By JR on Thursday, April 14, 2016
Would global warming increase both flooding AND drought?
That is the prediction that Warmists are attached to. But it runs counter to basic physics. Warming should cause more evaporation off the oceans which will in turn fall as rain. So more flooding would be a reasonable expectation during warming. So how is warming supposed to cause drought too?
The short answer is "models" but leaning on them is leaning on a broken reed. A more substantial answer is that warmer weather will not only evaporate water off the seas but also off the land. So you may have an initial drying effect on the land. But wherever the evaporation comes from, it will end up as rain. So there can be no net loss to the land. The clouds will give its moisture back plus more moisture from the oceans. And the earth surface is two thirds ocean so we are looking at a LOT more rain.
So a recent paper (below) has caused heartburn among Warmists. It says that the slight warming of the 20th century did NOT cause drought -- which is what one would on basic principles expect and which is borne out by other studies -- e.g. "the proportion of Europe experiencing extreme and/or moderate drought conditions has changed insignificantly during the 20th century". And another finding for a dry region during C20: "We found no evidence for a decrease either in mean annual rainfall or in the incidence of drought".
Note also the greening of the Sahel in late C20 and early C21. Instead of getting drier, the semi-desert Sahel got greener.
So professional Warmist Joe Romm has rubbished the paper. He says the paper is discredited and shows that it's conclusions are contradicted by "findings" from Warmist studies. That contradiction should bother us not at all so let us look at the academic criticisms of the paper:
ANY scientific paper is open to criticism, No study is perfect. So the issue is whether the criticisms discover a fatal error or make an improbable generaliztion. The criticisms Romm refers to are here. And they are far from totally dismissive. A few quotes:
In a recent study (Donat et al., 2016, Nature Climate Change, doi:10.1038/nclimate2941) we found that, when aggregating over the dry and wet regions of the world, precipitation changes are consistent between models and observations over the past 60 years. Nevertheless, it is true that modelling and analysis of precipitation changes are still related to a number of uncertainties, especially when it comes to regional changes in precipitation. This is partly related to the large temporal variability in local precipitation time series, but also shortcomings in the models with simulating processes related to precipitation."
I am not too surprised that there is disagreement for the 20th century as there is a strong component of random variability evident in the observational record. The picture of the “wet getting wetter and the dry getting drier” is one that is very likely to emerge over the course of this century but has not been evident, or expected, during the 20th century.
If this paper’s conclusion about model overprediction holds up to further scrutiny it will be extremely interesting; my own work focuses on model-data discrepancies so I am particularly interested. But due to the above aspects of the study, I am not convinced that this particular conclusion will hold up. We shall see, as I am sure this result will attract lots of attention."
So the study is not at all as risible as Romm claims. It is just one indication of what is going on.
Northern Hemisphere hydroclimate variability over the past twelve centuries
Fredrik Charpentier Ljungqvist et al.
Accurate modelling and prediction of the local to continental-scale hydroclimate response to global warming is essential given the strong impact of hydroclimate on ecosystem functioning, crop yields, water resources, and economic security1, 2, 3, 4. However, uncertainty in hydroclimate projections remains large5, 6, 7, in part due to the short length of instrumental measurements available with which to assess climate models. Here we present a spatial reconstruction of hydroclimate variability over the past twelve centuries across the Northern Hemisphere derived from a network of 196 at least millennium-long proxy records. We use this reconstruction to place recent hydrological changes8, 9 and future precipitation scenarios7, 10, 11 in a long-term context of spatially resolved and temporally persistent hydroclimate patterns. We find a larger percentage of land area with relatively wetter conditions in the ninth to eleventh and the twentieth centuries, whereas drier conditions are more widespread between the twelfth and nineteenth centuries. Our reconstruction reveals that prominent seesaw patterns of alternating moisture regimes observed in instrumental data12, 13, 14 across the Mediterranean, western USA, and China have operated consistently over the past twelve centuries. Using an updated compilation of 128 temperature proxy records15, we assess the relationship between the reconstructed centennial-scale Northern Hemisphere hydroclimate and temperature variability. Even though dry and wet conditions occurred over extensive areas under both warm and cold climate regimes, a statistically significant co-variability of hydroclimate and temperature is evident for particular regions. We compare the reconstructed hydroclimate anomalies with coupled atmosphere–ocean general circulation model simulations and find reasonable agreement during pre-industrial times. However, the intensification of the twentieth-century-mean hydroclimate anomalies in the simulations, as compared to previous centuries, is not supported by our new multi-proxy reconstruction. This finding suggests that much work remains before we can model hydroclimate variability accurately, and highlights the importance of using palaeoclimate data to place recent and predicted hydroclimate changes in a millennium-long context16, 17.