How to Think About Global Warming and Intense Rainfall

I sure am glad I haven't been home in Los Angeles over the past week. The normally sunny Southland area has gotten a staggering five plus inches of rain – enough to cause risks of landslides and knock out power for thousands.

And this on top of a previous winter storm from earlier this month, which I did have the joy of experiencing. That one dropped 1.5 inches in a twenty four hour period in downtown Los Angeles. My roof leaked like crazy, and I hear it has been leaking again more recently.

So just how unusual has the weather been? Well, without any expertise in the region's climatology beyond what common sense can tell, I turn to the reporting of the LA Times:

As of Sunday, more than 10 inches of rain had fallen this season in downtown Los Angeles, about 4 inches more than normal.... The wet weather is expected to continue into today, with the chance of showers diminishing this afternoon.

Smaller storms are expected later in the week.

So, an unusually rainy season, without a doubt. What do we make of this, from a global warming perspective?

Well, it turns out that among climate scientists, there's a broad general expectation that in a warmer world, once you have a storm, that storm will be capable of producing more precipitation.

This follows from basic physics, explained admirably here:

An increase in global temperatures will lead to an intensification of the hydrological cycle. This is because an increase in surface air temperature causes an increase in evaporation and generally higher levels of water vapor in the atmosphere. In addition, a warmer atmosphere is capable of holding more water vapor. The excess water vapor will in turn lead to more frequent heavy precipitation when atmospheric instability is sufficient to trigger precipitation events. Intense precipitation can result in flooding, soil erosion, landslides, and damage to structures and crops.

This is merely the theoretical expectation – it says nothing about any individual rainfall event. To repeat the climate change scold's refrain: I don't want to be accused of saying that global warming caused the weather in Los Angeles this month.

Indeed, it's particularly important to be cautious in this regard given other changes that global warming may bring about. For instance, the heavy rain-bringers to LA are technically known as extratropical cyclones – but these sorts of storms are actually expected to track further northward, on average, in a warmer world. In other words, away from Southern California.

So there's no clear picture here – in general, my sense is that my new home will have much more to worry about from drought and loss of water supplies than from occasional intense rainfall events.

But at the same time, it's important to understand our weather in context. And in that context, not only can we say that global warming should be pushing us towards more rainfall extremes on average – we can also remember that fact every time we're stuck at home for the weekend while it pours.