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.

Emissions

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.

Temperature

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.

Arctic Meltdown

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."

Ice Sheets

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.

Sea-Level Rise

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."

Ocean Acidification

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.

Thawing Tundra

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.

Projection: In 2007, IPCC reported with “high confidence” that “methane emissions from tundra… and permafrost have accelerated in the past two decades, and are likely to accelerate further.” However, the IPCC offered no projections regarding permafrost melt.

Reality: Scientists estimate that the world’s permafrost holds 1.5 trillion tons of frozen carbon. That worries scientists: The Arctic is warming faster than anywhere else on earth, and researchers are seeing soil temperatures climb rapidly, too. Some permafrost degradation is already occurring.

Large-scale tundra wildfires in 2012 added to the concern.

Why the miss? This is controversial science, with some researchers saying the Arctic tundra is stable, others saying it will defrost only over long periods of time, and still more convinced we are on the verge of a tipping point, where the tundra thaws rapidly and catastrophically. A major 2005 study, for instance, warned that the entire top 11 feet of global permafrost could disappear by century's end, with potentially cataclysmic climate impacts.

The U.N. Environmental Programme revealed this week that IPCC’s fifth assessment, due for release starting in September, 2013, will again "not include the potential effects of the permafrost carbon feedback on global climate."

Tipping points

The IPCC has been silent on tipping points – non-linear "light switch" moments when the climate system abruptly shifts from one paradigm to another.

Projection: IPCC has made no projections regarding tipping-point thresholds.

Reality: The scientific jury is still out as to whether we have reached any climate thresholds – a point of no return for, say, an ice-free Arctic, a Greenland meltdown, the slowing of the North Atlantic Ocean circulation, or permanent changes in large-scale weather patterns like the jet stream, El Niño or monsoons. The trouble with tipping points is they’re hard to spot until you’ve passed one.

Why the miss? Blame the computers: These non-linear events are notoriously hard to model. But with scientists recognizing the sizeable threat tipping points represent, they will be including some projections in the 2013-14 assessment.

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.