Influência do homem na precipitação

[Rog]

Estudo prova influência do homem nos níveis de precipitação

Investigadores canadianos e norte-americanos relacionam a relação entre a actividade humana e os níveis de precipitação registados no século XX, depois de encontrarem provas da existência de alterações do padrão de precipitação provocadas pela acção do homem.


No estudo, a esta semana publicado na revista ‘Nature’, os cientistas compararam as alterações observadas nos níveis de precipitação registados ao longo do século XX com as alterações simuladas em 14 modelos climáticos. "A acção humana teve uma influência detectável nas alterações observadas na precipitação média e estas mudanças não podem ser explicadas pelas variações climáticas internas ou pela acção da natureza", refere cujos resultados sugerem que a acção humana contribuiu de forma significativa para o aumento de precipitação observado nas latitudes médias do Hemisfério Norte, também designadas por Zona Temperada; para uma situação de seca nos trópicos e sub-trópicos do mesmo hemisfério; e provocaram o aumento da humidade nos sub-trópicos e trópicos profundos do Hemisfério Sul.

A influência humana no clima já tinha sido detectada na temperatura do ar à superfície, na pressão do nível do mar, na temperatura da atmosfera livre, na altura da tropopausa (a camada intermediária entre a troposfera e a estratosfera) e no índice de aquecimento do oceano.

Fonte: Ciênciapt.net

 

[Vince]

Mais um estudo que certamente provocará muita polémica e discussão a juntar a muitas outras.

Aqui vai o destaque da Nature, a ver se depois temos acesso ao estudo.

Rainfall changes linked to human activity
Greenhouse-gas emissions have made the Northern Hemisphere wetter.
Human activity has made the weather wetter in a large slice of the Northern Hemisphere, say researchers. It has also made the regions just south of the Equator wetter, and those just north of it drier.

Agriculture and human health have already been affected, Francis Zwiers of the Canadian Centre for Climate Modelling and Analysis in Toronto and his colleagues report in Nature1.

This is the first evidence that human activity has altered rainfall patterns. "We expected rainfall patterns to change, but there's been no conclusive evidence that we are seeing human effects," says climate researcher Nathan Gillett, of University of East Anglia in Norwich, UK, one of the study's authors. "This study shows we are."

To show human influence, the researchers compared observed changes in rainfall during the twentieth century with those predicted by 14 climate models, divided into three groups. One group contained estimates of human greenhouse-gas emissions, one included only natural factors such as volcanic aerosols, and a third contained both.

The models including both human and natural influences gave the best fit to the observed trends. In the zone between 40 and 70 °N, which includes much of North America and most of Europe, rainfall increased by 62 millimetres per century between 1925 and 1999. The researchers estimate that between 50 and 85% of this increase can be attributed to human activity.

Similarly, most of the wetter weather between 0 and 30 °S is down to human activity. Previous studies had not detected a human influence because the amount of global rainfall was analysed, which meant increases in some areas were masked by decreases in others.

Drought and flooding

One place that has got drier is the Sahel region of Africa. Sandwiched between the Sahara and the rainforest belt, the Sahel experienced severe drought — and famine — between the 1950s and 1980s.

"If you look for the regions where both models and observations show a decrease in rainfall it's obviously made it harder to grow crops in those regions," says Gillett.

"In the far north we've seen an increase in average rainfall," he adds. "That will have increased stream flow in rivers, which may have increased flooding in those regions."

"This is a very important paper," says climate researcher Myles Allen of the University of Oxford, UK. "It identifies the fingerprint of human influence. This means that the precipitation trends they identify may be harbingers of more to come."
http://www.nature.com/news/2007/070723/full/070723-4.html

The Rain in Spain Stays Mainly in the Plain. Or Does It?
Climate change explains shifting rainfall patterns: wet places getting wetter and dry places drier

Models of climate change can predict and explain shifting rainfall patterns globally, says a new study. In a study set to come out in Nature tomorrow, an international group of scientists reports that they simulated atmospheric behavior using several different models and used them to forecast anthropogenically driven changes in average annual rainfall at different latitudes from 1925 to 1999. The predictions matched actual rainfall measurements during the 75-year period, both in the magnitude (amount) and the trend (increase or decrease) of precipitation. The move comes just five months after the release of an Intergovernmental Panel on Climate Change (IPCC) report, which contained accurate predictions of temperature variations due to global warming using the same models.

Between 1925 and 1999 precipitation between 40 and 70 degrees north latitudes increased at the rate of 62 millimeters (2.44 inches) per century. The northern tropics and subtropics, between 0 and 30 degrees, became drier at 98 millimeters (3.86 inches) a century, while it got wetter in the corresponding zone between the equator and 30 degrees south at a rate of 82 millimeters (3.23 inches) per century. The models, which factor in natural effects such as solar winds and volcanic eruptions, along with anthropogenic forcings like greenhouse gases and aerosols, match these precipitation variations accurately in trend and reasonably well in magnitude.

These trends would further desiccate many of world's great deserts like the Sahara and the Arabian (both in the northern subtropics), whereas tropical rain forests like those in Amazonia and Africa straddling the equator and the southern tropic zone would get wetter. Most of Europe, and Canada, lying above 40 degrees north as well as southern Greenland are expected to get more drenched. This sets up a competition of sorts between higher snowfall, which increases the Arctic ice cover, and the higher temperatures that melt it. "Overall, I expect warming to win in the long run," says study co-author Gabriele Hegerl of Duke University.

"A warmer globe means more water vapor in the atmosphere, which increases the potential for rainfall," she says, explaining the increase in total global rainfall over the past several decades. "The way [the moisture] turns into rain is more complex, however," she adds, which causes both increments and decrements in local rainfall. The step from moisture to clouds involves cooling, seed particles (including pollutant aerosols) and global wind patterns that blow the moisture from its place of origin to its place of condensation. There are even factors, like change in forest cover, that are known to influence local rainfall but are not very well represented in any of the models. All these complications have traditionally rendered attempts at modeling rainfall—which is much harder than modeling temperature changes—futile. "We were surprised by how well the results matched [real-life data]," Hegerl says.

This, however, is not the end-all of climate modeling. Almost all of the rainfall data available today are over land, whereas oceans cover 70 percent of Earth's surface. Difficulty in measuring rainfall over the oceans has precluded any analysis of this immense area. Furthermore, for reasons still unknown, of the 10 or so models used, different ones make accurate predictions at different latitudes; no single model works over all latitudes, and the mean of all of them is closest to observed data. And lastly, although the models get the precipitation trends spot-on, they "significantly underestimate the magnitude of change [in rainfall]," Hegerl admits, explaining that better modeling is near the top of the agenda for the researchers.

So what's next? Now that the link between shifting rain trends and increasing greenhouse gas and aerosol emissions has been confirmed, scientists are looking to explore connections between climate change and other atmospheric metrics such as cloud cover.
http://www.sciam.com/article.cfm?articleID=F91E2F10-E7F2-99DF-32220A4B6FA37D73&chanID=sa007

 

[Vince]

Sobre todos estes estudos e toda as polémicas sobre as alterações climáticas, a minha opinião pessoal é que eu tenho que respeitá-los, quem sou eu com toda a minha ignorância para colocar em causa o trabalho de tantos e tantos cientistas muitas vezes brilhantes. Aqui há uns meses acho que foi o Lula do Brasil que sintetizou isso muito bem. Era mais ou menos qualquer coisa deste género: Se tivermos um familiar gravemente doente e 7 ou 8 reputados médicos a indicarem-me que uma determinada operação é a melhor opção e outros 2 ou 3 disserem o contrário, em quais eu vou confiar ? Eles é que são os especialistas... O que não quer dizer que não devemos ter todos um olho critico sobre as coisas obviamente.


No entanto há uma coisa que me enerva brutalmente, sempre me enervou, e já falei mais vezes disso. É o alarmismo, o sensacionalismo.

Por exemplo, este estudo a publicar na Nature, deve ser um estudo muito sério. Mas haveria necessidade de ilustrar o mesmo com esta imagem ? Que os Media em geral sejam sensacionalistas compreende-se, é assim com tudo, mas na Nature era de evitar ...

Ontem o Jeff Masters, um dos fundadores do WUnderground e especialista em ciclones tropicais falava do mesmo. A propósito dum artigo sobre a relação entre os oceanos mais quentes e a intensidade dos furacões ("Warmer Oceans, Stronger Hurricanes"), o Jeff Masters disse que o artigo era excelente, tinha coisa muito boas, mas que por outro lado resvalou para o disparate.

O Jeff Masters ridicularizou o artigo porque o artigo que falava de super-furacões gigantes no futuro e vinha ilustrado com um furacão com umas 5000 milhas de diametro, com um eyewall de 200 milhas...

Obviamente ao Jeff saltou-lhe a tampa ao ver tamanha barbaridade e explicou muito bem que tal cenário era um completo disparate, totalmente ridiculo, porque simplesmente não havia espaço para na Terra se desenvolverem tais sistemas... e em vez de "Hurricane" no gozo apelidou tal coisa de "SciAmicane"

E atenção, que o Jeff Masters é um conhecido warmer... ou seja, até entre os warmers se nota irritabilidade, muitos começam a ficar fartos do exagero, dos disparates e do sensacionalismo.

The July 2007 issue of Scientific American has an article called "Warmer Oceans, Stronger Hurricanes" (referred to as "Warmer Water, SUPER HURRICANES" on the cover). The article is written by Dr. Kevin E. Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research, and a lead author on the landmark 2007 climate report issued by the UN's Intergovernmental Panel on Climate Change (IPCC). The article makes the case that "evidence is mounting that global warming enhances a cyclone's damaging winds and flooding rains." The article presents some solid evidence to substantiate that point of view, which I will share below. However, I was disappointed in the general tone of the piece, which was over-hyped and did not paint an objective view of the current scientific thinking on the global warming/hurricane issue.

The hype
First off, the reader is hit with a dramatic full-page artist's depiction of the global super-hurricane of the future--a massive 5000-mile diameter Caribbean storm the size of North America. The storm's 200-mile eye is wider than the Florida Peninsula! Whoa, I said when looking at the whopper "SciAmicane". No doubt many readers perusing the magazine, trying to decide whether to buy it, had the same reaction and plunked down their $5 to read about this grim threat. OK, lets talk reality here. The largest tropical cyclone on record, Supertyphoon Tip of 1979, had a diameter of 1380 miles--less than one third the size of the SciAmicane. A storm like the SciAmicane cannot physically exist on Earth unless the oceans were to super-heat to about 122° F (50° C). Only an asteroid impact or similar calamity could create such a hypercane. Even the most extreme global warming scenarios do not heat the oceans to 122°, so the SciAmicane is there to sell magazines, not to illustrate what global warming might do to hurricanes.

Figure 1. Comparison of sizes: the Earth, the largest tropical cyclone on record (Supertyphoon Tip of 1979, 1380 miles in diameter), and the recently discovered hurricane-like vortex on Saturn (the Saturnicane). The "SciAmicane" is about the same size as the Saturnicane--5000 miles across.

The article also calls attention to 2004, when "an unprecedented four hurricanes hit Florida, and 10 typhoons made landfall in Japan". I've erroneously made this statement, too, but the truth is that Japan was hit by only four typhoons in 2004. Ten tropical cyclones that were of typhoon strength at some point during their life did hit, yes, but six of these had decayed to tropical storm or tropical depression strength by the time they hit Japan. The article then refers to a "consensus explanation" emerging to explain recent hurricane activity patterns, and "that explanation forebodes meteorological trouble over the long term." I'd say that the issue is still very much under dispute. In fact, the consensus statement on hurricanes and climate change adopted by the World Meteorological Organization in December 2006, in response to the recommendations of a panel of 125 hurricane researchers was thus: "Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point." Trenberth's article gives a list of four publications to read in the "more to explore" section, but none of these include the recent articles that call into question the strength of the global warming/stronger hurricane connection. (I apologize for not reviewing the many excellent articles that have appeared on this subject of late!)

The good science
There's quite a bit of good science in the article, which is worth reading if one keeps in mind its biases. In particular, I like the discussion of how global warming has affected precipitation and atmospheric water vapor. The 0.6° C (1.0° F) rise in Sea Surface Temperatures (SSTs) globally since 1970 has increased water vapor in the atmosphere by 4%, thanks to increased evaporation. This in turn has led to an 8% increase in global precipitation. Trenberth makes the point that no given hurricane can be blamed on global warming, but one can say 8% of a given storm's rainfall is due to global warming. There's also a nice discussion about how weaker than normal trade winds over the tropical Atlantic in 2005 caused less evaporational cooling than normal, allowing the ocean to heat to record temperatures. Finally, the conclusion of the article is one I certainly agree with:

We would all be wise to plan for more extreme hurricane threats.

Both theory and computer models predict a 3-5% increase in hurricane winds per degree C increase in tropical SSTs, and there is concern that the actual increase may be much more than this.

Jeff Masters
http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=726&tstamp=200707#commenttop

 

[Minho]

Percebo a irritação do Jeff Masters artigos destes só servem para desacreditar a ciência que hoje em dia já é tão maltratada... Basta ver os comentários na rua sobre o Verão quente que nos aguardava, dá pena ver como as pessoas gozam, descredibilizam e lhes reforça ainda mais a convicção que os meteorologistas são todos um bando de lunáticos que lançam uns búzios para fazerem previsões...