There is a strong argument to be made that the recent flooding in Colorado is the result of global warming. Here are three things one could say about the flooding. Think of these as alternative hypotheses to explain that event:
1) Weather has extremes. Sometimes, instead of raining just a bit, it rains a hella lot and you get a big giant flood.
2) Weather has extremes etc. etc. but global warming tends to make some of the extremes more extremes, so instead of getting just a big flood, you get a big giant flood.
3) The storm that brought well over a foot of rain to one mountainous area was qualitatively distinct; it happened because of a configuration in the weather patterns that might have happened at any time over the last several centuries but only very very rarely, but because of global warming, this sort of thing happens far more frequently. The weather patterns in the Northern Hemisphere have shifted in a way that makes the rain event in Colorado a fairly likely thing to happen somewhere in the world several times a year, and it happened to happen in Colorado this time around. Prior to global warming caused changes, this effect would be very rare, now it is common.
The difference between these ways of looking at the weather is very important, because under option 3, we have a problem. Just as people who live along the Gulf Coast or the mid-Atlantic or south need to worry about hurricanes as a thing, or people who live in the middle of the US have to worry about tornadoes as a thing, or people who live in Minnesota have to worry about killer cold as a thing, it may be the case that people who live at latitudes in the Northern Hemisphere now have to worry about this new weather pattern, which some call "weather whiplash," as a thing. When you build your mountain roads in the Rockies, you've got to figure that there is a reasonable chance that during the next few decades there will be a foot of rain in the catchment of the stream that road runs along. Either build the road differently, or plan to replace it now and then. Mountain valley settlements in high mountains like the Rockies may need to measure out a new "high water line" for the creek they overlook and plan for that water line being reached within the lifetime of the inhabitants of the village, once or more.
Similarly, just as dense concentrations of rain are more likely under option 3, dense concentrations of dry conditions are also likely. In other words, weather whiplash is like my old broken sprinkler.
Until recently I had one of those sprinklers that wave back and forth with a couple dozen high power streams of water. The water comes out of a bar, and the bar oscillates back and forth and back and forth so there is a long, linear, gentle rain storm that passes back and forth across the lawn over the zone covered by the sprinkler. But when my sprinkler got old it would get stuck sometimes. The bar would stop oscillating, and the streams of water would create a long linear rain storm on one strip of the law while the rest of the lawn simply got dryer. The broken sprinkler did something that resembles the weather in the middle-ish part of the United States for a week or so during September 2013. The midwest got a "flash drought" during which no rain fell but it as hot and breezy, while the Rockies and other areas got lots of rain from a big storm that sat there for days and days without moving. The main part of the storm was in Colorado but New Mexico got extra rain as well, and after the storm left Colorado it moved north in the Rockies and wet down Wyoming and Montana a bit as well (causing only some flooding).
The jet stream is often a long, linear, fast moving necklace (well, more than one necklace as there is more than one jet stream) that encircles the earth at some distance from the equator. It is associated with the movement of air masses around the globe. These air masses alternately pick up and drop moisture. When the air mass is dry, it dries out the land beneath. When the air mass is wet, and it mixes with some other air along a front, it drops rain. But the rainfall (and correspondingly, the dry spots) are somewhat like an oscillating sprinkler that is not broken. A given area is likely to experience alternating rain and dry.
Some regions experience more dry than wet, some regions are wetter, but the rainfall across a given region is typically doled out in chunks, some of which can be very heavy, but rarely more than a few inches in a given storm.
Lately, the jet stream seems to have been very frequently changing its configuration. Instead of being a relatively straight circle around the globe it is all kinked up in the big "waves." Where there are waves, several things happen. First, the movement of air along the jet stream slows down, and this interacts with other air masses. More importantly, it seems, is that the kinks create large very slow moving or stationary low and high pressure systems. The high pressure systems are south of the jet stream, the low pressure systems are north of it, but since the jet stream is kinked, these low and high pressure systems end up being next to each other. So, we get tropical stuff moving north, and subarctic stuff moving south, and there are vast differences in moisture and temperature. This can result in two things at the same time. Some regions have dry conditions and some have lots of precipitation. The key thing is this: Since these systems are very slow moving, or sometimes, just plain stuck, like my sprinkler, the dry conditions persist for many days, and the wet conditions persist for many days. Thus, Colorado.
I wonder if is possible that the position of the waves in the jet stream will end up being more frequently located in certain spots. I have no reason to say this empirically, but since air mass movement is linked to the position of mountains and oceans and stuff, it seems a reasonable question to ask. If that ends up being the case, than we could end up with a new climate regime wherein certain areas tend to get repeated stalled rain systems (not every year, but just more frequently than average) while other regions get repeated stalled dry conditions. That might be good news, because it might be easier to adjust to weather whiplash with more predictability. But if this sort of pattern was to be strong, we would probably see it already, so don't count on it. Most likely, a climate pattern where very rainy weather shows up out of nowhere and sits on top of you for a week while elsewhere dry conditions persist for a few weeks in a row is not good for agriculture or for mountain villages and roads.
I got a new sprinkler. It wasn't easy. This time of year it is hard to get sprinklers because they tend to stock up on them in the spring. Also, with the drought conditions were've been experiencing over the last few weeks in my neighborhood, there has been a run on the few sprinklers that are left. Climate change made it hard for me to find a sprinkler! (First World Problem #212124). But eventually I got one. I'm not sure how hard it will be to get a new climate.
But if this sort of pattern was to be strong, we would probably see it already, so don’t count on it.
I'm not so sure about that. Even if there were a strong preference for certain positions of the wave pattern, I expect it would take at least a decade if not more to establish this with confidence, and the high-amplitude standing wave configuration was rare until a few years ago. There is a reason the NWS uses 30-year baselines for determining climatology.
Even if the position is approximately random, however, certain positions will be worse than others. Mountains tend to enhance precipitation along the windward slopes. At Colorado's latitude, this usually means the west side (if you have ever driven across the Rockies, you may have noticed a distinct difference in vegetation at altitudes above 7500 feet: deciduous trees west of the Continental Divide, evergreens to the east). In this case, there was a southeasterly flow tapping into the seasonal monsoon which normally stays further south; when this flow hit the Front Range, lots of rain fell. Shift that pattern east to Missouri, and you will probably have some flooding, but not the widespread devastation we saw in Colorado, because the rain won't fall as quickly.
Well, that is a very good point that a given spot that tends to enhance rain, for instance, will have more effects regarding rain. That would be true though even if the Rossby waves were random. And yes, it might take a while to notice it, but if that is the case, it would be the kind of effect that matters to trees rather than to, say, decisions about building sports stadia with covered roofs vs. not.
But also, if we are entering an era of convoluted jet streams, we may not have settled in. It may be that as this transition progresses, standing waves end up standing in certain spots because of mountain waves or sea-land interactions (which in turn would probably be affected by major oscillations, so there might be an "El Nino Rossby Wave of Wetness" over the Rockies for five years, then it moves off and becomes the "La Nina Rossby Wave of Wetness" over Central Europe.
It would add a new and interesting dimension to the boring old seasonality thing.
"The weather patterns in the Northern Hemisphere have shifted in a way that makes the rain event in Colorado a fairly likely thing to happen somewhere in the world several times a year, and it happened to happen in Colorado this time around. Prior to global warming caused changes, this effect would be very rare, now it is common."
It also happened in Alberta, Canada, a vast region of Siberia/China (Google 'Amur River' - the flooded area is equal to Germany and France combined), central Europe, and northeastern India.
And all these events were similar to Colorado in that they involved 'average' systems that simply got 'stalled'.
I'm pleased to see you writing about this, Greg, because I'm seeing the very same thing. Floods from rapid snowmelt or hurricanes are to be expected. 1,000+ year events happening from typical (but stalled) systems are another sort of animal altogether.
BTW, Colorado's rainfall amounts exceeded anything to be expected in a 1,000-year event - by more than 200%:
I should have made it clear that the abnormal flood events I listed all occurred in 2013. There are many, many other examples.