While the Venus discoveries ignited a burst of interest in AGW, they did nothing to shore up its scientific merits. In 1971, Russian climatologist Mikhail Budyko gave a speech to a large international conference in Leningrad in which he presented an alarmist AGW scenario. Reaction was strongly negative. It was clear that AGW was very much a minority opinion among scientists at this time. A quick look at Venus reveals that the atmosphere is opaque due to globe-spanning sulfur dioxide clouds. These clouds are 30 to 40 km thick. No sunlight reaches the surface.<ref>NASA's "[http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html Venus Fact Sheet]" gives the planet's diurnal temperature range as zero, i.e. no difference between night and day on the surface.</ref> It follows that the planet has no greenhouse effect whatsoever. In a runaway greenhouse, the greenhouse effect vaporizes additional water, which in turn increases the greenhouse effect. This leads to even more water vaporization, etc. Venus has very little water of any kind, so the positive feedback Sagan had proposed is impossible.<ref name="pierre" /><ref>Anthony Watts explains why Sagan's theory is "beyond absurd" in "[https://wattsupwiththat.com/2010/05/06/hyperventilating-on-venus/ Hyperventilating on Venus]," May 6, 2010.</ref> Whatever the chemistry of the Venusian atmosphere, water vapor absorbs a wider spectrum of radiation than carbon dioxide does and remains the primary greenhouse gas in the terrestrial atmosphere, as Tyndall showed long ago.
Venus research proved to be an incubator for AGW showboaters. James “Boiling Oceans” Hansen, who emerged as the public face of AGW in the late 1980s, was a NASA Venus researcher in the 1960s. Hansen's claim that continued CO<sub>2</sub> emissions may will trigger a runaway greenhouse on Earth is disproven by the events of the Paleocene-Eocene Thermal Maximum (PETM), a huge natural spike in CO<sub>2</sub> that occurred 55.5 million years ago.<ref>During PETM, global temperature rose by 5–8 °C for about 200,000 years. The accelerated growth of plantlife eventually allowed for a return to pre-PETM conditions. That is to say, negative feedback overcame positive feedback, contrary to the runaway greenhouse theory.*Kunzig, Robert, "[http://news.nationalgeographic.com/news/2013/13/130729-runaway-greenhouse-global-warming-venus-ocean-climate-science/ Will Earth's Ocean Boil Away?]", ''National Geographic'', July 30, 2013.</ref> The event lasted for about 200,000 years, during which there was a global temperature rise of 5–8 °C.<refbr>*McInherney, F.A..; Wing, S. (2011). "A perturbation of carbon cycle, climate, and biosphere with implications for the future". Annual Review of Earth and Planetary Sciences. 39: 489–516. Bibcode:2011AREPS..39..489M. doi:10.1146/annurev-earth-040610-133431</br>*Gabriel J. Bowen & James C. Zachos, "Rapid carbon sequestration at the termination of the Palaeocene–Eocene Thermal Maximum", ''Nature'' Geoscience 3, 866–869 (2010) doi:10.1038/ngeo1014</ref> Overlooking Hansen's history of following one kook theory after another, the mainstream media treated him as the voice of serious science.<ref>Sullivan, John, "[https://johnosullivan.wordpress.com/2012/05/20/top-scientists-vent-on-nasas-sub-prime-greenhouse-gas-hoaxere-optional/ Top Scientists Vent on NASA’s Sub Prime Greenhouse Gas Hoaxer]", May 20, 2012. "The oceans will begin to boil," Hansen announces in [https://wattsupwiththat.com/2012/01/12/quote-of-the-week-dr-james-hansen-of-nasa-giss-unhinged/ this video]. In his book, ''Storms of my Grandchildren'', Hansen writes: "[I]f we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty."</ref>
===From cooling to warming===