The record-breaking heatwave that hit western Canada and the Northwest United States last month is estimated to have been a 1 in 1000 year event, and "virtually impossible" without human-caused climate change, according to a new study.

A group of international climate scientists from the US, Canada, the UK, the Netherlands, France, Germany and Switzerland has released the results of a 'rapid attribution' study on last month's deadly heatwave in North America.

The research – published this week by the World Weather Attribution (WWA) group – found that the extreme temperatures observed during last month's heatwave made it a 1 in 1000 year event for today's climate.

But today's climate is around 1.2ºC warmer than the start of the industrial revolution. So, how much did global warming contribute to last month's record-breaking heatwave?

The study reveals that what is an improbable heatwave in today's climate would have been almost impossible in the past (at least 150 times less likely) and may become more likely in the future.

Last month's extreme heatwave "was about 2ºC hotter than it would have been if it occurred at the start of the industrial revolution," according to the WWA analysis.

It also found that the temperatures observed over a large region of western Canada and the Northwest US in late-June would have been "virtually impossible without human-caused climate change".

Looking into the future, the study shows that heatwaves like this one "would occur roughly every 5 to 10 years" if our planet reached 2ºC of global warming, which is only 0.8ºC warmer than today's climate.

Event attribution is a relatively new and emerging field of climate science, which aims to identify the human fingerprint on individual extreme weather events in near real-time. You can read more about the World Weather Attribution group here.