US$300 billion (S$409 billion) will be needed to stop the rise in greenhouse gases and buy up to 20 years of time to fix global warming, according to UN climate scientists. That is equal to the gross domestic product of Chile, or the world's military spending every 60 days.
The sum is not to fund green technologies or finance a moonshot solution to emissions, but to use simple, age-old practices to lock millions of tonnes of carbon back into an overlooked and over-exploited resource: the soil.
"We have lost the biological function of soils. We have got to reverse that," said lead scientist Barron Orr of the UN Convention to Combat Desertification.
"If we do it, we are turning the land into the big part of the solution for climate change."
Dr Rene Castro Salazar, assistant director-general of the UN Food and Agriculture Organisation (FAO), said of the 2 billion ha of land worldwide that has been degraded by misuse, overgrazing, deforestation and other largely human factors, 900 million ha could be restored.
Returning that land to pasture, food crops or trees would convert enough carbon into biomass to stabilise emissions of CO2, the biggest greenhouse gas, for 15 to 20 years, giving the world time to adopt carbon-neutral technologies.
"With political will and investment of about US$300 billion, it is doable," Dr Castro Salazar said. We would be "using the least-cost options we have, while waiting for the technologies in energy and transportation to mature and be fully available in the market".
The heart of the idea is to tackle the growing problem of desertification - the degradation of dry land to the point where it can support little life. At least a third of the world's land has been degraded to some extent, directly affecting the lives of two billion people, said Mr Eduardo Mansur, director of the FAO's land and water division.
Marginal lands are being stressed by accelerated climate change and a rate of population growth that could lift the global tally to almost 10 billion people by 2050, he said. Much of that growth is in areas such as Sub-Saharan Africa and South Asia where land is already highly stressed.
"The idea is to put more carbon into the soil," said Mr Orr. "That's not going to be a simple thing because of the natural conditions. But keeping the carbon in the soil and getting that natural vegetation, grazing land and so on thriving again - that's the key."
Last month, at a United Nations conference on desertification in New Delhi, 196 countries and the European Union agreed to a declaration that each country would adopt measures needed to restore unproductive land by 2030.
The UN team has used satellite imaging and other data to identify the 900 million ha of degraded land that could be restored. In many cases, the revitalised areas could benefit the local community and host country through increased food supply, tourism and other commercial uses.
Key to returning dry lands to vegetation is the use of fertiliser, said Mr Mansur. "Fertilisers are essential for increasing productivity. Good fertiliser in the right quantity is very good for the soil."
But decades of poor agricultural practices in both rich and poor nations have resulted in misuse, either from using the wrong products, using too much fertiliser, or in some areas, using so little that the soil loses its nutrients.