My house has been much hotter in the summer in the last decade than it used to be. Global warming? Probably not. The two huge Douglas fir trees south of the house died and had to be cut down. The effect of that loss on the temperature of the house in summer was immediate and dramatic. Local often trumps global. When it comes to our home and yard, two big trees may matter more than global warming.
Global warming is happening, however. It’s been happening since the glacial maximum about 20,000 years ago, so it is nothing new. How fast it is happening and how much is caused by people isn’t the subject of this article. Nor will I address the global aspect of global climate change. I will instead consider the small and personal. What effect will climate change have on your yard and garden next year and in the next few years and decades? Here’s my synthesis, based upon information from many sources. (See chapter 3 end notes and references in The Resilient Gardener .)
That the overall trend for the planet for the last 20,000 years has been one of warming is incontrovertible. However, climatic trends are full of irregularities and hiccups. Any period of a thousand years in which the overall trend is in one direction has periods of years, decades, and sometimes even centuries in which the climatic trend reverses temporarily. The Little Ice Age, from about A.D. 1300 to 1850, is named for the seriously colder weather in North Atlantic Europe and America. It was a period of several hundred years that was part of the overall global warming trend that has been occurring since the last ice age.
Even if the globe is warmer on average in years to come, that doesn’t mean your yard will be any warmer, even on average. Climate change causes changes and irregularities in the patterns of ocean currents and winds. The local effects of those changes are huge compared with the few degrees cited as likely increases in average global climate in the next few decades. A change in wind patterns that brings Arctic inland air masses to you instead of mild ocean air will matter much more than a few degrees higher average global temperature.
Even major trends are region specific, not globally uniform. And water may matter more than temperature. The Medieval Warm Period was lovely for Europe. Famines and diseases were rare. Populations swelled. Civilization and cities expanded and flourished. The same period was devastating for Mexico and the American Southwest, which experienced horrific droughts–droughts that probably contributed to the collapse of civilizations and the vanishing of entire populations. In the Sahara, the Medieval “Warm” Period was cooler, not warmer, and marked by more prolonged droughts. In many other parts of the world, the main impact of the global warming of the Medieval Warm Period also seemed to be droughts. Says Brian Fagan in The Great Warming, “with respect to California, it’s sobering to remember that the past seven hundred years were the wettest since the Ice Age.” Pointing to prolonged droughts that lasted decades and even generations, Fagan says that, viewing the overall global situation, not just that of Europe, “it is tempting to rename the Medieval Warm Period the Medieval Drought Period.” The American Northwest, California, the Southwest, the inter-mountain West, and the lower Hudson River Valley are all vulnerable to increased aridity and droughts.
Finally, for agriculture, the regularity of weather patterns may matter much more than averages or overall climate change trends. In spring of 1350, it started raining in northern Europe and rained for the next several months. That spring marked the beginning of five years of colder, wetter, stormier, more erratic weather that was the obvious beginning of the Little Ice Age, which lasted another 550 years. Most of the famines in northern Europe during the Little ice Age were as much or more associated with erratic, rainier, stormier weather than with temperatures. In the Year without a Summer in New England (1816), the freezes were unremarkable, as New England freezes go. What mattered was that they happened right through the entire summer.