Photograph from China Daily, Reuters

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Extreme weather, like this unusually severe downpour in Chengdu, China, has likely increased.

Photograph from China Daily, Reuters

Friday’s Big Global Warming Report: 5 Takeaways

Climate scientists express increased confidence of human link to global warming.

In the last century, the air and the water has warmed, snow and ice has melted, and the seas have risen. The world’s climate scientists already knew that, but they expressed renewed confidence in those troubling trends in a major report Friday that has come to represent the global scientific consensus around global warming.

The release of the Intergovernmental Panel on Climate Change summary report kicks off a series of climate reports set for release this year and next.

“Many of the observed changes are unprecedented over decades to millennia,” begins the report, which summarizes the worldwide changes worldwide wrought by climate change and its likely future effects. “Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.” (See related “Quiz: What You Don’t Know About Climate Change Science.”)

Amid changes in the air and seas, average global surface temperature data show an increase of 1.5 degrees Fahrenheit (0.85 Celsius) from 1880 to 2012, the report says. (Related: “UN Climate Report Relevance Debated Amid Rollout.”)

“The warming in the climate system is indeed unequivocal,” says report co-chairman Thomas Stocker of Switzerland’s University of Bern, speaking at the report’s release in Stockholm. “It is not just one decade that is warmer, but a succession of decades.” (Related: “Does Global Warming Pause Debate Miss Big Picture?”)

Five takeaways from the new report:

1. On the extreme weather front, the report concludes it is “very likely” that cold days and nights have decreased, while warm days and nights have increased, since 1950. More extreme precipitation has also likely increased worldwide, particularly in North America and Europe. That means that the top one percent of heaviest rain or snow storms are heavier now, as compared to then. In other words, when it rains hardest, it pours harder.

2. The oceans have warmed with “virtual certainty,” the report concludes, at a rate of about  0.2 degrees Fahrenheit (0.11 Celsius) per decade since 1970 in the upper 246 feet (75 meters) of surface water. Ocean warming accounts for more than 90 percent of the heat added to the atmosphere by global warming in that time, with most of it pumped into the top 2,300 feet (700 meters) of the oceans. “That doesn't mean the oceans are saving us,” Stocker says. “It means it would be much worse without the oceans.”

All the excess carbon dioxide emitted by burning fossil fuels in that time has “very likely” also increased acidification of the ocean—which threatens corals, shelled sea creatures, and the oceanic food web—by 26% since the beginning of the industrial era. That means a pH drop of 0.1 delivered to the global oceans by humanity.

3. What about those polar bears? Sea ice (as well as glaciers and ice sheets) has declined overall since 1970. The loss of sea ice very likely accelerated since 1993. Only in Antarctica is sea ice cover growing, something predicted by climate change forecasts. In the Arctic, the average sea ice extent decreased around 3.5 to 4.1 percent per decade from 1979 to 2012. Similarly, permafrost temperatures have increased across most regions, although the amount varies, since the 1980s.

4. Sea level rise has happened, and will happen in the future, as a result of global warming, the report finds. On average, sea level has risen 7.5 inches (0.19 meters) since 1901, and will rise higher with “virtual certainty” in this century. Barring a collapse of Antarctic ice sheets, sea level rise is not likely to exceed 3.22 feet (0.98 meters) by 2100, the report says. (Read “Rising Seas” from the September issue of National Geographic magazine.)

5. More than half of the global warming observed since 1950 has a human cause, largely from the greenhouse gas effects of gases such as carbon dioxide emitted from burning fossil fuels. All of the effects seen in the report look “virtually certain” to continue in the future as long as emissions continue.

“If we don’t take action, then temperatures will increase,” says Michel Jarraud, Secretary-General of the World Meteorological Organization. He warned at the Friday morning briefing on the release of report that current industrial and consumer greenhouse gas emissions are on the high side of various future “scenarios” in the report that project warming in this century.

The report uses precisely-defined, and oft-debated, language to express its confidence in impacts or observations about climate change, such as:

  • “virtually certain,” which means a  99–100 percent confidence.

  • “very likely,” which means a  90–100 percent confidence.

  • “likely,” which means a 66–100 percent confidence.

So, when the report authors concludes that heat waves have “likely” increased on some continents since 1950, they are expressing 66 percent or better confidence in the finding.

“We do not go for headlines, but we make scientific statements,” Stocker said at the briefing, before concluding: “Climate change challenges the two primary resources of humanity, land and water. It challenges our planet, our only hope.”

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