Q: My MacBook Air uses a 45w power adapter.  Can I use a 60w power adapter?

No. It will be at your risk if you try.

Yes. The power rating of the adpater is the maximun it can provide. The actual power that is supplied by the adpater is determined by what is required.

You can always use an adapter that has a higher power rating than is required as long as the voltage rating is the same.

Power Adapter/Wattage

Power adapters for Intel-based Apple notebooks are available in 45W, 60W, and 85W varieties. Although you should always use the proper wattage adapter for your Apple notebook, you can use an adapter of a higher wattage without issue.

http://support.apple.com/kb/HT2346