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Friday, September 27, 2013

Michael Mann & Dana Nuccitelli: IPCC report underlines the need for climate action

IPCC report underlines the need for climate action

Cairns flooding
The predictions of sea level rise have been revised upwards in the latest IPCC report.Credit: Cairns Regional Council
The IPCC report to be officially released later today finds, again, that humans are to blame for increasing global temperatures. It adds an exclamation mark to the call to reduce greenhouse gas emissions.
IT HAPPENS EVERY SIX YEARS or so: the Intergovernmental Panel on Climate Change (IPCC) publishes its assessment of the current state of scientific understanding regarding human-caused climate change. That assessment is based on contributions from thousands of experts around the world through an exhaustive review of the peer-reviewed scientific literature and a rigorous, several-years-long review process.
One significant change from the 2007 IPCC report involves climate scientists' confidence that humans are causing global warming. In 2007, the IPCC stated with 90 per cent confidence that human greenhouse gas emissions were the main cause of global warming since the mid-1900s. Now the IPCC states with 95 per cent confidence that humans are the main cause, even with the new inclusion of the cooling effect from sunlight-scattering human aerosol emissions.
As with all IPCC conclusions, this confidence is based on an evaluation of the peer-reviewed scientific literature. For example, a paper published in 2012 by climate change attribution experts Tom Wigley and Ben Santer concluded with 95 per cent confidence that humans have caused at least half the global warming since 1950, and most likely all of it. Similarly, the IPCC report concludes that humans have caused at least 50 per cent and most likely 100 per cent of the global warming over the past six decades, with external natural factors like the sun and internal natural variation like ocean cycles each contributing approximately zero to the warming during that time.
The robust body of scientific evidence summarised by the IPCC report is the reason behind the 97 per cent expert consensus that humans are causing global warming.
However, in the weeks leading up to the publication of the 2013 IPCC report, climate contrarians have been working overtimepublishing opinion articles full of myths and misinformation, to mislead and confuse the public about its sobering message. Most of these opinion pieces have focused on the mythical global warming 'pause' (more accurately described as a speed bump), which in reality merely refers to a temporary slowing in the warming of air temperatures at the surface of the Earth.
As the IPCC report notes, the oceans absorb approximately 93 per cent of the warming of the global climate. When we account for all the data, including the heating of the oceans, air, land, and melting of ice, there has actually been more global warming over the past 15 years than during the previous 15 years, with the climate accumulating heat at a rate equivalent to four Hiroshima atomic bomb detonations per second during that time.
The IPCC also notes that the global surface warming speed bump is based on cherry picking.
The Intergovernmental Panel on Climate Change was established by the United Nations Environment Program and the World Meteorological Organisation in 1988 in order to review and report on the published climate science.
In 2007 it won the Nobel Peace Prize, along with former US Vice President Al Gore,"for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change"
In 2010 it came under fire for including two errors in the report. Since then it has established an official protocol for dealing with suspected errors.
    * More than 9,200 scientific publications cited
    * 209 lead authors and 50 review editors from 39 countries
    * More than 600 contributing authors
    * More than 2 million gigabytes of data from climate models
    * 1089 expert reviewers from 55 countries and 38 governments
    * 54,677 review comments
The surface warming trend from 1997 to 2012 is lower than the average projection made by climate models, but the surface warming trend from 1993 to 2007 was significantly higher than projections. That's mainly because during the 1990s, ocean cycles acted to accelerate human-caused global warming, while in the 2000s, more heat has been transported to the deep ocean layers. These cycles average out over time, and the long-term human-caused global warming trend is crystal clear.
The IPCC also expresses strong certainty that the Earth is experiencing the impacts of that warming in the form of melting ice, rising global sea levels and various forms of extreme weather. In fact, in order to correct for several overly conservative estimates in the 2007 IPCC report, the 2013 report has increased its projections for the future rates of several climate impacts such as Arctic sea ice melt and sea level rise.
The 2013 IPCC report has taken a conservative approach with regards to the climate's sensitivity to the increased greenhouse effect. The estimate of the amount of warming we expect from a doubling of atmospheric carbon dioxide in a best-case scenario from 2°C to 1.5°C. This change brings the estimated climate sensitivity range back to the same values estimated in the IPCC reports published in 2001 and earlier.
This lowered best-case estimate is based on one narrow line of evidence: the aforementioned surface warming speed bump. Yet there are numerous explanations for this temporary change (unaccounted for effects of volcanic eruptions and natural variability in the amount of heat buried in the ocean) that do not imply a lower sensitivity of the climate to greenhouse gases. Moreover, other lines of evidence contradict a climate sensitivity lower than 2°C. It is incompatible, for example, with paleoclimate evidence from the past ice age, or the conditions that prevailed during the time of the dinosaurs, as we discussed in a previous ABC piece.
Despite its conservative approach, the IPCC has expressed much higher confidence than ever before that humans are the dominant cause of the current global warming. We will see far more dangerous and potentially irreversible impacts in the decades ahead if we do not choose to reduce global carbon emissions. There has never been a greater urgency to act than there is now.
The latest IPCC report is simply an exclamation mark on that already-clear conclusion.
Michael Mann is Distinguished Professor of Meteorology at Penn State University and author of two books, "The Hockey Stick and the Climate Wars: Dispatches from the Front Lines" and "Dire Predictions: Understanding Global Warming". He was an expert reviewer of the 2013 IPCC report. You can follow him on Twitter @MichaelEMann.
Dana Nuccitelli is an environmental scientist and climate blogger for The Guardian and Skeptical Science. You can follow him on Twitter @dana1981.

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