Guess it depends on what you mean by the Earth. We are all part of the Earth in some sense. With very few exceptions, everything we make was made from materials either mined or gathered from the Earth. So your global consciousness thing is in a sense already here. To badly mangle the words of Carl Sagan, "We are all made of Earth stuff... and so are our toys."

I do not think that we will be able to ask the earth a question. It would exist on a different level of consciousness, to ask the question would require interaction and manipulation of the system from outside the system. In the same way that a neuron cannot ask the brain a question, an ant cannot ask it's colony "hive mind" a question. I do believe that the earth itself (including us and all living creatures as part of the system) does have a form of consciousness already though. It can react to changes and threats and communicate it's state amongst many other things.

This should really be a response to palmagma and yeroca's comments but I just tried that and 1. My response was too big for a comment and 2. When I submitted my reponse it failed and I lost what I had said. Ok, so slightly retracting what I said before. I think that it would be possible for one person to ask the Earth a question, but not more than one person. In the same way that if neurons were intelligent enough to ask the brain a question by changing their pattern of firing, one neuron could ask the brain a question. But if all neurons did this the brain would cease to function as a brain. I agree that a system needs inputs, outputs and a processing unit for communication to be viable. The Earth has several things that could be used as inputs and outputs (with some processing going on). For example, an alien intelligence might create storms as an input and measure the rate of growth of a major city as an output. You may be able to glean some kind of health of the earth, even vague analogies to happiness, pain etc. But you cannot ask any meaningful questions in this way (on a side note it is quite interesting that the "hive mind" is not necessarily more intelligent than any of its components). So how could you ask the Earth a meaningful question? One possibility might be to use evolution as the processing unit. So if for example you wanted to ask the Earth "Do alien life forms exist?", you might introduce a disease such that if any of the last 5 generations of your ancestors was considered by popular consensus to have made any small advances in any area relating to the problem then they would live twice as long as anybody else and have up to 10 children. Then wait a few hundred thousand years. Come back. Pick the individual that lives the longest and "read off" the answer. I apologize for the long ramble, maybe I could have picked a better example. I quite like the idea of using evolution as the processing power though.