How odd is the recent cold snap and what does climate change have to do with it?
There are very few spells in the past century of such intense cold in April
BarcelonaLate frosts are not a rare phenomenon, but the recent cold snap has been truly remarkable. Out of the 153 Meteocat stations which have over ten years of data, 124 have broken their record for the month of April. Even looking at older records, the precedents for such low temperatures in the fourth month of the year are scarce.
In Lleida, which holds records since 1960, there have only been two April nights which were colder than Saturday night, none of them during the 21st century. We have to go back April 21, 1991 and to April 13, 1986 to find temperatures slightly lower than -1.8 ºC registered on Sunday. If we add the last ten days of March, things don't change much. Interestingly, March 21, 2018 registered -1.3 ºC in Lleida, but then there had not been any other temperatures below -1 ºC since 1985. At Girona airport, the April record was equalled, not beaten: the precedent in this case is also from 1986, but very similar temperatures occurred in 1996 and 2001.
This Sunday the temperature anomaly in Catalonia as a whole reached -7.4 ºC, if the data of the last thirteen years are taken as a reference. There have not been many days in the last decade in which the temperature has been so far from normal. The last time there was such a strangely low temperature in Catalonia was with the early cold snap at the end of October 2018. Then the anomaly reached 8 degrees. On the other hand, February 3, 2020 was 7.8 degrees higher than would be expected.
Climate change
What are the implications of climate change in all of this? The first thing to say is that late frosts were more frequent before than they are now. The data clearly show that going below freezing in a city like Lleida after March 15 happened more frequently a few decades ago than in recent decades.
As we have already seen, however, this cold episode is quite exceptional, even if one takes into account the context of four or five decades ago when cold was more common, and there is a key aspect of climate change that makes it more detrimental to agriculture: during the last decades flowering and ripening started earlier. Today, a frost arriving on April 1 finds more trees in bloom than 30 or 40 years ago.
This is known thanks to the dedication of some phenological observers such as Josep Borrell from Serra d'Almos, who for decades have been recording data on flowering, ripening, leaf fall and bird arrivals. Today we know that on average pears ripen 40 days earlier than they did four decades ago; apricots, 28; peaches, 22, and apples, 21. Olive trees flower 20 days earlier than 45 years ago. This makes an April 1 frost now much more damaging than an April 1 frost a few decades ago. Since 2013, Meteocat has been doing a very thorough job of monitoring phenology, with an extensive network of observers keeping an eye on changes in nature.
The other idea of how climate change may have been present in this anomalous cold snap is more uncertain and more complex. For some years now, some studies have suggested that the jet stream may be slowing down. The jet stream is a corridor of winds that occurs at mid-latitudes and at a height of about 10 kilometres. It is a current that moves from west to east and is basically the result of the temperature difference between polar and tropical latitudes.
When the jet stream slows down, air movements between latitudes are easier; that is, it is easier for cold air to move to lower latitudes and warmer air to move towards places close to the Arctic Circle. The fact that the Arctic is warming faster than the rest of the planet causes the temperature difference between the poles and tropical latitudes to decrease, and this would lead to a scenario with a weaker jet stream and more undulations. This would lead to more chaotic weather that could favour situations of extreme temperatures, including intense cold spells.
It is impossible to determine whether climate change has had anything to do specifically with the current cold episode, but it is true that in recent years we have had quite a chaotic late winter, with March being colder than February and late snowfall on relatively low land. On March 20, 2018 up to 12 cm of snow fell on Tibidabo, in what was one of the latest snowfalls in the history of the Fabra Observatory. In 2020 and 2021 the median March temperatures were equal to or lower than February temperatures in Catalonia as a whole. And last year, in mid-March and April there were also two episodes of very low temperatures for the season. The one in April was especially remarkable, and caused temperature anomalies of more than 5 degrees on the 16th and 17th.