Just €280bn - that is the money needed to stop the rise in greenhouse gases and buy up to 20 years of time to fix global warming, according to United Nations climate scientists. It is 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 tons 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 Barron J Orr, lead scientist for 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."
Rene Castro Salazar, assistant director general at the UN Food and Agriculture Organisation (FAO), said of the two billion hectares of land around the world that has been degraded by misuse, overgrazing, deforestation and other largely human factors, 900 million hectares 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 $300bn, it is doable," 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. It will stabilise the atmospheric changes, the fight against climate change, for 15 to 20 years. We very much need that."
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 Eduardo Mansur, director of the land and water division at the FAO.
Marginal lands are being stressed around the globe by the twin phenomena of accelerated climate change and a rate of population growth which 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 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, thriving again, that's the key."
Last month, at a UN conference on desertification in New Delhi, 196 countries plus the EU 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 hectares of degraded land which could be realistically 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 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 - from using the wrong products, too much fertiliser, or in some areas too little - resulting in the soil losing its nutrients.
"The problem is big and it is growing," said Mansur. "The main cause of emissions from agriculture is poor land management. But the solutions are known: sustainable land management, sustainable water management, sustainable soil management."
He stresses the problem is not about reclaiming desert, but restoring wasteland that was productive before human intervention. "Don't mix desertification with desert," he said. "A desert is an ecosystem. There are deserts on the planet that have to be preserved."
Nor is it merely a matter of planting trees, since each area has to be considered in terms of the people who live there and how they can live on the land sustainably.
Kenya, for example, plans to plant two billion trees on 500,000 hectares to restore 10pc of its forest cover, but it is also working on ways to adapt to the changes in climate.
"We have to improve our livestock and crops to be drought resistant or drought tolerant," said Kennedy Ondimu, director of environmental planning and research at the country's environment ministry. "We have to look at developing our indigenous vegetables and indigenous livestock gene bank apart from embracing hybrid crop varieties and livestock varieties. We need to prioritise animal breeding," he said.
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