Even taking into account how poor the state of science journalism is in this country, and what a sorry rag the San Fransisco Chronicle is, this piece on the danger posed to San Francisco by invasive pythons is well below the curve.
To summarize, there is an introduced population of Burmese pythons in the Everglades, 3100 miles away. A USGS lab used an off the shelf climate model to predict where in the US Burmese Pythons could deal with the climate, and San Francisco (along with pretty much the southern half of the US) showed up as one potential habitat.
The idiot who wrote the article then follows this train of thought to show that pythons will be eating pedestrians on Market Street by 2020:
•One python showed up at Lake Okeechobee, 100 miles north of the Everglades, and in the same direction it would travel if its single-minded goal was to reach Fisherman's Wharf.
•Pythons can travel up to 20 miles a month, and the road to SF is 3000 miles, so it will be here in 150 months, or about 12 years.
•Pythons eat mammals.
•"Human beings - like rodents, beavers and deer - are mammals, government scientists confirmed."
•Giant snakes falling from office buildings and eating you.
The sad thing is, reading the article, I am not sure if he is joking, but I am sure there are some people who will think this is a real threat.