Wake of the flood (first reprise)

Thailand is experiencing its worst monsoon flooding for at least 50 years. The NASA Earth Observatory image above shows the waters piling up to the North of the capital Bangkok, which is already beginning to experience flooding (visit the EO page to see a comparison with earlier floods, and The Guardian for a striking set of flood pictures). The Thai government yesterday declared a five day weekend to allow the city’s inhabitants to make preparations. The intense monsoon season has also brought extensive flooding to Cambodia and northeastern India over the last couple of months, and destroyed a significant part of SE Asia’s rice crop. On the other side of the planet, heavy rain and flooding has been affecting Mexico, El Salvador, Costa Rica, Honduras, Guatemala, and Nicaragua. Jeff Masters reported that in the ten days up to October 20th, Huizucar in El Salvador received an astonishing 1.513 metres of rain.

At first glance, it looks like a continuation of the remarkable series of extreme weather events — especially heavy rainfall and flooding — that we’ve seen over the last few years. But apart from the human suffering and economic dislocation being experienced around the world, it appears there’s another interesting consequence of all this precipitation — it’s causing global sea level to fall.

I stumbled across this idea in a NASA news item released back in August, and referred to it in the last edition of The Climate Show, but I think it’s worth developing the idea a little further. Here’s what NASA had to say:

[Josh] Willis said that while 2010 began with a sizable El Niño, by year’s end, it was replaced by one of the strongest La Niñas in recent memory. This sudden shift in the Pacific changed rainfall patterns all across the globe, bringing massive floods to places like Australia and the Amazon basin, and drought to the southern United States.

Data from the NASA/German Aerospace Center’s twin Gravity Recovery and Climate Experiment (Grace) spacecraft provide a clear picture of how this extra rain piled onto the continents in the early parts of 2011. “By detecting where water is on the continents, Grace shows us how water moves around the planet,” says Steve Nerem, a sea level scientist at the University of Colorado in Boulder.

So where does all that extra water in Brazil and Australia come from? You guessed it–the ocean. Each year, huge amounts of water are evaporated from the ocean. While most of it falls right back into the ocean as rain, some of it falls over land. “This year, the continents got an extra dose of rain, so much so that global sea levels actually fell over most of the last year,” says Carmen Boening, a JPL oceanographer and climate scientist. Boening and colleagues presented these results recently at the annual Grace Science Team Meeting in Austin, Texas.

The story included a couple of graphs to illustrate the point. Here’s the drop in sea level, as recorded by satellite:

And here’s where the GRACE satellites showed it ended up.

When I first read the NASA article, I was amazed. Perhaps I shouldn’t have been. After all, lots of things can cause sea level to rise and fall — thermal expansion, ice melt (or freeze), and so on. During ice ages lots of ocean ends up on land in the form of the great ice sheets that are built by accumulating snowfall. But this is, to coin a phrase, a really neat demonstration of an interesting effect — one that we can only begin to appreciate because of the application of truly remarkable technology.

So what’s happening now? Here’s the latest sea level chart from the University of Colorado (data to Sept 19th):

There’s been a slight upwards tick over the last few months, but no dramatic surge back towards to the trend line. That suggests to me that the processes that caused the drop in the first place are still operating. The population of Thailand might agree…

Eventually, as Josh Willis says at the conclusion of the NASA article, the floodwaters will run off the land and return to the ocean, La Niña will swing back to El Niño, and sea level rise will resume its upwards trajectory. With La Niña likely to stay in place through the southern summer, it will be some time before El Niño returns and imposes its own pattern on the world’s weather. But when it does we could be in for a wild ride as sea level surges and global temperatures reach new peaks. We live in interesting times.

[Update 27/10] Flash flooding has ripped through Liguria and Tuscany in Italy. Both the Guardian and BBC videos open with water roaring down the streets of (I think) Monterosso — a place I visited in September. Dublin has also experienced torrential rain and flash flooding in the last few days: video here. Intensification of the hydrological cycle anyone?

[Grateful Dead]