Perception of climate change
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Contributed by James Hansen, March 29, 2012 (sent for review March 4, 2012)

Abstract
“Climate dice,” describing the chance of unusually warm or cool seasons, have become more and more “loaded” in the past 30 y, coincident with rapid global warming. The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures and the range of anomalies has increased. An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (3σ) warmer than the climatology of the 1951–1980 base period. This hot extreme, which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area. It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small. We discuss practical implications of this substantial, growing, climate change.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: james.e.hansen{at}nasa.gov.
Author contributions: J.H. designed research; J.H. and M.S. performed research; J.H., M.S., and R.R. analyzed data; and J.H. wrote the paper.
The authors declare no conflict of interest.
See Commentary on page 14720.
See Author Summary on page 14726 (volume 109, number 37).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1205276109/-/DCSupplemental.
Freely available online through the PNAS open access option.
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