The question “did climate change cause this unusual weather event” is frequently asked. In almost all cases the answer is “no”! But, if the the question is “did climate change contribute to this unusual, or extreme, weather event” the answer is often “based on intuition I would say: probably yes ”.
“Cause” vs. “contribute” are really two different, but related, questions. By way of illustration we can start with a couple of very fundamental facts. Warm water evaporates more quickly that cold water, and warm air holds more moisture than cooler air. Thousands of global temperature measurements tell us that the oceans and the atmosphere are warming, therefore moisture is being added to the atmosphere more rapidly and the air has the capacity to hold more moisture. Therefore it stands to reason global warming may contribute to the intensity of individual rainfall events and may even cause them.
However, the ability to quantify any effect of climate change upon rainfall, and to scientifically prove a cause and effect relationship is a more difficult task. Two recent papers in the on-line version of the esteemed British journal, Nature, have addressed this topic. The first, “Human contribution to more intense precipitation extremes”(1) compared 50 years of precipitation records (1951 – 1999) over much of the northern hemisphere with the results of several computer models used to predict 20th century precipitation. It was only when the effects of greenhouse gases was added to the models that agreement with the actual measured precipitation was obtained. The study showed an increase of 7% in the probability of extreme precipitation on any given day during the later half of the 20th century in the Northern Hemisphere, a far greater percentage than would be expected from the natural variability of weather. A second finding was that existing climate models tend to be conservative in predicting precipitation, typically underestimating the amount that was actually measured.
With a more narrow geographic scope, another paper “Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000” (2) examined the data from a single, regional event, the wettest autumn since records began in 1766, and found that the chances of such heavy floods as the region experienced in the fall of 2000 were roughly doubled by the increased level of greenhouse gases in the atmosphere. Nine out of ten of their computer model simulations showed that anthropogenic greenhouse gas emissions increased the probably of such flooding as experienced in 2000 by over 20%, while two-thirds of the simulations showed an increased probability of over 90%.
The fact that both of these groundbreaking studies took nearly 10 years to complete, even when an base of actual data existed, shows the tremendous amount of computer time that the highly complex climate models require. While such models may not be perfect, they are rapidly becoming more accurate, and as demonstrated here, capable of more highly refined results; and they are definately the best available tools for understanding climate change. Note that in both cases, these studies are not saying that specific weather events were directly caused by climate change, but rather that the probabilities of their occurrence was increased by climate change. It is also noteworthy that the second study, dealing with the 2000 flood risk in England and Wales, was conducted by the ClimatePrediction.net project at Oxford University, using a computer grid composed of approximately 60,000 donations of unused computer cycles on individual personal computers around the world.
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1.) Human contribution to more-intense precipitation extremes: http://www.nature.com/nature/journal/v470/n7334/full/nature09763.html
2.) Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000: http://www.nature.com/nature/journal/v470/n7334/full/nature09762.html
