The Intergovernmental Panel on Climate Change today published their much anticipated 5th Assessment Report entitled ‘Climate Change 2013: The Physical Science Basis’ and what they refer to as an unprecedented view of the climate system. In summary the report concludes that there is now even greater clarity about human impact on climate change. Under the different scenarios considered, those that assume strong mitigation see a chance of keeping global mean warming under 1.5°C. However, those that envisage continued carbon dioxide emissions (CO2) or postponed reductions, indicate that options of limiting global warming to 2°C may become unattainable.
Whilst many will already have read much of the media spin on the leaked and incomplete version, for those who prefer to read the facts rather than the fiction, here are just a few of the key conclusions.
· Warming of the climate system is unequivocal, 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.
· Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010.
· 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 rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.
· The atmospheric concentrations of CO2, methane (CH4), and nitrous oxide (N2O) 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.
· The atmospheric concentrations of CO2, CH4 and N2O have all increased since 1750 due to human activity.
· 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. This evidence for human influence has grown since the last report - AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.
· Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
· Changes in the global water cycle in response to the warming over the 21st century will not be uniform.
· The global ocean will continue to warm during the 21st century. Heat will penetrate from the surface to the deep ocean and affect ocean circulation.
· Global mean sea level will continue to rise during the 21st century. The rate of sea level rise will very likely exceed that observed during 1971–2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets.
· Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped.
The full report and summary for policymakers can be downloaded here.