What does it mean when someone tells us that this spring has been unusually cold? How should we interpret the news that snowfall from the past winter was not typical? In fact, what is perceived as normal depends on perspective, writes Andreas Hoy.
Winter in most parts of Europe was colder than usual, and snowier in some places this year. Did it feel especially severe to you? Or did it feel milder than the winters you remember from your childhood?
Your answer may depend on how many winters you have experienced. Someone who is 18 may have found this winter quite cold, but someone who is 80, who can think back on more winters over the decades, might have found it relatively mild.
Both perspectives are correct, legitimate and informative – a lesson that is important to underline now, as the World Meteorological Organization (WMO) and many national weather services have recently shifted how they define our “normal climate”.
To make a judgment about how usual or unusual the weather is or was requires a point of comparison – the then to contrast with the now. Short-term “weather perspectives” put things such as heatwaves, rainstorms and temperatures into context to reflect what is typical in current times, so that we can all properly plan.
But “climate perspectives” provide context over a longer time, revealing, for example, that earlier generations experienced different “normals”.
In terms of climate, these baselines serve as the foundation of understanding. To give people up-to-date perspectives, the WMO has begun updating its “standard normal” every 10 years. Reference periods representing the short-term weather perspective were adjusted this year to reflect our “new normal”.
From now on, the current weather will be compared with what was normal over a more recent 30-year period, from 1991 through 2020 (rather than from 1981 until 2010).
Yet, to keep up with the accelerating change and the increasing number of extreme weather events, the WMO has also introduced a “two-tier approach” to use both the most recent 30 years (1991-2020), and to retain a historical base period (1961-1990).
For historical perspective, the WMO uses baselines that go back centuries. Its recent report, “The State of the Global Climate”, for example, uses various baselines, including the 1850-1900 time period with which to compare the global mean temperatures that underpin the IPCC Special Report on Global Warming of 1.5 °C.
These baselines are vital, but they complicate communication. They potentially create confusion over what the “new normal” means, and they can make changes seem less dramatic than they really are.
What does it mean when someone tells us that this spring has been unusually cold? How should we interpret the news that snowfall from the past winter was not typical? We can only make sense of these things if we know the reference periods to understand the changes we are experiencing. Information from the Stockholm observatory, one of the longest existing and most reliable climate stations in the world, shows why contexts are so essential.
This first figure shows the long-term temperature development in Stockholm using the 1991-2020 “weather perspective” in yellow, and a 1961-1990 “climate perspective” in red.
Both lines follow the same pattern, but they each show quite different pictures of what would be “too cold” or “too warm” anomalies from what would be considered normal over time.
Both show large warming from the 1990s, but from the 1991-2020 “weather perspective”, the past would have been just “too cold” all the time, while the current temperatures are “normal”.
On the other hand, the 1961-1990 “climate perspective” provides a reasonable historic perspective of climate conditions typical for the 20th century, but with recent years (correctly) appearing as exceptionally warm.
This second figure shows the monthly temperature anomalies of the last year, 2020, again using both perspectives. While both reference periods agree that 10 out of 12 months were “too warm”, the more recent reference period show the warming as less extreme.
May and July may have been perceived as quite cold in Stockholm in 2020, yet those months would have been felt as quite normal in previous times – a context supported by the very small anomalies compared to the earlier reference period.
The work that I and others are doing through the ClimVis Europe project recognises this need for greater clarity in such communication. The project aims to create a meaningful European climate data visualisation tool that combines past, present, forecasted and future observed and modelled data for people who are not experts in climatology.
This work has shown how important it is to give people ways to clearly understand the context.
The entities that relate climate and weather information – such as local, regional, and national state offices and institutions; national weather services; institutions that deal with agricultural and forestry interests; and, especially, weather forecasters on the nightly news – must step up to their responsibilities in this regard.
They often make their own decisions about how to present weather information – and without making clear what context they are using.
No wonder some people are confused about the extent and significance of global warming – and the importance of policy responses.
This then is a plea for everyone who communicates information about climate or weather – weather forecasters, and the local, regional, national and international services and institutions. Communicate carefully and clearly.
We want and need the full story.
This article was written in the context of the ClimVis Europe pilot project, financed by the Swedish Institute. ClimVis is a partnership between the SEI centres in Stockholm and Tallinn, the University of Krakow and the Institute of Monitoring of Climatic and Ecological Systems Tomsk, under consultation by the Royal Netherlands Meteorological Institute and the Swedish Meteorological and Hydrological Institute.