2021 was a crazy year in all of our lives (yet again) but it was also a notable year for weather & climate. It was a top 10 warmest year for all of Minnesota and specifically in the Twin Cities, it was our 5th hottest ever (of 148 years). I go into month-by-month detail HERE, but in this article I will concentrate on the significance of the year overall & what that means for weather extremes in the future & past.

Let's talk about the year as a whole first & then I will break down the notable weather/climate event of that month. Note that of the top 10 warmest years (below): 6 of the 10 are just in the last 15 years! 8 of the top 10 are just in the past 34 years. That alone tells you something is definitely UP in our climate when 60% of our top 10 years make up only 10% of the 148 years of climate data.

Let's look just how rare it is to have this many top 10 warmest years (the top 7% of our records) so frequently since 2000.

Without jumping too deep into the mathematics behind statistics & normalized data, we can calculate the odds (or probability of occurrence) of each event. You'll of course notice that the #3 to #6 rankings are basically once in a generation occurrences that have all happened within one generation period.

This is one of the many ways we are able to extract the human-caused (anthropogenic) element of climate change out of the numbers.

Again without boring you with too many details, we know that most of Minnesota has warmed about 1.5° C (or nearly 3° F) since 1880. While a seemingly small number, this translates into a significant shift. If you know anything about statistics, it's about 1.3 standard deviations. That means the warm/hot extremes become exponentially more likely and the cold extremes become exponentially less likely. I illustrate this below...

The basic idea to garner from the illustration above, is that when you move the middle of the data (the average, or median, i.e. middle number) over (right, or warmer) a seemingly small amount, you add a lot more heat and 'chop off' a lot more cold.

This is the primary driver behind the fact that warm extremes are more frequent now. In other words, 2021 would be a ONCE in 30 year event, historically, but with just a couple degrees of warming, we've made it at least a 1 in 10 year event, to as much as a 1 in 3 year event. The warming of the last century has made a hot year like 2021 at least FIVE times more likely. It's not the 'new normal' - yet, but will in the future as we continue our trajectory upward.

Likewise, we can look at some COLD examples (since we're dealing with our first bout of REAL cold this season!). Let's take the number -20° in the Twin Cities. That's a low temperature that used to happen about 5 times PER winter, prior to 1900. Now? We've only been that cold a TOTAL of 9 times since 2000. That translates to 1 in every 2 or 3 years. So, we'll still get some 'extreme cold' but much less frequently- on an exponentially decreasing scale. A number like that matters, because, for example, the emerald ash borer, one of our latest invasive pests, can't survive those extremes. EAR (Emerald Ash Border) larvae start to die off at -20° and almost none survive -30°. These temps still routinely happen in northern Minnesota, but not often enough farther south. If we're only that cold every two or three years here, that's enough to kill off some (but not all) and allow the population to come back strong. Northern Minnesota, at least for the time being, can consistently kill it off with temps that, at least once a winter, do indeed still drop to -30°.

That -30° F threshold: for the Twin Cities used to happen 1 in 3 years (still not common) but now has become a 1 in 35 year event! The last time we were that cold was the infamous winter of 1996. Basically, we will so rarely ever get to -30° that the EAR is here to stay (in some form). I don't doubt we will once again hit -30° at some point this century, but that will probably be it: a one off event (& a few people will say: "look, global warming isn't real- it was cold!"). It's not about just the numbers: it's the frequency.

Remember, we had our hottest summer EVER recorded (yes beating 1987 and any of the 1930s). There was only ONE month this year that was colder than normal, that was February. Climate IS the big picture.

Let's take a look at 2021's significant events by MONTH, HERE.

-Meteorologist, Sven Sundgaard