Climate change driven by humans made heatwave 'twice as likely'

Climate change resulting from human activities made the current Europe-wide heatwave more than twice as likely to occur, say scientists.

Researchers compared the current high temperatures with historical records from seven weather stations, in different parts of Europe.

Their preliminary report found that the "signal of climate change is unambiguous," in this summer's heat.

They also say the scale of the heatwave in the Arctic is unprecedented.

The scale and breadth of the current heat being experienced across Europe has prompted many questions about the influence of global warming on extreme events.

To try and see if there is a connection, researchers looked at data from seven weather stations, in Finland, Denmark, Ireland, the Netherlands, Norway and Sweden.

They chose these locations because they all had digitised records dating back to the early 1900s, unlike the UK. The team also used computer models to assess the scale of human-influenced climate change.

The researchers found that in the weather stations in the Netherlands, Ireland and Denmark, climate change has generally increased the odds of the current heatwave by more than two-fold.

There are several different definitions of what exactly makes a heatwave but the researchers in this study have gone with the hottest consecutive three-day period in a year. This has allowed them to compare the data from the seven different locations over the past 100 years or so.

"In many parts of Europe three day heat is not very exceptional and you could argue that it would be better to look at longer," said Dr Friederike Otto from the University of Oxford, one of the study's authors.

"But we've looked at longer periods and it doesn't change the result very much."

The researchers also say the warmest three days in succession this year may not yet have happened but they believe that even if next week is warmer, it won't change the overall impact.

Scientists are loath to say a specific event was "caused" by climate change - however they believe that this new study joins a growing list of solid links between rising temperatures and extreme events.

One thing the researchers can't say right now is whether the high pressure system that has been blocked over Europe for almost two months was caused by climate change. The scientists, from the World Weather Attribution group say they will address this question when they formally publish their findings in a scientific journal later this year.

They can't be that definite. However the study does give figures for what are termed "return periods" or the chances of something happening again.

They estimate that in southern Scandinavia it's likely there will be a similar heatwave every 10 years, while further south, in the Netherlands, it's likely to be once every five years. This ties in with projections from several scientists that the type of heatwave we've had this summer could occur every second year by the 2040s.

"The logic that climate change will do this is inescapable - the world is becoming warmer, and so heatwaves like this are becoming more common," said Dr Friederike Otto, from the University of Oxford.

"What was once regarded as unusually warm weather will become commonplace - in some cases, it already has," she added.

While acknowledging that the current heatwave in the Arctic is unprecedented in the historical record, the researchers were not able to clearly resolve the impact of human influence.

That's because summer temperatures there vary a good deal from year to year so the trend was impossible to estimate from the observations, the authors said.

Despite their reservations about the Arctic they argue that their initial findings should prompt more action on cutting carbon from governments.

"We are not taking the right measures," said Dr Robert Vautard, from the CNRS in France.

"We are discovering climate change rather than doing something against it."

It involves some serious number crunching!

This new research is called an attribution study - the researchers work out how often these type of extreme heat events have happened at each of the weather stations they looked at.

They compare those findings to modelled results of the climate without the influence of human emissions of carbon dioxide. This way they can work out how much climate change has tipped the odds of a rare event happening.

The list continues to grow.

The major European heatwave of 2003 was among the first events to be linked though it took scientists several years to do it - eventually they concluded that human induced climate change had made the event 500% more likely!

These days the attributions studies are much faster - just last year scientists concluded that the flooding in Houston, Texas was made 38% more likely by climate change while the so-called "Lucifer" heatwave in Eastern Europe was made 10 times more likely. This new study was completed in less than a week.