March 7, 2011 — Climatologists use a 30-year period to calculate climate normals and although that timeframe is changing with a new decade, signs of Global Warming are still apparent. Bryn Jones, a meteorologist at The Weather Network, takes an in-depth look at this trend and the outlook for the future.
Bryn Jones, meteorologist
Climatologists use a 30-year period as a standard to calculate climate normals (i.e. averages or means). Though other period lengths can and have been used, 30 years is long enough to statistically eliminate year to year variations which may taint an objective assessment of the actual climate one is trying to measure. It does this while retaining the advantage of not being so long as to eliminate weather/climate stations with record lengths capable of offering valid insights into the climate simply because they had not been in operation for, say, 100 years. By using 30 years instead of 100 or some other large value we significantly increase the number of stations available for use in climatological studies around the world. We need to recall that widespread, routine and objective measuring of the weather and climate locally and around the world is a relatively recent phenomena only going back 100 to 200 years or so; and mostly since World War II. So, 30 years is a statistically sound but also advantageous period length for the calculation of climate normals though this latter benefit is of less importance than it was historically as the record length of many stations worldwide is now greater than 30 years.
Another standard in climatology is to only calculate climate normals once every 10 years. In the years before digital data bases and widespread use of computers, this cut the down the time and effort needed to derive normals manually by not having to do it every year while providing time to also quality check the data for mistakes prior to permanently archiving it. Besides, there would really be nothing to gain doing running 30-year means as one would only be changing the normals by very small amounts and do so by ingesting year to year variations (i.e. adding/dropping one year at a time) which is what we were trying to avoid in the first place when it was decided to use 30-year means.
Climate Normals, Climate Change and Global Warming
In many parts of the globe the calculation of 30-year normals updated every 10 years is not unduly impacted by climate change as the change is happening slowly enough to modify the difference between, say, the 1981-2010 normals and the 1971-2000 normals in a controlled and moderated manner. However, there are parts of the world where the change is happening so rapidly that pronounced deviations are observed when comparing a given year’s climate to a longer term 30-year average.
If, for example, over a 30-year period there was greater warming at a station in the last 10 years than the previous 20 (i.e. the warming was accelerating) then, numerically, only about one third of the values used in calculating the 30-year normal would tend to the high side while two thirds would be not so high. The result is a 30-year average which is lower than the average we would derive if we had only looked at those warm 10 most recent years. So, if we compared some year’s warm winter to the 30-year normal we should see a pronounced signal that that year was on the warm side.
Ten years later, when it was time to calculate new 30-year normals in a warming climate scenario we would now have 20 years on the milder side (i.e. the twenty most recent) and 10 years (i.e. the ten oldest) tending to the colder side. If we now compared that same year’s warm winter from above to the new 30-year normals one would expect that the signal would not be as pronounced, though it would still be there. And this is indeed what is happening in much of the world.
However, in some parts of the world, the rate of change is so great and is continuing through subsequent years after the last year in the most recent 30-year normals’ period that we continue to see strong signals of warmer than average temperatures whether we compare them to 1981-2010 normals or 1971-2000 normals or the 1968-1996 (29-year) normals used by the Earth System Research Laboratory in the United States.
It is no surprise that places where this is happening are in the high latitudes across portions of the Arctic. This was predicted as early as circa 1980 when it was reasoned (and simulated by computer models) that the loss of “permanent” snow and ice at those latitudes would allow for greater absorption of the energy from the sun. This in turn would make land and water in those areas warmer than normal and so melt more snow on average over the course of the summer while shortening the length of winter so that less snow would accumulate so that the sun could warm temperatures even more, and so on. This positive feedback effect for more pronounced warming over high latitudes compared to what we expected to happen over lower latitudes toward the equator was well predicted. During that era of research, consensus and confidence in this aspect of the predicted warming was high. It underscores that however inexact climate prediction may be there is some skill and we can glean some useful information from such research.
Impact in Canada
In Canada, we see the strongest example of the above effects in the eastern Arctic in winter. Nunavut, especially the north and eastern areas comprising Baffin Island, Devon Island, Ellesmere Island and adjacent islands in the eastern Arctic Archipelago, have most consistently exhibited at least modest to pronounced warm summer and winter temperature anomalies in all but 6 years out of the last 15 years or so compared to 1968-1996 means. This area extends west into the western Archipelago across Prince of Wales Island, Bathurst Island, and Melville Island into Victoria Island with somewhat less frequency. The central Canadian mainland Arctic covering western Nunavut and the eastern Northwest Territories show less of this signal; fluctuating somewhat more either side of normal on year to year basis, while the western Arctic including the Yukon shows yet greater year to year variations in temperatures above or below average. As it turns out, this is predominantly a winter phenomena, though spring and autumn have similar tendencies.
More complex than that
Why should the winter be more sensitive to the loss of snow and better absorption of sunlight in the Arctic when there is not much sun, if any, to begin with at that time of year? Like everything in the climate system, the whole story is more complex than this and there are other considerations.
Digging deeper we find that the positive feedback effect noted above is much more pronounced in winter and the shoulder seasons of spring and autumn than during the summer. Today, the snowpack across the Arctic does largely melt during the summer (especially near the climate stations which generally are in coastal areas near sea level) but it did so historically as well so that areas with no snow on the ground in a typical August in the 70’s were still that way in, say, the 90’s. Consequently, there have been lesser snowpack changes to impact August temperatures between those 2 decades. This would be similar for July but less so for June and September. So, despite the fact that there isn’t much sun in the Far North during the winter the positive feedback basically dominates an increasingly longer span of the year as the snowpack season shortens. This, in turn, tends to confirm that the positive feedback effect noted above is the key operative here as the lesser amounts of snow on the ground reflecting back sunlight during the summer months has not changed as much as the loss of snow on the ground during the shoulder seasons leading into winter has.
Combine this with related phenomena, such as less Arctic sea ice and decreasing permafrost, and we might conclude that it is not so much that the Arctic is getting warmer as it is not as cold as it used to be! Yes, the summers have warmed but not as much as the rest of the year in many places. Much of this is the result of the Far North being able to absorb more sunlight due to a decreasing area of white, reflective land and ocean surfaces which would otherwise bounce the sun’s energy back out to space rather than letting the earth’s surface absorb it.
There is another possibility here, at least in part. Perhaps there is something else operating here; something which occurs over a period of a few decades or so which we poorly understand. Such “multi-decadal” processes could be tainting the statistics and we should not discount this possibility. What we are seeing in the eastern Canadian Arctic may be some sort of multi-decadal signal superimposed on the broader warming signal. We need to remember that climate change is the result of many processes which have always been in operation combined with the impacts humans are having on the climate.
Will this trend continue? No, otherwise we’d end up with an atmosphere like that on Venus. Eventually, the earth-atmosphere-ocean system should reach a new semi-equilibrium which is warmer than that of today but no where near hot enough to melt lead such as on Venus! Consequently, with successive decades we should see the strong signals of warmer than average temperatures in the eastern and northern Arctic of Canada on almost a year to year basis become less pronounced and less frequent. By then, that region will likely look very different than it has historically.