This is an interesting question. Cities are increasing in size and in the future, most scholars believe that this trend will continue. Here are my predictions.
First, with the growth of cities, we will see an increase of the population as well. Recently, we just passed the 7 billion mark. We can confidently say that today there are more people on earth than ever before. This will cause a need for much more energy - oil, natural, gas, and alternative energy. This will motivate companies to be more daring in the acquisition of these materials, which will have a negative effect on our environment. A perfect example is BP. After their oil spill last summer, the United Stated gave them the green light to continue! We will see more accidents.
Second, as cities grow, there will be more need for food. For this reason we will see food prices rise. We will also need more farms to produce food. I suspect that we will use more land to get food and this too will have a negative impact on our environment. Forests, which we desparate need, may be cut down to create more farming fields.
Third, we will need more building commodities, like steel, copper, and other elements. We will have to strip the earth of more resources and this, too, will have a negative impact. The population increase is a problem waiting to happen.