The record-breaking heatwave scorching the US west this week would have been “virtually impossible” if not for the climate crisis, a team of scientists has determined.

Millions of Americans from the Pacific coast to the Rockies baked under unseasonably warm and even dangerous temperatures this week, with temperatures up to 30F (17C) above average for the time of year.

The climate crisis, caused primarily by the burning of fossil fuels, has made this kind of heatwave four times more likely to occur over the last decade, according to a new rapid analysis released Friday.

“These temperatures are completely off the scale for March,” said analysis co-author Ben Clarke, who is an extreme weather and climate change researcher at Imperial College London, in a statement.

In a world without global warming, the current heatwave would have also been milder, with temperatures about 1.4F (0.8C) cooler, says the analysis by World Weather Attribution, an international consortium of climate researchers.

“These findings leave no room for doubt. Climate change is pushing weather into extremes that would have been unthinkable in a pre-industrial world,” said Friederike Otto, a climate science professor at Imperial College London, who also worked on the study.

To carry out their rapid analysis, the scientists examined forecasts for a five-day period, from 18-22 March. To quantify the impact of global warming on the week’s extreme temperatures, the researchers analyzed weather and forecast data, and also used climate model simulations to compare how heat events have changed in today’s climate.

Fueled by an area of high pressure in the atmosphere, the heat dome has shattered temperature records in 140 cities stretching from California to Missouri, according to the Weather Channel, while leaving California, Nevada and Arizona under extreme heat warnings on Thursday.

More heat is in store for the coming days. The mercury is expected to continue ticking upward in the south-west, and the heatwave is expected to creep toward the plains and the south later this week. By the end of the week, 100 cities could set all-time temperature records for the month of March, with temperatures climbing as high as 30F (17C) above average for the time of year, the new analysis says.

Heat is the deadliest form of extreme weather in the US. Weather officials this week raised concerns about an increase in heat-related illnesses, especially among vulnerable populations, and advised people to remain hydrated and stay inside when they can.

The heat has also taken a toll on local economies, with multiple California and Tahoe-area ski resorts being forced to close or shrink operations amid rapid snowmelt and high temperatures this week.

“In the US west, the seasons that people and nature were used to for centuries are disappearing, putting many, including outdoor workers and those without air conditioning, in danger,” said Otto. “The threat isn’t distant – it is here, it is worsening and our policy must catch up with reality.”