“Climate change threatens irreversible and dangerous impacts, but options exist to limit its effects,” so states the briefest summary of the latest Intergovernmental Panel on Climate Change (IPCC) Assessment Report. The IPCC released the Synthesis Report of its Fifth Assessment Report over the weekend, which brings together all of its findings since the Fourth Assessment Report in 2007. While the report was as bleak and as stark as expected based on the interim reports issued over the last 13 months, it maintained the possibility that the situation could be mitigated – providing we can muster the political and collective will to do so.
The Synthesis Report provides an overview of the state of knowledge concerning the science of climate change. The work of over 800 scientists, it is the most comprehensive assessment of climate change ever undertaken. It also pulls no punches when it states: “Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems.” The challenge of understanding and managing risks and uncertainties are important themes in the report, and it uses a percentage based system of uncertainty to describe the likelihood of the outcomes discussed.
While it is impossible to summarize the 116-page report in the space available here, it provides detailed findings on sea level rise, atmospheric greenhouse gas concentrations, ocean warming, ice sheet loss and detailed analyses of human-induced drivers of global warming. Thomas Stocker, Co-Chair of IPCC Working Group I, states, “Our assessment finds that the atmosphere and oceans have warmed, the amount of snow and ice has diminished, sea level has risen and the concentration of carbon dioxide has increased to a level unprecedented in at least the last 800,000 years.” As an example, the report finds that since 1750 atmospheric concentrations of greenhouse gases have increased dramatically: carbon dioxide levels have increased by 40 percent, methane by 150 percent and nitrous oxide by 20 percent.
The report also outlines mitigation strategies for the effects of climate change. It observes that warming caused by carbon dioxide emissions “is effectively irreversible over multi-century timescales unless measures are taken to remove CO2 from the atmosphere.” Referring to world leaders’ pledge to curb global warming to no more than 2°C (3.6°F) by 2100, it notes: “Ensuring CO2-induced warming remains likely less than 2°C requires cumulative CO2 emissions from all anthropogenic sources to remain below about 3650 GtCO2 (1000 GtC), over half of which were already emitted by 2011.”
The report’s authors call for swift action to reverse the trends. Chair of the IPCC R. K. Pachauri stated, “The scientific case for prioritizing action on climate change is clearer than ever. We have little time before the window of opportunity to stay within 2°C of warming closes. To keep a good chance of staying below 2°C, and at manageable costs, our emissions should drop by 40 to 70 percent globally between 2010 and 2050, falling to zero or below by 2100. We have that opportunity, and the choice is in our hands.” In response, U.S. Secretary of State John Kerry, said: “This is another canary in the coal mine. We can’t prevent a large scale disaster if we don’t heed this kind of hard science.” The next meeting of world leaders to discuss action on climate change will occur in Lima, Peru from the 1st to 12th December, 2014.