I recently did a talk at the Yard, Garden, and Patio Show in Portland, entitled ‘Plants for a Changing Climate’. Some shared interest that I post my talk so I will describe it in this blog and I hope you find it informative. The first half of the talk was describing the weather over the past 60 years. The second half was plants. I’ll just be talking about the weather here. I’ll try to keep it as cut and dried as possible- I won’t veer off though I have a habit of that. Heh.
There are many ways to approach the climate over time. And I won’t bore you with intricacies but I will show how I arrived at my information. To be simple and precise I chose to compare our extreme temperatures as a way of showing change. I won’t be focusing on monthly averages or even yearly day to day averages. To make it more comparable, this is how the USDA arrives at their zones so it seemed appropriate. The weather record in Portland is not exactly as long as you would think so I used two records to make up for that. The official record at Portland International Airport only extends as far back as October, 1940. Thats when the airport was moved from Swan Island to its current location. Why there isn’t a published record on Swan Island, I don’t really know, but I have seen it mentioned in old literature. The second record which extends back to 1875 is that of downtown Portland. I’ll talk about why that is useful but also limited. Be aware that these two records are urban- well not as much the airport but they are our official records and there is quite a bit of difference between them and the suburbs. I’ll also review the distinct microclimates in the metro area.
20ºF, 100ºF- Life above and below
In my search through records I had to delineate certain temperatures to make it more simple. For instance, I chose 20º above (zone 9) and 20ºF and below (zone 8 and colder) and that was fortuitous. Think of zone 8 as the line we cross into what we consider arctic air in our region. It turns out that is exactly how our winters fall, to give away a simple fact our average annual low at the Portland Airport in the last 30 years is 20.3ºF (-6.9ºC). And our winters have a distinct percentage above and below that line. This is arbitrary, I know but it really works for our discussion. Also, I reviewed our annual highest temperature and would you believe that is exactly 100.0ºF for the past thirty years? Tracking the ultimate high might mean less to gardeners than freezing temperatures, but I wanted to see if that foretold a trend in extremes as well.
30 Year average
The official USDA zone maps are compiled by averaging the yearly low temperature extremes at official reporting stations. This is done in 30 year increments and the last thirty year review was 1980-2010. In my review I chose to look at 1986 to 2015 in order to see if there had been any more change in a more modern appraisal. The information is there so, hell, why not? The official temperatures at official reporting stations are compiled and then isothermic lines are drawn to delineate the zones in the region. For instance, in the Portland Metro area those official reporting stations are: Pearson Airport- Vancouver, Scappoose Airfield, Hillsboro Airport, Portland International Airport, Troutdale Airport and Aurora Regional Airport. Those 6 stations are relevant to our immediate area. By the way if you look at our current observations page at the NOAA website those are what come up and those are the only stations with official records. Of course there is a significant amount of difference between each station. Topography, elevation, exposure to the Columbia River Gorge and distance from it all play into the local climate.
Microclimates: Macro differences
Microclimates in our region are distinct in several different ways. First, elevation has several effects. You might think that the higher you are the colder you are, well thats only partially true. Honestly, cold air sinks and the most extreme cold temperatures in our area are recorded at the lowest elevations. Once you get above about 700′ the temperature again begins to drop. So, slopes between 700′ and about 200′ in the metro area (and Willamette Valley) are considered thermal belts. If thats difficult to picture just remember that is where you see wine grapes grown in our region- there they avoid late and early valley frosts and the cold air sweeps downhill and does not collect. The most important aspect of elevation in western Oregon is that the higher you are the wetter you are. Elevation is the key. The Portland Airport at 21′ above sea level records the least amount of rain in the metro area at 35.10″ annually. In the west hills of Portland above 800′ that average spikes up to 55″ and more. As far as precipitation is considered elevation is everything. By the way downtown Portland at 110′ elevation averages 42.30″- so you can see how sensitive that is. A few feet and wow. Also, there is much more snow at higher elevations and this increases even more dramatically.
The longest records and what they tell us
The longest record of Downtown Portland- taken at the KGW Television Station currently tells us a lot about the trajectory of our climate. Computing the averages 30 years at a time from 1875 until 1955 and guess what? Our climate was colder. There were warm years and very cold years. What is interesting about this time frame is two striking things. First the 19th Century was very much colder than the 20th century- we were exiting the Little Ice Age which ended in approximately 1910- most likely that was caused by what is known as the Maunder Minimum a period of less sun spot activity and some pretty honking big volcanic eruptions (Tambora 1815) contributed to significant cooling. Then in the late 18th Century the industrial revolution began spewing CO2 into the atmosphere and sun spot activity changed. A combination of the two shows a turn in climate. And you can see a change. An example that shows warming sensitivity is average annual snowfall. In 1900 downtown Portland averaged 18.1″ of snow per year. Thats an awful lot. Currently, we only total an average of 3.1″. Also, there were regularly widespread regional arctic outbreaks that lasted for weeks at a time. In fact the Columbia River and the Willamette both froze over regularly. That speaks not just of intensity but duration of freezes. The last record of both rivers freezing completely is 1933. Also, exceeding 100ºF, rare in the 19th Century became much more frequent following 1910. This is a trend that has repeated all over the west so we know some sort of transition was taking place. Just for fun there was only one year that never dropped below freezing, that was 1934- and as you know from history the 1930’s showed great perturbation in climate- not just the dust bowl but in the PNW as well.
The Ultimate Extremes
I chose the last two thirty year increments to really delve into details. And before that just how cold was it? Downtown Portland’s coldest temperature officially was -2ºF in January 1888- a month that saw more than three feet of snow- something unfathomable now. At the Portland Airport the coldest temperature was -3ºF in early February 1950- a winter of intense cold and and snow. In an aside the coldest temperature recorded that winter night in downtown Portland was a relatively warm 7ºF- remember microclimates are everything. It could be that by that time the urban heat island was beginning to emerge. The urban heat island is just that- an area like an island in urban cores where glass and asphalt absorb heat and then release it gradually. It becomes very apparent in averages. Surprisingly the warmest temperature prior to 1955 downtown was 107ºF in 1942 a high that hasn’t been achieved since.
1956-2015- Downtown Colder then…
To really get a handle on our climate I chose the last two 30 year increments to see if that showed valuable change. I figured the last 60 years is about relevant to the older plants in gardens (heh). And what I discovered was not a huge surprise. In 1956 to 85, downtown Portland had an average annual low of 19.3ºF- Zone 8b. There was just one winter below 10ºF (Zone 7) 6ºF in 1968. 13/30 winters were Zone 8 or colder and that averages out to 43.33% in that time frame. Conversely, 17/30 winters were above 20ºF- or 56.66%. Now, this is in the urban core so it is only surprising in that it is way warmer than most people would surmise. The warmest winter was the El Nino year of 1958 where the lowest temperature was a paltry 31ºF. Barely a frost. The average annual high was 99ºF with an ultimate high of 106ºF in the great heatwave of August 1981. In 1985 to the present the downtown recording station showed slight warming. In that time the average annual low rose to 22.3ºF (Zone 9a) and zone 9 winters increased slightly to 60% of average. The highest temperature remained stagnant and the highest recorded temperature was 105ºF in July, 2009.
1956-2015 Volatile weather at the Airport
The average low temperature at PDX in period 1956-85 was exactly 15ºF (Zone 8a). There were 21/30 years in those 30 years with winters zone 8 or colder. (70% of winters). Five of those winters were zone 7 (10ºF) or colder with ultimate lows of 6ºF in 1957 and 1964. Just 9 winters were zone 9. What is striking is that in the last 30 years the percentages have reversed. Fully, 56.66% (17/30) winters were were zone 9. and just 43.33% of winters of zone 8 or colder. The most striking statistic is that the coldest and last official zone 7 freeze at PDX was February, 1989 with a low of 9ºF (a particularly brutal freeze with highs in the teens for 3 days). Thats 27(!) years since a true zone 7 winter. Something has changed. Nothing like that expanse of time shows up anywhere in the previous records. The highest temperature was 107ºF in 1965 and again during the phenomenal heat wave of August 1981 where 107ºF occurred twice in one week. So, the average annual low- as I said rose to the current 20.3ºF (Zone 8b)- unless you want to believe we are .3ºF into zone 9. The warmest winters were 1999 and 2002 and where the lowest temperature was just 26ºF. (This year the lowest was 24ºF and last year it was 23ºF).
Microclimates show much greater extremes.
Those are urban records and I took a look at the records from the other reporting stations and they show quite a bit of difference. In Hillsboro for instance the coldest temperature of 4ºF was achieved in 1989, 1998 and it dropped to 7ºF as recently as December, 2013. Cold air sinks and away from the Columbia Gorge in the Tualatin Valley where there is less air mixing and greater radiational cooling as well, it gets quite a bit colder. This pushes Hillsboro down to Zone 8a (11ºF) and it averages a zone 7 freeze at least twice every ten years. Another surprisingly cold part of the metro area is Vancouver as well as rural Clark County. Cold air collects and in 2013 the mercury dropped to 7ºF in the last big freeze. This area though officially zone 8a (14ºF) averages one out of ten years with a zone 7 freeze. In both of these locations that should be taken into account when choosing longer lived plants (shrubs, trees).
Our current climate
So, what is our climate like in the 21st Century? And remember that averages are made up of extremes. There will be colder winters as well as warmer winters- that is part of climate. The way it affects gardens is real. And you should always record the temperatures in your own garden- you may live in a frost pocket. Remember that exposure to subfreezing winds (from the Gorge) knocks off a lot of plant hardiness. I like to remember that at 20ºF a 30mph wind causes the equivalent damage to a wind-free 10ºF- thats how zone 8 plants can be damaged or killed at a zone 9 temperature. And gardeners here know that sodden plants can be injured in combination with low temperatures. We can expect one day a year with a high temperature below 32ºF. One day a year with a low below 20ºF, 44 days below freezing, one day a year with a high above 100ºF and 16 days above 90ºF.
The future looks very different
In the last 100 years our temperature in the PNW has risen by 1.3ºC. And you can see that has had a distinct effect. We will continue to warm. 2015 was our warmest year ever, we broke the record for most days above 90ºF (29) most days with highs above 80ºF and most lows above 60ºF, 6 of the 12 months were the warmest on average ever recorded. And the number 2 year? 2014. Its a trend- last year was a glimpse into the future. Experts expect that our climate will warm in the PNW by 3-5ºC by the year 2060. Thats kind of hard to imagine. Snowpack will become more volatile, low elevation snow will likely disappear and water will take on more importance. More than ever we should learn about our climate and prepare.