How to make sense of unusual cold when the world is hotter than ever

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Left: Global annual average carbon dioxide (black curve) and air temperature (red curve) since 1850. Right: Average temperatures of central England in summer (red curve) and winter (curve). Temperatures relative to the 1850–1900 average. Credit: Neven Fakkar / Data: Met Office HadCRUT5 and HadCET

Earlier this year, the UK’s weather and climate service, the Met Office, announced that average global temperatures in 2023 will 1.46°C above pre-industrial levels. This made it the hottest year on record, 0.17 °C higher than the previous record set in 2016.

However, shortly after that announcement, the Met Office also predicted several days of blast of cold Arctic air, which would bring sub-zero temperatures, snowfall in many parts of the UK. When the cold wave came the temperature dropped -14°C in the Scottish Highlands and -11°C in England too,

Ten days later we reached a village in the Scottish Highlands. A pleasant 19.9°C, the hottest January temperature ever recorded in Britain – by a full degree Celsius. This may seem more in keeping with the global warming trend. Yet just ten days after that record heat, much of Britain has again been hit unusually hard cold and snowy weather,

It’s not just the UK. This winter has seen record-low temperatures Canada, America And China,

This may seem confusing. Why are weather and climate producing such opposite signals? This is because they describe atmospheric characteristics on quite different time-scales.

you can’t feel the climate

I don’t think there is anyone on Earth who can actually experience “global annual average” temperatures. No one really knows what a degree of extra heat over a century feels like, especially as there can be a 10°C difference in temperatures between day and night in the UK, for example, or on a hot summer day and The difference between a hot day and a day can be 20 degrees Celsius and more. Cold winter night.

This means that we generally have difficulty sensing or remembering seasonal averages and how they change with the passing years. We can see climate change in environmental changes such as the retreat of glaciers or plants flowering earlier, and we can track the changes with instruments. But climate change is very difficult to “feel.”

In contrast, we remember weather better on daily and weekly time scales – especially extreme weather such as cold, heat waves or strong storms.

hot one day, cold the next

Weather vs. climate: how to feel unusually cold when the world is hotter than ever

How winters are changing in Oxfordshire. Credit: Neven Faker / Data: Met Office HadUK-Grid

Weather phenomena are much faster and more variable than climate properties which are defined and change over longer time scales. The weather may be hot one day and cold the next, but the annual average climate does not suddenly swing from hot to cold.

Climate is basically the accumulation of weather over a period of time. For example, weather information may refer to the local temperature at noon or 4 pm, the daily minimum, average or maximum temperature, or the weekly average. Whereas the climate is more long-term.

Climatic information may refer, for example, to the average temperature over a month, or the average over a seasonal (three-month) period, years or decades. In climate analysis, we typically look for anomalies with respect to a “baseline” – perhaps a long-term average of 30 or 50 years of data.

line rotates upward

We can use over a century of data to detect patterns, such as close relationship In the left graph (above) between global atmospheric CO₂ and near-surface temperature. Of course, there are some variations of about 0.1°C or so – fluctuations in the red line – because climate does not change smoothly overall. That’s why 2016 was exceptionally warm, and the years that followed were slightly cooler.

These variations become more apparent when we zoom in and examine a smaller regional area or smaller time units. For example, the right-hand graph above shows temperature data for central England (HadCET) record, the world’s longest running instrumental temperature record which began in 1659. This graph, which shows both winter and summer average temperatures for central England, shows more significant variability by both measures over the same period since 1850 – on the order of 1°C. The internal variability of these seasonal means briefly overestimates long-term climate change at this regional scale before the 1960s.

Looking at just the right-hand graph – 174 years of data – you’d be hard-pressed to see recent climate change. But zoom out to the global annual average data in the left graph, and the long-term trend becomes clear.

We can zoom in even further to see daily winter weather variability In the English county of Oxfordshire (HadUK-Grid), The histograms below show daily minimum temperatures (left panels 2.a and 2.c) and daily mean temperatures (right panels 2.b and 2.d) from two different 21-year periods.

They show that the probability of experiencing sub-zero weather is still significant even in the recent 2002-2022 period. However, the “tail” of daily minimum temperatures to the left of the average is thin, so extreme cold temperatures are less common. The mean daily minimum has increased by 0.59 °C (numbers in blue) in the recent period by about 1 °C to 1.6 °C, while the daily mean has increased by 1.29 °C – both increases are greater than global warming over this period. Time.

These are signs that Oxfordshire has been warming for a long time, and its winters are warming slightly faster than the rest of the world. Global climate change also increases the likelihood of extreme temperatures in winter. This doesn’t prevent chills in winter, but it does reduce their likelihood.

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