"Nonetheless, there is something worrying about one of Griffin's other statements, which said that "I have no doubt . . . that a trend of global warming exists".

Griffin seems to be referring to human-caused global warming, but irrespective of that his opinion is unsupported by the evidence.

The salient facts are these. First, the accepted global average temperature statistics used by the Intergovernmental Panel on Climate Change show that no ground-based warming has occurred since 1998. Oddly, this eight-year-long temperature stasis has occurred despite an increase over the same period of 15 parts per million (or 4 per cent) in atmospheric CO2.

Second, lower atmosphere satellite-based temperature measurements, if corrected for non-greenhouse influences such as El Nino events and large volcanic eruptions, show little if any global warming since 1979, a period over which atmospheric CO2 has increased by 55 ppm (17 per cent).

Third, there are strong indications from solar studies that Earth's current temperature stasis will be followed by climatic cooling over the next few decades.

How then is it possible for Griffin to assert so boldly that human-caused global warming is happening?"
http://www.news.com.au/couriermail/story/0,23739,21920043-27197,00.html

As to question:
"How then is it possible for Griffin to assert so boldly that human-caused global warming is happening?"

I think it is quite possible for number of reasons. First when the general public uses the term "global warming" they could using it's informal/nonscientific meaning which is "human-caused global warming". If instead one is using the scientific meaning, there is no doubt we are in a period of warming- whether one talking about hundred century tread or recently in last century or so.
But even if Griffin was using the common usage of human caused warming definition of "global warming" there is evidence that human activity is causing global temperature to rise. Human activity has to either cause cooling or warming and of these choices there seems to be a fair amount of evidence that the net result of human activity is more warming than cooling. As an example, if the question instead was do large volcanic eruption cause warming or cooling? The scientific answer is of course, global cooling.

Human activity over last Century has caused some warming [there is no scientific evidence that it has caused all the measured warming]. The most easily measurable affect of human activity on average temperature is Urban Heat Islands. UHI has in the immediate regional affect of a urban area over periods of decades of time and as much as say 10 C increase in average temperature. Urban area comprise a relatively small area of this planet but nevertheless it's enough area that some increase global temperature would be caused by this.

Another article:
Remember in January when the National Oceanic & Atmospheric Administration (NOAA) and its good friends in media trumpeted that 2006 was the warmest year on record for the contiguous United States?
...
Meanwhile, you probably missed the latest about 2006. As NOAA reported on May 1 - with minimum mainstream-media fanfare - 2006 actually was the second- warmest year ever recorded in America, not the first. At an annual average of 54.9 degrees F, it was a whopping 0.08 degrees cooler than 1998, still the hottest year.

NOAA explained that it had updated its 2006 report "to reflect revised statistics" and "better address uncertainties in the instrumental record." "
http://www.news.com.au/couriermail/story/0,23739,21920043-27197,00.html