The Changing Western Climate: How It’s Likely to Affect You, Your Garden, and the Climate Zone Maps You Depend Upon

By: Jim McCausland

Jim McCausland is a writer and editor for Sunset magazine and a senior editor for the Sunset Western Garden Book….

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Despite seemingly warming climate patterns, some recent winters along parts of the West Coast have been extremely cold; here, an unusually heavy snowfall blankets the Miller Botanical Garden in Seattle in November 2007. Photograph by Richie Steffen

Despite seemingly warming climate patterns, some recent winters along parts of the West Coast have been extremely cold; here, an unusually heavy snowfall blankets the Miller Botanical Garden in Seattle in November 2007. Photograph by Richie Steffen

People who cultivate plants, it seems, have always had to take climate change more seriously than most. During the Medieval Warm Period (AD 800-1300), Vikings colonized Greenland, where they farmed and fished until the Little Ice Age (1300-1850) froze them out. The great drought of 1276-1299 probably drove the ancestral Pueblo people, who grew much of what they ate, out of their cliff dwellings in Colorado, Utah, Arizona, and New Mexico. And extended drought in Mexico and Central America coincided with the collapse of the Mayan culture.

In those earlier times, gardeners could only cope and hope. But these days, we have the advantage of historical perspective, 150 years of worldwide weather records, and computer modeling to shape our hopes and fears. Those resources tell us to look forward to an average temperature increase of 3° to 8°F by mid-century, with rising sea levels, more (and more intense) hurricanes, and unpredictable weather all around.

The Really Big Picture

It helps to understand how delicately balanced the earth’s climate system must be. To survive, we need a planet doused in temperature-stabilizing free water and generally mild conditions. Earth’s average temperature is about 59°F, with extreme swings from -129° to 134°F. This is only possible because the earth is just the right distance from the sun; our twenty-four-hour day distributes heat evenly by rotating the planet like a chicken on a rotisserie; atmospheric gasses like carbon dioxide, methane, and water vapor hold in some heat, but (so far) not too much; and the oceans, which cover seventy-one percent of the globe, gain and release heat gradually enough to keep the climate temperate.

The system is so perfectly balanced for our survival that University of Washington astronomer Dr Donald Brownlee and UW paleontologist Dr Peter Ward make the case in their book, Rare Earth (Springer, 2000), that these conditions are probably not repeated anywhere in the universe.

A little imbalance in the system could be catastrophic, especially if it pushes us over some climatic tipping point that fundamentally alters the way weather systems function. Many people see the earth’s present warming trend as evidence that something is out of kilter. The big question is whether this change is human caused (and possibly correctable), or a continuation of much larger natural cycles (like ice ages and interglacial warm spells) that go back through all of history, and will continue whatever we do. Or, perhaps, both.

Déjà Vu

The planet has always experienced heating and cooling cycles. By most estimates, the last ice age peaked 19,000 years ago, and ended about 12,000 years ago. From 11,000 to 7,000 years ago, the climate warmed about 3°F in the Pacific Northwest. Dr Linda Brubaker, a dendrochronologist at the University of Washington, tracked environmental changes that occurred during that warming period by reading charcoal and pollen samples deposited in lakebeds. She reported that, as temperatures increased, prairies and savannas expanded, and the composition of lowland forests changed, with Douglas firs and pines increasing, and hemlocks being driven upslope. Tree lines rose, forest fires increased, and snowpack decreased.

Eventually, the climate cooled again, occasionally warming (as it did in the Medieval Warm Period, and as it’s doing now) and cooling (as in the Little Ice Age). By 1850, Western glaciers reached their maximum extent. At that time, Glacier National Park had 150 glaciers and snow/ice fields. By 1981, only forty were left, the rest having succumbed to melting, sublimation, and reduced snow accumulation in winter.

Many think the current round of global warming is human caused. Al Gore makes a well-considered, well-documented, and readable case for this in An Inconvenient Truth (Rodale Press, 2006), also an award-winning movie.

Others think that, whereas human causes might feed into climate change, vastly larger natural forces probably govern the shift. Climatologist George Taylor offers several potent examples: the influence of El Niño (periodic ocean warming west of Peru and Ecuador) and La Niña (cooling in the same area), whose cyclical influences on climate were only well understood in the 1980s; the Global Conveyer Belt (circulation of ocean currents between the Atlantic, Indian, and Pacific oceans), which was not recognized as a major climate driver until the 1990s; the Pacific Decadal Oscillation (a twenty- to twenty-five-year cycle of ocean heating and cooling linked with El Niño and La Niña); and fluctuating levels of cosmic radiation, which directly influence cloudiness and, thus, temperature.

Whatever the causes might be, changes are coming that will affect the way you garden.


Writing for the May 2007 issue of San Diego Horticultural Society’s newsletter, Steve Brigham coined the phrase “global weirding,” which describes how “even a small warm-up of average temperatures worldwide can produce not just hotter temperatures, but also more extreme weather throughout the world, including droughts, floods, heat waves, and freezes.” He makes a good case. During the past three years, various parts of the West have experienced record rain, drought, and low humidity, plus a twenty-year freeze that burned even native laurel sumacs (Malosma laurina) from Pt Conception to San Diego. As I write this from the Seattle area in late April, I’m looking outside at the fourth consecutive day of snow showers (and, no, this is not normal).

In a study released by Goddard Institute for Space Studies (GISS) early this year, NASA reported that global average temperature, which is “about 1°F warmer than the 1951-1980 mean, continues the strong warming trend of the past thirty years . . . The eight warmest years in the GISS record have all occurred since 1998, and the fourteen warmest years in the record have all occurred since 1990.” They note that this is all the more serious because two other factors—solar radiation and the El Niño-La Niña cycle—are in cool phases right now.

For gardeners, even small temperature increases make a difference. Most everbearing strawberries, for example, produce all summer along the coast, where summer temperatures are mild. Inland, the same varieties only deliver two crops: the first in late spring, and the second after the weather cools in fall.

Tomatoes experience the same thing: most varieties stop setting fruit when daytime temperatures stay consistently above 90°F, and all stop at temperatures over 100°F.

Wine grapes, complex creatures that they are, depend on a combination of winter minimum temperatures, and both day- and night-time summer temperatures for production and flavor. That magic combination has changed over the past several decades. Napa, Sonoma, and St Helena have all been pushed into warmer grape growing zones (per UC Davis’s Winkler Heat Summation scale), while Ukiah and Mendocino County have cooled. Growers see the differences. Can you taste it yet?

For spring-flowering perennials, milder temperatures stretch out their flowering, while heat tends to push it to an early conclusion.

On the low end, every gardener understands frost. If you’re growing citrus, you know that a few hours below 27°F can cost you the farm. If you’re growing apples, peaches, pears, or lilacs, the same winter chill can only help, especially if you live in a borderline climate where mild winters can compromise flowering during the following spring.

Even mid-range temperatures matter—and not just to honey bees, which start working at about 55°F (45° for mason bees and certain strains of cold-tolerant honey bees). Tomatoes stop setting fruit when night-time temperatures stay below 55°F. The incredible Amazon water lily (Victoria cruziana), whose floating pads can support a baby, needs a steady 72°F minimum to prosper. Guzmania, a bromeliad, and firecracker flower (Crossandra infundibuliformis) suffer below 59°F, and tropicals such as caricature plant (Graptophyllum pictum), velvet plant (Gynura aurantiaca), and some bananas decline when temperatures remain below 55°F.


Water—who controls it, who uses it, and how much we pay for it—is a central issue for people in the West. As population expands in the West’s dry summer climate, demand eventually outstrips the water supply. If global warming starts reducing the snow pack, the situation will become critical faster.

Snow matters because, in much of the West, we depend upon a vast system of reservoirs to store part of our water, and the melting snow pack recharges those reservoirs as summer advances. Whenever there is a smaller-than-average snow pack, our water supply is diminished in the hottest, driest part of summer—when we need it most. Water rationing calendars often follow.

To reduce that dependence, major Western water suppliers like Seattle Public Utilities (SPU) are reducing demand with tightened building codes (low-flow shower heads and low-volume toilets, for example), subsidies for water-efficient appliances, and aggressive pricing for water itself (use more, pay much more). These tactics have been phenomenally successful: per-capita water use is down, and, in spite of population increases around Puget Sound, SPU is delivering twenty million gallons of water per day less than it did a few years ago.

For gardeners, the best defense against water restrictions is appropriate planting. If your garden can survive on rainfall alone, rationing is moot. Native plants obviously fit the bill, but so do plants that come from any of the planet’s five wet-winter, dry-summer mediterranean climates (much of the Mediterranean basin, most of California, southwestern South Africa, central Chile, and Western Australia). To learn more, contact the Mediterranean Gardening Society (

If you’re wedded to green summer lawns and plants that need regular water through the warm season, now is the time to become an expert on drought-tolerant buffalo grass, blue grama grass, drip irrigation, and cisterns that collect roof runoff (a 2,000-square-foot roof sheds more than 1,200 gallons of water during a one-inch rainstorm).

Climate change may lead to hotter, drier conditions and result in more disastrous wildfires such as the July 2006 Sawtooth Fire in San Diego County. Photograph by Wes Schultz, courtesy California Department of Forestry and Fire Protection - See more at:

Climate change may lead to hotter, drier conditions and result in more disastrous wildfires such as the July 2006 Sawtooth Fire in San Diego County. Photograph by Wes Schultz, courtesy California Department of Forestry and Fire Protection


Much of the natural Western landscape depends on periodic wildfires for renewal. In parts of the California chaparral, such fires have historically come through once every decade. In the West’s forested mountains, the fire regime runs from thirty-one years in dry forests to several hundred years in moist coast-side forests. But after a century of fire suppression, fuel loads have grown to unmanageable proportions in forests that would normally have burned long ago, and bark beetles have moved through aging or drought-weakened woodlands, unchecked by the fires that restock the forest with younger, more resistant trees. When fires finally do start, they’re bigger, burn hotter, and do more damage than smaller, more frequent fires.

In addition, the wildfire season itself is expanding in California, as is the number of houses abutting wildland. Combine that with drought, and the fact that hot air spawns more lightning storms than cooler air, and you have a formula for trouble that will only be magnified if summer average temperatures increase.

The best defense against this is to plan for wildfires, building and planting accordingly. Wide, low volume perennial buffers help make a fire-resistant house more defensible. That’s important, because when fires come, fire crews do triage, putting the greatest effort into protecting the most defensible houses. 

Looking into the future

Having grown up near Los Angeles, I’ve seen how easy it is for the human species to foul its own nest. During the 1950s, my town had fifty smog alerts per year—an average of one day in seven when our breathing was labored and our eyes burned and watered from sulfur dioxide.

But, after watching Mt St Helens blow up in 1980, vaporizing several glaciers in minutes and polluting both air and water over half the state, I became convinced that nature could change things faster, more unexpectedly, and on a larger scale than we can imagine.

But the point is not whether man or nature has more potential to create disaster. It is that while we cannot control volcanoes, hurricanes, cosmic rays, and the Global Conveyer Belt, we have a great deal of influence over the kinds of cars we drive, the quantity of resources we use, and the kinds of plants we choose for the garden.

In the end, these are all moral stewardship decisions whose importance is increased if they influence climate change—and still important if they don’t.

The Sunset 2007 climate zone map for the western- most regions of the US. Copyright © Sunset Publishing

The Sunset 2007 climate zone map for the western- most regions of the US. Copyright © Sunset Publishing

Climate Zone Maps

Many years ago, George Taylor asked a friend whether she knew the difference between climate and weather. She answered that climate tells you what clothes to buy, and weather tells you what clothes to wear.

The difference is critical to those who publish climate zone maps. Because climate looks at long-term weather averages (thirty-year minimums), climate maps are not affected by the short-term weather that determines today’s heat or cold, rain or sunshine.

Global warming influences weather, but it is, above all, a climate issue. And, since it has logged only a 1°F average temperature increase in the last century, it has not advanced enough yet to move climate zone boundaries on maps.

Yet the questions keep coming, because zone maps keep changing, and because relatively short-term weather patterns can seem like climate shifts. Two years ago, for example, the National Arbor Day Foundation published a revised and warmed-up hardiness zone map ( based on only fifteen years of statistics. It supports a global warming trend, but is based on too short a time frame to be taken seriously by climatologists.

The Sunset 2007 climate zone maps, published in the latest edition of the Western Garden Book, have also been revised. However, the changes, most of which were made in the Southern California interior and in parts of Canada, reflect better data and interpretation, not actual climate shifts.

At this writing, the USDA Agricultural Research Service’s newly revised Plant Hardiness Zone Map is out for review, and should be published and posted online within a few months. The revision was done by Taylor’s Oregon Climate Service and Chris Daly’s PRISM Group, both working under a USDA contract to Oregon State University. Substantial changes were made in the Western United States, where the USDA map has always been weak. But Daly emphasizes that climate zone lines were adjusted primarily because the PRISM mapping system provides a more accurate rendition of the relationship between climate and topography than anything used before.

Both Taylor and Daly agree with NASA that there has been a definite warming trend over the past three decades. But, because that trend shows up more in summer highs than in winter lows, it will have almost no affect on their maps, since they are solely based on winter minimum temperatures. As Daly puts it, “the last thirty years could be indicative of a climate change signal—or not.” Time will tell.

Current Signs of Climate Change

Increasing carbon dioxide: Charles David Keeling started measuring carbon dioxide (CO2) in 1958, when it formed 315 parts per million of the atmosphere. When he died in 2005, it was at 380 parts per million. Normal level for an interglacial period had been about 280 parts per million. As CO2 (a greenhouse gas) increases, temperatures generally rise. About fifty-five million years ago, CO2 levels were about 2,000 parts per million, and temperatures averaged about 9° higher than they are today.

Slowing of the Gulf Stream: The Gulf Stream has slowed about thirty percent since the 1990s, and most of the giant North Atlantic maelstroms, which suck Gulf Stream waters to the ocean floor to return south, have vanished.

Hurricanes: In 1998, Hurricane Mitch slammed Honduras, which rarely gets hurricanes. Mitch was the strongest hurricane in 200 years.

Disappearing Polar Ice: In 2000, a Canadian ship navigated the Northwest Passage without having to break ice. The Greenland ice sheet has been melting since 1979.

Changing Rainfall Patterns: Rainfall amounts, worldwide, have increased by about ten percent in the last century. At the same time, dry continental interiors have expanded.

Rising Sea levels: The 150-acre Carteret Islands have been eroding since the 1960s, and will probably be submerged by 2015. Tuvalu (the ten square-mile Ellice Islands), one of the smallest independent nations in the world, has made arrangements to evacuate to New Zealand as they become flooded by rising seas, creating the new category of “environmental refugees.” Kiribati (the 313-square-mile Gilbert Islands) is expected to be rendered uninhabitable by salt-water flooding by the end of the century.

Increasing Temperatures: Global air temperature records go back about 150 years—almost to the end of the Little Ice Age. In that time, temperatures have risen 1.1° to 1.4°F. About forty percent of that warming is probably the result of solar radiation. Arctic air temperatures have risen about 3° to 5°F in the last thirty years.

Weeds and Climate Change

Higher levels of carbon dioxide—a side effect of global warming—are pumping up weed growth. Weeds grown under urban conditions of higher temperatures and more carbon dioxide grew up to four times higher than weeds in a country plot forty miles outside the city in a USDA-ARS study. “As the climate and carbon dioxide levels change, we can no longer assume the weed control strategies we used in the past will continue to work,” said Lewis Ziska, a USDA researcher who led the study. “Not only are some of the nation’s most invasive weeds spreading, but they are becoming more difficult and costly to control.” (Excerpted from Nursery Management & Production Weekly E-mail, April 8, 2008)