The 1992 Earth Summit failed. Will this year's edition be different? Quick: How many people know that there's a U.N. Convention to Combat Desertification? No? Once upon a time, it was a huge deal - one of the three major international agreements to come out of the 1992 Earth Summit in Rio de Janeiro. The other two dealt with climate change and biodiversity. Losing the war on dunes. (JOE PENNEY) The logic was simple enough. As farming spreads around the world, more land is in danger of being overexploited and crumbling into desert. The 1992 convention aimed to study this issue and halt the degradation. Yet in the 20 years since, the problem has only gotten worse. In 1992, about 15 percent of the world's land was degrading. Today, it's 24 percent. Nations have only just recently, in 2009, agreed on a set of "metrics" for desertification. Effectively, there's been zero progress. And as a new report card (pdf) in the journal Nature finds, this is true of most of the agreements and promises that were made at that 1992 Rio summit. The world has made little headway in slowing climate change, little headway in halting species extinction. On June 20 of this year, the world's leaders will convene again in Rio to reboot the Earth Summit. But how confident can we be that this new incarnation of global environmental talks, 20 years later, will be any different from the last one? Back at the 1992 summit, there were already signs of trouble. Rich countries and poor countries bickered endlessly in Rio over who should pay for various environmental protections. Yet, in the end, the world's nations agreed to keep thrashing these issues out in U.N. forums - this was the origin of the series of global climate talks at Kyoto, Copenhagen, Durban, and so forth. The hope was that lasting solutions would slowly emerge over time. That didn't quite happen. The Nature report card gives the world a fat red "F" on averting global warming. True, there has been decent progress on measuring greenhouse-gas emissions and conducting more scientific research, but carbon emissions keep rising and rising. The United States never agreed to the Kyoto Protocol, a treaty to limit carbon emissions, and now countries like Canada and Japan want out, too. Meanwhile, the world only gets a "C" on measures to reduce tropical deforestation and to prepare for the coming impacts of climate change. Or take the Convention on Biological Diversity, also launched in 1992. The world gets a "C" in protecting ecosystems - about 10 percent of the world's ecologically valuable land has been protected by 2010. But there's another big "F" in reducing the speedy pace of extinctions around the world. Scientific journals are now rife with warnings that the Earth could soon face its fifth-ever "mass extinction event," driven by human activity. "Plausibly we are a little better off than if we didn't have all of this diplomacy," David Victor, a scientist at the University of California, San Diego, told Nature. But the evidence is hard to find. And so now, twenty years later, the U.N. is warning - in a gloomy 525-page report - that the environment has reached the breaking point. The stats pile up oppressively: Greenhouse gases are set to double in the next 50 years. Nearly 90 percent of water and fish samples are contaminated by pesticides. About 20 percent of vertebrate species are under threat of extinction. Coral reefs have declined by 38 percent since 1980. Of the world's 90 most important environmental goals set over the years, the U.N. report finds, significant progress has been made on just four of them, despite dozens of treaties and talks and conventions and meetings. One of the few big exceptions was the 1987 Montreal Protocol, in which world leaders agreed to phase out CFCs and other chemicals that were chewing through the protective ozone layer. That's a rare instance of successful diplomacy, as the "ozone hole" above Antarctica has stopped growing and is soon set to shrink. So can the 2012 Earth Summit be any more successful than past incarnations? The details that gummed up the 1992 talks - haggling over who would pay for what - haven't disappeared. When it comes to global warming, poorer nations still want wealthy industrialized countries to pay for carbon cuts, while countries like the United States are unwilling to take drastic measures unless nations like China and India do more. The warnings from scientists and environmentalists may sound far more dire these days, but the basic political hurdles are as large and obtrusive as ever. Related: Check out George H.W. Bush's 1992 speech at Rio. The tone is vastly different from that of today's generation of Republicans. A sample: "We stand ready - government and private sector - to help spread "green technology" and launch a new generation of clean growth."