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On Earth Day 2013, a planetary report card on global warming

Planetary carbon dioxide concentrations are the highest they've been in the past 800,000 years, an ignominious milestone for Earth Day 2013. Still, the world is making some progress toward addressing global warming.

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Over much of the past decade, researchers and climate negotiators had focused on 450 p.p.m. as a target for stabilizing CO2 levels in the atmosphere. Doing that, they thought, would yield roughly a 50-50 chance of holding the rise in global average temperatures to about 2 degrees Celsius (3.6 degrees Fahrenheit) by the end of the century – a target that international climate negotiators settled on after reviewing the potential effects of higher temperatures.

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Five years ago, however, climate scientist James Hansen and colleagues published a paper that looked at environmental changes that global warming already was bringing and pointed to a 300 to 350 p.p.m. range as the target most likely to avoid the worst effects of global warming. If he's right, that means it could be a lot harder to keep the warming to within the 2 degree target.

Dr. Hansen retired from the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York this month to play a more activist role on climate issues.

Meanwhile, the Global Carbon Project reported in December that CO2 emissions are increasing at nearly the highest growth rates envisioned by the Intergovernmental Panel on Climate Change, a United Nations-sponsored scientific advisory body.

Within the Global Carbon Project's broad emissions numbers, however, one apparent hopeful spot has emerged, the project's international team of scientists says. Global CO2 emissions from land-use changes appear to have declined in absolute terms, as has their proportion of overall emissions.

Despite a sharp spike around 1997, when extensive peat and forest fires burned in Indonesia, CO2 emissions from land-use changes have eased from about 1.4 billion tons in 1990 to about 900 million tons in 2011. The team credits new efforts to combat deforestation – healthy forests lock up CO2 that plants take from the air – as well as replace trees that were felled. Where land-use changes accounted for 36 percent of global CO2 emissions in 1960 and 18 percent in 1990, the proportion stood at 9 percent in 2011.

Brazil, where deforestation has been rampant, has been one of the bright spots, especially over the past several years, notes Greg Asner, a researcher at the Carnegie Institution for Science's Department of Global Ecology at Stanford University in Palo Alto, Calif. Low commodity prices reduced the pressure to convert forests to farmland, he says, and Norway poured a lot of money into the Amazon Fund, designed to stave off deforestation. Brazil has also stepped up efforts to combat illegal logging.

These factors have combined to dramatically reduce deforestation in Brazil, which lost 27,777 square kilometers of tropical forest in 2004. Since then, fewer square kilometers have been lost nearly every year, reaching a preliminary figure of 4,656 square kilometers lost in 2012.

"That's still a lot," Dr. Asner says of the latest losses. "But it's real progress. We thought we were going to lose the system faster than it's being lost now."

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