstreaks, revealed that insects are comparable to plants in their responsiveness to temperature. This is in contrast to migratory birds, which change their behavior less readily. If the behavior of butterflies is representative of a general pattern involving more insect groups, there is a possibility that birds will be ecologically mismatched with their spring food sources. Butterflies are also changing their distribution, with more southern species increasing in abundance and more northern species declining in abundance. (pages 133 - 152)

DOI: 10.7208/chicago/9780226062211.003.0010
[aquatic insects, mayflies, warming water, fish, fishing, krill, sunfish, Cape Cod, Malaysia]
Mayflies and other aquatic insects represent an important food source for fish, birds, and other species when they emerge in great abundance from lakes, ponds, rivers and other bodies of water during certain days and nights of the spring and summer. Fishermen take advantage of these emergence times in choosing their lures to increase their chances of catching fish. No information has been found to analyze how these insect emergence times are being affected by a changing climate; however, the changing climate is likely to affect not only insects, but also the fish that depend on them. Warming water, for example, might have a negative effect on fish species that live in cold waters. Fish represent an important food source to people throughout the world, particularly in developing countries. The author describes his wife Margaret's approach to fishing in ponds in New Hampshire and in the ocean off Cape Cod. The goal of Margaret's fishing, which she learned growing up in rural Malaysia, is always to bring something home to eat. This willingness to catch and eat smaller fish and even the tiny marine crustaceans that constitute krill is an approach that might be a more sustainable approach for providing food for the world's people. (pages 153 - 164)

DOI: 10.7208/chicago/9780226062211.003.0011
[Mosquitoes, blood, Eastern Equine Encephalitis, horses, disease, Massachusetts State Laboratory, insecticide, birds]
Thoreau admired mosquitoes for their persistence, even though he was often bothered by their attacks when camping. We now know that female mosquitoes that bite people to obtain a blood meal transmit many human diseases, such as malaria, yellow fever, and dengue fever. In Massachusetts, mosquito eggs and larva overwinter in bodies of water, and adults emerge in the spring after the ice melts. Mosquitoes have been intensively studied by the Massachusetts State Laboratory, due to their role in transmitting a dangerous viral disease called Eastern Equine Encephalitis (EEE). The virus persists in bird populations and has outbreaks in people and horses every few years in southeastern Massachusetts where there are large swamps. The State Laboratory collects mosquitoes in special traps over the course of the spring and summer to determine their abundance and the presence of the virus. In years when the insects are abundant and infected with EEE, areas of southeastern Massachusetts are sprayed with insecticide to reduce mosquito populations. This large data set has the potential to be useful in climate change research; outbreaks of EEE seem to be occurring with greater frequency in recent years, possibly due to a warming climate and increasing populations of overwintering birds. (pages 165 - 180)

DOI: 10.7208/chicago/9780226062211.003.0012
[Amphibians, vernal pools, Great Meadows, Blanding's turtle, spotted salamander, spring migration, Concord Golf Course]
Climate change will negatively impact many amphibian species, particularly those that live in vernal pools that fill with water in the spring and dry out during the summer. Similarly, the endangered Blanding's turtle will be harmed if its habitat at Great Meadows National Wildlife Refuge dries out in warmer summers. Salamanders may be vulnerable to a warming climate if they are unable to forage at night due to higher temperatures and lower humidity; spotted salamanders might be particularly affected by climate change, but they are difficult to study as they live in the ground and are rarely sighted. Spotted salamanders are only regularly noticed when they undergo their spring migration to breed in vernal pools and may be more common than previously thought. The author describes the problems faced by the spotted salamanders at the Concord Gold Course. More research is needed on climate change as it affects amphibian species in general and on its impact on the spotted salamander in particular. (pages 181 - 198)

DOI: 10.7208/chicago/9780226062211.003.0013
[Boston Marathon, running, winter sports, Bernd Heinrich, thermoregulation, wind speed, wind, heat prostration]
This chapter uses an opening anecdote of the author's experiences training for and running the 1970 Boston Marathon to introduce the topic of fluctuating temperatures during the annual city event as a possible indication of climate change. The warmer winters associated with climate change will continue to shorten the winter sport seasons in New England and across the world and cause more frequent cases of dehydration and heat prostration among athletes in the hotter summer months. The experiences of runner and University of Vermont professor Bernd Heinrich are used to describe the regulation of body temperature in both animals and humans, especially during long-distance running. The remainder of the chapter evaluates the importance of temperature, precipitation, and wind in determining record times in the Boston Marathon, concluding that race times are fastest on cold days with a tailwind. The hotter race days in Boston projected for the end of this century are predicted to result in slower winning times. (pages 199 - 211)

DOI: 10.7208/chicago/9780226062211.003.0014
[climate change wedges, environmental stewardship, climate change scenarios, simple living, biodiversity, greenhouse gases, coastal flooding, Japan, consumption, population control]
In the final chapter of the book, the author presents an image of Henry David Thoreau traveling in Concord fifty years in the future and offers two possible future scenarios depending on what path the earth's population takes for conservation of its climate and resources. In the first scenario, careless environmental stewardship has led to depleted bird, fish, and wildlife populations, coastal towns engulfed by rising sea waters, shorter winters, and dying forests. In the second, Thoreau is presented with a population that lives in smaller houses, uses public transportation and renewable energy, and has reduced its consumption of animal products. The author holds up climate change evidence from Concord as a microcosm of what is occurring over the entire planet and warns of the dangers of continued inaction on local and governmental levels. Also discussed are two systems of wedges, or partial solutions for addressing the production of carbon dioxide and other greenhouse gases; the first system accepts current patterns of economic and population growth and relies on new technology, and the other focuses on reducing consumption and population control. As an example of a personal approach to reducing consumption, the author describes living simply in Japan. A final perspective developing from Thoreau's writings emphasizes that individual action on climate change is both entirely simple and vital to the survival of both people and the natural world. (pages 212 - 228)

Afterword: Citizen Science

Appendix: Species Mentioned


Further Reading