The response was vehement. Corporations and individuals who opposed all government regulation began to spend many millions of dollars on lobbying, advertising, and “reports” that mimicked scientific publications, in an effort to convince people that there was no problem at all. Environmental groups, less wealthy but more enthusiastic, helped politicize the issue with urgent cries of alarm. But the many scientific uncertainties, and the sheer complexity of climate, made room for limitless debate over what actions, if any, governments should take.
There were some things that virtually all experts agreed on as of 1988. A rather straightforward calculation showed that doubling the level of carbon dioxide in the atmosphere... which would arrive in the late 21st century if no steps were taken to curb emissions... should raise the temperature of the surface roughly one degree C. However, a warmer atmosphere would hold more water vapor, which ought to cause another degree or so of warming. Beyond that the calculations got problematic. Cloudiness was likely to change in ways that could either enhance or diminish the warming, and scientists did not understand the complex processes well. Moreover, humanity was emitting ever increasing amounts of smoke and other pollution; again scientists were not sure how this might affect climate. Only better observations and computer models could attempt to project the outcome.
Scientists intensified their research, organizing programs on an international scale. Was the global temperature rise due to an increase in the Sun’s activity? Solar activity began to decline, but the temperature soared faster than ever. Did computer models reproduce the present climate only because they were tweaked until they matched it, making them worthless for calculating a future climate change? Improved models successfully predicted the temporary cooling due to a huge volcanic explosion in 1991 and passed many other tests. In particular, the modelers could now reproduce in detail the pattern of warming, changes in rainfall, etc. actually observed in different regions of the world over the past century. Nobody had been able to build a model that matched the historical record and that did not show significant warming when greenhouse gases were added.
The physics of clouds and pollution remained too complex to work out exactly, and modeling teams that made different assumptions got somewhat different results. Most of them found a warming of around 3°C when the carbon dioxide level doubled, late in the 21st century. But some found a rise of 2°C or perhaps a bit less, a costly but manageable warming. Others calculated a 5°C rise or even more, an unparalleled catastrophe.
Meanwhile striking news came from studies of ancient climates recorded in Antarctic ice cores. For hundreds of thousands of years, carbon dioxide and temperature had been linked: anything that caused one of the pair to rise or fall had caused a rise or fall in the other. It turned out that a doubling of carbon dioxide had always gone along with a 3°C temperature rise, give or take a degree or two — a striking confirmation of the computer models, from entirely independent evidence.
The world’s governments had created a panel to give them the most reliable possible advice, as negotiated among thousands of climate experts and officials. By 2001 this Intergovernmental Panel on Climate Change (IPCC) managed to establish a consensus, phrased so cautiously that scarcely any expert or government representative dissented. They announced that although the climate system was so complex that scientists would never reach complete certainty, it was much more likely than not that our civilization faced severe global warming. At that point the discovery of global warming was essentially completed. Scientists knew the most important things about how the climate could change during the 21st century. How the climate would actually change now depended chiefly on what policies humanity would choose for its greenhouse gas emissions.