Extreme weather, the climate crisis and El Niño … it’s climate change 101.

When our skin is wet, it feels cooler – the cooling effect is evident if we’ve had a swim or worked hard and built up a sweat. How that happens is also why global overheating and extreme weather go hand in hand.

It occurs because air can hold water vapour (an invisible gas, unlike the minute droplets that form steam or clouds). So, when we’re looking through air, we’re looking through water vapour as well as the likes of oxygen and nitrogen. Additionally, the warmer the air, the more vapour it can hold.

The reason our skin feels cool is the process of evaporation, as the liquid becomes vapour in the air (e-vapour-ation!) and sucks energy (heat) from our skin to turn itself into a gas.

No prizes for guessing the reverse happens when air is ‘full’ of water vapour (at saturation, or 100% relative humidity). At that point, cool it a little or add more vapour and clouds form and then rain.

So, hold these thoughts:

• hotter air can hold more water vapour
• it takes heaps of energy to evaporate water, and
• more heat (energy) makes air more vigorous.

Scientists and aircon engineers know how all this works and use jam-packed, clear-as-mud charts (seriously, you should see them) to do their calculations. For example, the charts show that air at 21°C can hold some 7% more water vapour than at 20°C.

Factor in that more heat equals more energy equals more wind in the weather, and we see why warmer weather systems deliver even more rain even more quickly.

But it doesn’t end there. Imagine a warm, moist, westerly coming off the Tasman Sea and cooling against the mountains. No longer able to hold so much moisture, it dumps a heap of rain on the West Coast. Sound familiar?

Now, consider this: while it requires significant energy to evaporate water, the process reverses when water vapour condenses into clouds. This releases the stored energy back into the air and, bingo, hotter air flows down off the mountains, resulting in phenomena like Canterbury’s well-known nor’westers. These winds are particularly drying because they’ve not only dumped a heap of their moisture as rain but heated up in the process, creating even more capacity to hold water vapour and so more drying ability.

Thus, when climate scientists say climate change will bring more floods in some places and more droughts in others, that’s the sort of process they’re describing.

Now, enter El Niño

El Niño (warming) and La Niña (cooling) occur in the tropical Pacific Ocean, cycling over several years in what The Guardian’s political editor, Damian Carrington, calls “the biggest natural climate phenomenon on the planet”.

In La Niña, prevailing easterlies push warm surface water against Australia and Indonesia, leaving much of the Pacific surface cooler, which helps soak up atmospheric heat. But every few years, El Niño kicks in, easterlies weaken, and warm water spreads right back to South America, creating a vast warm surface which, in 2016, contributed to air temperatures reaching record highs worldwide.

We’re entering another El Niño phase, and NIWA tells us that “During El Niño, New Zealand tends to experience stronger or more frequent winds from the west in summer, which can encourage dryness in eastern areas and more rain in the west.” Heard that before?

Leading Australian climate scientist Joelle Gergis puts it more forcefully: “The last thing we need right now is a bloody El Niño!”

With global warming plus El Niño, we shouldn’t be surprised if we’re facing a period of extreme weather on steroids. Hold tight.

Interested in more info? Try Under the Weather: A Future Forecast for New Zealand by Climate Commissioner James Renwick.