Is global climate warming or cooling?

Apologies if this has been posted previously:

Easterling, D. R., and M. F. Wehner (2009), Is the climate warming or cooling?, Geophys. Res. Lett., 36, L08706, doi:10.1029/2009GL037810

Is the climate warming or cooling?

David R. Easterling

National Climatic Data Center, NOAA, Asheville, North Carolina, USA

Michael F. Wehner

Lawrence Berkeley National Laboratory, Berkeley, California, USA

Summary from me:  In other words, there is nothing special about the recent decrease in the rate of global mean temperatures.  It has happened before and the overall prognosis is for continued warming. 

Quote:

Originally Posted by dawei 

Interesting. The researchers used models under a business as usual scenario and got negative trends from 2001-2010 and 2016-2031...all without factoring in solar or volcanic activity. Even ENSO is "not well simulated".

Any highly coupled nonlinear system will respond nonlinearly to a change in its external forcings.  That's why it's nonlinear.  Climate skeptics are largely ignorant of all physics, so they don't "get" this concept, and any nonlinearity in response of the climate is seen not as supporting the fundamental notion that climate is nonlinear with understood physics, but that the physics of climate are misunderstood and the system is in fact linear.  (Work through that sentence, it does make sense.)  The key difference between climate being linear and nonlinear is that if it is linear, which it is not, then any decrease in global mean temperature has to come from a decrease in the forcing, or some other unknown property of the system.  In contrast, what  the GRL paper shows is that because climate is nonlinear, short-term decreases represent fundamental variability in the (nonlinear) system itself, rather than any change in the forcing(s). 

I've given up trying to make this point to skeptics.  All they hear is a whistling noise as the concepts fly past their heads. 

Quote:

Originally Posted by dawei 

 

But isn't the fact that it is nonlinear because of the other forcings at work in the system? 

 

I mean...isn't natural variability due to the fluctuating strengths of the dozens of different factors in the climate that can affect temperature?

That's not the way I understand it.  But then, I have the intuition of a pineapple. 

My impression is that the system is nonlinear not because there are multiple forcings at work, but because there are coupling mechanisms between the forcings in terms of feedbacks (both positive and negative).  These multiple connections between the forcings and feedbacks make the system as a whole nonlinear because an increase in any forcing can cause a change in all the feedbacks, which can all (feedbacks and forcings) affect each other as well.  (In terms of the governing differential equation, the system has all these chaotic transient terms running around in the solution whenever something changes.  Although that is an overly simplistic view since a lot of times the transient terms are deterministic (in the sense you know what they are and how they decay) and in climate they are not.)

Moreover, since climate is never stable (christ I sound like a skeptic, but hear me out), there are always these transient terms in the system.  This is climate variability, and understanding it is a key emphasis in the US Global Change Reseach Program (USGCRP), since in order to determine if a specific change detected is due to the change in radiative forcing from anthropogenic CO2 or not, you have to know whether that change might be due to some other transient term running around. 

So, maybe we're saying the same thing and you are visualizing this a little differently, but to me it's not that things that force the system are all changing, it's that the feedbacks are all coupled to the forcings, and by changing one forcing you change all the feedbacks and the coupling terms between the feedbacks and *all* the forcings (e.g., by raising atmospheric temperature you muck around with water vapor, and that affects nearly everything from the solar forcing to the radiative forcing to the outgoing longwave flux from deep convection).  This causes all sorts of nonlinear behavior and by triggering an increase in a negative feedback, it grows nonlinearly for a time and causes an apparent cooling.  But then, that transient dies down, or triggers a second positive feedback that will dominate after a time.  I think this is what the paper in GRL is demonstrating. 

Quote:

Originally Posted by dawei 

Ah I get you now. So the cause of the cooling was mostly from negative feedbacks of forcings, and not from random variation of the climate itself...

I was under the impression that it was just the result of the chaotic nature of climate, unrelated to any anthropogenic forcings--such as that shown in the -.05 to .05 K distribution of the pre-industrial control model.

Yes!  And no!  What I was saying was that whenever you change a forcing, whether by "natural" means or anthropogenically, you trigger all the nonlinearities in the system.  So the cooling periods in the GRL paper are most likely related to the change in the anthropogenic forcing.  However, you are also correct that in the real world, since there are both changes in the anthropogenic forcing and the natural forcings, any observed cooling periods could represent either natural variability or be caused by the anthropogenic forcing.  There's really no way to distinguish the two by observation.  What modeling studies such as the GRL one show is that just because you observe a cooling period in the real world does not necessarily imply the increase in radiative forcing from anthropogenic CO2 is irrelevant

If I went back over what you had originally written, I think I would find you were saying essentially the same thing as me, only using a little different terminology.  So I trampled all over you because it's a slow day and I am doing something so incredibly boring (that isn't really working anyway) that anything is more interesting.  But I don't want to go back and re-read things because I don't want to be embarrassed any further.