Arctic explorers have taken the first-ever samples of ocean water at the north pole after a gruelling two-and–a-half month expedition across the polar ice.

Headed by former bank manager Ann Daniels, the Catlin Arctic survey team achieved what last year's expedition - led by polar explorer Pen Hadow - failed to do: reach the north pole and take water samples to measure the impact of a changing climate.

Pen Hadow, the survey's director and last year's expedition leader, said: "It's not possible to imagine what this team has had to do to pull off this extreme survey. I consider them to be the world's toughest to have done this."

The survey hopes to measure how fast the Arctic Ocean is acidifying due to rising CO2 levels and what effect it has on the region's animals and plants. Setting out in early March, the three-strong explorer team trekked over 483 miles across sea ice off the coast of Greenland to the geographic north pole.

Daniels said: "It has been an unbelievably hard journey. Conditions have been unusually tough and at times very frustrating with a frequent southerly drift pushing us backwards every time we camped for the night. On top of that we've had to battle into headwinds and swim across large areas of dangerously thin ice and open water."

The team also struggled with ice cracks forming under their tent and thin ice and fierce north winds.

Last year's Catlin Arctic survey, which found evidence that Arctic ice was thinner than expected, was beset by technical difficulties, and the team had to be airlifted off the ice before reaching the pole.

On their journey to the north pole, the Catlin team drilled, collected samples from water as deep as 5,000m, and measured ice thickness.

As the adventurers forged north, a separate team of scientists undertook measurements and samples at an ice base north of Canada in -45C temperatures. Between the two groups, the survey has collected over 2,200 pieces of data from plankton collections, ice core samples and around 350 water samples. The samples will now be sent to British Columbia in Canada for analysis.

Globally, oceans have seen a 30% increase in acidity on pre-industrial levels, the fastest rate of change in 55m years. Scientists say that carbon emissions from human activity is to blame. The Arctic Ocean appears to be acidifying faster than warmer regions because cold water absorbs more CO2.

"As it's been collected for the first time, this data will be viewed as baseline information for further studies, providing insight into the impact of carbon dioxide absorption [in the Arctic]," said Dr Tim Cullingford, science manager for the Catlin Arctic survey.

The survey hopes to present the findings of the expedition before the end of this year.