Rain storms

Technology and meteorology have improved our understanding of causes of floods; it is now known that the upper-atmosphere jet streams can contain ‘rivers’ of rain. (‘River’ is suitably emotive for the media, but meteorologically is inaccurate.) A moderately recent example of a severe flood caused by such an event happened in 1862. It was driven by a period of prolonged intense rain for around 40 days from a northern jet stream ‘river’ that had drifted unusually far south. This inundated Sacramento and the associated valleys of California. It similarly severely affected all the neighbouring states. For example, it formed a lake in Arizona 60 miles long and 30 miles wide. If the weather patterns duplicate this flood in our time, the effect would be even greater, because the Sacramento Valley floor has sunk over the last 200 years. The flood would therefore be much deeper. Once again, I can point to the role of technology leading to an increased danger, as the sinking of the ground level is the result of arterial water extraction for agriculture. In 1862, the event caused economic ruin across the entire region; we can expect the same financial impact from future floods caused by this type of source.

My examples so far are for relatively short-term natural events, with long-term consequences for us. I have already mentioned that droughts were devastating and are blamed for the loss of civilizations from the Americas to Asia. But less sudden changes in weather patterns can be equally destructive and long term. A minor change in average temperature disrupted a monsoon weather pattern that had delivered rainfall and fertilized North Africa until about 5,000 years ago. A rapid switch from fertile savannah to the sands of the Sahara Desert emphasizes how delicately balanced is our reliance on climate. (The term ‘Sahara Desert’ is overkill, as ‘Sahara’ means ‘desert’.)

We contribute to sensitivity to such changes by farming practices and destruction of natural forests, because deforestation can reduce rainfall. Once lost, forests may not recover. The scenario of an equivalent desertification of the South American jungles initially seems ridiculous, but the parallel with the sudden changes that happened to the Sahara is unfortunately much closer in time, and more probable, than we might expect.

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