By now just about everybody on the planet will have heard of Hurricane Harvey, and we all feel deeply sympathetic to the people of Houston. This was a dreadful time for them, which raises the question, why did this happen? As the disaster abates, the words “Global Warming” keep coming up. Global warming did not cause that Hurricane, it did not cause it to land on Houston, and with one reservation, it almost certainly did not cause hurricanes to be more common. However, global warming would have made the ocean a little warmer than usual, and that will have increased the intensity of any hurricane that was generated, made it more expansive, and more powerful. While it might have been the most newsworthy event, it was by no means the worst event attributable to an effect of global warming.
Hurricanes and Typhoons are just local names for tropical cyclones, and they originate because the earth is a rotating sphere, and because surface temperatures are uneven, therefore in places air rises because it is warmer, and in other places it falls. In the former you get low pressure, while in the latter, high pressure, and because there are pressure differentials, air flows towards and away from these systems respectively. Air moving in the north-south directions has different velocities in the east-west directions because of the different rotational velocities, and this generates some circular air motion (the Coriolis force) the direction depending on whether the air is being sucked in or being pushed out. In the normal course of events this would generate modest circulation, which would affect nobody badly.
However, there is an additional aspect. When the circulation goes over water, it evaporates moisture, and when this is sucked upwards in a low pressure event, eventually the air gets colder and the water comes out as water droplets, which generate clouds, and if there is enough moisture, rain. Of course, this is somewhat oversimplified, especially in mid-latitudes where you get fronts, etc, to complicate matters as air at different temperatures starts to mix, but the above, while oversimplified, at least lets us see what happened with Harvey. The reason the tropical storms are so bad, when you get away from the equator so as to get some effect from the Coriolis effect, is that the warmer the water, the more moisture gets sucked up. Water has a rather high latent heat of evaporation, so when it condenses out, that energy has to go somewhere. The warmer air rises, generating lower pressures below, and hence more suction, which means more water sucked up, leading to even more air being sucked in, leading to the extremes of rotational kinetic energy that we see.
So, the warmer the water, the more energy is available to power stronger winds, and more rain comes down. Harvey was particularly bad because it stalled over Houston. Normally, tropical cyclones run out of strength as they cross land, because there is no further moisture to power them, but Harvey had half of itself over land, and half over the Gulf of Mexico, so it was able to keep itself going longer than you might expect. So the hurricane would have been a little stronger than without the global warming, it would have dropped much more rain than without the global warming, but its path greatly accentuated the damage. Irma will do the same wherever it hits.
What global warming will also do is increase the number of tropical cyclones around the world. That is simply because by increasing the surface temperatures of the seas, there is more energy available for a weather event, hence more of the systems that would normally just qualify as storms or cyclones get upgraded to the tropical cyclone status. Worse, they do not have to be in the tropics. In Wellington, where I live, this winter the Tasman was 1.5 degrees C hotter than usual for this time of the year, and when a resultant system somehow met some colder sub Antarctic air, we got a storm with wind speeds that qualified for a category 3 hurricane, with a lot of rain, but it was cold. So, what we can expect in the future is many more of these storms, and not just in the tropics. The storms do not need to be hot; they merely need to have been powered initially with warmer seawater.
I mentioned that Harvey was not the worst event. At the same time, the monsoon over parts of India and Bangla Desh, thanks to increased sea temperatures, gave record rainfall that put about half the country under water, thus probably wiping out a large fraction of the country’s crops. It also killed about twelve hundred people and severely affected the lives of forty-one million people. And Bangla Desh in one of the poorest countries on the planet. There may be a tendency to think Houston, being part of the richest country on the planet, will get over this, and it probably will, but these changing events are going to happen everywhere, and as with Bangla Desh, many places will not be able to cope easily. It is the richer countries that have to start doing things to control these disasters, if for no other reason than they are the only ones with the means to make an impact. We really need to work out how to deal with such events, because they will occur, but better still, we need to take real action to minimize the number that do happen, and that means really doing something about global warming. Those who deny its existence should be made to exchange positions with people in Bangla Desh