A lot of tropical news this week. The 2009 hurricane season in the Atlantic has stirred to life quickly with two (Update Sunday: 3 !) tropical storms forming on Saturday. It’s not at all unusual to have little hurricane activity until August. The season runs from June 1st to November 30, but the prime season is from Aug, 1st to mid September. American forecasters have an old saying that there will almost always be a hurricane on the weather maps when Labour Day arrives.

These storms form in very warm ocean. The National Climate Data Center (NOAA) released the July global land and ocean temperatures on Saturday. Ocean temps were the warmest on record for July. The land and ocean temps were the 5th warmest on the instrumental record. This follows June 2009 which also came in as warmest.

Another interesting bit of tropical news this week is a new paper published in Nature on hurricanes of the past. One of the great debates in science right now is the question of whether climate change will bring more hurricanes or fewer. The debate has raged between two opposing groups. Kerry Emanuel of MIT has produced interesting evidence that we have seen an increase in hurricanes already due to the warming of the past 50 years.

Chris Landsea of NOAA has produced evidence that we are just detecting more tropical storms, and that there has not been an increase. I had a chance a couple of years ago to hear both of them present at the AMS meeting in San Antonio. I left with the firm conviction that the question remains open. Understand here, that this debate is not about climate change in general. Despite what you read on the Internet, science has moved on from that.

One thing that does seem very certain now is that hurricanes in the warmer world of late this century, will be wetter. Perhaps considerably wetter. The kind of catastrophic flooding we saw in Taiwan this past week, will likely be more common in the future.

Why you ask? Water vapour.

If the average temperature of the air over the oceans rises 1 degree F, the air can hold 4% more water. (This is one reason why more snow is likely in Antarctica as it warms, not less. A 3C rise in temp. by late this century would bring an increase of around 22% in the amount of water held in the atmosphere! (You won’t see that bit of science on these junk science sites)

Sea surface temperatures are a major factor in hurricane formation. If the sea surface temperature is below about 27C then hurricanes are not likely. Upper level wind shear and atmospheric water vapour are other important ingredients.

Other factors like wind shear in the upper atmosphere act to inhibit hurricanes. The El Nino that develops every 4-7 years in the Pacific, increases the wind shear over the Atlantic, and we usually see fewer storms. Will there be more wind shear in a warmer world? Possibly. Conditions could combine to produce about the same number of storms in the future. (Much wetter ones though)

Micheal Mann of Penn State University is the lead author of a fascinating paper in this weeks NATURE. His team used soil/silt cores in a series of locations to estimate past hurricanes. If a hurricane hits a coastline, the overwash of sea water will leave a deposit that can be identified in the cores. They used these sediment cores to estimate hurricane activity over the last 1500 years. In addition, they used a statistical model that factored in variables like sea surface temperature to estimate storms as well.

They found that during a period of rather warm Atlantic Ocean water around 1000 years ago, we saw as many hurricanes as we have over the past 15 years. This is a good confirmation that warmer seas, do give more hurricanes and perhaps more intense ones.

Chris Landsea of NOAA argues that the increase in storms over the past century is just an artifact of spotting them more easily with satellites and aircraft. One thing seems likely here, the hurricanes did increase in the past during a period of warmer oceans.

Whether or not a warmer world caused by human means, instead of natural ones, will do the same is still open for debate. The science, however, might just be beginning to tilt in favor of Mann and Emanuel.

Either way, with sea level now rising 3mm per year, and increasing, future hurricanes, will be wetter and cause more destruction. The current thinking is the IPCC will be adjusting their forecast of sea level rise up considerably in the next report.

This back and forth in the peer reviewed literature is how science advances. When we can answer the question of hurricanes in a warmer world, we will have gleamed another piece of fundamental knowledge of how are planet works.

I end with a book recommendation. Kerry Emanuel of MIT is one of the leading experts on hurricanes. He has written a fabulous book called Divine Wind. It combines poetry and science. It’s one of the best general audience  science books ever written.

Note this is a dual post- I wrote it as a guest post on Skywarn 256’s Weather Blog as well.