Normally, when I cite the above as the process, I get grief from folks who say I am mis-interpreting things, as usually I am boiling a complex argument down to this summary. The great thing about alarmist Trenberth’s piece is that no interpretation is necessary. He outlines this process right in a single paragraph. I will label the four steps above
Given that global warming is “unequivocal” , to quote the 2007 IPCC report , the null hypothesis should now be reversed, thereby placing the burden of proof on showing that there is no human influence . Such a null hypothesis is trickier because one has to hypothesize something specific, such as “precipitation has increased by 5%” and then prove that it hasn’t. Because of large natural variability, the first approach results in an outcome suggesting that it is appropriate to conclude that there is no increase in precipitation by human influences, although the correct interpretation is that there is simply not enough evidence (not a long enough time series). However, the second approach also concludes that one cannot say there is not a 5% increase in precipitation. Given that global warming is happening and is pervasive, the first approach should no longer be used. As a whole the community is making too many type II errors .
Are you kidding me — if already every damn event in the tails of the normal distribution is taken by the core climate community as a proof of their hypothesis, how is there even room for type II errors? Next up — “Our beautiful, seasonal weather — proof of global warming?”
Remember that the IPCC’s conclusion of human-caused warming was based mainly on computer modelling. The IPCC defenders will not admit this immediately, but press them hard enough on side arguments and it comes down to the models.
The summary of their argument is this: for the period after 1950, they claim their computer models cannot explain warming patterns without including a large effect from anthropogenic CO2. Since almost all the warming in the latter half of the century really occurred between 1978 and 1998, the IPCC core argument boils down to “we are unable to attribute the global temperature increase in these 20 years to natural factors, so it must have been caused by man-made CO2.” See my video here for a deeper discussion.
This seems to be a fairly thin reed. After all, it may just be that after only a decade or two of serious study, we still do not understand climate variability very well, natural or not. It is a particularly odd conclusion when one discovers that the models ignore a number of factors (like the PDO, ENSO, etc) that affect temperatures on a decadal scale.
We therefore have a hypothesis that is not based on observational data, and where those who hold the hypothesis claim that observational data should no longer be used to test their hypothesis. He is hilarious when he says that reversing the null hypothesis would make it trickier for his critics. It would make it freaking impossible, as he very well knows. This is an unbelievingly disingenuous suggestion. There are invisible aliens in my closet Dr. Trenberth — prove me wrong. It is always hard to prove a negative, and impossible in the complex climate system. There are simply too many variables in flux to nail down cause and effect in any kind of definitive way, at least at our level of understanding (we have studied economics much longer and we still have wild disagreements about cause and effect in macroeconomics).
So we frequently hear that “while this event is consistent with what we expect from climate change, no single event can be attributed to human induced global warming”. Such murky statements should be abolished. On the contrary, the odds have changed to make certain kinds of events more likely. For precipitation, the pervasive increase in water vapor changes precipitation events with no doubt whatsoever. Yes, all events! Even if temperatures or sea surface temperatures are below normal, they are still higher than they would have been, and so too is the atmospheric water vapor amount and thus the moisture available for storms. Granted, the climate deals with averages. However, those averages are made up of specific events of all shapes and sizes now operating in a different environment. It is not a well posed question to ask “Is it caused by global warming?” Or “Is it caused by natural variability?” Because it is always both.
At some level, this is useless. The climate system is horrendously complex. I am sure everything affects everything. So to say that it affects the probability is a true but unhelpful statement. The concern is that warming will affect the rate of these events, or the severity of these events, in a substantial and noticeable way.
It is worth considering whether the odds of the particular event have changed sufficiently that one can make the alternative statement “It is unlikely that this event would have occurred without global warming.” For instance, this probably applies to the extremes that occurred in the summer of 2010: the floods in Pakistan, India, and China and the drought, heat waves and wild fires in Russia.