We will reach the 20 year mark with no trend at the end of 2017. It will at that point be impossible to reconcile climate models with reality and the mainstream consensus on how the climate system responds to greenhouse gases will begin breaking apart.
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.”
The chart on this page reproduces an important diagram from Chapter 9 of the IPCC report. The gray line shows the surface temperature record (HadCRUT4 from the UK Met Office) from 1860 to the present. The black line shows the average of climate model runs covering the same interval. The black line in effect sums up mainstream views on how the climate works. Leading theories of major climatic mechanisms are programmed into models, which are then used to simulate the evolution of the climate. All models remain within a fairly narrow neighbourhood of the mean. This implies that the models share an overall central tendency and do not wander too far from it. In that sense the black line can be described as the mainstream thinking of contemporary climate science.
The data prior to the year 2000 represent historical reconstructions. Modellers were able to “peek at the answer” since they could not only observe inputs to the climate system (such as historical greenhouse gas levels, volcanic activity, solar changes and so forth) but also the simulation targets, namely average temperatures, when tuning their models. The match over the historical interval is therefore not proof of model accuracy since the models were forced to line up with observations.
But as of around 2000, the models are run prospectively, and this is where they begin to fail. Prior to 2000, the gray and black lines continually touch and cross, diverging and converging as they track each other over time. Whenever they drift apart for a few years they quickly turn and close up again.