Well of course! Did you expect anything else? NG is still a fossil fuel. You pull it out of the ground, where it's been trapped for some millions of years. Fossil fuel == CO2 that was "sequestered" where it wouldn't affect the climate. It's just very lightly "processed" fossil fuel so in the ff hierarchy it's the lowest-carbon flavor.

Coal is the most processed - highest carbon. Oil is in between.

The Sierra Clubbers are scientifically correct: eventually, we'll have to make do with what we get from the sun and a few leftovers that are inherent to the planet (nuke, geothermal, etc.). Eventually is a long time because fossil fuel reserves depend to some extent on ability to pay, but dumping all that fossil CO2 back into the atmosphere is certainly shortening the time WE (as a species adapted to cooler weather and lots of food) have left to enjoy things. So we'll go out with a bang! Then, slowly, we and our detritus will become part of the geologic record (if anything in the future is interested in looking) and fuels will begin to be fossilized again. In a billion years or so, as the earth cools and the sun expands, plate tectonics will stop, the oceans will boil, and all life on Earth will end anyway unless it can adapt to Venusian conditions. Then in a couple more billion years, the sun will expand to reach Earth's orbit and vaporize the place, shortly before going nova and shrinking down to a white dwarf.

The hypocritical part comes from them enjoying the fruits of fossil fuel use (how many have 4WDs to get to the trailhead?) while, in effect, demanding that the Rest Of Us regress to the stone age while we still have alternatives. A stone-age (or even sustainable medieval with some "appropriate" technology like railroads - had to sneak that in) human culture would have a global population of around a billion at most, probably closer to a few hundred million. Dear SC members: are you volunteering to be among the 6+ billion who have to go, or are you (as I suspect) planning to be the rulers of the remaining peasants?