A Skeptic's Guide to Global Warming
Can an alternative hypothesis explain global warming?
- Human activity is raising greenhouse gas concentrations.
- Such gases warm the Earth's surface.
- Theory predicts warming similar to observations.
- Realistic climate models can approximately reproduce observations if human influence is included (figure, right panel).
- The models can not if human influence is excluded (figure, left panel).
Nevertheless, it is possible that observed global warming is due to an alternative mechanism. Such an alternative needs to explain two things:
- Why greenhouse warming is not as strong as models and theory say.
- How some other mechanism has caused the warming instead.
Some scientists have argued that climate models overestimate climate sensitivity (see AR5 Sec. 10.8.2.2 for discussion). An interesting alternative mechanism for warming is related to variations in the number of cosmic rays hitting the atmosphere. These particles may help seed cloud formation, which could then change the amount of solar radiation absorbed by the Earth (see AR5 , Box 10.2). Another possibility is that internal variability of the climate system creates century-long trends in global mean temperature.
No climate model has reproduced 20th century warming by incorporating these alternatives rather than greenhouse gas increases. Whatever the shortcomings of climate models, they have been more extensively tested than the non-existent simulations demonstrating a different mechanism. Any skepticism about model-based evidence for a human cause applies doubly to other hypothesized causes backed by little more than speculation.
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References
AR5: Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2013, Stocker, T. F., D. Qin (eds.).
Last modified: 22 July 2018