How many times have you seen, read or heard some climate ‘expert’ or other say that mankind’s greenhouse gas emissions are largely responsible for the unprecedented warming we have seen over the past century, and especially what we have seen over the past 30 years. It is as if to some that nature has stepped back leaving mankind to take over the climate. In reality, whatever one’s predictions for the future, such claims are gross exaggerations and misrepresentations. Natural and human climate influences mingle and even today natural effects dominate.
I’ve been reading the paper by Hurst et al on measurements of Stratospheric Water vapour (SWV) over Colorado. In the past year or so SWV has become a very interesting topic in climate science as it is clear that small variations in SWV play a larger role in the Earth’s radiation budget than had been supposed.
Solomon et al looked at variations in SWV and concluded that it had been declining since about 2000 and this decline may have partly compensated for the temperature rise expected in the past decade due to AGW. It can’t be the only cooling factor involved that has kept the global annual average temperature constant within the errors for the past decade.
For me, there was another conclusion drawn in the Solomon paper that fascinated me, albeit at a lower level of statistical confidence (but still respectable.) It is that the data show that SWV started to increase around 1980 and continued to increase until about 2000 and that it may have contributed about a third of the warming observed in that period.
A third is no small factor and yet another indication that we are finding out new things about climate change all the time that belie the claim that the science is settled.
Looking at HadCrut3 the temperature rise seen since 1980 is about 0.5 degree C with, I suppose, an uncertainty of about 0.05 deg C either way. Removing the SWV component brings that down to about 0.35 deg C which is less than the global temperature rise seen between 1910 and 1940. So much for the claim that the recent warm spell has seen temperature rises that are unprecedented. If the SWV contribution is correct its removal has another ramification. It takes the current global temperature to below that of the Medieval Warm Period (MWP). (I know some argue that the MWP was not global to which I would counter that new research shows a much wider influence for the MWP than once supposed. I would also point out that today’s warming is likewise patchy and not globally uniform.)
But what else has been going on since 1980?
The variations that occur in the Earth’s climate over such periods are called decadal changes and their study is in its infancy. Climate models only capture some aspects of observed decadal variations.
The observed global temperature is clearly a mixture of various influence occurring over various timescales. The sun has a a decadal influence of about 0.1 deg C (some solar physicists say slightly larger) and the exact nature of that influence is uncertain given the recent paper by Haigh et al describing the effect of differential spectral variations from the sun on the earth’s atmosphere. Despite this – for I think the jury is still out – 0.1 deg C seems to be a good ballpark figure that is often quoted as the major climate driver before 1950. Clearly, whenever AGW became dominant (1960- 80 according to the IPCC), the sun’s influence did not suddenly go away, especially at a time of historically high solar activity. Scafetta and West maintain that the sun can account for 25 – 30% (possibly more) of the temperature rise since 1980 starting an interesting debate.
So perhaps we can conservatively account for another 0.1 deg C per decade since 1980 which, in our hypothetical game of accounting for the recent changes on Earths temperature might account for up to 0.2 deg C, leaving about 0.2 deg C unattributed since 1980.
So what else happened around 1980?
The North Atlantic Oscillation went strongly positive around 1980, and so did a few years later the Pacific Decadal Oscillation. The scientific literature contains a range of estimates of the influences of these factors but, again being conservative, 0.1 deg C for both of them doesn’t seem too outrageous.
With the NAO and PDO we are down in the noise of the post-1980 warming period having only considered four factors. There are many more that will have exerted a warming or cooling influence since 1980, including AGW.
Clearly things are not that simple. The spatial patterns of climate variations have to be considered and compared to what is being seen today although there exists considerable uncertainties in interpretation. Since 1910 warming has been more extensive over land than over the ocean, where it has been relatively homogenous, except for some weak cooling in the North Atlantic and some extra warming in some parts of the Southern Ocean.
Whatever is happening with decadal fluctuations, the temperature of the Earth has to start increasing. The current explanations for the standstill seen in the past decade or so are wearing thin. At issue isn’t wether CO2 is a greenhouse gas, or that it’s accumulating in the atmosphere, or that the world has warmed. What is at issue are questions of climate sensitivity, feedback mechanisms and the relative strengths of other variations.
Have we just lived through a period where a handful of natural decadal and longer variations have conspired to act in the same direction pushing temperatures up? When I read a peer-reviewed paper in a major journal (Solomon’s paper was published in Science) that plucks from the air an explanation for a third of the warming seen since 1980 via a previously little regarded mechanism, I begin to wonder.