The answer is because none of the models take into account either technological change or massive intentional human impact on the environment. [Yes, you read that correctly.]
Point one: technology
I recall watching the old Walter Cronkite specials on the future way back in the late 1960s. We were going to have household robots and you would get in your car, turn it over to the traffic control system, and doze your way to the Beach. Well, half the drivers on the road each weekend appear to be dozing their way to the Beach, but none of that ever happened.
Robotics in the Star Trek android/Terminator sense has so far turned out to be a dead end. And neither dear old Wally or his advisors even sensed the oncoming information revolution.
Rapid technological change (especially knowing which technologies will actually be feasible on a large scale) is essentially unpredictable.
Let's consider three examples from this week's news.
I posted the other day an update on India's Tata Nano. All the environmentalists are giving Mr. Tata crap for developing a cheap car that will enable millions of Indians to get off those damn motor scooters, and thus potentially screw up all the carbon emission forecasts. Except that Mr. Tata is apparently going to have the last laugh, as industry insiders report he will soon announce a Tata Nano fueled by the world's first compressed-air engine that generates little or no pollution.
So technology has potentially allowed Tata Motors to reduce carbon emissions in India by selling more cars, because the compressed-air engine will actually produce less pollution than the gasoline-powered motor scooters in use today.
Example two: from this week's issue of Discover, concerning ultra-capacitors:
Capacitors have the handy ability to store and release electrical energy very quickly—much more quickly than the batteries and fuel cells already being used in electric and hybrid-electric cars. Unfortunately, typical capacitors have been able to store only tiny amounts of charge, making them useless for driving the power-hungry engines these cars use. Not so ultracapacitors. While they still can’t store as much total energy as a fuel cell or a battery, ultracapacitors—also known as electrochemical capacitors—can supply the burst of energy needed to accelerate up a hill or around another car on the highway. They can also soak up energy that would otherwise be lost during braking, storing it for later use.
Such ultra-capacitors are now being field-tested; Toyota and Honda are looking closely.
Acceleration has always been an issue in hybrid or all-electric cars. Potentially the solution is closer than we think.
Example three; from the New Scientist regarding graphene:
The carbon supermaterial graphene is already known for its exotic electronic properties. Now two studies suggest that the material is also one of the strongest, most elastic and stiffest materials known to science.
Graphene crystals are atom-thick sheets of carbon atoms connected together in hexagons, like chicken wire.
Graphene flakes are produced every time we put pencil to paper – the graphite in pencils is simply a 3D structure comprising multiple stacked layers of graphene. And yet graphene was only isolated for the first time in 2004.
In the graphene "gold rush" since then, scientists have scrambled to uncover the material's properties and discover potential applications. The large surface-to-volume ratio and high conductivity already suggest uses in ultra-small electronics.
But researchers are also looking at bringing graphene out of the microscopic world:
"We are limited only by the size of graphene flakes available," says Booth. "There is no reason that the method will not scale up to much larger flakes."...
Graphene could be added to polymers to form super-strength composites, Booth says.
One of the most significant problems with automobiles is controlling weight (more pounds equal more energy necessary to move them). Suppose that with compressed-air engines, ultra-capacitors, and super-strong graphene we could build pollution-free automobiles that were light as a feather but safe in collisions while using very little energy....
And this is only one facet of the technological explosion occurring right now.
Point two: intentional massive human impact on the environment.
So far what we hear about in the media has to do with averting climate change, or reducing our carbon footprint (I personally like to leave carbon footprints on the ceiling to amaze my guests), but the reality is that we're very near to being able to intervene in the environment in a BIG way.
Again, back to Discover, looking at the sidebar to an article on ocean acidification and its threats, Three Bold Plans to Save the Seas:
1 One proposal, first suggested in the late 1980s by oceanographer John Martin of the Moss Landing Marine Laboratories in California, involves seeding ocean surfaces with iron to promote phytoplankton blooms that will soak up carbon dioxide, eventually exporting it into the deep ocean. The plan has the added theoretical benefit of reducing atmospheric carbon....
2 A second tactic under consideration at places like the Carnegie Institution of Washington and the University of California at Santa Cruz is to neutralize the seas—possibly with limestone from, say, the White Cliffs of Dover....
3 Last year a team of scientists led by Kurt Zenz House, a doctoral candidate at Harvard University, proposed something they call engineered weathering, inspired by a natural process in which slightly acidic freshwater is neutralized by exposure to alkalizing minerals. Under House’s proposal, hydrochloric acid would be harvested from the ocean by a specialized electrochemical treatment and then exposed to silicates, resulting in a net alkalizing shift....
There are, of course, technological drawbacks to all of these plans. And then there is the difficulty of funding them or achieving international consensus to pursue one or more of them.
But they are all inherently feasible with existing technology, and they represent only the smallest sample of scientists THINKING BIG.
Thinking small will get us lives that are, ultimately, nasty, brutish and short (with no apologies to Thomas Hobbes--he's dead and it has long since been in the public domain).
Thinking big will leave us scrambling to deal with the new consequences of our own actions.
But that, combined with the potentials of rapidly changing technology, suggests to me that we are mostly being far too passive about the potential for dealing with climate change.
Humanity has always succeeded best not when it adapts to given circumstances, but when it is the adapting agent.