UNDERESTIMATING: Today, ice loss in Greenland and Antarctica is trending at least 100 years ahead of projections compared to IPCC's first three reports. Pictured: Rajenda Pachauri, chair of the Intergovernmental Panel on Climate Change (IPCC). Image: Flickr/kk+
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Scientists will tell you: There are no perfect computer models. All are incomplete representations of nature, with uncertainty built into them. But one thing is certain: Several fundamental projections found in Intergovernmental Panel on Climate Change reports have consistently underestimated real-world observations, potentially leaving world governments at doubt as to how to guide climate policy.
At the heart of all IPCC projections are "emission scenarios:" low-, mid-, and high-range estimates for future carbon emissions. From these "what if" estimates flow projections for temperature, sea-rise, and more.
Projection: In 2001, the IPCC offered a range of fossil fuel and industrial emissions trends, from a best-case scenario of 7.7 billion tons of carbon released each year by 2010 to a worst-case scenario of 9.7 billion tons.
Reality: In 2010, global emissions from fossil fuels alone totaled 9.1 billion tons of carbon, according to federal government's Earth Systems Research Laboratory.
Why the miss? While technically within the range, scientists never expected emissions to rise so high so quickly, said IPCC scientist Christopher Fields. The IPCC, for instance, failed to anticipate China's economic growth, or resistance by the United States and other nations to curbing greenhouse gases.
"We really haven't explored a world in which the emissions growth rate is as rapid as we have actually seen happen," Fields said.
IPCC models use the emission scenarios discussed above to estimate average global temperature increases by the year 2100.
Projection: The IPCC 2007 assessment projected a worst-case temperature rise of 4.3° to 11.5° Fahrenheit, with a high probability of 7.2°F.
Reality: We are currently on track for a rise of between 6.3° and 13.3°F, with a high probability of an increase of 9.4°F by 2100, according to the Massachusetts Institute of Technology. Other modelers are getting similar results, including a study published earlier this month by the Global Carbon Project consortium confirming the likelihood of a 9ºF rise.
Why the miss? IPCC emission scenarios seriously underestimated global CO2 emission rates, which means temperature rates were underestimated too. And it could get worse: IPCC projections haven’t included likely feedbacks such as large-scale melting of Arctic permafrost and subsequent release of large quantities of CO2 and methane, a greenhouse gas 20 times more potent, albeit shorter lived, in the atmosphere than carbon dioxide.
Five years ago, the summer retreat of Arctic ice wildly outdistanced all 18 IPCC computer models, amazing IPCC scientists. It did so again in 2012.
Projection: The IPCC has always confidently projected that the Arctic ice sheet was safe at least until 2050 or well beyond 2100.
Reality: Summer ice is thinning faster than every climate projection, and today scientists predict an ice-free Arctic in years, not decades. Last summer, Arctic sea ice extent plummeted to 1.32 million square miles, the lowest level ever recorded – 50 percent below the long-term 1979 to 2000 average.
Why the miss? For scientists, it is increasingly clear that the models are under-predicting the rate of sea ice retreat because they are missing key real-world interactions.
"Sea ice modelers have speculated that the 2007 minimum was an aberration… a matter of random variability, noise in the system, that sea ice would recover.… That no longer looks tenable," says IPCC scientist Michael Mann. "It is a stunning reminder that uncertainty doesn't always act in our favor."