In 2100, what will be the world's #1 superpower?

[Citizen James]

The problem is that human behavior is chaotic (in a mathmatical sense), the further ahead you try and project the harder it is to get a decent estimate.  Things like the weather or the stock market are childs play in comparison.

Some possible scenerios:

The new crusades
The terrorists suceed in transforming the war on terrorism into a war of peoples.  The west makes it into a new crusades.  Japan, though not predominantly Christian, still gets involved in the war  after devistating attacks in Toyko.  Result: A phyric victory for the west - Paris in ruins, major damage in countless western cities, much of the world's oil supply under a radioactive glow.  Beneficiaries:  Brazil, Argentina, cand Russia, who have their own oil supplies and would be at most tangential to the war while being fairly well along in their industrialization.   China makes a decent run for it, but is too corupt and inefficent to hold superiority all the way up to the century mark without sweeping reforms.

American Theocracy/fascism.
  America swings hard to the fundementalist right and continues to gut the bill of rights in the name of national security.  War with China looms after North Korea is "cleansed with the holy fire" of nuclear weapons.  Japan sits tensely in the middle trying to convince all parties that they are more or less "Swiss".  Result:  The reminants of the US (Minus LA, San Fransisco, Seatle, and most of the west coast and major western cities), turn severely isolatinist and are viewed as paraiahs by most of the rest of the world.  China ceases to exist, along with much of southeast asia.  Japan gets through largely unscathed from direct attacks, but with terible aftermath of fallout has to deal with major health issues.  Beneficiaries: the EU.   Though the enviorment and climate change become the #1 most important issues, they are best suited technologically and furthest from the devistation to hold their own.

Deux ex machina
Who says the future has to be bad?  Technological advances could revolutionize the way the world works - safe, cheap unlimited sources of energy; inexpensive means of turning seawater into drinkable water; cures for AIDS and cancer; and so on.   Beneficiaries - everyone, but especially the developers (most likely the US, Japan, or the EU).

Diablo ex machina
Of course, there is also the law of unintended consequences.  One of the factors in the downfall of the Roman empire was side effects from their technology - more specificly their plumbing was made with lead leading to widespread lead poisioning.   What happens if a new technology becomes widespread, and the consequences are either ignored or slow to manifest.  Beneficiaries - nobody, though less developed nations would presumably be less effected.

China Reborn
Survivors of the Tien a Mein square massicre come to power in China and enact massive democratic reforms.  Programs are undertaken to produce transperancy in government leading to greater dectection and elimination of coruption.  There are growing pains, but with a large and increasingly well educated population, along with widespread natural resources, they overtake the still strong US in terms of overall power and influence.

India matures
The world's most populus democracy, continues to industrialize and educate it's people.  With technial know-how and sizable resources, they rise quickly to world power status.

And those are just a few vague possiblities.  There are plenty more where those came from.