The world's leading climate scientists said in a United Nations report that there was no longer room for doubt that climate change was occuring, and the dominant cause has been human actions in pouring greenhouse gases into the atmosphere.
The IPCC declared today in the “Summary for Policymakers” of its Fifth Assessment Report on climate science, from hundreds of the world’s top scientists, there is a 95 to 100 percent chance global warming is caused by humans. “Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes,” the Intergovernmental Panel on Climate Change wrote in the report that also called on governments to “step up” their responses.
Climate-change deniers have promoted the fact that temperatures have risen more slowly in the past 15 years as a refutation of man’s responsibility for global warming. Scientists say this explanation is oversimple and doesn’t take into account a long-term warming trend.
Warming of the climate system is "unequivocal," states the report, and since the 1950s, many of the observed changes are "unprecedented over decades to millennia." The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.
Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years.
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.
The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. CO2 concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.
Now, the IPCC has for the first time embraced the concept of a “carbon budget” and calculates that humanity has already burned through slightly more than half of its carbon budget (531 metric tons by 2011) since the onset of the Industrial Revolution in the 18th century.
The New York Times reports, “at the rate energy consumption is growing, the trillionth ton will be released somewhere around 2040…. More than three trillion tons of carbon are still left in the ground as fossil fuels.”
So, humanity has roughly 30 years to quit fossil fuels and shift to a new foundation of clean, renewable sources such as solar and wind power.
Once again, the world's leading scientists on climate change have spoken. Their conclusion is that global warming is real, that it is already causing enormous problems for our planet, and that it will only get worse unless we take bold action to cut greenhouse gas emissions and transform our energy system.
It is long past time for Congress to address this crisis.
U.N. Secretary-General Ban Ki-moon said the study was a call for governments, many of which have been focused on spurring weak growth rather than fighting climate change, to work to agree a planned U.N. accord in 2015 to combat global warming.
"The heat is on. Now we must act," he said.
U.S. Secretary of State John Kerry said the report was a wake-up call, "Those who deny the science or choose excuses over action are playing with fire," he said, referring to skeptics who question the need for urgent action.