Global panel says there's no room for doubt on climate change


This post has been updated to clarify conclusions about likely average temperature increases.

In a long-awaited report, the world's top climate scientists say that there's no longer room to doubt or deny that climate change is happening and that human activity is the largest contributing factor.

Today, the U.N.'s  Intergovernmental Panel on Climate Change released a summary document for policymakers, which the BBC describes as the first installment of a collection of scientific climate change studies that'll come out over the next year. And the report's conclusions are not good — some are pretty grim. 
The IPCC lays out the report's conclusions in layman's terms via a series of press releases. Here are some of the highlights (many of these conclusions are laid out in one press release, which is available here):
  • While scientists were already certain that human activity is driving global warming, the report concludes "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
  • Since pre-industrial times, atmospheric carbon dioxide concentrations have increased by 40 percent, largely because of fossil fuel emissions and land-use changes. The oceans have absorbed about 30 percent of the emitted carbon dioxide, which is causing ocean acidification.
  • Depending on the greenhouse gas emissions scenario that plays out, the global average temperature increase will likely pass 1.5 degrees Celsius by the end of the century. The report also says that the end-of-the-century increase could be as low as .3 degrees Celsius and 4.8 degrees Celsius, according to an article in The Guardian. Scientists consider a 2-degree increase to be the tipping point beyond which serious changes will happen on a global scale.
  • "Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent," says one of the releases.
  • Arctic sea ice will very likely continue to shrink.
  • Increasing atmospheric carbon concentrations are also likely, and that'll lead to increasing ocean acidification, says the IPCC.