Abstract
2014 was nominally the warmest year on record for both the globe and northern hemisphere based on historical records spanning the past one and a half centuries1,2. It was the latest in a recent run of record temperatures spanning the past decade and a half. Press accounts reported odds as low as one-in-650 million that the observed run of global temperature records would be expected to occur in the absence of human-caused global warming. Press reports notwithstanding, the question of how likely observed temperature records may have have been both with and without human influence is interesting in its own right. Here we attempt to address that question using a semi-empirical approach that combines the latest (CMIP53) climate model simulations with observations of global and hemispheric mean temperature. We find that individual record years and the observed runs of record-setting temperatures were extremely unlikely to have occurred in the absence of human-caused climate change, though not nearly as unlikely as press reports have suggested. These same record temperatures were, by contrast, quite likely to have occurred in the presence of anthropogenic climate forcing.
Highlights
2014 was nominally the warmest year on record for both the globe and northern hemisphere based on historical records spanning the past one and a half centuries[1,2]
Press accounts reported odds as low as one-in-650 million that the observed run of global temperature records would be expected to occur in the absence of human-caused global warming
We attempt to address that question using a semi-empirical approach that combines the latest (CMIP53) climate model simulations with observations of global and hemispheric mean temperature
Summary
Using the most defensible of the three noise null hypotheses (ARMA), we find (Fig. 3) that the recent run of record-breaking NH and global mean temperatures (13 of the 15 warmest years and 9 of the 10 warmest years having occurred since 2000) is extremely unlikely to have occurred due to natural variability alone, with odds of 1-in-170,000/1-in-5000 (NH) and 1-in-10,000/1-in-770 (global) respectively (Table 1). That observation is consistent with the notion that this record was somewhat of an outlier, having been substantially boosted by an unusually large (by some measures the largest) El Niño on record The fact that it took an usually long time (7 years, until the 2005 global temperature record) for the 1998 record to be matched or exceeded is related, in our analysis, to the fact that it occurred earlier than expected given the trajectory of anthropogenic warming. Our findings underscore the profound impact that anthropogenic forcing has already had on temperature extremes
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