Why are the leaves falling off my climbing rose?
Sometimes as climbers get bigger they lose alot of the leaves off of their lower and bigger canes. This is mainly due to the higher foliage doing all the bush needs it to do for life support. Some climbers do tend to get what is called "leggy". Be sure to check the leaves for any signs of black spot fungus or powdery mildew as those two funguses can cause problems if left untreated. Also, if it is realy hot where you are the rosebushes will sometimes drop leaves as they cannot support them all with the heat. So they drop some to keep the ones left healthy and doing well.
Some of the Climbers tend to shed some foliage as they put on new foliage and extend their long canes so it could just be the natural process of things. It could also be heat stress and lack of water. If the bush becomes heat stressed it will start to shed its foliage in an effort to cool off and only keep the foliage it really needs to sustain itself. If you have been keeping it well watered then it could just be heat stress. Another thing that will cause the leaves to fall of is an attack by a fungus such as one called Black Spot here. If there are black specks on the foliage before it falls off, then that is what is going on. Here in the USA we have a product called Mancozeb that will stop this type of attack in its tracks. The spraying does leave a bit of a yellow powdery residue on the foliage, which is part of how it works. Once you do get the black spots on the leaves they will not heal and go away but the Mancozeb should kill the fungus and the new foliage will be clean again. Not being able to see the bush in person or photos of it makes it hard to give you an exact answer, however these are some of the things that could cause the leaves to fall off. What time of year is it for you there, are you still in the actual growing season?
Stan the Roseman