CSIRO chief Larry Marshall has apologised for describing the emotion of the climate debate as almost "more religion than science".

Key points: The CSIRO boss apologises for referencing religion in climate debate

The CSIRO boss apologises for referencing religion in climate debate He said he was referring to the 'passionate zeal around the issue'

He said he was referring to the 'passionate zeal around the issue' Dr Marshall defended this CSIRO shake-up, despite international backlash

Dr Marshall had told the ABC the backlash from his decision to restructure the organisation made him feel like an "early climate scientist in the '70s fighting against the oil lobby" and that there was so much emotion in the debate it almost "sounded more like religion than science".

He also said he would not be backing down on his controversial shakeup of the organisation's climate divisions, telling the ABC he was yet to be persuaded.

At Senate estimates Thursday afternoon he backed away from those comments.

"I'd like to apologise for any offence I may have caused to anyone with respect to my reference to religion," Dr Marshall said.

"I was merely referring to the passionate zeal around this issue, not any other reference, and I deeply apologise."

The redirection of climate science priorities at the CSIRO has drawn international condemnation, with thousands of climate scientists signing an open letter protesting against the changes.

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All up, 350 jobs will "change" — a plan that has drawn the ire of an Intergovernmental Panel on Climate Change co-chair and even the World Meteorological Organisation which has made an unprecedented statement condemning the decision.

He confirmed that half of the CSIRO's climate measurement and modelling staff would go as part of an organisational shakeup.

"We're reducing our labour in modelling and measurement by about half — but we're trying to be smarter about how we do things, trying to be more collaborative and leverage much more of a bigger portion of the innovation system that also works in climate," he said.

Labor senator Kim Carr questioned how CSIRO could assess its adaptation strategies without having the measurement capabilities to observe their effect.

"It's not a binary issue … we're continuing our measurements — so we're not stopping measuring but we're not the only people doing measurement," Dr Marshall said in response.

CSIRO 'chosen to shift emphasis to mitigation'

During the Senate estimates hearing Dr Marshall was quizzed about the backlash to the cuts and was asked if he thought the international scientists were wrong.

"We're not saying that modelling and measurement are not important. We're saying that modelling and measuring isn't more important than mitigation and we've chosen to shift our emphasis to mitigation," he said.

Who is Larry Marshall? Before heading up the CSIRO, Dr Marshall was venture capitalist in Silicon Valley

Before heading up the CSIRO, Dr Marshall was venture capitalist in Silicon Valley His firm specialised in developing Australian companies in Asia and the US

His firm specialised in developing Australian companies in Asia and the US He has a doctorate in physics

He has a doctorate in physics He spent 25 years working overseas before taking up the top job at the CSIRO in 2015

Dr Marshall detailed exactly which areas would be pared back under the changes — with 65 jobs to go in the Oceans and Atmosphere teams, up to 60 from Land and Water and 40 from manufacturing.

But he insisted redundancies would be a last resort.

"This change is a refresh and redirection of capability of CSIRO, not a cut to staffing level," he said.

"After this process, over two financial years a number of team members should be the same or slightly higher.

"But worst case, up to 350 team members could be affected and if they can't be redeployed or reskilled they will leave."