Addressing the Landcare conference in Sydney, Professor Mark Howden from the Australian National University (ANU) criticised the way companies can continue to pollute by buying offsets.
"Our first priority in terms of emission reductions should be stopping the fossil fuel combustion burning," he said on Wednesday.
Australia's carbon offset scheme allows companies to continue burning fossil fuels by buying offsets without reducing the amount of their greenhouse gas emissions.
Professor Howden said companies should not be allowed to offset without first reducing their emissions.
"We should not be using, at this stage of climate change, once off or potentially once off carbon sequestration on our landscapes to give a free ride to other industries," he said.
Prof Howden said big emitters should not be able to just trade their way out of trouble.
"There is a case for using carbon offsets in terms of hard-to-abate sectors," he said on Wednesday.
"We can't use that ... which allows them to continue doing what they're doing, not investing in alternatives, simply because they can get cheap carbon credits, some of which might be dodgy."
Prof Howden - who is director of the ANU's Institute for Climate, Energy and Disaster Solutions - told the conference agricultural productivity has already been "hit big time" by climate change.
He said recent studies showed on average across the globe climate change had driven agricultural productivity down about 20 per cent.
"The challenges of feeding the world but looking after our land resources will just increase over time if climate change is let rip," he said on Wednesday.
Increasing global temperatures were already having an impact on food supply, he said, with the battle for water destined to get worse as climate change intensifies.
"If we think we've got challenges in terms of competition for water between agriculture, environment, urban and industrial uses (now), this is just going to get far, far more intense."
The chair of Landcare Australia Doug Humann told the conference the recent state of the environment report has left many reeling, and highlights the need for greater investment in the conservation sector.
He told AAP the program "has been starved of funds in recent years, it's lost funds".
"We need to reinstate the level of funding for landcare, the National Landcare Program. It needs to be directed to landcare projects on the ground," he said.
"The twin crises facing land and water management in Australia are climate change and biodiversity loss."
Conservationists, farmers and climate experts are meeting face to face for the first time in four years for the Landcare conference.
During a farm visit to a Landcare project at Camden on the outskirts of Sydney on Tuesday, Mr Humann said the charity was reliant on volunteers and needed an injection of youth.
"What we need is more young people involved," he said. "It's always going to be heavily dependent on volunteers, and we encourage that, but it needs that government support as well."
One of those volunteers is Tony Biffin, who helped to plant some of the 22,000 trees that line the banks of the Nepean River at the Camden town farm in Sydney's southwest.
The dairy farmer spends about six hours a week volunteering at the council-owned property, where he helps manage the beef cattle herd.
"It comes from a love of ... the land here and the farm," he told AAP.
"It's all about sustainability and more sustainable agriculture."
Climate Change Minister Chris Bowen has been contacted for comment.
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