Prediction of the long-term trend (aka global warming) is different from prediction of climate variability, meaning shorter-term changes, including El Nino. The two are closely linked, with the trend affecting variability, but our understanding that the climate is warming due to increased greenhouse gases from fossil fuel burning is different from our ability to exactly forecast how the climate will behave over the next few months. The former is well established, while the latter is still a developing science. That said, over time, probability forecasts for climate variability can be successful--check out Michelle's post on climate predictions for more about the "long game."