It seems like a new report on climate change is coming out every month. Not only did the U.N. Intergovernmental Panel on Climate Change release its latest state of the science assessment last fall. The American Association for the Advancement of Science, the world's largest general scientific society, has also put out a report, albeit a much shorter one, that seeks to educate the public on "what we know" about climate change. Additionally, each year the World Meteorological Organization — a U.N. agency like the IPCC — releases a report summing up the state of the world's climate. Here's a roundup of key findings from this year's report, which came out just today:
1) 2013 tied with 2007 for the sixth-warmest year on record. The global temperature, averaged over the surface of our planet, was 0.5 degrees Celsius (or 0.9 degrees Fahrenheit) warmer than it was on average between 1961 and 1990. Only 2010, 2005, 1998, 2003 and 2002 were hotter. 2013 was just slightly warmer than it was between 2001 and 2010, the hottest decade on record. WMO arrives at this ranking by combining information from three temperature datasets: two from the U.S. and one from the United Kingdom.
2) Thirteen of the 14 warmest years on record have occurred in the 21st century. The only year outside the 2000s that is in the top 14 is 1998, which was unusually warm because a strong El Niño occurred that year. Still, individual years don't matter as much as the long-term trend. To that end, this chart, which assigns each decade a color, shows how the warmest 50 years stack up against one another. Notice that the years don't line up in perfect chronological order, but years of the same color (and thus decade) are clustered closely together, with years from the most recent decade topping the chart.
3) The Earth is still warming, even though the rise in air temperatures has flattened. Climate change contrarians often suggest that global warming has "paused" or experienced a "hiatus" since the late 1990s. While it's true that air temperatures haven't increased like they did prior to the late 1990s, scientists say that argument ignores what's happening across the rest of the planet. Scientists say that the Earth is still absorbing more energy, as sunlight, than it's radiating back to space. If that excess energy doesn't end up in the air, it has to go elsewhere on the Earth. The WMO report points to the oceans, which have sucked up far more heat during the "hiatus" period than they previously did. Between 2000 and 2013, the oceans added roughly three times as much heat as they did between 1980 to 2000, the report said. The amount of heat added varies depending on how deep you go in the oceans, but this chart offers one useful perspective on the general trend.
4) Man-made climate change contributed to at least some extreme weather events in 2013. Traditionally, scientists have had to exercise caution about pinning the blame for any single day or period of extreme weather on climate change, or even humans' contribution to the changing climate. Just as a period of heavy snow in one area doesn't necessarily provide evidence against climate change, a period of intense heat in an area doesn't necessarily provide evidence for it. Scientists now are increasingly able to probe whether a weather event bears at least some fingerprint of human influence (namely that of greenhouse-gas emissions from burning fossil fuels or deforestation).
Some research cited in the WMO report suggests that Australia's record-hot 2013 couldn't have happened without human influences. Not only was 2013 the warmest calendar year on record for the Land Down Under, but also the summer of 2012-2013 was its warmest summer on record. In a recent study that the WMO report cites, researchers used models to simulate Australia's climate with and without greenhouse-gas emissions from humans. The researchers concluded that Australia's summer heat wave was five times more probable with the added greenhouse gases (in conjunction with natural climate influencers) than without them. And Australia's record-warm 2013 as a whole, the WMO report said, would have been "virtually impossible" without the greenhouse gases that humans have added to the air.
5) Arctic sea ice grew last year both in terms of volume and reach, but the long-term trend is still downward. It's not terribly surprising that Arctic sea ice extent — the area of water that is at least partially covered in ice — bounced back in 2013; the ice had melted to a record-low extent in September 2012. But Arctic sea ice extent actually underwent record growth between September 2012 and the end of the 2012-2013 growth season in March. Still, we can't necessarily conclude that the Arctic's sea ice is "recovering." In the end, 2013's maximum sea ice extent fell below the long-term average, as this chart shows:
The red line, which represents 2013, is just below the 2000s average and well below the average of the previous two decades. Overall, Arctic sea ice extent's annual peak is decreasing an average of 2.6 percent per decade, the WMO report said. It's too early to make any conclusions about how this year will compare to 2012 or 2013, but so far 2014 definitely trends below previous decades' averages. Sea ice volume, which better reflects how much ice there actually is, also was higher in 2013 than in 2012 (as well as 2010 and 2011 for that matter), WMO's report said. But in the long term, it's also declining.
6) The Antarctic Ocean keeps gaining sea ice. In fact, in 2013 the Antarctic hit a new record for a single year's peak ice extent. The gains haven't been drastic — 2013's maximum ice extent was just 2.7 percent higher than the long-term average from 1981-2010. But scientists are still trying to understand why this phenomenon is occurring, but they're generating some interesting potential explanations for it. The Arctic and Antarctic have drastically different climate systems, and these differences may help explain why the Antarctic keeps gaining ice.
You can read the report summary here.