The volume of rainfall from Tropical Storm Harvey is probably linked to climate change associated with global warming that increases the amount of moisture in the atmosphere, the UN weather agency said today.

The slow-moving storm has brought catastrophic flooding to Texas, killed at least nine people, led to mass evacuations and paralysed Houston, the fourth most-populous US city.

The US National Weather Service had to introduce a new colour on its graphs to deal with the volume of rain.

"Climate change means that when we do have an event like Harvey, the rainfall amounts are likely to be higher than they would have been otherwise," Clare Nullis, spokeswoman for the World Meteorological Organization, told a United Nations briefing in Geneva.

"Climate change does very likely increase the associated rainfall," she said, adding in some locations Harvey's rainfall may approach 1.2 metres, and that the US National Weather Service had to introduce a new colour on its graphs to deal with the volume of rain.

"Climate change doesn’t cause tropical cyclones. They’ve always been there. The relationship between climate change and the frequency of hurricanes and tropical cyclones is not clear, there’s still a lot of research going on into that," Nullis said.

US President Donald Trump plans to visit Texas today.

In June, Trump announced the United States would withdraw from the Paris climate change pact, drawing international condemnation. This month he rolled back environmental regulations on government-funded building projects in flood prone areas as part of his proposal to spend $1 trillion to fix aging US infrastructure.