Kyle Armour, a doctoral student in physics at the University of Washington is boldly challenging that certain assertions of the Nobel Prize-winning International Panel on Climate Change, in their current state, may be flawed.  He argues that the UN's suggestion that stopping aerosol emissions will stop warming is misleading [press release].  These conclusions are noteworthy, given the controversial state of warming research and legislation aimed to "stop" global warming.

At issue is various climatology models, collected from published research, that attempt to simulate the effects of changing global climate variables. These variables include changing the levels of an "aerosols" (atmospheric dust) like sea salt or soot from burning fossil fuels; or greenhouse gases (GHGs) like CO₂ or methane.  The effects of these variables are dubbed "forcings" (aerosol forcing, GHG forcings, solar forcing, etc.).  Various forcings sum up to predict a net climate change and its contributors by approximate percentage.  

Models are typically fit to current data, but the narrow range that many climate variables have been constrained to in the modern era limits them.  They're also limited by how many variables and effects on those variables they consider.  Last, but not least, they're limited by how accurately and completely we can measure certain variables (e.g. total global aerosol levels).

In this case, Kyle Armour says that current models are flawed in that they fail to consider how high the uncertainty is regarding the amount that aerosols contribute to climate change.  

He says that the aerosols could contribute a lot to climate change, or only a little.  

In the "best case" scenario they would only contribute a little to net warming, thus they would not be masking the effects of GHG-related warming.  If all emissions of aerosols and GHGs stopped (a cessation of fossil fuel burning, and mammalian livestock farming, in short) the aerosols would quickly exit the atmosphere.  GHGs would remain for years at elevated levels, but the net result would be a slight decrease in temperatures by about half a degree Fahrenheit, given that the aerosols were the chief culprits.

In other words, the current temperature, which is about 1.5 degrees Fahrenheit above pre-Industrial Revolution levels would dip to only 1.0 degrees Fahrenheit above that base level -- but wouldn’t return entirely for many years.

Society can obviously not just instantly cut emissions, Mr. Armour acknowledges, but he says that such a scenario would offer justification to emissions cuts.

However, it's also possible that aerosols offer a larger contribution and are masking the effects of GHGs.  In this case, even if emissions stopped, temperatures would continue to rise and likely reach 3.5 degrees Fahrenheit above pre-Industrial Revolution levels, as the GHGs would persist in the atmosphere.  Such a temperature increase would likely cause some of the more severe predicted climate change effects (though it could offer benefits as well).

In other words, Mr. Armour is arguing that uncertainty in the aerosol components of models may lead to the IPCC significantly underestimating the amount of warming that will occur under various scenarios.

Mr. Armour says that keeping this uncertainty in mind is critical and the IPCC needs to do a better job in doing so in its next report.  He states, "This is not an argument to say we should keep emitting aerosols. It is an argument that we should be smart in how we stop emitting. And it's a call to action because we know the warming we are committed to from what we have emitted already and the longer we keep emitting the worse it gets."

One interesting conclusion of the study not explored by Mr. Armour is the question of maximum forcing.  Clearly historically temperatures rose due to increased GHGs, but leveled off (reach equilibrium) or reversed as the global system dampened the warming effects.  (In other words the Earth remained habitable, if a bit hotter, and didn't become some sort of arid, barren fireball.)  This equilibrium may be reached by a number of mechanisms -- radiative heat loss into space/changes in ocean currents/changes in atmospheric water vapor, etc.  The question is what is the "maximum" reachable temperature?  

If Mr. Armour is correct and we may already be locked in to a large temperature rise, the question is whether we'll reach this maximum.  If so, the climate change will already be enacted.  While this will be unfortunate in some ways (population would have to shift, growing areas would shift, etc.) and fortunate in others, humanity would already be forced to adapt to the change.

If indeed a maximum with dampening is destined to be reached, stopping emissions would do little good (unless we can somehow remove a significant quantity of GHGs from the atmosphere, which does not seem currently feasible).  Thus the question of whether fossil fuel and farming emissions should be cut, and if so how much, largely rests on a data set that is largely unknown and uncertain.  Mr. Armour's key conclusion is in noting this, and in noting that the IPCC needs to do a better job informing policy makers (politicians) of this uncertainty.

Mr. Armour's work has been published [abstract] in the journal Geophysical Research Letters.