Hurricane Intensity in North Atlantic – Signal Emerging from Noise

December 18, 2013

Mother Jones:

For more than a decade, the question of how global warming is affecting the scariest storms on the planet—hurricanes—has been shot through with uncertainty. The chief reason is technological: In many parts of the world, storm strengths are estimated solely based on satellite images. Technologies and techniques for doing this have improved over time, meaning that there is always a problem with claiming that today’s storms are stronger than yesterday’s. After all, they might just be better observed.

..just maybe, a new scientific paper has managed to get past this long-standing data problem. The study, just out in the Journal of Climate from hurricane and satellite expert Jim Kossin of the National Climatic Data Center and his colleagues, seeks to create a completely consistent database of hurricane satellite images that will finally allow for apples-to-apples comparisons. How? “We can’t take bad data and make it good, because that’s adding information that we don’t have,” explains Kossin. “But we can take the good information and make it worse.”

That’s the surprising solution that the scientists implemented in their paper. Data that was too “good”—for instance, because the satellite images were too high in resolution—was degraded to what Kossin calls the “lowest common denominator”: one satellite image of each storm taken every three hours, with a pixel size no greater than eight kilometers by eight kilometers. Using this technique, Kossin and his colleagues at NCDC created a 28-year record of storm images across the world’s seven hurricane basins, from 1982 to 2009. Then they used a computer algorithm to compute each storm’s maximum strength, removing human error and unpredictability from the equation.

The result? The scientists found that globally, hurricane wind speeds are increasing at a rate of a little more than two miles per hour per decade, or just faster than six miles per hour over the entire period. There are some key caveats, though, the biggest being that the trend they found was not statistically significant at usually accepted levels. (For nerds: the p valuewas 0.1). But there were strong and significant trends in some hurricane basins of the world, especially the North Atlantic (the region encompassing the Caribbean, Gulf of Mexico, and open Atlantic north of the equator), where storms have been strengthening at the rate of nearly nine miles per hour per decade (see chart above). But other basins offset that, including the western North Pacific, which showed a negative trend.

The punch line, then, could hardly be called overwhelming. But as Kossin explains, that may be precisely what you expect to see once you’re finally analyzing the troublesome hurricane data reliably. These results, after all, are quite consistent with the idea that the signal of hurricane intensification might be just now emerging from the “noise” of natural climate variability. “What we’re observing could very easily fit into an assumption of this greenhouse gas forced trend in the tropics and the effect that it has on tropical cyclone intensity,” says Kossin.

Perhaps the best news is that if scientists continue adding to the new database of homogenized satellite images—starting with the years 2010-13, which were not part of this study—the chance of finding a significant trend (or showing that there just isn’t one to be found) will increase. “I think every year, we’ll get a little bit closer to the truth,” says Kossin.


6 Responses to “Hurricane Intensity in North Atlantic – Signal Emerging from Noise”

  1. Cy Halothrin Says:

    One small flaw with this post is that hurricane “categories” only refer to wind speed. While that is certainly one significant measure of a storm’s strength, the potential destructiveness of a hurricane is also affected by its size (diameter) and the amount of rainfall (which can actually be much higher in a given region if the hurricane is moving slowly).

    Hurricane Sandy in 2012 was a category 3 at its peak (when it hit Cuba), and only category 2 during its most destructive stage over the USA. However, it was the largest Atlantic storm on record, reaching a maximum diameter of 1100 miles (1800 km) which is about three times average size. While the death toll wasn’t high, the flooding caused major damage, a record-breaking US$68 billion.

    As to be expected, the WTFUWT folks seized on the fact that Sandy was “only category 2” to prove that it wasn’t a problem at all. I have this perverse image of Anthony Watts clinging to a raft in a raging flood using his iPad to send a message to his fans that “the warmists” are exaggerating and it’s only a light rain.

    • greenman3610 Says:

      I’ve been listening to an active conversation on this especially since Haiyan. Consensus seems to be that the emerging evidence supports stronger storms, but everyone agrees there is not a long enough record, for tornadoes or hurricanes, to nail the trend exactly.
      What atmospheric physicists are telling me is that, more heat, more moisture –>something’s gotta give. atmosphere more active.
      If I’ve dropped a brick off every building in manhattan, and it always falls to the ground, I have a pretty good idea that if I drop one off that NEW building, it most likely will also fall to the ground. No one has run that exact experiment yet, but, it seems very likely the results will be the same. Physics makes certain demands.

  2. Cy Halothrin Says:

    Intensity vs Frequency…

    I couldn’t say if more storm intensity equals less frequency or vice versa – I’m not a climatologist. One thing that does occur to me though is that in a warmer world, we could be a seeing a longer hurricane season. Typhoons don’t occur in winter (now) because the sea surface temperatures are too low. But if warm enough, wouldn’t it possible to have a year-round hurricane season?

    Of course, if temperatures are so warm that we have a year-round hurricane season, I imagine that many other weather patterns will change. It will be a very different world from the one we know now.

    At the end of the day, the big problem is not that a warmer world will necessarily be a worse one. The deniers like to claim that a more tropical world will be lovelier (though they claim at the same time that the world is cooling – they can’t seem to make up their mind). But lovely weather or not, the real problem is that we are inducing climate changes in decades that would require thousands of years to occur naturally. And we’re doing this in a world where we’ve built an awful lot of unmovable infrastructure barely above sea level. To make it worse, with seven billion (and rising) people in the world, we have more mouths to feed and shrinking space to do that in. A rapidly warming climate means famine, and climate refugees (who will not be welcomed) to the Arctic region. It does not look pretty.

  3. The chief reason is technological …

    Not only. As analyzed here, the period of observation is too short. This is AMO: evolution from a phase cold to warm. This factor can be decisive here. Moreover warming – always – acts in two ways of: hurricanes, extra-tropical cyclones, tornadoes, etc. .. The amount of energy increases (eg hurricane strength increases) but gradients decrease (strength and frequency of hurricanes decreases).

  4. Dan Staley Says:

    I’d like to see how this compares to the other two measures of storm energy.

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