Why the weather is becoming wackier

Is it 'weather change' we should worry about, rather than climate change? The answer lies in your definition of 'weather and 'climate'

by HENRY HENGEVELD

In early November, I participated in an evening public forum on climate change in Yellowknife. During the follow-up question period, an elderly gentleman arose and declared, “I don’t know about this climate change stuff, but I have noticed that our weather is changing.”

Then he asked, “Can you explain that?” 
Small wonder that he asks! In 2005, temperatures in Yellowknife during early November were about seven degrees warmer than normal. This winter, daily highs during the same period averaged about five degrees colder than normal. As the travel brochures advertise, November in Yellowknife is also supposed to be the season for clear skies and glorious displays of northern lights.

Instead, skies were perpetually overcast and snowy, and the only “aurora borealis” I saw while there was on postcards. And the local citizens, having found the record warmth of the preceding winter rather pleasing, grumbled.

Unusual fall weather was also evident in other parts of Canada. In Vancouver, residents spent much of November boiling their drinking water because of the erosion effects of excessive rainstorms sweeping in from the Pacific. By the end of the month, the cold north winds that caused the Yellowknifers to grumble had expanded deep into the Prairies and swept over the Rockies, putting the chill on coastal B.C., an area normally dominated by warm, wet winds from the tropical Pacific.

My son, the arborist, finished the last week of the month helping to plow some 15 inches of snow off Vancouver parking lots instead of pruning trees. That isn’t supposed to happen in Vancouver, at least not in November. In sharp contrast, the citizens of Ontario ended November enjoying unseasonably balmy temperatures in the mid-teens, like Vancouver should have been getting.

Journalists, of course, love such weather drama, especially when other news is slow.  Perhaps because of the media coverage, many Canadians have, like the gentleman in Yellowknife, become convinced that our weather is becoming wackier. 

So is it “weather change” we should worry about, rather than climate change? The answer, of course, depends on what we mean by “weather” and “climate.” The online Merriam-Webster dictionary informs us that weather is “the state of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness.” That is as good a definition as any.

Climate, on the other hand, is described as “the average course or condition of the weather at a place, usually over a period of years, as exhibited by temperature, wind velocity, and precipitation.”

Most climatologists would want to apply this to many other atmospheric variables as well, and to soils, lake and ocean conditions. Furthermore, they would add “and its variability,” since data on the normal variability of weather tell us how much we can expect it to fluctuate from one day to the next, and (for any given day) one year to the next. It also provides information on how often extreme weather events are likely to occur, and how intense they might be.

In other words, as Environment Canada’s weather guru, David Phillips often says, climate tells you what weather you can normally expect at any given place and time of year. Weather is what you actually get!
  
Following the guidelines of the international climate community, Environment Canada describes the climate of a particular location or time of year on the basis of the last three complete decades of weather data, currently the 1971-2000 period. Analysts develop statistical “normals” for this time period and make these available on line  (http://www.climate.weatheroffice.ec.gc.ca/). This helps inform Canadians about what kind of weather to plan for in their diverse activities, including what kind of crops to plant and how to manage them.

Even without climate change, climate normals can vary between one 30-year period and the next. That is because the effect of natural variations in ocean currents and atmospheric wind patterns on our climate can vary slowly over multiple decades. The Pacific Decadal Oscillation, for example, causes a slow variation on the climates of western Canada over a cycle of 50 years or so. Much like an ocean swell can cause the pattern of waves we see coming into shore change over time, these oscillations have a muted seesawing effect on the day-to-day weather we can expect.

Climate change, on the other hand, is a long-term change in expected weather that continues for many decades and even centuries. This has an effect on weather similar to that a rising ocean tide might have on the distance that each wave washes up the shoreline. The higher the mean ocean level gets, the further the waves reach inland and the less they retreat again to expose the beach.

Likewise, climate change causes both the average “weather” and the frequency and intensity of its extremes to change. For example, if the climate becomes warmer, then current extremes of very hot days become more frequent while extreme cold days become less frequent.

For instance, the warmest ten per cent of summer days (one way of defining “hot days”) in Hamilton, Ontario, under current climate conditions, reach or exceed the threshold of 29.5˚C. If the summer climate for Hamilton warms by an average 3.4˚C by 2050 -- as projected by climate model studies -- and variability doesn’t change, then the average number of “hot days” would increase from the current nine to 39 events (about every third day!).

During the hot summer of 2005, Hamilton experienced 33 days with temperatures of 29.5˚C or greater.
There is also another way that a change in climate can affect our weather behaviour. The flow of winds across the Earth is strongly influenced by large high-pressure areas that act like boulders in a stream, deflecting the air currents around them. These pressure systems have seasonal patterns that determine where the dominant storm tracks occur, and how much, as they move from west to east or meander north and south, bringing cold, snow-laden air one moment and warm rainstorms the next.

As climate changes, so will these patterns. In fact, there are indications that, as the climate warms, the jet stream that controls the storm tracks may meander more. That means more frequent switches from cold to warm and back again, like we saw this past November.

So climate warming can be expected to cause wackier weather -- and we may already be seeing the early results! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.