Season's 1st tropical storm forms in eastern Pacific

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Season's 1st tropical storm forms in eastern Pacific

Forecasters say Hurricane Aletta is not a threat to land.

Slow-moving tropical storms and hurricanes also lead to strong winds blowing for a longer duration over the same place and possibly more storm surge, Kossin told the AP.

The unusually slow-moving Hurricane Harvey was a recent example.

Dr Kossin said: 'Tropical cyclones over land have slowed down 20 per cent in the Atlantic, 30 per cent in the western North Pacific, and 19 per cent in the Australian region.

"We've kind of hypothesized that this type of behavior may happen, this slowing down of the forward speed of the cyclones", said Colin Zarzycki, a climate scientist at the National Center for Atmospheric Research who has reviewed at Kossin's study.

In the second study, published in the Journal of Climate, a team led by Ethan Gutmann, with the National Center for Atmospheric Research in Boulder, tried to see how 22 hurricanes that hit in the last 13 years might be different if they hit in the future under a warmer climate.

Kossin admits that there are probably both natural and manmade factors influencing the slowing of storms and recommends further studies using climate models to determine how much greenhouse gas emissions are responsible for affecting the storms' speeds. A slow storm increases the risk of damaging floods. Between 1949 and 2016, tropical cyclone translation speeds declined 10 percent worldwide, the study says.

And at the same time, a hotter atmosphere can hold more moisture.

"The poles tend to become disproportionately warmer than the tropics do under global warming", Kossin said.

But Kossin, in his paper, writes that he wouldn't expect big changes in his results due to different means of measurement, since "estimates of tropical-cyclone position should be comparatively insensitive to such changes".

"What we're seeing nearly certainly reflects both natural and human-caused changes", Kossin said.

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