MORE FLOODS & MORE BIG SNOWSTORMS
"There are rivers of moisture in the atmosphere. Only you can’t see them."
This is the way my former boss described how hurricanes are steered by upper-air winds. Dr. Neil Frank was Director of the National Hurricane Center, and he had an amazing ability to explain complex things in easy to understand ways.
It’s all about moisture.
There’s even moisture up there when skies are clear. Of course, there’s a lot more moisture around when it’s about to rain or snow. But just how much moisture is up there? Is there actually more moisture around now than in decades and centuries before? In a word, YES.
That extra moisture has helped produce more rain in storms all over the world. And when it’s cold enough, it has helped produce bigger snowstorms.
HOW DO WE KNOW?
It’s easy to say that there’s more moisture in today’s world. One way to tell is to measure the rain and snow. Let’s stay home and use Philadelphia as an example (many other cities around the world have similar results):
Here are our recent weather records -- all since 2009.
WETTEST SINGLE DAY
2nd & 3rd BIGGEST SNOWSTORMS
We have kept official records in Philadelphia since 1871. That’s 145 years. Yet all of those records happened in less than EIGHT!
And it’s not just happening here. Heavy downpours have clearly increased across the U.S., especially in the Northeast.
Can we measure direct evidence of an increase in moisture? It wasn’t possible before weather satellites. It is now. The satellites measure something called "Precipitable Water" (PWAT). That’s defined as "the total water vapor contained in a vertical column." That’s from the ground to the top of the atmosphere. The more water vapor, the more precipitable water there is. And that extra water vapor is what leads directly to increased rain or snow.
WATCHING THE RIVERS OF AIR
It’s a beautiful thing to see. Animations of those "rivers" of moisture show tropical moisture spreading westward across the Atlantic from Africa. The yellow and brown colors show the "thickest" moisture areas…
Water vapor has increased in recent years, according to more and more sophisticated "remote sensing" of the atmosphere.
Notice that the greatest increase is in the tropics. But the yellow to orange colors off the U.S. East Coast show significant increases too. That’s the general picture. Now imagine a coastal storm, with a long path of onshore winds. We’re now adding extra moisture. This is confirmed by measurements of precipitable water in individual storms. I’ve seen many National Weather Service forecast discussions in recent years mentioning “record” or “near-record” PWAT ahead of a storm. I saw it in January of this year ahead of our giant snowstorm. I also saw it this past Thursday ahead of the system that brought 3-5 inches of rain in parts of Delaware and South Jersey.
We can look back at storm after storm where record rain or snow has fallen and see extremely high PW ahead of it.
SO, WHY HAS PRECIPITABLE WATER INCREASED? AND WILL IT CONTINUE TO INCREASE IN THE FUTURE?
"Observations and climate model results confirm that human-induced warming of the planet is having a pronounced effect on the atmosphere’s total moisture content."
That’s the first line of a report nearly 10 years ago from the Lawrence Livermore National Laboratory. It’s been around since 1952, and is managed by the University of California for the U.S. Department of Energy.
Yes, global warming has already contributed to an increase in PWAT, which has then contributed to more record rain and snow. And future warming is likely to cause even higher levels of PWAT, which will likely contribute to even more record rain and snow. It won’t necessarily happen everywhere, for every storm, of course.
AND TO MAKE MATTERS WORSE
There’s this other thing about water vapor. It is considered to be a "positive feedback" that will increase global warming beyond what increases in carbon dioxide (CO2) will do. You see, water vapor is the most effective of all greenhouse gases, which warm the earth. Of course, almost all of it is natural, but not all.
Here’s how it works: the atmosphere warms due to human causes, then water vapor increases, which then helps increase temperatures, which helps increase water vapor even more, which then helps increase temperatures even more……and on, and on, and on…..How much of a positive feedback will it be? That’s one of the actual debates going on among climate scientists.
Water vapor isn’t the only positive feedback, either. Melting arctic ice leads to more heat being absorbed (ice reflects sunlight much more effectively than the darker ocean), which then melts more ice, which then causes more heat being absorbed, which then melts more ice…and on, and on, and on….
So, now we have carbon dioxide (and other greenhouse gases) leading to warming, plus the positive feedbacks of water vapor AND melting arctic ice. This is why many climate scientists are concerned that future warming will be even more (and faster) than many current computer models show. Climate skeptics often suggest that future warming will be slower than current models show. The positive feedbacks suggest that there’s at least a reasonable chance the models will underestimate future warming.
Glenn "Hurricane" Schwartz
Chief Meteorologist, NBC10 Philadelphia