Climate change can already be seen in the weather, but record temperatures could also occur by coincidence

The warm and dry early summer may be the result of climate change – or not. Climate change can also make rainfall forecasting increasingly difficult.

What a warm and dry early summer it was! But is a hot May in Finland a sign of climate change? “Yes and no”, says Jouni Räisänen, a university lecturer from the Institute for Atmospheric and Earth System Research (INAR) at the University of Helsinki.

“The dry early summer was a result of a persistent high pressure system.  But why did we have this high pressure? The answer is: coincidence, most likely. Most of what happens in the atmosphere is completely random.”

On the other hand, if it weren’t for climate change, with a 90 percent probability May 2018 would not have been as warm. In May 2018, the average temperature in Helsinki was 14.5 °C, a new record by far. It was over 4 °C higher than the mean value from 1981 to 2010.

The situation can be interpreted in different ways. If the most dire predictions come true, Earth’s average temperatures would rise by 4 °C by the end of the century in comparison to pre-industrial times. In Finland, the warming would be even larger. In that situation, a May as warm as this would be completely ordinary.

This might not happen, however.

“Should the goals of the Paris Agreement be achieved and the global mean warming limited to two degrees Celsius above the pre-industrial level, the average May mean temperature in Finland would be only two degrees higher than the current average”, Räisänen says.

A warm May might not become an annual event then, but as the atmosphere continues to warm, Finland will see very warm months more often in the future. If people are currently sweating in May once a century, then a two-degree rise might cause them to sweat once a decade.

Climate change is slowly gathering pace

Climate change is often brought up in weather discussions when something occurs in the weather that people consider extraordinary. Räisänen reminds us that no individual weather event can yet indicate that we have crossed a critical threshold.

“Climate change proceeds slowly and steadily, and it can be summed up with the following six words: it is usually warmer than usual.”

This can be seen in long-term observations of the weather. Although there is variation from day to day and year to year, observations of average temperatures reveal that Helsinki, for example, has never been warmer than in this century. Only year 2010 was colder than the annual mean for the 20th century.

Climate change will also increase the frequency of extreme weather conditions, but Räisänen states that this should not be interpreted as meaning that the frequency of all possible weather extremes is increasing. For example, extremely cold winters and chilly summers are both becoming rarer. Rather, scorching hot summers and mild, rainy winters will likely become more common.

Weather forecasting is constantly developing

Climate change may bring new challenges to predicting the weather. As the climate becomes warmer, the amount of water vapour in the air will increase, which may result in showery rain becoming more prevalent. Predicting showers is more difficult than large areas of rain.

These showers may also result in a self-perpetuating process. When a sufficiently large amount of water vapour condenses in the atmosphere, the air is warmed, which enhances the rising motion, which results in yet more showers.

Räisänen reminds us that we are currently talking about long-term developments. Models and methods for predicting the weather are also developing constantly, and in any case, weather forecasts will become more accurate in the future.

Should we learn from this?

All decisions made by the mankind in the next ten years or so will affect the planet’s conditions far into the future. Some of the carbon dioxide released into the atmosphere will linger for thousands of years. Even if the planet stopped warming, sea levels would continue to rise long after that.

According to one prediction, CO2 emissions will triple by 2100. However, Räisänen does not want to believe that events will proceed according to the worst possible scenario.

“If were were heading towards this scenario, the consequences of climate change would be far more severe by 2050 than they are currently. There must be some limit to human stupidity, though.”


Will the next month of May be warm as well?

The above graph presents the likelihood of May 2019 being as warm as in 2018, when the average temperature was 14.5 degrees Celsius.

The graph has three different models based on observations and predictions, where each presents the average temperature for May and provides different probabilities for an average temperature of 14.5. The probabilities can be seen on the coloured area on the right side of the dotted vertical line.

The blue curve depicts the weather in May based on the climate between 1901 and 1980. The orange curve depicts the years 1980 to 2017, and the pink curve depicts 2018. In addition to the observations, the pink curve also accounts for the predicted effects of climate change.

Based on these three models, the probability of May 2019 being as warm as or warmer than May 2018 is approximately one per mil, 0.5 per cent or 1 per cent.

Therefore, the likelihood of another May 2018 occurring in the following years is rather small. Still, the unpredictable nature of the atmosphere means that next May could be just as warm or warmer. The probability of new record temperatures has increased manifold when comparing to a situation where climate change didn’t exist.