Additional info from Skeptical Science
No, it hasn't been cooling since 1998. Even if we ignore long-term trends and just look at the record-breakers, that wasn't the hottest year ever. Globally, 2010 and 2005 were tied for the hottest years on record.
Though humans love record-breakers, they don't, on their own, tell us much about trends — and it's trends that matter when monitoring climate change. Trends only appear by looking at all the data, globally, and taking into account other variables — like the effects of El Niño or sunspot activity — not by cherry-picking single points.
There's also a tendency for some people just to concentrate on air temperatures when there are other, more useful, indicators that can perhaps give us a better idea of how rapidly the world is warming. Oceans, for instance — due to their immense size and heat storing capability (called "thermal mass") — tend to give a much more "steady" indication of the warming that is happening. When we look at ocean temperatures, records show that the Earth has been warming at a steady rate before and since 1998 and there are no signs of it slowing anytime soon.
Adapted from © John Cook and Skeptical Science