CO2 casts off shackles to power up Atlantic hurricanes

NOAA’s GOES-13 satellite captured this visible image of Hurricane Sandy battering the U.S. East coast on Monday, Oct. 29 at 9:10 am EDT. Sandy’s center was about 310 miles south-southeast of New York City. Tropical Storm force winds are about 1,000 miles in diameter, and are set to intensify in the 21st century. Credit: NASA GOES Project

Changes in greenhouse gases and other air pollution will likely make Atlantic storms that could hit the Caribbean and Eastern US more intense through this century. That’s according to research from Gabriel Vecchi at the US National Oceanographic and Atmospheric Administration (NOAA) in Princeton, New Jersey, and Gabriele Villarini at the University of Iowa. They’ve found that more greenhouse gases strengthen these storms but other pollutants known as aerosols or particulates, which include soot, do the opposite. Increases in both types of pollution through the 20^th century therefore cancelled each other out. But with more recent efforts to limit aerosol pollution succeeding, Atlantic storms now look set to become more destructive. “Both reductions in particulate pollution and increases in greenhouse gases are going to co-operate, we think, to give us more intense hurricanes in the Atlantic,” Gabriel said.

Gabriel has long studied Atlantic storms, and together with Gabriele recently found that how often they happen will likely only increase during the first half of the 21^st century. “The number of storms in a season is only part of the story,” Gabriel told me. “A big question for society is the intensity.” So it was natural, he added, to follow on by looking at how strong and long-lasting they are. Scientists have already looked at their intensity for narrow “time-slices”, for example from 1985-2005 and then predicting from 2080 to 2100. “People haven’t explored how we go from the late 20^th century to the late 21^st century,” Gabriel said.” That’s because to do this research they need complex and very detailed ‘high resolution dynamical’ climate models, which use up scarce time on the world’s most powerful computers. For the same reason, previous studies only look at a few possible scenarios for how much of the greenhouse gas CO2 humans will produce by burning fossil fuels.

Patterns matter

Air from the tropics mixes with air high in the atmosphere above the Atlantic to influence the chance of hurricanes in that area. NOAA’s Gabriel Vecchi was able to predict Atlantic hurricane intensity throughout the 21st century because this creates a comparatively simple link between Atlantic and Tropical sea temperatures. Credit: NOAA

Though simpler ways to predict storm intensity might not be as detailed as these models, Gabriel and Gabriele realised they could give a fuller picture. So they exploited a connection between sea surface temperatures and a measure of storm intensity called power dissipation index, or PDI. The best conditions for hurricanes are when the Atlantic warms more than the atmosphere high above it. That means the scientists could work out PDI from the difference between these temperatures. Meanwhile air high above the Atlantic mixes especially well with air high above tropical oceans, which in turn mix well with the air directly above the surface of those tropical oceans. That connection links temperatures high above the Atlantic closely to temperature at the surface of tropical oceans. So closely, that the scientists could calculate PDI using just the differences between tropical and Atlantic sea surface temperatures. “For hurricanes it’s patterns of temperature that matter,” Gabriel explained. “It’s not how much the Atlantic warms by – it’s how much more than the tropics.”

To get temperatures to predict how PDI might change, Gabriel and Gabriele turned to 17 ‘general circulation’ climate models. From 1949-2008, they ran them with real aerosol and greenhouse gas emissions to check the temperatures and therefore PDI were close to what actually happened. The scientists then ran the models with three different emission scenarios, where the amount of greenhouse gas humans produce varies to cause average global temperature rises between 2°C and 5.6°C by 2100. But by contrast to more detailed approaches, in a research paper in the Journal of Climate published online last month, they looked at the whole period from 2006-2099. Finally, they ran two more experiments without the influence of aerosols, one simulating the past and one the future, to understand the parts these emissions played.

Clean air, foul weather?

Atlantic hurricane intensity, or power dissipation index (PDI), is set to increase as CO2 emissions grow and aerosol pollution in the atmosphere reduces in the 21st century. Credit: Gabriel Vecchi

Despite PDI staying relatively stable in the 20^th century, in all three future scenarios it tended to increase over the 21^st century. That tendency was especially strong in the warmest predictions. But without aerosols that tendency weakened in future predictions, while in the simulation of the past without aerosols PDI increased more than it did in reality. Gabriel says this is due to the gain and loss of the aerosols’ dampening effect “Over the past century, increases in greenhouse gases alone should have increased hurricane intensity noticeably,” Gabriel said. “However, over the past century, and particularly since World War II, there was a big increase in aerosol pollution over the Atlantic, which cooled it compared to the tropics. That decreased hurricane intensity. But greenhouse gases have been rising steadily, while aerosol pollution rose steadily until the mid-1970s, when clean air controls went into effect. So there should have been a minimum net hurricane intensity sometime in the 1970s and 1980s, which is what we observed.”

Gabriel now hopes to use these findings to make detailed predictions. While Hurricane Sandy’s recent devastation might underline the importance of such efforts, he emphasised it showed how important it is we prepare for such events. “New York, New Jersey and Connecticut are very developed areas, but they can still have weather events that have a very disruptive influence,” Gabriel said. “We are vulnerable to weather, and regardless of their relation to climate change we need to take these things seriously.”

Journal reference

Villarini, G., & Vecchi, G. (2012). Multi-Season Lead Forecast of the North Atlantic Power Dissipation Index (PDI) and Accumulated Cyclone Energy (ACE) Journal of Climate DOI: 10.1175/JCLI-D-12-00448.1