What’s with all the tornadoes?

For most of us, summer is the perfect time to get a bit of rest and recuperation. But across the Atlantic, last summer was anything but relaxing as a series of hurricanes have pummelled coastlines in the Americas. Storms Harvey, Irma and Jose have now been and gone, but what was behind the spate of super-storms?

Hurricane Irma (NOAA/CIR, 2017)

We’ve taken a look at why last summer saw so many serious hurricanes, asking why they are still so damaging and looking towards a future that might be even worse. First, a quick introduction to the science of hurricanes. All hurricanes begin with an area of low atmospheric pressure over a warm patch of ocean. The low pressure sucks in air from all sides while the high ocean temperature encourages evaporation and then rain cloud formation. These processes repeat themselves and rising water vapour pushes aside the newly formed clouds. As the clouds move, they cool enough for the vapour to condense and fall as heavy rain. This is a classic convection current. However, air moving towards the central area of low pressure (which is felt as wind) doesn’t travel in a straight line. Instead, due the Coriolis Effect (a consequence of earth’s rotation), incoming air in the northern hemisphere bends slightly to the right. The resultant inward swirling is what produces the familiar spiral of cloud that you see on satellite images. Most Atlantic storms begin this sequence in warm waters to the west of Africa before being nudged towards the Americas by gentle prevailing winds in the upper atmosphere.

So why were so many hurricanes in the news then? Harvey is a good example here because we’re closer to knowing its full effects and because it typifies the features of many destructive storms. Harvey first made headlines due to its power. It had made the usual journey west from Africa, intensifying as it passed over patch after patch of warm water. By the time it made landfall on the Texas coast winds in the storm had reached 130mph. This placed it in the second most powerful category of hurricane (4), accounting for the ominous tone of the coverage.

However, it was not Harvey’s ferocious winds nor the storm surge (a huge mass of water pushed beyond the Texas coastline by the powerful gusts) that produced the most damage: Harvey’s most potent weapon was rain. Two areas of high pressure pinned the storm in place over the city of Houston, meaning that this single metropolitan area was subjected to unending downpours for two days. Up to 50 inches (1.27m) of rain made Harvey the largest single wet weather event ever recorded on the US mainland which led to Houston and the surrounding areas experienced wide scale flooding. The cost of this event is expected to break records with figures of up to $180 billion.

So the powerful nature of the storm and the significant damage it caused explains why Harvey got so much news coverage. With Irma estimated to cost on a par with that of Harvey it’s time to ask another important question: how can multiple once-in-a-generation natural disasters occur in the same month? The answer is complicated, one underwhelming but nonetheless necessary point to make is that this is peak hurricane season in North America. The warm seas of the late summer provide plenty of fuel for major storms and the intensity (windspeed) of Harvey was far from unprecedented. It’s not unlikely for there to be multiple big storms in a single summer either, just less probable. Could the changing climate be a possible link to this hurricane-heavy summer? This would be difficult to verify since attributing single weather events (such as Hurricane Harvey) to a long-term alteration of underlying conditions makes little sense. Still, among organisations who study hurricanes (including the IPCC) there is a broad consensus; the warming of the planet, triggered by human actions, is making strong storms like Harvey and Irma more likely to occur. There’s no fancy explanation for this – knowing as we do that hurricanes form over warm oceans, the link is there for all to see. What’s more, the seismic downpours that made Harvey such a notable storm are also made more likely by a warmer climate. That’s because the ability of air to hold moisture increases with temperature and Kevin Trenberth, a scientist at the US National Centre for Atmospheric Research even speculated that perhaps 30% of Harvey’s rainfall was a “human contribution” via anthropogenic warming. So last summer’s spate of storms was probably a glimpse of what is to come: a future in which we will see not only more of the strongest storms, but more record-breaking rainfall as a result. There is, however, another side to this story. Harvey is a perfect example of human decisions making hurricanes – even those influenced by climate change – much more devastating than they ought to be. It is well known that Houston sits on a flood plain and past floods (including after storm Allison in 2001) are a testament to this. But the city has done little to rectify its natural susceptibility. In fact, Houston has continued to develop in a way which makes flooding more likely. As an example, around 7000 homes built in the city since 2010 lie in known flood zones and, a story replicated across America according to a paper from Environmental Scientist Nina Lam. There are, however, some mitigating factors. A letter sent to President Trump in April 2017 by the head of the Texas General Land Office (responsible for managing public land across the state) asked for funding to aid the construction of a protective “Hurricane Wall” near Houston. That at least shows a willingness to improve preventative measures among decision makers in Texas. However even so no realistic level of protection would have stopped all flooding in Houston. The fact the death toll remained below 100 also reflects well on evacuation and rescue efforts. Nonetheless, the refusal to implement policies which minimise flood and hurricane damage should not be ignored. Considering that said policies can be as simple as investing in public transport rather than car parks, purchasing adequate protection is a far cheaper option than paying for total reconstruction when the once in the generation storm does arrive.

So let’s answer the key questions one final time. Why were so many hurricanes in the news last summer? The ideal conditions made them large and powerful. Why did so many of these intense storms occur at once? It could be a coincidence, but global warming makes storm-filled summers much more likely. And lastly, why are hurricanes still so damaging? Because we are not even using the knowledge we have to mitigate disasters as best we can. To end on something of a high note, this last point does at least give room for improvement meaning that even in a stormier world there remains hope that the hurricanes of the future will be far safer for humanity.

NOAA/CIR. (2017). Hurricane Irma. [Image] Available at: <https://commons.wikimedia.org/wiki/File%3AGeocolor_Image_of_Hurricane_Irma.jpg&gt; [Accessed 25th March 2018].

David Verry is a second-year BSc Geography with Quantitative Methods student. He can be found on Twitter at @dnverry.

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