We know there has been natural global cooling - ice ages and the like, so it would make complete sense for there to have been natural global warming at some point too.

We also know in the UK the romans (circa 100BC) grew grapes almost up to the scottish borders, something not possible today because it's too cold.

So, the climate has always been changing, and while it's almost certain that humans have made an impact on the environment, I find it very hard to believe that the results will be catastrophic.

You're wrong.

High frequency trading means that more trades happen in general. This extra competition to fill orders drives down the difference between the buy and sell prices and greatly reduces arbitrage situations (ie, the difference in price between the same stock listed on different exchanges and possibly in different currencies).

So, if you buy or sell a something, you're giving less money to the market-makers and you're getting a more "correct" price. It levels the playing field.

And it's true that arbitrage and hifi trading are a zero-sum game. That's why it's an arms-race at this point.

Almost! It's actually about the location of the data center, i.e, close to the exchange. A fast, low latency connection to the exchange gives you a crucial edge over the competition. It means when things change you can get your trades in before your competition does. This is ever more important in the up-and-coming automated trading systems.