In a surprise move, US Senate Republicans joined Democrats and went on record Wednesday acknowledging that climate change is real.

The symbolic amendment, attached to a controversial bill authorizing contruction of the Keystone XL oil pipeline, was approved 98-1 after Senator James Inhofe, seen as the top climate change denier in Congress, announced he was supporting the legislation.

Inhofe however strongly rejected any suggestion that human activity was responsible for climate change.

The move can be seen as a critical step forward for US lawmakers, but it is not a revolutionary one.

The 16-word measure states: "It is the sense of the Senate that climate change is real and not a hoax."

It makes no mention of the impact of human activity, including the use of fossil fuels, on global warming.

Two other measures attributing climate change to human activity failed to pass the 60-vote threshold in the 100-member Senate, although one got five Republican votes and the other received 14.

"This is a small victory but an important one," Senator Barbara Boxer, top Democrat on the Senate's environmental panel, told reporters.

"It means that there's a softening of the attitude of the deniers. They're losing ground in the face of public opinion."

Senator Brian Schatz, who introduced the failed measure with the most direct attribution of climate change to human activity, said Wednesday's progress "exceeded my expectations."

"There is an emerging bipartisan group of people who believe that climate change is real and caused by humans and solvable," he said.

The measures were introduced by Democrats keen on highlighting differences with some Republicans on the simmering issue of climate policy.

Republicans hold the majority in both the Senate and the House of Representatives following November congressional elections.

But with the 2016 presidential race on the horizon, Republican leaders may have agreed to hold votes on the measures now in order to avoid potentially embarrassing climate votes in the midst of a White House race.

Applause rippled through the chamber when Inhofe declared he was co-sponsoring the measure saying climate change is not a hoax.

"Climate always changed," Inhofe said, noting there was archaeological, historic and "biblical evidence" of that.

"The hoax is, that there are some people who think they are so arrogant to think that they are so powerful that they can change the climate. Man can't change climate," Inhofe insisted.

During his State of the Union address Tuesday, President Barack Obama chided Republicans for refusing to acknowledge scientific conclusions that human activity is impacting the climate.