Because El Nino and La Nina IS a short term factor. In the same way as the volcanic eruption of Pinatubo was responsible for the short term cooling spike seen in 1992/1993.So if we now think that El Nino is important for climate, why blame el nino for a single year? and why not talk about the La Nina's after 1998 that would affect the trends just as well. Right now we have a considerable El Nino, does that change trends?
There's nothing new about this being seen as important for climate; it has consistently been a feature of the IPCC reports.
We do indeed have an El Nino forming at present; and if that continues -- and if we don't get a big eruption -- then there is a strong probability that 2010 will set a new record for the hottest measured global anomaly. It doesn't even need to be as strong as the 1998 El Nino event, given the boost from the long term trend.
The evidence at this point is that these events don't affect trends themselves; they are rather short term changes in the climate mode, not a cummulative effect that could give a trend. They are a significant source of short term variation.
Some of the recent rise in temperature since 2008? Of course. But longer term warming? No. The short term changes in temperature are much stronger than the long term trend, and so it is entirely proper to recognize the role of such things for making one year warmer than another.And can the current ENSO (El Nino Southerly Oscillation) activity be responsible for some of the recent warming?
Cheers -- sylas