Weather and climate records are not mutually exclusive

The atmosphere is a chaotic system with an infinite number of configurations – the weather pattern on any specific day has never before been exactly the same everywhere. Thus, it is extremely difficult to link a specific weather event to climate change. And yet, when significant weather occurs, discussion immediately jumps toward climate change – especially in recent years.

In 8 sunny days during early April 2020, there were 4 new daily maximum temperature records at the University of Reading Atmospheric Observatory, in a dataset which runs almost continuously since 1908. The UK also experienced its warmest April 10th on record, with 24.6°C beating the 23.3°C recorded in 1909.

Daily records are more controversial than monthly, seasonal, or annual records, because they are inherently more noisy due to the chaos which I mentioned earlier. Some calendar days have unusually cold or unusually warm records purely by chance, meaning it is “easier” to set a new record on certain days if the weather happens to take up a conducive pattern on those days. In the aforementioned example of the April 10th record, the record for April 9th is 25.5°C and the 11th is 26.0°C – so the weather on the 10th would not have set a new record for either of the neighbouring days, and is therefore not particularly unusual on its own. As an aside, in my experience daily records aren’t given as much “air time” in the UK as in the US; I’m not sure why.

The ability to break a record is also a function of the length of the observational record – the longer the record, the greater the sample of possible weather configurations – i.e., the greater the sample of “climate”. Thus, the potential for breaking new records should decline with increasing dataset length after an initial spike.

Thus, we could expect to break maximum and minimum records in a stationary climate for these reasons; some records will always be broken just because “shit happens”.

So why the climate link?

Climate is essentially just a smoothed out version of the noise. A lot gets made of the saying “weather is not climate” as a motivator for not even speaking about climate change when severe weather occurs. Equally, the accusation is often made that severe cold is dismissed as “just weather”, while heat is taken as “evidence” of climate change. Neither is really true.

Let me go back to Reading’s recent maximum temperature records. These may have just occurred because the weather setup was, in essence, perfect, and had never happened before in the dataset at that specific time of year – there was a particularly strong anticyclone, uninterrupted sunshine, and desiccating continental air leading to rapidly-drying soils helping to intensify surface warming. It is entirely plausible that date-maxima occurred because of the weather. There have been amazingly warm spells in early April many years ago, which – of course  – were not linked to climate change.

But, we know the globe is warming – rapidly – and in recent years record maxima have been occurring much more often than record minima.

In the above tweet, I look at the current (as of 2019) daily-record maxima and minima for the calendar year, and calculate the ratio of how many have been set since each year in the record. As you can see, since the mid-1990s more than 3 times the current record maxima have been set versus the current record minima. In fact, the 2019 data point is missing on this plot because there were no record minima that year but 10 record maxima – producing a division-by-zero error. [The apparent drop-off at the end doesn’t mean record maxima are declining, it’s just due to very low sampling since that’s only a few years of data].

In these last 30 years or so, Reading, along with the rest of the globe, has been rapidly warming. Therefore, from our climate standpoint, we are seeing much more record warmth than record cold, and we know this is consistent with what we’d expect to see in a warming world. Thus, when record warmth occurs, it is evidently consistent with a warming world. It doesn’t on its own prove climate change, and it doesn’t necessarily mean it couldn’t have happened without climate change.

There has recently been extreme warmth in Florida, which has received a lot of climate change attention. Simultaneously, an Alaskan Ridge weather regime led to an extreme plunge of cold air across the central US which produced numerous cold records.

From a weather perspective, the two go hand-in-hand due to an amplified mid-latitude flow configuration. Usually, when somewhere is extremely warm, somewhere else is unusually cold because jet stream “waviness” is a key contributor to extreme weather patterns. For example, you can see the Rossby wave-pattern across the Northern Hemisphere in my “#GlobalHeatwave” tweet from 2018. Evidently, the same weather patterns were present in both 1976 and 2018, but as this tweet pointed out, the climate was different.

Florida’s recent warmth doesn’t mean anything on its own, but it is consistent with the strong warming trend that has been observed there over recent years. It’s for precisely this reason why extreme weather which is consistent with a long-term trend gets more attention than the opposite – it is immediately implicated with the trend.

The analogy of loaded dice is one I always return to – you can still roll a 1 (severe cold in a warming world), and that doesn’t mean the dice are not loaded, but you’re much more likely to roll a 6 (heatwaves). Equally, you can roll a 6 on normal dice (heatwaves happen anyway). You can repeatedly roll a 6 on normal dice by chance, too (extreme weather happens without climate change).

Since climate is just the summation of the weather over a longer time frame, the two are not mutually exclusive but inextricably linked. Expecting more record warm days due to our warming world means each one we see is symptomatic of larger change, much like smelling smoke when you already know the house is on fire.

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