From the Environmental Defense Action Fund:

Here are 10 startling facts we learned in 2009 that underscore the climate threat:

A study published in the journal Science reports that the current level of carbon dioxide (CO2) in the atmosphere – about 390 parts per million – is higher today than at any time in measurable history -- at least the last 2.1 million years. Previous peaks of CO2 were never more than 300 ppm over the past 800,000 years, and the concentration is rising by around 2 ppm each year.

The World Meterological Organization reported that 2000-2009 was the hottest decade on record with 8 of the hottest 10 years having occurred since 2000.
2009 will end up as one of the 5 hottest years since 1850 and the U.K.'s Met Office predicts that, with a moderate El Nino, 2010 will likely break the record.
The National Snow and Ice Data Center reported that while a bit more summer Arctic sea ice appeared in 2009 than the record breaking lows of the last two years, it was still well below normal levels. Given that the Arctic ice cover remains perilously thin, it is vulnerable to further melting, posing an ever increasing threat to Arctic wildlife including polar bears.

The Arctic summer could be ice-free by mid-century, not at the end of the century as previously expected, according to a study by the National Oceanic and Atmospheric Administration.

Recent observations published in the highly respected Nature Geosciences indicate that the East Antarctica ice sheet has been shrinking. This surprised researchers, who expected that only the West Antarctic ice sheet would shrink in the near future because the East Antarctic ice sheet is colder and more stable.
The U.S. Global Change Research Program completed an assessment of what is known about climate change impacts in the US and reported that, "Climate changes are already observed in the United States and… are projected to grow." These changes include "increases in heavy downpours, rising temperature and sea level, rapidly retreating glaciers, thawing permafrost, lengthening ice-free seasons in the ocean and on lakes and rivers, earlier snowmelt, and alterations in river flows."

According to a report by the US Geological Survey, slight changes in the climate may trigger abrupt threats to ecosystems that are not easily reversible or adaptable, such as insect outbreaks, wildfire, and forest dieback. "More vulnerable ecosystems, such as those that already face stressors other than climate change, will almost certainly reach their threshold for abrupt change sooner." An example of such an abrupt threat is the outbreak of spruce bark beetles throughout the western U.S. caused by increased winter temperatures that allow more beetles to survive.

The EPA, USGS and NOAA issued a joint report warning that most mid-Atlantic coastal wetlands from New York to North Carolina will be lost with a sea level rise of 1 meter or more.

If we do not reduce greenhouse gas emissions by the end of the century, some of the main fruit and nut tree crops currently grown in California may no longer be economically viable, as there will be a lack of the winter chilling they require. And, according to a study published in the Proceedings of the National Academy of Sciences, U.S. production of corn, soybeans and cotton could decrease as much as 82%.