This is from
NASA:
"Unlike the surface-based temperatures, global temperature measurements of the Earth's lower atmosphere obtained from satellites reveal no definitive warming trend over the past two decades. The slight trend that is in the data actually appears to be downward."
As is this:
In the latest (March 13, 1997) edition of Nature, two scientists, James Hurrell and Kevin Trenberth, report that sea-surface temperatures monitored by buoys and ships at various locations in the tropics show, for the same period as the satellite record, a warming trend of +0.12 deg. C/decade, in apparent disagreement with the satellites.
Two decades is not three decades, i.e. thirty years, the time period described by Bethell. Prior to the 1998 El Nino, the trend was low (but Spencer and Christy had to make some fixes to the data due to errors discovered by others). Combined, warming trends emerged in their analysis and analyses performed by other research groups, and these trends are continuing. They are congruent with surface warming.
Bethell made a typical error of someone writing to make rhetorical points and not really interested in the accuracy of his scientific interpretations.