The United Nations panel known as the Intergovernmental Panel on Climate Change meet once every six years, reviewing the latest independent analysese of climate change using direct observations of Earth’s climate, palaeoclimate archives, theoretical studies of climate processes and simulations using climate models. The 95 percent certainty that humans have caused most of the warming of the planet's surface is an increase in certainty from the 90 percent certainty in the last assessment report, which came out in 2007.
The Summary for Policymakers of the IPCC’s Fifth Assessment Report was released September 27, and warns that a pause in warming over the past 15 years is too short to reflect long-term trends. Present-day carbon dioxide levels are at an "unprecedented" level which has not been seen for at least the last 800,000 years. Continued emissions of greenhouse gases like carbon dioxide will cause further warming and changes in all aspects of the climate system. Containing these changes will require sustained reductions of greenhouse gas emissions.
Sea level is also set to continue to rise at a faster rate than over the past 40 years, with water levels expected to rise between 26 cm at the low end and 82 cm at the high end. Over the last twenty years the Greenland and Antarctic ice sheets have been melting, glaciers have receded in most parts of the world, and Arctic sea ice has continued to shrink in terms of extent.
The Arctic has lost 75 percent of its summer sea ice volume
Last year's Arctic sea ice minimum exceeded the previous record low, with the ice declining the previous two years as well; this year there has been a short-term 'recovery'. The Arctic has lost 75 percent of its summer sea ice volume over the past three decades and climatologists are not just concerned about the extent of the sea ice, but also its thickness, which has been in constant decline. Each year the Arctic sea ice reaches its minimum in September. Arctic sea ice extent reached its lowest point this year on September 13, 2013 when sea ice extent dropped to 5.10 million square kilometres. September averaged an ice extent of 5.35 million square kilometres, placing 2013 as the sixth lowest ice extent, both for the daily minimum extent and the monthly average. September ice extent was 1.17 million square kilometres below the 1981 to 2010 average.
Sea ice in Antarctica reached a winter maximum extent of 19.47 million square kilometres this year. Antarctic sea ice extent for August 2013 averaged 6.09 million square kilometres, which was 1.03 million square kilometres below the 1981 to 2010 average for August, but above the level recorded last year. 2012 had the lowest September ice extent in the satellite record. In any given year the weather can act to either preserve more or melt more sea ice at both poles.
The Arctic ice cap grows each winter as the sun sets for several months and shrinks each summer as the sun rises higher in the northern sky. Each year the Arctic sea ice reaches its annual minimum extent in September. It hit a new record low in 2012. This summer's low ice extent continued the downward trend seen over the last thirty-four years. Scientists attribute this trend in large part to warming temperatures caused by climate change. Since 1979, September Arctic sea ice extent has declined by 13.7 percent per decade. Summer sea ice extent is important because, among other things, it reflects sunlight, keeping the Arctic region cool and moderating global climate.
Each of the last three decades has been warmer at the Earth's surface, warmer than any period since 1850, and most likely warmer than any time in the past 1,400 years. Model simulations indicate that the change in the global surface temperature by the end of the 21st Century is likely to exceed 1.5 degrees Celsius, relative to 1850.
The new report alters one figure from the 2007 report: the temperature range given for a doubling of CO2 in the atmosphere was 2.0C - 4.5C in the 2007 report and this range has been changed to 1.5C - 4.5C.
This video depicts a scenario in which carbon dioxide concentrations reach 670 parts per million by 2100, up from around 400 ppm today: http://bit.ly/1h8zPg9
The IPCC Summary for Policymakers: http://bit.ly/15UJlPg
National Snow and Ice Data Center: http://bit.ly/15OwqE2