Cross-posted at: Dailykos

Recently we saw Kerry Emanuel of the Massachusetts Institute of Technology and meteorologist Tom Knutson demonstrate that global warming may indeed be making hurricanes much more powerful:

Is global warming making hurricanes more ferocious? New research suggests the answer is yes. Scientists call the findings both surprising and “alarming” because they suggest global warming is influencing storms now — rather than in the distant future.

My global warming hero Rasmus helped to explain why this might be so:

The analysis…indicates a systematic tendency where the strongest winds associated with tropical storms have been in the most recent decades. The graphic also shows a deepening of the minimum pressure over time, i.e. an indication of increased severity…

But the one huge problem in my mind has consistently been that we have almost no data on hurricane intensity that it older than 150 years.  Now we do:

What we really need to know is: is this new data suggesting something that is outside the historical trend.  Are we seeing something new?  Hurricane activity varies greatly from year to year and from decade to decade and tells us very little about long-term trends. We can only begin to see new trends by comparing present data with historical and pre-historic trends.  But the data goes back not more than 150 years and in some cases, most cases, not even that long.

Sounds like that’s going to change. Live Science reports that Claudia Mora of the University of Tennessee has devised a way to measure the intensity of a hurricane season by looking at tree rings in the southeastern pine forests.

In the new study, researchers examined growth rings in pine trees near Valdosta, Georgia, finding hurricane signatures that matched known storms.

“We’ve taken it back 100 years and didn’t miss a storm,” she said.

In my former life, I was an archaeologist and I still LOVE this stuff!

We’ve known for a very long time that one can, essentially, see the weather from ancient times by looking at tree rings.  A tree ring (and associated isotopes and chemical signatures) can tell you if it was a wet or dry year, if there was a high occurrence of lightening, if there was a forest fire, if there was a volcanic eruption and so on.  Here in the southwest, our detailed knowledge of tree ring data is the best in the world.  You can literally find out what the weather was like in the summer of 982AD, for example.  Not so in most other parts of the world.  Until now.  

Mora and her group have begun looking at tree ring data to determine historical trends in hurricane intensity:

Mora’s team examined early-year and late-year growth. Most hurricanes strike late in the year, with August and September being the busiest months. They probed woody tissues for sudden drops in a particular oxygen isotope called oxygen-18.

Hurricane depletes the air of oxygen-18, so a hurricane’s rain has less of it than other downpours. A shallow-rooted tree like the longleaf pine draws from a storm’s rain within a couple weeks, leaving a storm’s calling card in the new tissue.

Nerd or not, this is just cool. And important.

Mora’s team then applied the technique to trees in other parts of the Southeast and uncovered a vault of hurricane information going back 227 years. Active periods were noted in the 1870s and 1950s, corresponding to spikes known from historical records.

Additional work revealed droughts and wet years in trees going back to 1450. One storm, known as the Great Hurricane of 1780, put its John Hancock in the trees.

The trees say hurricane activity from 1580-1640 was low, which matches with what scientists expect based on other clues to the climate from that time.

“What we’re trying to do is understand frequency of hurricanes and how variable their occurrence is over the long-term,” Mora said. “We’re trying to come up with a reliable way to establish this.”

Her paper is due to be published this week.