The wildfire season globally has lengthened by almost 20% in 35 years as the average temperature has risen, a climate change study says.

"Fire weather seasons have lengthened across 29.6 million square kilometres of the Earth's vegetated surface," said a paper in the journal Nature Communications on Tuesday.

This resulted in a 18.7% increase in season length overall.

"We have shown that combined surface weather changes over the last three-and-a-half decades have promoted global wildfire weather season lengthening," said a team of scientists in the United States and Australia - two countries particularly affected by wildfires.

The researchers used climate data and three indices of fire danger, to produce an overview of fire season length from 1979 to 2013.

Weather is the largest driver of fire risk - temperature, humidity, rainfall and wind speed all influence the frequency and intensity of wildfires. These factors, in turn, are all affected by a changing climate.

Wildfires play a crucial role in clearing forests, improving grazing and boosting new plant growth, but also pose a threat to human life and property, and nature.

About 350 million hectares are burnt every year, said the report. Fighting wildfires cost the United States about $US1.7 billion per year over the past decade, and Canada about $US1 billion.

Total wildfire costs in Australia in 2005 were estimated at nearly $US9.4 billion, or 1.3% of gross domestic product, said the report.

Over the study period, the global land area with "unusually" hot years increased by 6.3%per decade, said the team.

"Fire weather season length and long fire weather season affected area significantly increased across all vegetated continents except Australia," they reported.

And they warned: "If these trends continue, increased wildfire potential may have pronounced global socio-economic, ecological and climate system impacts."