I’ve tended gardens around the West for much of my adult life, from the tomatoes and basil I nurtured through a Laramie winter in a solar greenhouse to the climbing roses I inherited in our yard in southern New Mexico’s Chihuahua Desert.

Now I’m writing a book for Rocky Mountain gardeners, drawing on my education in plant ecology and experience as a Western gardener, making use of references on plants and gardening, and the past century or so of Western climate data.

There’s one large problem with this gardening information however: it’s all based on the past, which may no longer be a good predictor of the future.

For several decades, scientists have been warning with increasing alarm that levels in of carbon dioxide in the atmosphere are rising more rapidly than is normal, and that increased levels of this gas, called a "greenhouse gas," because it acts like the glass in a greenhouse to trap and reflect incoming solar heat, will raise global temperatures.

The theory, dubbed "global warming," is controversial. Everyone -- from scientists who may know something about the complicated systems that maintain our planet’s climate, to the politicians, who usually don’t -- has weighed in on the validity of global warming.

But the data are increasingly difficult to ignore. Earlier this month, the World Meteorological Organization, a body of the United Nations that normally spendsits time on decidedly non-headline-grabbing tasks such as coordinating weather data collection among countries, issued an urgent press release.

In careful language, this apolitical body noted that monthly and annual average temperatures around the globe have increased gradually for the past 100 years. In fact, the rise in Northern Hemisphere temperatures during the 20th century may be the largest rise of any century in the past thousand years.

In Switzerland, according to the press release, the month of June was the hottest in at least the past 250 years. This year’s pre-monsoon heat wave in India brought highs from113 to 120 degrees F, topping the average weekly temperatures by 36 to 41 degrees, and killing some 1,400 people.

A handful of recent observations demonstrate that the rise in global temperatures is already reflected in altered distributions of animals and plants. In California, for instance, entomologists have mapped changes in butterfly ranges that show cold-loving species’ territories shrinking uphill.

Here in Colorado, pikas, the high-mountain rabbit relatives that whistle from precarious perches on rockslides, have recently disappeared from seven of 25 sites where they were once plentiful, as alpine habitats grow warmer and drier.

On the western Great Plains, blue grama grass, a cool-season grass that does not grow well when nighttime temperatures rise, has declined in 23 years of warmer average temperatures. At the Arnold Arboretum in Boston, tender species like camellias, never successfully grown outside the greenhouse now thrive outdoors.

The evidence is impossible to dismiss. Imagine camellias, those lush flowers epitomizing hot Southern climates, flourishing in Boston!

Although the global warming theory is named for its predictions of rising global temperatures, the heart of the theory is less about warming than about unpredictability: As the globe heats due to climate change, the number and intensity of what meteorologists call "extreme weather events" is expected to increase.

That means more severe weather, cold as well as hot, rain as well as drought, more tornadoes, hurricanes, cyclones, blizzards, torrential rains. As the World Meteorological Organization points out, that data is compelling, too.

In this year already, we’ve surpassed several records for extreme weather. The month of May brought 562 tornadoes to the Great Plains and Southeast, resulting in 41 human deaths. The record had been 399 tornadoes in June of 1992.

Here in the Upper Arkansas River Valley, a record-breaking, multi-year drought still grips the landscape, even after the generous gift of snow brought by the record blizzard that paralyzed Colorado’s Front Range in March.

Record highs, record snowfall, record drought, and record storms. With the number of climate records recently shattered, clearly the past isn’t a reliable guide to the future, for gardeners or anyone else.

"All we really know is that the weather is going to be different than it has been in the past," writes Peter Del Tredici of Boston’s Arnold Arboretum in an essay for the New York Times. "From the garden where I work, the plants seem to be telling us something we may not want to hear: the world is changing."

Writing from a record hot spell in south-central Colorado, I wish that I didn’t agree.

Susan Tweit is a contributor to Writers on the Range, a service of High Country News in Paonia, Colorado (hcn.org). She is a naturalist and the author of several books about the West.