Weather n Climate

Is Hurricane Harvey a Case of Climate Change

With rainfall still continuing over southeastern Texas, USA, The Guardian reports that flooding due to tropical storm Harvey has claimed at least 35 lives by August 31, 2017. In an unfortunate development, two explosions have taken place at a chemical plant in Crosby, Texas that suffered a power shortage due to the flooding caused by Harvey.

The plant run by Arkema manufactures organic peroxides used in the production of plastic and paint products, which can be extremely combustible.  With floodwaters still rising over Houston, Texas, the mayor of Houston Sylvester Turner had declared a city-wide curfew in Houston since 5 am on August 30 to curb potential criminal acts in abandoned properties.

The floods have required unprecedented relief efforts, with new shelters being set up for residents after the George R. Brown Convention Centre in Houston took in more than 10,000 people despite having a capacity of only 5,000. Smaller numbers have proliferated in numerous other centres, with the Toyota Centre, home to the Houston Rockets NBA basketball team as an example. Local and federal agencies have rescued over 13,000 people so far across southeastern Texas, although the numbers of people choosing to remain in their homes is unknown. One-fifth of the oil refineries in the US have been shut down due to Harvey, causing gasoline prices in the US to reach their highest point since July 2015 (C. Phipps, 2017).

Tropical Cyclone – a violently rotating wind system

Hurricane Harvey that emerged over the Atlantic Ocean as a tropical storm on August 18, 2017 weakened by August 22 as it travelled westwards through to the Gulf of Mexico. On August 23 the tropical storm system regenerated and began to intensify over the Gulf of Mexico to be identified as a Category 4 hurricane by the evening of August 25 with wind speeds reaching 215 kmph.

Moving slowly, Harvey made landfall in southeastern Texas in the US at around 10 pm in the evening of August 26. By August 27, Hurricane Harvey had been downgraded to a tropical storm, although the rainfall was still very intense, bringing flooding rains to Houston, Texas and adjoining areas. By August 28, the storm system began moving back towards the Gulf of Mexico, and had recorded over 1016 mm of rainfall over southeastern Texas between August 23 to 29 (NASA, 2017), which is among the highest ever recorded volumes of rainfall ever for the region.

A Case of Climate Change?

Although the Houston floods have been referred to as an event occurring once in 500 years by some in the US, such catastrophic precipitation events might be more ubiquitous than assumed. A similar flooding event took place in Houston just last year in 2016, when 508 mm of rainfall over just two days led to flash floods (E. Holthaus, 2016). Flash floods are quite frequent in the areas around Houston, but the recent past has witnessed many such weather anomalies within a short span of time.

Houston has in the recent past seen 167 per cent more heavy downpours than it did in the 1950s, says meteorologist Eric Holthaus, adding that it has seen four 100 year flooding events since early 2015 (R. Meyer, 2017).

Similarly Hurricane Katrina that struck New Orleans in the US was referred to as a 100 year event, although scientists differ over the matter (J. Worland, 2015). Rather than putting out such phrases they can refer to the odds of such an event occurring over a region, with a once in a 100 year event implying a probability of a 1 in 100 chance of such an event occurring in a year in a region. Similarly once in 500 years could imply a 1 in 500 chance of the event occurring in the region in a particular year. Declarative statements could mislead the public as to the facts of the case.

Fig: Forecast for flooding in the US as on 8/30/2017 as based on flood gauges
Source: NOAA

The point is not the severity of the event, but its ubiquity and recurrence, with the most recurrent flood related casualties in the US annually occurring in southeastern Texas according to the National Weather Service, USA. This is the most flood-prone region in the US – also known as Flash Flood Alley. This creates ambiguities over whether these extreme precipitation events can indeed be considered an anomaly in the region. The academic community is asking whether climate change is behind the events in Houston, or is this part of normal fare in Flash Flood Alley?

 

On August 29, the World Meteorological Organization (WMO) released a statement – “Climate change means that when we do have an event like Harvey, the rainfall amounts are likely to be higher than they would have been otherwise”. Clare Nullis, a UN spokeswoman said this in a conference (The Guardian, 2017). The concomitant factor that can be held chiefly responsible for the accentuation of Harvey’s effects is the higher than normal warming of the seas, which leads to faster evaporation with the air holding a greater amount of water vapour.

The theory in favour of climate change states thus that as temperatures rise across the world due to global warming, more moisture is available in the air, which makes the dumping of the moisture more intense.

The scientific basis for this theory is the Clausius-Clapeyron equation, which proves that for each rise in temperature by a degree Celsius, there is a corresponding 7 per cent increase in water vapour in the Earth’s atmosphere. The equation uses mathematical equations to characterize phase transition in matter.

The fact that supports this theory is that surface temperature over the Gulf of Mexico is half a degree Celsius warmer than the seasonal summer average. Also about 10 to 20 cm of sea level rise is observed since 1880 (K. Christianson, 2016), which with the melting of previously frozen water expands the total volume of water available for evaporation.

A cumulative global climatic pattern cannot be modelled on the basis of Harvey and a rise in global mean temperatures as yet. Certain anomalies can be observed that can be attributed to aberrant heating, but a global climatic model cannot be formed on the basis of instances and anomalies alone.

For example, the stalling of Harvey over Houston and the persistent torrential downpours thus can be attributed to a general slowdown in summer atmospheric circulation in the mid-latitudes due to above normal warming in the Arctic. However, that climate change could make certain weather systems move less slowly and thus exacerbate their effects is for now a theory bound in speculation, and more circumstantial evidence might point the world towards better understanding the effects of climate change.

What we have now are computer simulations that point towards many aberrant climactic patterns, but it would take a climactic trend more significant to popular consciousness than a half degree rise in temperatures to make the world notice that the patterns of global climate are indeed shifting.

Events like the one in Houston have to be seen in relation to a general trend in climactic patterns for them to be counted as a concomitant of climate change. The general predilection for them would be to be counted as part of the long standing climactic trends in a flood-prone region, but a closer examination reveals a few aberrations that might point towards more pertinent inferences.

Forecasting Tropical Cyclones

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