Greenland and Antarctica are melting, even though IPCC said in 1995 that they wouldn’t be.
Projection: In 1995, IPCC projected "little change in the extent of the Greenland and Antarctic ice sheets… over the next 50-100 years." In 2007 IPCC embraced a drastic revision: "New data… show[s] that losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003."
Reality: Today, ice loss in Greenland and Antarctica is trending at least 100 years ahead of projections compared to IPCC's first three reports.
Why the miss? "After 2001, we began to realize there were complex dynamics at work – ice cracks, lubrication and sliding of ice sheets," that were melting ice sheets quicker, said IPCC scientist Kevin Trenberth. New feedbacks unknown to past IPCC authors have also been found. A 2012 study, for example, showed that the reflectivity of Greenland's ice sheet is decreasing, causing ice to absorb more heat, likely escalating melting.
The fate of the world's coastlines has become a classic example of how the IPCC, when confronted with conflicting science, tends to go silent.
Projection: In the 2001 report, the IPCC projected a sea rise of 2 millimeters per year. The worst-case scenario in the 2007 report, which looked mostly at thermal expansion of the oceans as temperatures warmed, called for up to 1.9 feet of sea-level-rise by century's end.
Today: Observed sea-level-rise has averaged 3.3 millimeters per year since 1990. By 2009, various studies that included ice-melt offered drastically higher projections of between 2.4 and 6.2 feet sea level rise by 2100.
Why the miss? IPCC scientists couldn't agree on a value for the contribution melting Greenland and Antarctic ice sheets would add to sea-level rise. So they simply left out the data to reach consensus. Science historian Naomi Oreskes calls this – one of IPCC's biggest underestimates – "consensus by omission."
To its credit, the IPCC admits to vast climate change unknowns. Ocean acidification is one such impact.
Projection: Unmentioned as a threat in the 1990, 1995 and 2001 IPCC reports. First recognized in 2007, when IPCC projected acidification of between 0.14 and 0.35 pH units by 2100. “While the effects of observed ocean acidification on the marine biosphere are as yet undocumented,” said the report, “the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species.”
Reality: The world’s oceans absorb about a quarter of the carbon dioxide humans release annually into the atmosphere. Since the start of the Industrial Revolution, the pH of surface ocean waters has fallen by 0.1 pH units. Since the pH scale is logarithmic, this change represents a stunning 30 percent increase in acidity.
Why the miss? Scientists didn’t have the data. They began studying acidification by the late 1990s, but there weren’t many papers on the topic until mid-2000, missing the submission deadline for IPCC’s 2001 report. Especially alarming are new findings that ocean temperatures and currents are causing parts of the seas to become acidic far faster than expected, threatening oysters and other shellfish.
National Oceanic and Atmospheric Administration chief Jane Lubchenco has called acidification the "equally evil twin" to global warming.
Some carbon-cycle feedbacks that could vastly amplify climate change – especially a massive release of carbon and methane from thawing permafrost – are extremely hard to model.