While 1998 was an abnormally warm year, annual average temperatures have trended steadily upward in the decades since.
As a strong El Nino year, 1998 featured a significant spike in global temperatures. El Nino is the warm phase of a cyclic climatic pattern where sea temperatures in parts of the Pacific swing higher or lower than average. The 1998 El Nino stood out above the rising temperature trendline that is due to manmade global warming.
However, the long-term upward trend in globally-averaged temperatures has continued. In the past quarter century, the top ten hottest years on record have all occurred since 2010.