Scientists have known for decades (more than a century actually) that increases in greenhouse gases will cause the Earth to warm. What is less clear is how this warming will impact the weather we experience on a daily, monthly, or yearly basis. Recent research shows that we are already feeling the changes.

So, how might a warm planet be different from the planet we inherited? Increased temperatures can cause more heat waves, more droughts, more intense rainfall, higher water-vapor levels, sea-level rise, changes to ocean acidity, more intense winds, etc. Of course, some of these are not “weather” (ocean acidification and sea-level rise), but I include them because they are well-known and significant ways in which climate change expresses itself.



It is not correct to think these are future changes that will impact our children or their children. Rather, these changes can be detected now. And, as the years progress, we are detecting more significant changes.



This new paper provides up-to-date understanding of how extreme weather is changing in the USA. The paper looks at the USA, partly because there are excellent records there. We show that increases in intense precipitation have occurred in all regions of the continental USA and “further changes are expected in the coming decades”. It is a second of two papers that were published to the community of civil engineers so that future infrastructure can be designed with changing weather patterns in mind.

The physical mechanism that influences changes to precipitation is largely the moisture-carrying ability of the atmosphere. (For those of us who are sticklers for exactness, the atmosphere doesn’t “carry” moisture but this is the common phrase which represents the saturation pressure changes with temperature). Basically, when it gets warmer, the air “has” more water (humidity). But that water doesn’t stay in the atmosphere forever, it rains out quickly.



So, more moisture equals more rain. Not only that but when you increase moisture in the atmosphere, you tend to get heavier downpours. So, when it rains, it really rains. There are other aspects to changes in precipitation that are noted, for instance, changes to large-scale atmospheric wind patterns push wet and dry regions to different parts of the planet. So, this is complicated, a lot of things are happening at the same time.

Every part of the continental United States has experienced increased very intense precipitation events. The further northwest you go, the larger the increase. As you travel to the Southwest, the increases become much smaller.

But haven’t scientists also told us that the drier areas will become drier and the wetter areas will become wetter? How does this adage conform to the recent paper? Well, it turns out that if you get an increase in intense precipitation, it doesn’t mean that your location will be wetter. It may just mean that the rain you get falls in heavier downpours. Also, the “wetness” and “dryness” of an area doesn’t just depend on how much precipitation occurs. It also depends on temperature, areas that heat up will see their water evaporate more.

This combined temperature-precipitation connection with water availability is the reason so many scientists have pinned the current California drought on human emissions. You can’t look at current or past droughts as just a “precipitation” or a “temperature” problem… the two issues are interconnected. What the paper shows is that the overall precipitation has actually decreased in large parts of the Southwest, even though the heaviest rains have increased.



More importantly, the professionals in the civil engineering field have a road map for making changes to future infrastructure. At least everywhere in the USA, next-generation infrastructure should be more resilient to large precipitation events.