WORKING GROUP I CONTRIBUTION TO THE IPCC FIFTH ASSESSMENT REPORT CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS Final Draft Underlying Scientific - Technical Assessment
Climate Models and the Hiatus in Global-Mean Surface Warming of the Past 15 Years. Page 26 and on, excerpts :
The observed global-mean surface temperature (GMST) has shown a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years. Depending on the observational data set, the GMST trend over 1998 – 2012 is estimated to be around one-third to one-half of the trend over 1951 – 2012. For example, in HadCRUT4 the trend is 0.04 ºC per decade over 1998 – 2012, compared to 0.11 ºC per decade over 1951 – 2012. The reduction in observed GMST trend is most marked in Northern - Hemisphere winter. Even with this “hiatus” in GMST trend, the decade of the 2000s has been the warmest in the instrumental record of GMST. Nevertheless, the occurrence of the hiatus in GMST trend during the past 15 years raises the two related questions of what has caused it and whether climate models are able to reproduce it.
Owing to sampling limitations, it is uncertain whether an increase in the rate of subsurface-ocean heat uptake occurred during the past 15 years. However, it is very likely2 that the climate system, including the ocean below 700 m depth, has continued to accumulate energy over the period1998 – 2010.
The AR5 best-estimate ERF ( estimated radiative forcing ) forcing trend difference between 1998 – 2011 and 1951 – 2011 thus might explain about one-half (0.04°C per decade) of the observed GMST trend difference between these periods (0.06 to 0.08°C per decade, depending on observational data set).
This finding provides evidence that some CMIP5 models show a larger response to greenhouse gases and other anthropogenic factors ( dominated by the effects of aerosols ) than the real world. As a consequence, it is argued that near-term model projections of GMST increase should be scaled down by about 10%. This downward scaling is, however, not sufficient to explain the model-mean overestimate of GMST trend over the hiatus period.
In summary, the observed recent warming hiatus, defined as the reduction in GMST trend during 1998 – 2012 as compared to the trend during 1951 – 2012, is attributable in roughly equal measure to a cooling contribution from internal variability and a reduced trend in external forcing ( expert judgment, medium confidence ). The forcing trend reduction is primarily due to a negative forcing trend from both volcanic eruptions and the downward phase of the solar cycle. However, there is low confidence in quantifying the role of forcing trend in causing the hiatus, because of uncertainty in the magnitude of the volcanic forcing trend and low confidence in the aerosol forcing trend.
Comments by Steven Keeler follow:
The above few excerpts highlight the grounds for reaching a obvious conclusion for any independent reviewer taking the time to read the full summary. This 'body' of individuals have produced a document full of 'scientific garbage'. Somehow the IPCC is able to assert with a straight face that they are now, what, 95 percent certain of one factoid, yet the pages of this summary are riddled with details of uncertainty and only medium to low confidence ( all too often ) in the models, the data and what their colleges are trying to state. Garbage, contorted to give a proper agenda conclusion.
Judith Curry summary :
My original intention was to go through and try to map the IPCC’s logical argument. I quickly got dizzy owing to seemingly unwarranted assumptions and incomplete information (such as: did the climate models use the correct external forcing for the first decade of the 21st century, or not ?). I was then going to illustrate how any reasonable propagation of uncertainty of individual assertions/arguments through their main argument would produce much lower confidence in their overall conclusions. For example, they seem to have eliminated high CO2 sensitivity as a problem. Not to mention high confidence in increasing trend following 2012 (this high confidence comes right after blowing the prediction of the previous decade). And of course not to mention the relevant journal articles that didn’t get mentioned.
Apart from these obvious flaws, reading that text and trying to follow it is positively painful. Can someone remind me again how and why all this is supposed to be useful?
Climate Etc. is hosted by Judith Curry.
I am Professor and Chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology and President (co-owner) of Climate Forecast Applications Network (CFAN). I received a Ph.D. in Geophysical Sciences from the University of Chicago in 1982. Prior to joining the faculty at Georgia Tech, I held faculty positions at the University of Colorado, Penn State University and Purdue University. I currently serve on the NASA Advisory Council Earth Science Subcommittee and the DOE Biological and Environmental Science Advisory Committee, and have recently served on the National Academies Climate Research Committee and the Space Studies Board, and the NOAA Climate Working Group. I am a Fellow of the American Meteorological Society, the American Association for the Advancement of Science, and the American Geophysical Union.