Seca de São Paulo – Mudança Climática NÃO é a causa

The drought situation in São Paulo is critical. As of late October, the two principle reservoirs that serve the city were below 5% of capacity. Water pressures have been reduced to such an extent that people in the higher parts of the city are without water for most of the time. What is causing this?

The “Climate News Network” (website run by former Guardian & BBC journalists) they attribute this to deforestation and climate change¹. They say

The unprecedented drought now affecting São Paulo, South America’s giant metropolis, is believed to be caused by the absence of the “flying rivers” − the vapour clouds from the Amazon that normally bring rain to the centre and south of Brazil.

Some Brazilian scientists say the absence of rain that has dried up rivers and reservoirs in central and southeast Brazil is not just a quirk of nature, but a change brought about by a combination of the continuing deforestation of the Amazon and global warming.

This combination, they say, is reducing the role of the Amazon rainforest as a giant “water pump”, releasing billions of litres of humidity from the trees into the air in the form of vapour.

Meteorologist Jose Marengo, a member of the Intergovernmental Panel on Climate Change, first coined the phrase “flying rivers” to describe these massive volumes of vapour that rise from the rainforest, travel west, and then − blocked by the Andes − turn south.

Satellite images from the Centre for Weather Forecasts and Climate Research of Brazil’s National Space Research Institute (INPE) clearly show that, during January and February this year, the flying rivers failed to arrive, unlike the previous five years.

This explanation of deforestation causing the drought does not hold water. The following is an account of why this drought explanation is flawed.

The “flying rivers” or “rios voadores” is being studied as a Petronas-sponsored long-term project at http://riosvoadores.com.br/english/. Project leader Gérard Moss explains the nature of “flying river”.

The question is, where does the rain come from?

Most of the evaporation comes from the sea… The wind pushes this air over the Amazon Forest, a region where it rains quite a lot. The humid air eventually reaches the Andes, which force it south and that is what we are calling a “flying river”

So the most important part of the evaporation is from the sea. A minor part comes from evaporation the Amazon Forest. Yet the Climate News Network is under the impression that all of the evaporation comes from the Amazon. The same is true of the Ecologist, which seems to have used the same material. What is even worse, both sources claim that 22% of the Amazon has been lost. That would mean that the total evaporation from the Amazon region will have reduced by less than this figure and the total moisture content of the “flying rivers” by less than 10%. Even so, there is nowhere provided any data that shows the rainfall in the area is reduced. If the hypothesis were true, then the rainfall near the mouth of the Amazon would be largely unchanged, but as the “flying river” goes south into NE Bolivia and Paraguay, and the Brazilian states of Rondönia, Mato Grosso do Sul, São Paulo, Parana and Santa Catarina, there should be evidence of diminishing rainfall. But despite a quite expensive project employing a number of people and two light aircraft (one a sea plane) there seems to be no effort to gather the data that might falsify the data. Further, project leader Gérard Moss (who is a pilot and engineer) does not seem open to falsification of the hypothesis.
Starting at 7:10 he says:-

My dream is that the Flying Rivers project, through studying (the flying rivers) behaviour, will scientifically prove the amount of rainfall in the south and the Amazon forest. My dream is that we will finally stop exchanging the forest for grazing land and plantations. ….. (T)he project’s greatest challenge is to prove to all us Brazilians, that it’s no longer worth felling one single tree.

Gérard Moss is a pilot and engineer. He is the one who has the use of two aircraft. Further, since mid-2012, the project has been restricted to educational projects². One such project gives a useful tool that monitors the prevailing wind trajectories. The latest one I downloaded and superimposed the wind direction of the “flying rivers” in think blue arrows.

It would seem that the prevailing easterly winds have shifted south coming ashore in arid Bahia and doing a short loop round to São Paulo, completely missing the Amazon.

Unfortunately, the only map prior to October is for 23/07/14. This gives a similar picture of prevailing winds completely missing the Amazon.

I have a simple hypothesis that can easily be contradicted by archived data held by the website. The cause of the current water shortage lies in January and February, with the failure of the normal summer rains. My hypothesis that this failure was due to similar wind patterns occurring in January and February as found on 27^th October. This, naturally occurring, phenomena would have occurred at a similar time to the Gulf Stream shifting course – in the UK shifting north causing extreme storms in Southern England, with flooding in Somerset and the Thames Valley, and in the USA shifting south causing the extreme cold of the Polar Vortex.

There is, however, a further video by the BBC (in English) where Gérard Moss explains that half or more of the rainfall in São Paulo is from the Amazon, as opposed to the sea.

There are three potential sources of water vapour that could condense as rain in the city of São Paulo, but are not mentioned. First is sea evaporation that has not passed over the Amazon. Second is land evaporation from air currents that have not passed over the Amazon, like in the cases above. Third is evaporation from the “flying rivers” airflows after passing over the Amazon. There is up to 2,000 km between the end of the Amazon forest and São Paulo.

Summary

The current extreme drought in the city of São Paulo is not the result of Amazon deforestation for two reasons. First, the deforestation is insufficiently large to account for the drought levels. Second is that evidence points to a natural southerly shift in the current year in the easterly winds coming ashore in Brazil from near the Amazon delta to the much drier coast of Bahia.

But if the deforestation is not the cause of the draught, what are the likely causes? This will be the subject of a further post.

Update 1 14/12/14

I did not get round to the update. This is a background I wrote for the BishopHill discussion.

As my wife comes from Southern Brazil and I have visited the area a number of times, this caught my eye. Before linking the drought to climate change you need to consider the following geographical facts.

• The City of São Paulo is built on a plateau about 700 metres above sea level. This means that although the Tropic of Capricorn passes through the North of the City, the climate is relatively temperate. The highest recorded temperature is just 35.3 °C.
  
• The two principle reservoirs have been supplying the city since the 1920s when the population was less than a million, compared to twenty million today.
  
• The area of São Paulo and Rio de Janeiro was once covered by the Atlantic forest. This once extended well south into Argentina and inland to Paraguay – an area bigger than Western Europe. According to Wikipedia 88% has disappeared. (Climate News Network and The Ecologist put the figure at 91.5%) From extensive travel in Parana, Southern São Paulo state and Northern Santa Catarina this figure seems accurate. Most of the deforestation occurred in the twentieth century.
  
• The Rivers of São Paulo State mostly form part of the River Plate Basin – that meets the sea in Montevideo, Uruguay. This drains most of southern Brazil, the entire country of Paraguay, Northern Argentina as well as a corner of Bolivia¹. The principle river serving the city is the Tietê.
  
• São Paulo State is about the same area of the United Kingdom and has a population of 43 million.
  
• Upstate there is extensive agriculture, including soybean, sugar cane and cattle.
  
• This is the worst drought in 84 years, not ever recorded. The previous one was four years before the American dust bowel of 1934, so there might be common climate factors that have influenced the period.
  

My conclusion is that the seriousness of the current water crisis is due to the following factors, in order of importance.

1. Investment in water supply not keeping pace with demand.
   
2. A once-in-a-century drought.
   
3. Location of a megacity on a plateau, limiting the ability to cheaply extend the water supply.
   
4. Changes in rainfall patterns from deforestation.
   

Notes

1. Over at the BishopHill blog, commentator Entropic Man has started a discussion thread on the current drought in São Paulo, which he claims is due to deforestation and climate change. As the BishopHill blog is almost entirely given over to climate issues, the inference by Entropic Man is that human-caused climate change is responsible.
   
2. The website explains (in Portuguese)
   

   From the mid-2012, the project is restricted to educational, awareness actions and counts with the collaboration of the CPTEC in providing the data provided on the links of the weather mapsan important tool that allows the general public to see and track the trajectories of the flying rivers.
   

   Kevin Marshall