Climate researcher Dr. Gianluca Grimalda says that he risks losing his post at the Kiel Institute for the World Economy in Germany for refusing to use a plane as a means of transport back from Papua New Guinea.

Grimalda plans to return to Germany from his research trip without flying in order to reduce his carbon emissions by 90%, and instead intends to travel using ferries, coaches, trains and cargo ships.

However, he said the institute gave him three days notice to return by October 2 which would have forced him to fly, according to a statement issued by climate group Scientist Rebellion.

Grimalda told CNN that the Kiel Institute told him to "expect a second warning/request to show up in Kiel traveling by plane," after he failed to return and that they "will then issue the letter of redundancy" when he fails to show up a second time.

His salary for September has not been paid and his offer of unpaid leave has been turned down, he added in a statement.

The Kiel Institute told CNN that it "doesn't comment on internal personnel matters in public. When traveling on business, the Institute supports its employees in traveling in a climate-friendly manner."

When asked about Grimalda's claim that his September salary was unpaid, the Kiel Institute said that it would not publicly comment on an "internal personnel matter... for the protection of our employees."

An experimental economist and campaigner with Scientist Rebellion, Grimalda had been carrying out fieldwork in Bougainville, Papua New Guinea for six months, studying the relationship between globalization, climate change and social cohesion.

Grimalda's permission for traveling ended on September 10 but his research took longer than originally planned due to several security threats -- including one occasion when he was held hostage by a group armed with machetes and all his belongings were confiscated, Scientist Rebellion's statement said.

As the climate crisis intensifies, the impact of aviation has come under increasing scrutiny. Commercial aviation causes 2.5% of global CO2 emissions each year, with flight demand -- and emissions -- forecast to increase significantly in the coming years. As an alternative, climate activists such as Greta Thunberg have modeled greener ways to travel such as by ship or train.

"I am prepared to face all the legal and economic consequences of this decision," Grimalda said in a statement. "Ultimately it's also a mental health issue. My psychological state can't but be described as one of climate anxiety, and flying can't but worsen this condition."

It took Grimalda 35 days to reach Papua New Guinea from Germany earlier this year, as he traveled 15,000 kilometers (9,321 miles) overland to Singapore before taking a flight for the second leg of the trip.

He is currently still in Papua New Guinea before he leaves on a cargo ship on October 8 for his long trip back to Germany.