In the late 1990's the evidence suggesting that carbon emissions caused global warming was basically:
1. Carbon dioxide is a greenhouse gas. Proved in a laboratory a century ago.
2. Global warming has been occurring for a century, especially since 1975, and concentrations of atmospheric carbon have been rising for a century, especially since 1975. Correlation is not causation, but in a rough sense it looked like a fit.
3. Ice core data, starting with the first cores from Vostok in 1985, allowed us to measure temperature and atmospheric carbon going back hundreds of thousands of years, through several dramatic global warming and cooling events. To the temporal resolution then available (data points were generally more than a thousand years apart), atmospheric carbon and temperature moved in lock-step: there was an extremely high correlation, they rose and fell together. Talk about a smoking gun!
4. There weren't any other credible suspects for causing global warming. So presumably it had to be carbon emissions.
This evidence was good enough: not conclusive, but why wait until we are absolutely certain when we apparently need to act now? . . .
But starting in about 2000, the last three of the four pieces of evidence outlined above fell away or reversed. Using the same point numbers as above:
2. Closer examination of the last century using better data shows that from 1940 to 1975 the earth cooled at about 0.1C/decade while atmospheric carbon increased. But any warming effect of atmospheric carbon is immediate. By 2003 or so we had discovered global dimming, which might be adequate to explain this 35-year non-correlation. But what had seemed like a good fit between recent atmospheric carbon and global warming now looks shaky, in need of the recently-discovered unquantified global dimming factor to explain 35 years of substantial cooling. I reckon the last century of correlation evidence now neither supports carbon emissions as the cause nor eliminates it. Further quantitative research on global dimming might rescue this bit of evidence, or it might weaken it further.
3. As more ice core data was collected, the temporal resolution was improved. By 2004 or so we knew from the ice core data that in the warming events of the last million years the temperature increases generally started about 800 years *before* the rises in atmospheric carbon started. Causality does not run in the direction I had assumed in 1999 — it runs the opposite way. Presumably temperature rises cause a delayed rise in atmospheric carbon because it takes several hundred years to warm the oceans enough for the oceans to give off more of their carbon. . . .
The pre-2000 ice core data was the central evidence for believing that atmospheric carbon caused temperature increases. The new ice core data shows that past warmings were *not* initially caused by rises in atmospheric carbon, and says nothing about the strength of any amplification. This piece of evidence casts reasonable doubt that atmospheric carbon had any role in past warmings, while still allowing the possibility that it had a supporting role.
4. A credible alternative suspect now exists. Clouds both reflect incoming radiation (albedo) and prevent heat from escaping (greenhouse), but with low clouds the albedo effect is stronger than the greenhouse effect. Thus low clouds cause net cooling (high clouds are less common and do the opposite). In October 2006 a team led by Henrik Svensmark showed experimentally that cosmic rays affect cloud formation, and thus that
Stronger sun's magnetic field
=> Less cosmic rays hit Earth
=> Fewer low clouds are formed
=> Earth heats up.
And indeed, the sun's magnetic field has been stronger than usual for the last three decades. So maybe cosmic rays cause global warming. But investigation of this cause is still in its infancy, and it's far too early to judge how much of the global warming is caused by cosmic rays.
So three of the four arguments that convinced me in 1999 that carbon emissions caused global warming are now questionable. . . .
The evidence is not currently conclusive either for or against any particular cause of global warming. I think that it *is* possible that carbon emissions are the dominant cause of global warming, but in light of the weakening evidence I judge that probability to be about 20% rather than almost 90% as estimated by the IPCC.
