The climate peril from land degradation

Human influence on lands is a major contributor to climate change — and climate change, in turn, is harming ecosystems and threatening food security, a major United Nations report finds.

Why it matters: While cutting fossil fuel emissions is vital, Thursday's report shows how the uphill battle to meet the Paris agreement's temperature goals requires focus on land use, too.

• Periodic reports from a wide range of scientists convened under the UN's Intergovernmental Panel on Climate Change (IPCC) provide a benchmark for policymakers, diplomats and advocates.
• The latest analysis arrives amid new data on the loss of forests, which absorb CO2.
• Per AP, new details from Brazil's space agency show a "surge" in Amazon deforestation.

The big picture: The report presents a wide-ranging look at the effects of human activity and climate change on lands, and how land use creates emissions.

• "Climate change can exacerbate land degradation processes ... including through increases in rainfall intensity, flooding, drought frequency and severity, heat stress, dry spells, wind, sea-level rise and wave action, permafrost thaw with outcomes being modulated by land management," the report states.

By the numbers: It's likely that human activities "directly" affect more than 70% of the planet's ice-free land surfaces, and the authors estimate that about a 1/4 of those ice-free lands are "subject to human-induced degradation."

Where it stands: The IPCC estimates that agriculture, forestry and other land uses account for 23% of net human-caused greenhouse gas emissions, including 13% of CO2 and 44% of methane.

• Lands, however, remain a net CO2 "sink," or sponge, on a worldwide basis, absorbing the equivalent of almost 1/3 of emissions from fossil fuels and industry, the IPCC said.

Threat level: That absorption capacity, which report co-author Louis Verchot calls a "natural subsidy," is under increasing stress from deforestation, permafrost melt and other forces, even as higher CO2 levels spur plant growth.

"That subsidy could very easily be lost if we continue on current trajectories," Verchot, a scientist with the International Center for Tropical Agriculture, told reporters on a call Wednesday.

"If we continue to degrade ecosystems, if we continue to convert natural ecosystems, if we continue to deforest, if we continue to destroy our soils, we are going to lose this natural subsidy that we are getting that’s protecting us, in part, from ourselves and from the damage we are creating as we pump these greenhouse gases into the atmosphere."

Why you'll hear about this again: The report explores ways that land management and sustainable food practices can help fight global warming.

• "All assessed modeled pathways that limit warming to 1.5ºC or well below 2°C require land-based mitigation and land-use change, with most including different combinations of reforestation, afforestation, reduced deforestation, and bioenergy," per the report.
• It also emphasizes that reducing food waste would ease pressure to develop new agricultural lands.
• The report estimates that up to 30% of food produced is lost or wasted.

The report explores the benefits of changing diets to be more plant-based and moving away from meat, which curbs CO2 and methane, and reduces pressure on lands.

• On that last point, dietary changes could prevent up to 8 gigatonnes of CO2-equivalent emissions per year by 2050, but that's at the high end of a very wide range.
• Report co-author Cynthia Rosenzweig of NASA told reporters that the 8 gigatonne total would represent global adoption of a vegan diet.

But, but, but: Land-use changes that help stem emissions can come with barriers and tradeoffs, the report notes.

• Large-scale conversion of land for bioenergy and afforestation can create competition with food production, for instance, as well as land degradation risks of its own.
• Another co-author, Rutgers' Pamela McElwee, said afforestation — that is, creating forests in areas where they weren't previously — on a massive scale could create major food price increases.