Courtesy of Amanda Leigh Mascarelli, originally published in Nature.

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.

4. The hockey stick holds up

A follow-up to the infamous 1998 'hockey stick' curve confirmed that the past two decades are the warmest in recent history. Climatologist Michael Mann's contentious graph has become a symbol of the fierce debates on evidence for global warming, to the extent that an independent investigation into the study was performed at the request of US Congressman Joe Barton. The 2006 report that resulted from the Barton enquiry criticized Mann and colleagues for their reliance on tree-ring data from bristlecone pines as a proxy to reconstruct Northern Hemisphere temperatures over the past 1,000 years.

Although their earlier work had been largely vindicated, in September the same team revised their global surface temperature estimates for the past 2,000 years, using a greatly expanded set of proxies, including marine sediments, ice cores, coral and historical documents (Proc. Natl Acad. Sci. USA 105, 13252–13257; 2008). The team reconstructed global temperatures with and without inclusion of the tree-ring records: without their inclusion, the data showed that recent warming is greater than at any point in at least the past 1,300 years; inclusion of tree-ring data extended this period to at least 1,700 years. According to the Christian Science Monitor: "It still looks a lot like the much-battered, but still rink-ready stick of 1998. Today the handle reaches further back and it's a bit more gnarly. But the blade at the business end tells the same story."

5. Skeptics are still out there

Despite a near-universal scientific consensus to the contrary, climate change sceptics continued this year to insist that global warming is a farce. Although the Republican party officially acknowledges the role of humans in climate change, Alaskan Governor Sarah Palin remained unconvinced during her campaign as Senator John McCain's vice presidential running-mate, asserting "I'm not one though who would attribute [global warming] to being manmade".

The NBC late night show Saturday Night Live famously satirized Palin: when asked about her views on global warming, Palin's double Tina Fey responded, "I believe it's just God hugging us closer." More recently, climate bloggers have been up in arms over two articles published by the website Politico. One calls into question the science behind global warming and the other suggests that extreme cold-weather events coincide with appearances by former US Vice president Al Gore. Little wonder that American public opinion still fluctuates over whether climate change is a serious problem (Wired Science 14 May 2008; http://blog.wired.com/wiredscience/2008/05/the-climate-cha.html).

Amanda Leigh Mascarelli is a freelance science writer based in Denver, Colorado.

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Image Credit:
Tambako the Jaguar