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Still not worried enough about climate change? Read the latest climate assessment report from the United Nations Intergovernmental Panel on Climate Change (IPCC). This is the fourth and final scientific volume to lay the groundwork for the next international summits in Peru, 2014, and Paris 2015. U.N. member countries have promised to consider yet another global accord relating to climate change. Will the dire changes ahead lead at last to a meaningful action plan? Alas, probably not, especially now that Republicans control the U.S. Senate.

The latest U.N. report combines the findings of three earlier reports issued in the past 13 months. As a result, it does not offer new evidence per se; instead the report pulls together all the existing findings, and shows how urgent it is for the world to act.

The report opens with an explanation of the terminology used by its authors to describe the statistical analyses undertaken by its scientists and the probabilities they assign to various scenarios. The report explains, in Box 2: “Communicating the degree of certainty in assessment findings,” that “[t]he degree of certainty in each key finding of the assessment is based on the type, amount, quality, and consistency of evidence (e.g., data, mechanistic understanding, theory, models, expert judgment) and the degree of agreement” or confidence in the finding.

For “agreement” or confidence, the terms used are low, medium or high. The likelihood, or probability, of some well-defined outcome having occurred or occurring in the future is described quantitatively as virtually certain (99-100% probability); extremely likely (95-100% probability); very likely (90-100% probability), and so on.

Here are some of the key findings:

• 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, and sea level has risen.

• The period from 1983 to 2012 was very likely the warmest 30-year period of the last 800 years in the Northern Hemisphere, where such assessment is possible (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence).

• From 2000 to 2010 emissions were the highest in history. Historical emissions have driven atmospheric concentrations of carbon dioxide, methane and nitrous oxide, to levels that are unprecedented in at least the last 800,000 years, leading to an uptake of energy by the climate system.

• Many terrestrial, freshwater, and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances, and species interactions in response to ongoing climate change (high confidence).

• Surface temperature is projected to rise over the 21st century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions. The ocean will continue to warm and acidify, and global mean sea level to rise.

• Warming caused by CO2 emissions is effectively irreversible over multi-century timescales unless measures are taken to remove CO2 from the atmosphere. Ensuring warming remains likely less than 2°C requires cumulative CO2 emissions from all anthropogenic sources to remain below about 3650 GtC (1000 GtC), over half of which were already emitted by 2011.

• Many aspects of climate change and its impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped. The risks of abrupt or irreversible changes increase as the magnitude of the warming increases. Global mean sea-level rise will continue for many centuries beyond 2100 (virtually certain).

The message is dire, but not hopeless.  U.N. Secretary General Ban Ki-moon was quoted as stating, “Science has spoken. There is no ambiguity … Leaders must act. Time is not on our side.” IPCC chairman Rajendra Pachauri stated that, “We have the means to limit climate change. The solutions are many and allow for continued economic and human development. All we need is the will to change,” that change being a break from our addiction to oil, coal and gas.

The day after the report was issued, the U.S. White House Office of Science and Technology Policy tweeted a statement that read, “The IPCC’s new Synthesis Report is yet another wake-up call to the global community that we must act together swiftly and aggressively in order to stem climate change and its worst impacts.”

The Canadian government has yet to formally respond, or issue a statement acknowledging the report.

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