SUBSCRIBE

CONTRIBUTORS' GUIDELINES

CONTACT US

ADVERTISING

SEARCH

THIS WEEK IN
CALIFORNIA WILD

BACK ISSUES

Feeling The Heat

Liese Greensfelder

On a June day in 1996 in northwestern Nevada, Erik Beever headed his pickup off the pavement and dropped onto the dirt road leading to Summit Lake. After driving south for four hours through mountains and washes scattered with boulders and brush, he pulled over and hiked two miles cross-country to a field of rocks and small boulders that slumped across both banks of a canyon. Sixty years earlier a biologist had noted that this talus field harbored a population of pikas, pint-sized rock-dwelling mammals famed for their unique foraging skills and heart-melting cuteness. Widespread during the last Ice Age, Great Basin pikas (Ochotona princeps) have since retreated to scattered, island-like refuges higher in the mountains. In an effort to understand the factors at play in the persistence or extinction of such isolated populations, Beever was revisiting all the Great Basin sites where pikas had been discovered previously.

At the Summit Lake site, Beever, a wildlife ecologist with the U.S. Geological Survey in Corvallis, Oregon, found signs of pikas scattered all about. Sheltered beneath overhanging boulders were tell-tale haypiles of dried grasses and leafy plants that the little mammals had harvested and dried for winter fodder. Pellets of pika scat littered the ground under prominent rocks. But both hay and scat were weathered—perhaps a decade old—and Beever spent the rest of the day and much of the next searching in vain for fresh sign or live animals. Only later, after he’d analyzed all his data, did he realize that with the disappearance of these and other pikas, he had quite possibly documented one of the first known cases of a population of mammals falling victim to modern-day global warming.

In the past half-century, average annual air temperatures in the United States have risen by nearly 0.6 C (1 F), with much of the change occurring over the past two decades alone. Our climate is warmer now than at any time during the past 1,000 years. According to two internationally respected climate models, temperatures in the western United States are predicted to climb by another 3 to 6 C by the end of this century. After years of debate, scientists now almost universally agree that humans are to blame for turning up the heat.

The burning of fossil fuels and land-use changes such as forest clearing and paving over vast swaths of the Earth’s surface have combined to hike the atmospheric concentration of heat-trapping carbon dioxide (CO2) by 30 percent since the 1800s. It’s now at its highest level in at least 400,000 years. By the year 2100, atmospheric CO2 is expected to soar to two to three times the amount present in 1850. Thanks to the dogged work of scientists like Erik Beever, we know that global warming is nudging nature to alter its rhythms, uprooting species from their ancestral haunts, and pushing some plants and animals toward extinction.

Climate warming has been more extreme at higher latitudes. Residents of Alaska have seen roads subside into the landscape as permafrost thaws, and researchers headed to the North Pole have been startled to find open water where once there was sea ice. But in the western United States, most of the changes have been subtler, more like whispers foretelling what’s to come. No single report documenting change has by itself been able to finger climate as the culprit.

Yet the studies are adding up. And like migrating birds, almost all of them point in the same direction. In January, two papers published in the journal Nature consolidated the results of hundreds of biological investigations conducted by scientists worldwide.

The results were staggering.

The analyses found that changes which closely match predictions of what will happen in a warming world had already occurred among 80 percent of some 1,500 species studied. “We now have so much circumstantial evidence that the probability that all of the studies that are showing this evidence are wrong is exceedingly low. It would be like taking a coin and flipping it a bunch of times, and it keeps coming up heads,” says Terry Root, a Stanford University ecologist and co-author of one of the Nature studies. “You would assume that the coin was loaded.”

The coin is loaded. Heat is pushing the odds toward rapid change. Yet so far, at least, the changes have not been easy to document. For one thing, without meticulous records from the past, we can’t judge whether what we’re seeing today is something new or business as usual. But digging up reliable historical records is only the first step. Documenting change also requires tenacious fieldwork, scrupulous data collection and analysis, and, in many cases, just plain luck.

Luck eluded Raphael Sagarin for weeks when, in 1993, he first set out to study changes in Monterey Bay’s intertidal zone. As a graduate student in ecology at Stanford, Sagarin decided to replicate a legendary biotic census conducted between 1931 and 1933 at the Hopkins Marine Reserve, just a stone’s throw south of famed Cannery Row in Monterey. In the original survey, another graduate student, Willis Hewatt, ran a 108-yard line into the rocky zone between low and high tides at Hopkins. Then he tallied and identified every living invertebrate in 105 small plots along the transect, planting brass bolts into bedrock every so often to mark his line.

Hewatt’s first bolt, at the high-tide line, was still visible. But it took Sagarin weeks of searching—using a metal detector at low tide and paddling out in rented snorkeling gear at high tide to look down through swaying sea grass—to locate the weathered nub of Hewatt’s second marker, staked far into the bay. It was an exciting moment, Sagarin recalls. “We knew when we found it that we could re-establish the site.”

His efforts took two years and involved counting 58,000 individual invertebrates belonging to 105 different taxonomic groups. By the time they were done, Sagarin and co-workers realized that the flora and fauna of their study area had undergone a profound transformation since Hewatt’s time. After categorizing each species as southern, northern or cosmopolitan according to its range, they found that southern species such as strawberry anemones that were rare in Hewatt’s survey now flourished, while ranks of northern species including dovesnails had considerably diminished. In contrast to these one-way changes, the reserve’s 28 cosmopolitan species showed no clear trends: some populations had grown, some had declined, while others had remained relatively unchanged.

In the six decades between the two surveys, the annual mean shoreline water temperature at Hopkins warmed by 0.75 C, while the average summer maximum temperature rose by a sweltering 2.2 C. The changes Sagarin observed meshed perfectly with those that marine biologists predict for a warming regime: northern species that thrive in cooler water had disappeared to the benefit of southern species.

But before he could place blame squarely on climate change for these population shifts, Sagarin had to rule out other possible scenarios. Here, luck again played a role. Hopkins Marine Reserve is one of those rare places that has been virtually untouched by human activity over the past century. What’s more, its intertidal zone is composed of erosion-resistant granite. Boulders seen in photos taken by Hewatt in the 1930s are still clearly identifiable today. So the changes Sagarin observed could not be attributed to physical disturbance, harvesting, or erosion of the rock surfaces on which intertidal life depends both for anchorage and shelter. The evidence pointed directly at warming as the agent of change.

Population density changes such as these are just one response to warming. Because temperature triggers the timing of many events in the life cycles of plants and animals, hotter weather can also spur such changes as migratory animals jumpstarting their spring migrations or plants flowering earlier in the season. The study of lifecycle rhythms such as these that are influenced by climate and weather is called phenology.

Fortunately for biologists who study climate change, the world is full of amateur (albeit often unwitting) phenologists. They are the ones who can give you a 20-year record of the salmon fly hatch on their favorite trout stream, or who jot down when their wisteria is fully cloaked in purple blossoms each year.

In Europe, researchers have been unearthing troves of phenological records from decades and even centuries past and comparing them to present-day observations. Two of their many discoveries are that the growing season in Europe has lengthened by nearly eleven days in the past four decades and, in Britain, 20 bird species are now nesting an average of eight days earlier than in the 1970s.

But good phenological records are much harder to find in the United States. That’s particularly true in the West, with its broad unpopulated expanses and shorter history of settlement. A remarkable exception is the 37-year data set amassed by Joe Caprio, an agricultural climatologist from the University of Montana at Bozeman.

To determine how climate affects crops, between 1957 and 1994, Caprio enlisted more than 2,000 volunteers in twelve western states to record bloom dates and other seasonal information for purple lilacs and clonal honeysuckles, plants that can grow in almost any soil, in any climate, and at any latitude or elevation in the West. By coupling these records with data from agricultural crops like wheat and alfalfa, he eventually developed a formula based on the timing of the local lilac bloom each year that ranchers could use to predict harvest dates for their cash crops. By 1984 Caprio was already noticing an interesting trend in his data: lilacs in the north were tending to bloom earlier.

Caprio had retired by the time his phenological magnum opus came to the attention of Daniel Cayan, a climate researcher with the USGS and Scripps Institution of Oceanography in San Diego. Poring over Caprio’s mountain of data, Cayan realized how closely linked honeysuckle and lilac bloom dates are to temperatures in the two months immediately preceding bloom. A map of the plants’ bloom dates paints a clear picture of annual climate variability over the western states. “The marvelous thing was that although the actual date of bloom differs from one spot to another, whether it was earlier or later than the average was coherent,” Cayan says. “And that pointed to something that was a broader-scale influence, like climate.”

Cayan then went one step further. He compared local bloom dates with records from stream gauging stations that pinpointed the date of the onset of spring runoff into nearby rivers. Like lanes on a highway, his data ran side by side: when springs were warmer, both blooming and the first surge of snowmelt occurred earlier, and in cooler springs, the dates occurred correspondingly later. Cayan could also see that his data highway was winding its way uphill: between 1957 and 1994 as average temperatures throughout the region rose, they advanced the average date both of lilac bloom and snowmelt by about 7.5 days.

Although honeysuckle and lilac are non-natives, it’s reasonable to assume that warming has been toying with native plants as well. Adaptations like changes in bloom time may not initially harm some species. But as effects accumulate and as temperatures mount, they may trigger more drastic outcomes, such as the extinctions of local populations—also called extirpations—from the warmer reaches of their ranges. At the same time, those plants and animals that can will gradually escape to cooler higher latitudes or higher elevations.

One of the best documented cases of a range shift in response to warming is the study of the Edith’s checkerspot butterfly (Occidryas editha) conducted by Camille Parmesan, a population biologist at the University of Texas in Austin. (Like Terry Root, she is a co-author of one of the landmark papers on warming in January’s Nature.) A well-studied species made up of discrete, sedentary populations that range through the Pacific states all the way from northern Mexico to southern Canada, checkerspots were an ideal subject for Parmesan’s range-shift investigations.

After spending months scouring archives to compile a list of all sites where the butterflies had been found over the past century, Parmesan trekked to each site to check it out. Although she discovered extirpations throughout the butterfly’s range, Parmesan realized that they were occurring in an unmistakable pattern: four times as many populations had vanished in the butterfly’s southern range as in its northern range. What’s more, nearly three times as many populations had disappeared at elevations below 8,000 feet than at higher elevations.

A major range shift was clearly underway. Parmesan calculated that during the previous century the shift had already effectively moved the butterflies both 92 kilometers northward and 124 meters upward. This change was in virtual lockstep with the northward and upward shifts in the mean yearly temperatures (isotherms) during the same period.

Although many of us will be delighted to welcome new species into our backyards (some Nebraskans, for example, are excited to learn that a handful of armadillos have crossed into their state from Kansas), scientists are worried that range shifts could exact a dear toll.

“The thing that I’m really concerned about is the tearing apart of communities,” Terry Root says. “Species are going to be moving differentially, some are going to be moving a lot, others a little, one northeast, one northwest. The communities we know today could be really disrupted.” Adding those disruptions on top of habitat destruction, forest fragmentation and other land use changes could place us “on the verge of what could really be a very large mass extinction,” Root says. “It’s very scary.”

The golden toad vanished from Costa Rica's cloud forests in the 1980s. It's the only animal known to have gone extinct because of human-caused global warming—so far. photo: Ed Ross

So far, we know of only one species that has been completely wiped out by human-caused climate warming: the golden toad (Bufo periglenes), which hasn’t been seen in the mountaintops of Costa Rica’s cloud forests since 1987. But unfortunately for pikas in the Great Basin, they share some of the attributes that made the golden toad so susceptible to climate change. They’re highly specialized, they have a limited ability to move or disperse, and their habitat is scarce and disjointed.

It takes just a few hours of unprotected exposure to midday temperatures in the mountains to kill a pika, so their survival depends on access to cooler havens beneath their rocky outposts. A warming climate could degrade these retreats as well as wither the tender foliage pikas depend on. When Erik Beever compared conditions at all 25 historic pika sites—taking into account such variables as proximity to roads, elevation, amount of suitable habitat (talus fields), and intensity of grazing — he found that elevation played a crucial role. While pika populations at higher elevations were in good shape, pikas were disappearing from lower sites where they had little opportunity to retreat to higher, cooler elevations. And although there’s no way to prove right now that warming totally eradicated seven of these populations, it remains a prime suspect.

Are climate change and its grim effects inevitable? Camille Parmesan says not necessarily. If we can cut back CO2 emissions now, we may be able to limit additional warming to a couple of degrees. “It’s becoming evident that there’s a really big difference between two degrees and four degrees warming, but we don’t have hardcore analyses yet,” she says. “With two degrees warming we’re going to have some loss, there’s no question about that. But there are some things we can do management-wise to protect a lot of the things we want to protect.”

It wouldn’t take large personal sacrifices to cut back on emissions, Parmesan suspects—just practical lifestyle changes, like drying laundry outside on sunny days or turning the thermostat down a bit in winter and up a tad in summer. People should demand fuel efficient cars, and stop buying gas-guzzling SUVs. The decision rests with us, and we need to take action now. “You start getting up to four degrees warming, and it looks pretty dire,” Parmesan says. “There’s really no way to protect against that.”

Liese Greensfelder wrote “Denizens of the Dirt” in the Fall 2002 California Wild.