Highlights of the IPCC 4th Assessment

Following are highlights of the U.N. Intergovernmental Panel on Climate Change Fourth Assessment draft report, released in February, which builds upon new and more comprehensive data and research over the past six years.

Evidence of human impact

  • “Warming of the climate system is unequivocal, as is now evident from increases in global average air and ocean temperatures, melting of snow and ice, and rising sea levels.”
  • “It is ‘very likely’ (more than 90% probability) that anthropogenic (human) greenhouse gas increases caused most of the observed increase in globally averaged temperatures since the mid-20th century.”
  • “The atmospheric concentration of carbon dioxide in 2005 (379 ppm) exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores.”
  • “Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely (more than 66% probability) the highest in at least the last 1,300 years.”

Future projections

  • Global average air temperatures are expected to rise 2-10 degrees F if global warming continues unabated.
  • Warming tends to reduce land and ocean uptake of man-made carbon dioxide, thereby increasing the amount of emissions that remain in the atmosphere; warming also is increasing the acidification of oceans.
  • The IPCC estimates a 7-23 inch rise in sea levels by 2100. These estimates exclude the effects of ice flow and carbon feedbacks that will accelerate sea rise.
  • It is “very likely” that extremes such as heat waves, droughts, heavy rains and more intense hurricanes and cyclones will become more frequent. Arctic sea ice could disappear in summer by the latter part of the 21st century in some projections.

Article originally published Fall 2007.

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