Lucia over at The Blackboard has a couple of recent posts about Kevin Trenberth, called Trenberth’s missing trends and Trenberth’s Trick. These posts refer to a recent article by Kevin Trenberth called has global warming stalled?
So what does Lucia cricise about Kevin’s article? Well Kevin mentions that there are 3 periods in the last 30 years or so (1977-1986, 1987-1996, 2001-2012) where the trend has appeared to be flat and his article includes the following figure. The figure includes three horizontal lines covering the periods when the trend appears flat. What Kevin is trying to illustrate here is that although there are periods with no “warming”, the mean temperatures during these periods are still higher than mean temperatures prior to these periods. These aren’t trend lines, they’re simply horizontal lines representing the mean temperatures during these periods. Maybe Kevin could have made this clearer in his article, but that is still what they are (even Bob Tisdale acknowledges this in the comments to one of Lucia’s posts).
So what is Lucia’s criticism? Well one seems to be that Kevin hasn’t really discussed these trends and hasn’t even pointed out that during 2001-2012 the trend was actually negative. Well, firstly the lines on his figure aren’t trend lines, so criticising him for not discussing them seems a little unfair. Secondly, even though the trend for 2001-2012 is indeed negative (-0.016oC per decade) the error is +-0.186oC per decade. Now it is clear that the trend is most likely almost flat during this period, but to claim that it is negative – given such a large error – is a bit of a stretch.
Another criticism seems to be that we can explain the previous two flat periods as being a consequence of volcanic eruptions that produced some cooling. That we can’t explain the most recent flat period in the same way somehow indicates that this newer period is different and maybe indicative of the start of some kind of cooling trend. Really? That seems a bit of an extrapolation to me. All Kevin was pointing out was that natural variations can indeed produce periods during which the trend will appear flat. Lucia seems to agree with that for the periods 1977-1986 and 1987-1996. Maybe not certain what has produced the apparent flat trend during 2001-2012, but there was certainly a large ENSO event in the late 1990s. Given that all Kevin was trying to illustrate was that the periods during which the trends appear flat are still warmer than previous periods, it seems a little disingenuous to criticise him for not discussing things that weren’t even really relevant to what he was trying to illustrate. Also, if you have criticised climate scientists for a lack of rigour, you can’t really eyeball a couple of volcanoes during previous “flat trend” periods and then claim that you’ve explained why some periods have flat trends.
So, why do I think that Kevin’s article is actually pretty good? Well, when he attempts to answer the question “has global warming stalled?” he says
The answer depends a lot on what one means by “global warming”.
Indeed, I think this is one of the main problems with discussions about global warming. As Kevin himself mentions, for some it is equated to the “global mean temperature”. In my opinion, those who equate global warming with changes to the global mean temperature are simply illustrating that they don’t actually understand the fundamentals. Kevin then goes on to say :
“Warming” really means heating, and so it can be manifested in many ways. Rising surface temperatures are just one manifestation. Melting Arctic sea ice is another. ……. However, most (over 90%) of the energy imbalance goes into the ocean.
To me this is the fundamental point. Global warming simply means an increase in the amount of energy going into the climate system, not necessarily an increase in global surface temperatures. What causes this global warming? Currently the scientific evidence suggests that the increase in greenhouse gases in our atmosphere is reducing the amount of energy that is able to escape from our atmosphere and hence increasing the amount of energy in our climate system (i.e., it increases the difference between the incoming radiative flux and the outgoing radiative flux).
Satellites have actually measured this energy imbalance, so it is not simply some theoretical prediction. Kevin discusses this energy imbalance in quite some detail and even points out that
from 2005 to 2010 the sun went into a quiet phase and the warming energy imbalance is estimated to have dropped by about 10 to 15%.
So, even when the Sun went into a quite phase, the energy imbalance only reduced by 10 to 15%. Kevin finishes the article by saying
So the current hiatus in surface warming is a transient and global warming has not gone away: there is a continuing radiative imbalance at the top of atmosphere. But the global warming is manifested in a number of ways.
That’s why I think this is a good article. Kevin is actually discussing the fundamentals of global warming in quite some detail and quite clearly. To criticise it for a simple illustration indicating that – even during periods when the trend was flat – the mean was higher than during earlier periods, rather misses the fundamental point of the article. If you’re going to criticise an article about global warming it would be best if you made it clear that you understand what the term global warming actually means.