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While the debate continues on the degree to which human activity is causing global warming, and what we should do about it (which will not be the subject of this article), all the evidence points to the fact that temperatures around the world are on the rise. In the Pacific Northwest, a temperature rise of about 1 - 2°F (depending on who you read) over the last 100 years can be observed. Ever so gradually, a consistent low-elevation mountain snowpack is becoming a thing of the past; and most of our glaciers, especially those whose accumulation zones are at relatively low altitudes, are melting and shrinking.

Developing computer models to provide meaningful forecasts for climate changes is tricky business, and has a high margin of error. Nevertheless, experts are predicting the following for the Pacific Northwest: Temperatures should warm slightly overall, but not to the degree that is occurring in other parts of the world. A year-round increase in precipitation is expected. This combination is likely to result in an increased high-elevation snowpack, but a decrease in the low-elevation snowpack, which should allow glaciers with higher accumulation zones to stabilize and perhaps advance, while those at lower elevations will continue melting until they vanish or reach equilibrium at a smaller size.

One more disputable projection is that our climate is generally expected to become more "extreme", with rain, wind, and drought events occurring with greater severity than in the past. However, it is difficult to come up with a quantitative measure of "severity" compared to the past, especially in this part of the world where records don't go back very far. We must not be too quick to forget "extreme" events of the past, such as the Columbus Day Storm of 1962 that brought winds well over 100 mph to the Pacific Northwest, massive forest fires of 1910 that burned more than 3 million acres, and a number of huge lowland snowstorms including 1950, 1916 and 1880. And, no matter what wild weather events occur these days, the old records still never quite seem to fall.

The question for gardeners is: how will the influence of global warming on our climate affect our ability to cultivate certain plants? In the last 10 years, a growing faction of gardeners has taken an interest in exotic and "subtropical" plants that may or may not be too tender to grow here in the long term. Many of these plants have been attempted in the past, and failed during the cold winters of the 80's or even earlier. And few of us who were growing anything whose hardiness was remotely suspect in 1990 could forget the landmark freeze that occurred in December of that year, with frigid winds blasting out of the north. Almost all the eucalyptus in western Washington froze to the ground, and even some native plants were damaged.

So, will the cold winters of the past ever be back in western Washington? In the 1980's we saw a number of severe winters in which the temperatures dropped to 0-15°F and lower throughout the region, a pattern which, we know in retrospect, reached its grand finale in the December 1990 freeze. But we haven't seen any comparable winters since that time - while some of us got a little bit cold in January 1996, December 1998 and January 2004, it has been a long time since we have seen a real "landmark" winter of the same severity as the winters of the past.

This has led many to believe that global warming is somehow preventing arctic air from entering our region, and therefore, we ought to be able to grow an increasing number of marginally cold hardy exotic plants, expecting them to survive future winters, which will continue to moderate in severity. I, however, remain skeptical of this idea, and I think cold winters will return, for the following reasons.

1. There is a historical precedent for the mild period we are experiencing now. The period 1938 - 1948 experienced a period of relatively mild winters, roughly comparable in severity to the period 1996 - 2006. This period ended with two consecutive, very severe winters (1948-49 and 1949-50), something I hope we will not have to look forward to again, though it's certainly possible. The mildest winters on record throughout our region were not in recent years (though 1999-2000 is a close contender for sure), but in the 50's and 60's (most notably 1957-58 and 1966-67), interspersed with some very cold winters.

2. The severity of our winters does not depend on the overall warming of the Earth's climate, as much as the ability of arctic air to enter our region. Even if the Earth warms by a couple degrees, and the overall climate of the Pacific Northwest warms by a couple degrees, arctic air will still be present in the arctic; and even if this cold air is slightly warmer than it has been in the past, it will still be very cold for many years to come. Even if global warming melts all the permanent arctic sea ice, there will still be seasonal arctic sea ice (in winter when it matters), and perhaps more importantly the cold landmasses of Alaska, northern Canada and Siberia will remain as a storehouse of cold air in winter, even if some warming occurs. So, long story short, arctic air isn't going to simply go away. This arctic air is able to enter our region whenever the jet stream forms a "kink" pointing north to south over us or to our west. This phenomenon is no less likely to occur if our climate is a little warmer than it has been, or is now. In short, it's illogical to purport that global warming could block modified arctic air from entering our region, because this event is determined by other mechanisms: namely, the position of the jet stream and, frequently, the presence of a strong high pressure system in the North Pacific. If anything, the prediction of "more extreme events" may manifest itself as more variability in the path of the jet stream, and perhaps even a greater likelihood of a strong high forming over the North Pacific in winter, and thus more arctic intrusions in the future. This projection is only one possible scenario; of course, only time will tell.

3. If a chronological "global warming curve" and a "Pacific Northwest winter severity curve" were charted, they would not exhibit a clear correlation with each other. The global warming curve would depict a steadily rising line, with the earth growing warmer, on average, with each subsequent year. The "Pacific Northwest winter severity curve", on the other hand, would depict a roughly average period from 1890-1915, followed by a colder period from 1915-1935, a "warm" period (with milder than average winters) from about 1935-1975, a return to cold from about 1975-1995, and then a return to mildness for the last 10 years. This fluctuation does not show any correlation to global warming, but (and here I'm speculating wildly) it may be found to show some correlation to Pacific Decadal Oscillation, a phenomenon whose mechanisms are still not well understood, though it certainly seems to effect our climate.

4. My final point is that, while Seattle has not seen a landmark freeze since 1990, just about everywhere else in the greater Pacific Northwest region has. For example, the freeze of December 1998, which was only moderately severe in Seattle, can be considered a landmark freeze for the Rogue River Valley of Southwest Oregon. The freezes of January 2004 and January-February 1996 were both landmark freezes in Eastern Washington, where pretty much every location experienced record low temperatures, while the Seattle area escaped once again without experiencing such severe temperatures. The January 2004 freeze was also very cold for the Vancouver, BC area. In other words, it is only those of us in the Seattle and Portland areas who are enjoying this string of mild winters. Farther away, everyone outside of these areas has no reason to think our winters are getting milder. In short, Seattle and Portland have really lucked out in the last 15 years, deluding some of us who neglect to analyze all the data.

There are a few factors which may decrease the severity of future arctic blasts. As the Arctic ice cap melts and subpolar temperatures continue to moderate (which they have been, at a faster rate than temperatures in the Pacific Northwest), the pools of cold air from which our "arctic blasts" originate will, over many years, gradually become a few degrees milder than they have been in the past, though still cold enough to kill plenty of exotic plants when that modified arctic air enters our region. Consideration of this factor by itself might mean a severe arctic blast occurring now would be just a degree or two milder than our arctic blasts of the past. But those of us growing subtropical plants aren't going to notice much difference if the temperature in Seattle drops to 12°F instead of 10°F - it's still going to kill a lot of plants.

Another consideration is that El Niño may, in the future, dominate the ENSO with increasing frequency, with La Niña occurring less often, resulting in a slightly higher proportion of milder winters for the Pacific Northwest. However, despite a number of rather persistent or strong El Niños in recent years, there is no evidence at present that the ENSO phenomenon is trending towards more El Niños.

Finally, the 'urban heat island' effect has consistently proven to warm winter temperatures, especially at night. In fact, in analyzing historical winter minima, I have found that winter temperatures in parts of the Seattle area during "arctic blasts" may be as much as 5 - 7°F higher than they would have been without urbanization having occurred. Obviously, this makes urban areas milder than they were 100 years ago, but it does not affect microclimates that remain rural. Also, some larger cities like Seattle were nearly as urbanized 50 years ago as they are now, so little difference should be observed between cold temperatures over that time period.

In summary, while we're enjoying a relatively warm period now, I think the cold air will be back. Because of other factors, the cold air may be a degree or two milder than it used to be - and it may yet hold off for a year or several years - but it will be back. Of course, history has shown that the world's climate is quite unpredictable and frequently does not follow the expectations of scientists, let alone amateur climate enthusiasts like me. But that is my best guess for now.