What we've learned in 2008

Amanda Leigh Mascarelli looks at how far our understanding of climate change has come in the past twelve months.

1. Other greenhouse gases are also worrying

Scientists have long been aware of greenhouse gases other than carbon dioxide, but CO₂ has received most of the scientific and public attention owing to its prevalence in fossil fuel emissions and its long atmospheric life. However, scientific research published this year suggests that other heat-trapping gases also provide cause for concern. In July, scientists led by Michael Prather at the University of California, first proposed that nitrogen trifluoride, a gas produced in the manufacture of gadgets such as MP3 players and flat screen TVs, was likely to become a much greater contributor to climate change than previously assumed, mainly because of the growing demand for such products (Geophys. Res. Lett. 35, L12810; 2008). Their hypothesis was confirmed in October when Ray Weiss at the Scripps Institution of Oceanography, California, and colleagues found that the atmospheric concentration of the gas has increased 20-fold over the past three decades (Geophys. Res. Lett. 35, L20821; 2008). Also this year, several independent research groups reported a surge in emissions of methane (Geophys. Res. Lett. 35, L22805; 2008 and Nature 456, 628–630; 2008), a greenhouse gas twenty times more potent than CO₂. The exact source of the methane emissions remains a mystery.

2. Arctic summer sea ice is in rapid decline

Arctic sea ice saw some recovery this summer, compared with the record-breaking low set in 2007. However, the 2008 summertime minimum was still the second lowest level recorded since 1979, when the first satellite data of sea ice became available (National Snow and Ice Data Center 16 September 2008; http://nsidc.com/arcticseaicenews/2008/091608.html). In 2007, the Intergovernmental Panel on Climate Change projected that at the current level of emissions, summer sea ice could vanish completely anytime from 2040 to beyond 2100. But the extensive losses during the past two summers have led scientists to speculate that the Arctic Ocean may be ice-free in the summertime much sooner than anticipated. In October, scientists reported that the thickness of winter sea ice plummeted after the 2007 minimum, showing that the ice pack is not only shrinking but is decreasing in overall volume (Geophys. Res. Lett. 35, L22502; 2008). This is worrying because thin ice is more vulnerable to melting and creates a feedback effect: as the ice melts, dark, open water soaks up more of the sun's rays and further accelerates melting. Loss of Arctic summer sea ice could have not only regional, but global, effects and is widely regarded as a potential 'tipping element', in which a 'kick' to the system, driven even by natural variability, could lead to rapid, runaway warming.

3. Warming is already having an impact

The effect of human-induced warming on biological and physical systems, such as patterns of species migration and seasonal shifts, came into clear focus this year. An international team of researchers conducted a sweeping analysis of nearly 30,000 biological species and physical phenomena, such as timing of pollen release and bird nesting, and trends in ice melting. For the first time, researchers attributed pronounced, worldwide changes in these systems to human-caused climate change (Nature 453, 353–357; 2008). Spurred on by concerns that species and ecosystems may not survive such shifts, conservationists began to talk seriously about relocating species to help them adapt (Science 321, 345–346; 2008). And threatened by the loss of its icy habitat, the polar bear became the first species to be listed as climate-threatened under the Endangered Species Act, following a protracted legal battle by environmentalists.

http://www.nature.com/climate/2009/0901/full/climate.2008.142.html