The version of global warming theory being pushed by the IPCC is that anthropogenic emissions of greenhouse gases are causing a radiative energy imbalance of the climate system, leading to warming.
The radiative forcing history being used in the latest IPCC climate models looks something like the following, with red areas representing times when the climate system’s “stove is turned up”, that is, with heat accumulating in the system.
(Actually, the correct analogy would be that the stove setting remains the same, but the lid partially covering the pot is covering it a little more over time…but that’s too hard to explain.)
As can be seen, in the last 10 years the estimated forcing has been the strongest. Yet, most if not all temperature datasets show little or no global-average warming recently, either in the atmosphere, at the surface, or in the upper 700 meters of the ocean. For example, here are the tropospheric temperatures up though a few days ago:
So what is happening? You cannot simply say a lack of warming in 10 years is not that unusual, and that there have been previous 10-year periods without warming, too. No, we are supposedly in uncharted territory with a maximum in radiative forcing of the climate system. One cannot compare on an equal basis the last 10 years with any previous decades without warming.
There are 5 possibilities for the recent cessation of warming which are most discussed:
1) cooling from anthropogenic aerosols has been cancelling out warming from more greenhouse gases
2) natural cooling from internal climate fluctuations or the sun is cancelling out the GHG warming
3) increased ocean mixing is causing the extra energy to be distributed into the deep ocean
4) the temperature ‘sensitivity’ of the climate system is not as large as the IPCC assumes.
5) there is something fundamentally wrong with the GHG warming theory itself
Of course, some combination of the above 5 explanations is also possible.
The 1st possibility (aerosol cooling is cancelling out GHG forcing) is one of the more popular explanations with the climate modelers, and especially with NASA’s James Hansen. The uncertain strength (and even sign) of aerosol forcing allows the climate modelers to use aerosols as a tuning knob (aka fudge factor) in making their models produce warming more-or-less consistent with past observations. Using an assumed large aerosol cooling to cancel out the GHG warming allows the modelers to retain high climate sensitivity, and thus the fear of strong future warming if those aerosols ever dissipate.
The 2nd possibility (natural cooling) is a much less desirable explanation for the IPCC crowd because it opens the door to Mother Nature having as much or more influence on the climate system than do humans. We can’t have that, you know. Then you would have to consider the possibility that most of the warming in the last 50 years was natural, too. Goodbye, AGW funding.
The 3rd possibility (increased ocean mixing) is one of the more legitimate possibilities, at least theoretically. It’s popular with NCAR’s Kevin Trenberth. But one would need more observational evidence this is happening before embracing the idea. Unfortunately, how vertical mixing in the ocean naturally varies over time is poorly understood; the different IPCC models have widely varying strengths of mixing, and so ocean mixing is a huge wild card in the global warming debate, as is aerosol cooling. I believe much of past climate change on time scales of decades to many centuries might be due to such variations in ocean mixing, along with their likely influence on global cloud cover changing the amount of solar input into the climate system.
The 4th possibility (the climate system is relatively insensitive to forcing) is the top contender in the opinion of myself, Dick Lindzen, and a few other climate researchers who work in this field.
The 5th possibility (increasing GHGs don’t really cause warming) is total anathema to the IPCC. Without GHG warming, the whole AGW movement collapses. This kind of scientific finding would normally be Nobel Prize territory…except that the Nobel Prize has become more of a socio-political award in recent years, with only politically correct recipients. The self-flagellating elites don’t like the idea humans might not be destroying the Earth.
The longer we go without significant warming, the more obvious it will become that there is something seriously wrong with current AGW theory. I don’t think there is a certain number of years – 5, 10, 20, etc. – which will disprove the science of AGW….unless the climate system cools for the next 10 years. Eek! But I personally doubt that will happen.
As long as strong warming does not resume, the heat-hiding-in-the-deep-ocean explanation will provide refuge for many years to come, and will be difficult to convincingly rule out as an explanation since it takes so long for the deep ocean to warm by even a tiny amount.
Instead, there probably will be a tipping point (sooner than later) in popular perception when the public and Congress decide the jig is up, and they are no longer interested in hearing how we ‘might’ be headed for Armageddon.
The public already knows how awful scientists are at forecasting the future…especially a future of doom, which curiously seems to be the only future scientists know how to predict.