Global temperature records were broken again in May as NASA data showed the northern hemisphere also experienced the hottest spring on record.

Unusually heavy rain in Europe and parts of the US, as well as severe bleaching of coral reefs, was also seen in May.

The high temperatures have been particularly noticeable in high northerly latitudes and have led to very early melting of Arctic sea ice and the Greenland ice sheet.

The extent of average ice cover in the month was 580,000 square kilometers under the previous record low for the month, which was set in 2004.

The level of snow cover in the northern hemisphere was also extremely low.

Scientists say the strong El Nino weather phenomenon was partly to blame for the very high temperatures already seen this year.

However they also say that that the underlying cause of global warming remains atmospheric greenhouse gases from human activities.

Yesterday, scientists from Britain's Met Office warned that this year would likely see the concentration of carbon dioxide in the atmosphere breach 400 parts per million all year round for the first time at the monitoring station in Hawaii.

"The state of the climate so far this year gives us much cause for alarm," said David Carlson, Director of the World Climate Research Programme.

"Exceptionally high temperatures. Ice melt rates in March and May that we don't normally see until July. Once-in-a-generation rainfall events. The super El Niño is only partly to blame. Abnormal is the new normal."

"The rapid changes in the Arctic are of particular concern. What happens in the Arctic affects the rest of the globe. The question is will the rate of change continue? Will it accelerate? We are in uncharted territory."