It’s a law of physics, not a matter of opinion, that warmer air holds more moisture. Warmer air holds more water because the water vapour molecules in the air are moving at a higher average speed than those in colder air, making them less likely to condense back to liquid. According to the Clausius–Clapeyron equation, for each degree Celsius rise in air temperature, saturated air contains 7 percent more water vapour.
End of physics lesson, and starting point for the ensuing consequences. An atmosphere with more water vapour can make more precipitation. How the warming of the atmosphere translates into changes in global precipitation is not so linear, but the total volume of precipitation is likely to increase by 1-2 percent for each degree Celsius of warming.
To make matters worse, water vapour is itself a greenhouse gas that amplifies warming from our carbon dioxide emissions. In a positive feedback loop that is not at all positive in its impact on us, studies show that water vapour doubles the warming caused by carbon dioxide. In other words, if there is a one degree Celsius rise caused by carbon dioxide, the resulting increase in water vapour will cause the temperature to go up another one degree. And so warm air continues to warm up.
Water vapour cycles through the atmosphere quickly, unlike carbon dioxide that persists in the air for decades. However, the rate of water evaporation rises with temperature, so the amount of water vapour in the air at any one time (and the amount of warming it causes as a result) is strongly correlated to the amount of greenhouse gases we emit. Changes in weather patterns make it very difficult to predict where this extra water will fall. In a warmer climate, heavy rainfall will increase and be produced by fewer more intense events. The corollary is that this that these deluges lead to longer dry spells elsewhere. Evidence shows that regions that are already wet will get wetter, and dry regions will get drier.
Which brings us to the horror of Cyclone Idai that hit southern Africa a few days ago and the aftermath of widespread flooding and devastation to parts of Mozambique, Zimbabwe and Malawi. Hundreds of thousands, if not millions, of people are affected by one of the worst weather-related disasters ever to hit the southern hemisphere. In the words of Muleya Mwananyanda, Amnesty deputy regional director for southern Africa,
"As the effects of climate change intensify, these extreme weather conditions can be expected to revisit us more frequently. The devastation wrought by Cyclone Idai is yet another wake-up call for the world to put in place ambitious climate change mitigation measures."