IPCC fifth assessment Report: main findings

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THE IPCC fifth assessment Report was released in Stockholm at the end of September 2013. It is the most important publication on climate change and is made of thousands of pages whose realization involved the work of hundreds of scientists.
In this article we are going to show the Report’s main findings; in future articles we will examine more in depth some of the thematic deeply studied in the Report.
The executive summary is categorical in its conclusion: climate change has not stopped and man is the main cause. That link was deemed “likely” (which means probability of 66%) in 2001, and “very likely” (90%) in 2007.
Human influence on the climate system is clear…
This is evident from the increasing greenhouse
gas concentrations in the atmosphere, positive radiative forcing, observed warming, and
understanding of the climate system..
IPCC  5th Assessment Report, September 2013
The latest iteration identifies radiative forcing, the difference between the amount of heat coming into the climate and the amount reflected back, as the immediate cause of warming. Radiative forcing is expressed in watts per square metre (W/m2), a unit of energy. A rise indicates that heat is building up in the system.
Total radiative forcing from man-made sources has risen from 0.29-0.85W/m2 in 1950 to 0.64-1.86W/m2 in 1980 to 1.13-3.33W/m2 in 2011. The average has jumped from 0.57 to 1.25 to 2.29, respectively—a four-fold increase in 60 years.
IPCC’ scientist make clear that, in the short term, air temperature increase can be very volatile; so for example, the rate of warming over the past 15 years (period affected by the El-Nino Phenomena) is 0.05ºC per decade...smaller than the rate calculated since 1951. However, It is Ocean warming that dominates the increase in energy stored in the climate system, accounting formore than 90% of the energy accumulated between 1971 and 2010, of which about 60% stored in the upper ocean (0–700 m) and the remaining 30% stored in the ocean below 700 m.
As a consequence of all this, the Greenland and Antarctic ice sheets have been losing mass while sea levels are progressively rising.
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…
IPCC  5th Assessment Report, September 2013
The average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of the ice sheets, was very likely 226Gt [trillion tonnes] a year over the period 1971-2009 and very likely 275Gt a year over the period 1993-2009. The extent of the Arctic sea ice has shrunk by 3.5-4.1% a decade in 1979-2012, more than was estimated in 2007, and the summer sea-ice minimum is shrinking by about 10% a decade.
The rate of sea level rise since the mid-19th century has been larger than the mean rate during the
previous two millennia (high confidence). Over the period 1901–2010, global mean sea level rose
by 0.19 [0.17 to 0.21] m
IPCC  5th Assessment Report, September 2013
Meanwhile, the mean rate of global averaged sea level rise was 1.7mm a year between 1901 and 2010, 2.0mm a year between 1971 and 2010 and 3.2mm a year between 1993 and 2010.
Finally, the report uses four sets of scenarios for greenhouse-gas concentrations to claim that “global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5ºC relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2ºC for the two high scenarios.” The 2ºC mark is widely considered to be the dividing line between warming which is just about tolerable and that which is dangerous.
To have a two-thirds chance of keeping global warming below 2ºC, cumulative CO2 emissions from all anthropogenic sources must stay between 0 and about 1,000 trillion tonnes. As for 2011 the amount reached already 531 trillion tonnes. At current rates of greenhouse-gas emission, the threshold above will have been reached before 2040. As Thomas Stocker, the co-chair of the report depressingly put it: “we are committed to climate change…for many centuries even if emissions of CO2 stop.”


October 11th, 2013