While the Intergovernmental Panel on Climate Change (IPCC) still uses the iconic word “unequivocal” to describe warming of the climate system over the past century, a new word has slipped into its lexicon: the “hiatus.” They have begun referring, with a bit of hesitant throat-clearing, to “the warming hiatus since 1998.”
Both satellites and surface records show that sometime around 2000, temperature data ceased its upward path and leveled off. Over the past 100 years there is a statistically significant upward trend in the data amounting to about 0.7 oC per century. If one looks only at the past 15 years though, there is no trend.
A leveling-off period is not, on its own, the least bit remarkable. What makes it remarkable is that it coincides with 20 years of rapidly rising atmospheric greenhouse gas levels. Since 1990, atmospheric carbon dioxide levels have risen 13%, from 354 parts per million (ppm) to just under 400 ppm. According to the IPCC, estimated “radiative forcing” of greenhouse gases (the term it uses to describe the expected heating effect) increased by 43% after 2005. Climate models all predicted that this should have led to warming of the lower troposphere and surface. Instead, temperatures flatlined and even started declining. This is the important point about the pause in warming. Indeed, the word that ought to have entered the IPCC lexicon is not “hiatus” but “discrepancy.”
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