Gardeners have always been close observers of the weather. But recent erratic patterns as well as concerns about global warming have made the weather a front-page story. Many scientists attribute the high average temperatures of the 1990s to global warming. Then the winter of 1997 brought the latest manifestation of what is perhaps the best-observed weather event in history, El Nino. Did global warming exacerbate it? Again, some scientists think so. Finally, what might we expect from the weather in the future, and is there anything gardeners can do to prepare for more erratic weather patterns?
Could the extreme droughts, floods, and hurricanes of the last few years also be due to global warming? Or -- as the American Petroleum Institute insists -- since great weather fluctuations have always occurred, is the theory of global warming bunk? The answers are not simple.
Although "the abnormal is normal" when it comes to the weather, many scientists do indeed see evidence of a steady, long-term warming trend this century, speeding up in the 1990s. In an August 14, 1998, Science magazine article, James E. Hansen of the Goddard Institute for Space Studies and his colleagues concluded, "The issue should no longer be whether global warming is occurring, but what is the rate of warming, what is its practical significance, and what should be done about it." Hansen and other scientists believe this warming is the result of a strengthening greenhouse effect due to increased carbon dioxide in the atmosphere from burning fossil fuels (primarily coal, gasoline, and natural gas).
Global warming is caused by increasing levels of greenhouse gases including carbon dioxide, methane, and nitrous oxide. These gases and water vapor have always been part of the atmosphere. They permit sunlight to enter but block some of the warm, outgoing radiation from the surface of the Earth, much as the glass in a greenhouse holds heat in.
Evidence is mounting that the use of fossil fuels combined with deforestation and agricultural development produces great increases in the levels of these and other gases in the atmosphere. As a result, more radiation is blocked, and the average surface temperature of the Earth increases. In fact, the Earth's surface temperature has risen about 1F in the last century, and it is expected to rise even more rapidly in the decades ahead.
El Nino is a regular climatic event that occurs every four to seven years. Its effects begin when a pool of warm water appears off the Pacific coast of South America. Worldwide climate patterns can be altered depending on how big and warm the pool is. The winter of 1997's strong El Nino effect in North America shifted the jet stream to bring twice the usual rainfall to the West Coast, a warmer-than-usual winter to much of the continent, and drought and summer heat to Texas, Florida, and the Gulf Coast.
While most people were unaware of El Nino until the fall of 1997, scientists had been tracking it for many years. It is actually one of the world's best understood climatic events, thanks to sophisticated new monitoring systems that include satellites and a network of data-collecting ocean buoys. (Perhaps the real climate news of this decade is the accuracy of forecasting for El Nino. Compared to the difficulty meteorologists have in predicting tomorrow's weather, that in itself is a small miracle.)
Is it possible that global warming actually caused the extreme El Nino of 1997-98? This is just the sort of puzzle that Kevin Trenberth, head of the Climate Analysis Section of the National Center for Atmospheric Research in Boulder, Colorado, is currently working on. He emphasizes that this research still has no firm conclusions, but "because El Nino is involved with the movement of heat, it is conceptually easy to see how increased heating from the buildup of greenhouse gases can interfere," causing more frequent and severe El Nino effects.
According to Joel Michaelsen, a climatologist at the University of California, Santa Barbara, "1997's strong El Nino, combined with the exceptionally warm 1997-98 temperatures, provides a glimpse of what the climate might be like in a warmed Earth."
A classic chicken-and-egg question remains: Do changes in atmospheric circulation cause shifts in ocean currents and sea surface temperatures, or is it the other way around? Peruvian fishermen noticed and named El Nino long ago. Its opposite, or cold phase, only recently got its catchy title of La Nina. (Many scientists who study the phenomena simply refer to "warm events" and "cold events" in the El Nino -- Southern Oscillation or ENSO.)
The forecast for La Nina is based on the lesser-known effects of El Nino's opposite, or cold, phase. Because the 1990s have been the hottest decade ever recorded, it is uncertain how these predictions will play out.
Consistency in annual average minimum temperatures is the basis for the USDA's map of plant hardiness zones. One impact of global warming would be a northward shift for these zones sometime in the coming century. No one knows how much warming will occur, or where it will be felt the most, but predictions suggest the greatest increases will be in the high latitudes. For instance, the frost-free growing season in the Northeast has already lengthened by more than a week compared to the 1950s.
Adjusting the zones northward to accommodate warmer winters could be good or bad news. As hardiness zones and plants migrate northward, gardeners there may be able to grow plants previously not hardy, and gardeners in the South may be able to grow tropical plants. Furthermore, studies have shown that an atmosphere enriched with carbon dioxide will help some plants grow better. Crops (but weeds too) may need less fertilizer.
On the other hand, areas of the country that depend on a good, hard winter freeze to keep fungus and insects in check are likely to suffer more severe infestations. Where summers are normally dry, late rains will promote various fungi, and different insects will thrive in creeks and vernal pools that don't dry up as usual.
The consensus among scientists is that global warming will increase the probability of climate extremes -- drought, intense rainfall, and flooding. The first effect of global warming is to dry things out, because increased heat on the Earth's surface evaporates moisture. Soils become parched, and plants experience moisture stress. Droughts become more serious. At the same time, in the words of Robert Quayle, chief of the Global Climate Lab at National Climatic Data Center, as global warming increases, we can expect "more gully-washers. Warm air holds more moisture; so when it rains, it rains harder than usual."
Given that global warming is a worldwide phenomenon, people and governments need to work together. This country uses a large share of the world's fossil fuels, so we can start by calling for more fuel-efficient cars and home heating systems, and seeing that our gasoline-powered tools run efficiently.
Planting more trees may seem a small step, but growing trees consume carbon dioxide from the air and use it to build wood. It's not released to the air again until the tree dies. For information about how to help plant trees, contact Global ReLeaf, a program of the American Forests Association, at (800) 873-5323, or see their Web site at www.amfor.org.
Drier. If you live in a dry climate, plan and plant drought-tolerant gardens. Cut down on lawn areas, use ground covers, and avoid water-impermeable paving that restricts aquifer recharge. Irrigate with a drip-watering system. Drip systems use and move less water and apply it at the surface so evaporation is less.
Hotter. Summers promise to be hotter, especially in northern regions. Be mindful of plant heat stress on days when the temperature exceeds 86°F. That's the temperature when most temperate zone plants begin to suffer. Water your garden in the morning, before the day reaches its temperature peak. Use a heavy mulch of organic material (shredded bark, compost) to retain soil moisture, and avoid applying fertilizers during the heat of the day.
Wetter. Take measures to reduce and manage runoff from heavier-than-normal rains. Use mulches and ground covers, and deep-rooted plants to stabilize slopes. Fast-draining raised beds, always a good idea for vegetable gardens, may become obligatory in low-lying areas.
No one knows what the future holds, but we can assume our climate will become less predictable. As Robert Quayle says, "Any time there is a change, there are winners and losers. We just don't know yet who they will be."
Susan Chamberlin is a writer and garden historian based in Santa Barbara, California.
Photography by Mike MacCaskey/National Gardening Association