Weather extremes have been a lot in the news recently prompted by the Hurricanes Harvey and Irma wreaking destruction in the Caribbean. Some commentators say this is what to expect with man-made climate change, and that hurricanes are an example of extremes that are occurring right now along with heat waves and intense rainfall. The reality is not quite that dramatic, and the science does not stand up to the impression being given to the public.
Regarding hurricanes NOAA has issued a statement; “It is premature to conclude that human activities – and particularly greenhouse gas emissions that cause global warming – have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity.”
Hurricanes aside, the evidence for increased extreme events occurring at the moment is not very impressive.
The IPCC’s SREX report of 2012 said there is some evidence of change in some extremes going back to the 1950s. It is very likely that there has been an overall decrease in the number of cold days and nights for North America, Europe and Australasia, and with medium confidence Asia. There is also medium confidence for a change in the length and number of warm spells and heat waves. It is likely that more regions have experienced increases than decreases in heavy precipitation events, though there are large regional uncertainties. It is likely that there has been a poleward shift in northern and southern hemisphere extra tropical storm tracks.
There is medium confidence that some regions have experienced more intense and longer droughts in Southern Europe and Western Australia, but not in Central North America and North West Australia where droughts are less frequent and less intense. There is limited to medium confidence in changes in the magnitude and frequency of floods. SREX goes on to add that there is low confidence of any changes on a global scale with uncertainty as to even the sign of the changes. It is also likely that there might have been changes in coastal high water events.
Descriptions such as “likely” and “medium confidence” are not very firm scientific statements. They do not provide a reliable scientific base from which to discuss the possible effects of man-made climate change. The public, and many advocates, do not realise this weakness, but that doesn’t matter because the subtleties are usually ignored anyway.
An abstract from a paper by Sarojini, Stott and Black, published four years later in Nature Climate Change is no more impressive in the face of a growing realisation of the scale of uncertainties involved in identifying changes.
“Understanding how human influence on the climate is affecting precipitation around the world is immensely important for defining mitigation policies, and for adaptation planning. Yet despite increasing evidence for the influence of climate change on global patterns of precipitation, and expectations that significant changes in regional precipitation should have already occurred as a result of human influence on climate, compelling evidence of anthropogenic fingerprints on regional precipitation is obscured by observational and modeling uncertainties and is likely to remain so using current methods for years to come.”
It’s clear that looking at changes in occurrence rates and other changing parameters is going to take a long time. So some scientists have used a different technique. They compare observations to a virtual Earth on which there has been no man-made climate change. The justification is that there has been improvements in models, and besides they know what should be happening and that makes detecting it easier. The Nature Climate Changes paper adds, “Historical records are not conclusive, large uncertainties, have to wait a long time for the effect to show up. However expected changes may render risks based on historical data inaccurate.” In summary, you don’t get your data through nature, you get it through models.
The idea is to look at a particular heat wave and rerun it on an Earth where man-made climatic effects have been removed. If it’s stronger or longer in the real world than the virtual one the man-made climatic changes have increased its severity or frequency. If you wish you can add some numbers saying the event was made 50% or perhaps ten times more likely because of man-made climate change.
Of course it all depends upon the models and our ability to remove man-made changes from the observations. What’s worrying is that the climate models are very poor at projections and fail to adequately capture the real world. That’s something that has been admitted many times when discussing the so-called hiatus period in global surface temperatures. The hiatus lesson is that decadal natural variability is not understood. The result is, as the Nature Climate Change paper said, large uncertainties.
The process of the detection and attribution of extreme events to man-made climate change is a young one and despite papers published in peer-reviewed journals, which many mistake for statements of certainty, it is tentative. For the public however its power, skill and uncertainties are being misrepresented.
How many hurricanes feature in Al Gore’s Inconvenient Sequel movie I wonder?