In a first joint report on finance for nature in the G20 countries, they estimated that spending by the bloc - which includes large emerging economies - was $US120 billion in 2020.

Co-author Ivo Mulder, who heads the climate finance unit at the United Nations Environment Programme (UNEP), said the report's focus on the G20 and nature funding gaps could help those rich countries that have shown political leadership on the issue in recent months, including at the COP26 climate summit.

"The amount of money being invested in nature-based solutions is not nearly enough," Mulder told the Thomson Reuters Foundation.

"I would hope that G20 countries can lead by example and they're not doing that at the moment."

The report looked at how wealthy countries can better tackle the planet's climate, biodiversity and land degradation crises, such as by promoting sustainable farming and supply chains, or creating green spaces in cities to tackle rising heat.

Stepping up protection for natural areas such as parks, oceans, forests and wildernesses is seen as vital to maintaining the ecosystems on which humans depend, and to limiting global warming to internationally agreed targets.

Dozens of countries pledged to do more to conserve nature and make farming greener at November's COP26 climate talks, including a commitment by more than 100 members to halt and reverse deforestation by 2030

Last year, a United Nations report said global funding for nature conservation needed to triple this decade to about $US350 billion a year by 2030 and rise to more than $US536 billion by 2050.

The new study said the spending gap was larger and more difficult to bridge outside the G20 group of countries - not helped by the fact that only 2 per cent of the G20's $US120-billion investment in 2020 was directed towards overseas aid.

Funding from G20 countries represented 92 per cent of all global investment in nature, the report said, with the vast majority of this government money - 87 per cent or $US105 billion - allocated to programs inside their own borders, the report said.

Finance from the private sector also remains small at 11 per cent of the G20 total, or $US14 billion a year, even though it contributes about 60 per cent of gross domestic product in most G20 countries.

The report's authors called on the G20 to grab opportunities to boost their international investment in things like restoring degraded land and ecosystems, which can often be cheaper and more efficient than similar nature-based projects at home.

The report also noted that more than $US14.6 trillion was spent by 50 leading economies in 2020 to recover from the COVID-19 pandemic, of which only $US368 billion, or 2 per cent, was considered "green".

Governments need to "build back better" instead of following normal spending patterns after the COVID-19 crisis, it added.

Mulder said the report's findings could help governments implement their COP26 pledges and push forward with efforts to finalise a new global pact to halt and reverse biodiversity loss in Kunming, China, later this year.

Australian Associated Press