Global warming 'influencing weather extremes'

A tornado hit Albany in May. Photo / Supplied

Human-influenced global warming has played a role in the severe weather events in New Zealand and abroad over the past year, says a visiting climate expert.

Weather-related disasters in the past year range from a heatwave in Russia to flooding in Pakistan, China, India, and Queensland and drought in Brazil.

New Zealand also broke temperature and rainfall records and experienced a deadly tornado in Auckland.

Christchurch-born climate scientist Kevin Trenberth, now employed by the US National Centre for Atmospheric Research, said the effects of cumulative greenhouse gases building up in the atmosphere was most evident in rising ocean temperatures and ice melt.

He calculated that the sea surface temperature has increased 0.55C since the 1970s. This meant the water vapour in the atmosphere immediately above the ocean increased by 4 per cent.

A warmer, more moist environment was a breeding ground for storms and increased rainfall.

"The environment in which all storms form now is different from 30 or 40 years ago because of climate change.

"While we cannot say these events were due to global warming, it is highly likely these events would not have happened without global warming."

The high temperatures and warm ocean currents experienced in Australia and New Zealand were partly a product of the strongest La Nina event since the 1970s.

The La Nina or El Nino patterns influenced where rain fell and how strong rainfall was, but the additional factor of climate change could compound the intensity.

Dr Trenberth said the combination of natural swings between La Nina and El Nino with warming sea temperatures created new extremes.

"When natural variability is going in the same direction as global warming ... that's when we start breaking records.

"Global warming was a 5 or 10 per cent contribution which can be thought of as the straw that breaks the camel's back."

In 2010 the North Atlantic Ocean had its second most active hurricane season. Dr Trenberth's research connected this to record high sea surface temperatures in the Gulf of Mexico.

The warm water currents around eastern Australia were blamed for the devastating flooding and storms in Queensland in December and January.

A warmer world means extreme weather and increased rainfall will be the norm. For every 1C of sea surface temperature rise, atmospheric moisture over the oceans increased by 6 to 8 per cent.

After the deadly US spring storms, Dr Trenberth said: "When natural variability is compounded by human influences on climate this is what we get. Records are not just broken, they are smashed.

"It's as clear a warning as we are going to get about prospects for the future."