Are human ratings also progressing, could humans catch up with computers? (maybe playing against strong computers makes us strong)
Are team of humans able to defeat computers? Is any competition like this going on?
The best computers are much better than the best humans, and if I had to make up numbers I'd guess that they're improving at least ten times as fast as humans are (it's a wild approximation but, more or less, humans are improving by around 2 Elo points a year and computers are improving by around 20 points a year). There's no way humans will catch up.
Generally speaking, humans have already lost the battle against computers. Tablebases have "guaranteed" optimal play by computers in the endgame. Opening books have removed much of the computational burden in the opening. (It's MUCH easier to generate move or position lists for the first ten moves and index them than it is to compute them on the fly.) Computer hardware continues to get better.
Any situation that involves straight memorization is one that humans are going to lose to computers. With the opening and endgame "thoroughly optimized", the middlegame is the only place that humans really have a chance. Computers are deterministic. Compare these two situations with White to move:
[FEN "rn4rk/pp4pp/2bN4/q7/8/8/Q5PP/2BR3K w - - 5 10"]
[FEN "6rk/6pp/3N4/8/8/8/6PP/7K w - - 5 10"]
Ignore for a moment how contrived they are. In both cases, the correct move is the same,
Nf7#. However, even though a computer will pick up on the correct move, these positions are totally different from a computer's perspective. A human will look at this and recognize intuitively that the left half of the board is totally superfluous. A computer will have to compute that the positions are functionally equivalent.
However, as storage capacity increases, the number of these positions that aren't indexed slowly gets smaller. With the pasing of time, more and more of the middlegame approaches determinicity.
Consider also that it is much easier for computers to "learn" than for humans. It is trival (for given values of "trivial") to plug chess libraries in and give a computer access to all the accumulated knowledge therein. Humans cannot do that. We have to study for years to impress that upon ourselves.
TL;DR: As chess moves more toward exhaustive electronic storage and indexing of positions, humans will continue falling behind computers. Unless there is a breakthrough in our understanding of how memory and the brain work, there will be no reversing this trend.
I think that the whole theme of human against machine is unfair. After all, the computers use excellent algorithms and are trained on games played by human grandmasters and hooked up to high quality opening databases and endgame tables. An engine has a lot of human knowledge at the tip of its fingers and can access everything in a matter of seconds. From this point of view, chess engines are a combined success of human chess know-how and progress in computer science.
Today, the question of whether a human can beat a chess engine seems equivalent to asking if a boxer can beat a Terminator machine. I seem to recall that Kasparov said that humans can beat a chess engine, if the humans are given unlimited thinking time. It could be true but it has not been tested as far as I know. Considering that home computer driven engines reach Elo 3100, I don't see a human versus machine tournament any time soon.