Hurricanes are very large storm whose birth originates in the tropical oceans. They rotate about a central axis commonly called ‘the eye’. The oceans are a massive source of heat energy. Variations in sea surface temperature result in pressure differences in the atmosphere which cause storms to build up.
To be classified as a hurricane a storm must reach wind speeds of at least 74 miles per hour. The rotation of the Earth gives these storm their characteristic spiral shape. Cyclonic storms in the northern hemisphere rotate anticlockwise while those in the southern hemisphere rotate clockwise.
The main hazards from hurricanes are strong winds (up to 150 miles per hour for the very large storms) and high volumes of rain. Hurricane winds can uproot trees and destroy houses. Large amounts of rainfall in a short period of time can cause floods and rising groundwater tables.
The water-clogged landscape remains unstable, with increased risk of landslides and surface failures, for many years after a particularly large event. For example, there were increased number of landslides in Taiwan for 6 years after Cyclone Bhola.
Historically significant hurricanes
The effect hurricanes have on people’s lives is illustrated by the word hurricane itself; after the Caribbean god of evil, Hurrican. They are a devastating force of nature.
Cyclone Bhola (1970) is historically the worst event for deaths with reported numbers as high as 500,000 people dead, mostly in Bangladesh.
Hurricane Katrina, which struck the east coast of the U.S. in 2005 is the most costliest hurricane with overall damage exceeding $100 billion.
The 1979 Cyclone Tip was the most intense hurricane ever recorded with wind speeds around 190 miles per hour. It was also the largest hurricane with a diameter around 2,170 kilometres.
Hurricane Sandy (U.S. east coast, 2010) was an abnormally large event due to the fusion of a tropical hurricane and a winter storm. With rising temperatures due to global warming these hybrid-storms are expected to become more frequent.
What can we do?
Cyclone Mahasen hit Sri Lanka, Bangladesh and Burma last season in May and resulted in far less deaths than expected thanks to swift action from the local governments. This shows that rapid response can save lives.
Houses with appropriate shelters (as in the U.S.) need to be built in hurricane prone regions. Proper training should be given from early school level onwards on the appropriate actions to take before and after such events: e.g. having enough clean drinking water, tinned food, spare batteries for lights, mobile phone chargers, first aid kits etc.
Have a rapid response system in place from the national governmental level to manage pre-event evacuation (if needed) and post disaster recovery.
There is no clear agreement on what sort of effects climate change and global warming will have on hurricanes. However most scientists agree that the change will be for the worse whether it is in the form of increased number of hurricanes each year, increased size of the hurricanes or changes to the length of the hurricane season . We need to invest more into understanding the science behind these storms and the effects global warming will have on their magnitude and frequency of occurrence.
Food shortages brought on by extreme weather events have resulted in almost a quarter of Sri Lanka’s 21 million people becoming malnourished, says a World Food Programme (WFP) document.
“The increased frequency of natural disasters such as drought and flash floods further compounds food and nutrition insecurity,” says the document, the latest WFP country brief for Sri Lanka, released in June.
As per WFP’s most recent Cost of Diet Analysis, almost 6.8 million (33 per cent) Sri Lankans cannot afford the minimum cost of a nutritious diet and a large portion of this vulnerable population lives in poverty and is frequently subjected to extreme weather events.
In May heavy rains, brought on by Cyclone Roanu, affected 340,000 persons in 22 of the island’s 25 districts. “These people have very limited coping mechanisms, and these kinds of disasters can drive them deeper into poverty,” says minister for disaster management Anura Priyadarshana Yapa.
After the landslides and rains the government decided to shift out those living in high-risk areas but, according to public officials, they were faced with the problems of locating safe land and making income from agriculture.
“Most of those living on these high-risk areas rely on agriculture and we need to see how to secure their livelihoods,” head of the disaster management centre, Kegalle district, tells SciDev.Net.
The UN estimates that every year around 700,000 Sri Lankans are impacted by extreme weather, some repeatedly. “A sizeable segment of the flood affected population are squatters living in vulnerable areas prone to frequent flooding,” the UN Office for the Coordination of Humanitarian Affairs said following estimates made soon after the May floods and landslides.
“We need to develop long-term solutions, not stop-gap answers,” says Yapa, agreeing that there were serious problems arising from erratic weather patterns in Sri Lanka in recent years.
Recently a video has been circulating of Paul Beckwith from the University of Ottawa explaining how the northern and southern polar jet streams linked briefly across the equator this week.
Climate scientist Dr Caroline Holmes, who specialises in atmospheric dynamics, provides her independent view on this event below:
The essence of this video is the claim that the northern and southern hemisphere jet streams have connected via air crossing the equator, for a brief period of time; that this is ‘unprecedented’; and that it heralds dangerous consequences. To give my thoughts, I’ll address some of the concepts that come up in the video and some issues of language or terminology that are crucial for understanding it, and try to give an answer on whether I think this is a worrying development or not.
1)What’s so special about the equator?
Crash course covering two aspects: one is thermal, i.e., its to do with heat. As we know, the equator receives more heat from the sun than the poles do. Therefore it’s warmer. The atmosphere ‘tries’ to redistribute this heat from the equator to the poles, which is the reason for a lot of the circulation (i.e. large-scale movement of air and water) that we see in the ocean and atmosphere. Note, that in the northern hemisphere (I’ll call it NH from now on!) summer, the maximum heat from the sun is actually felt slightly north of the equator, and in southern hemisphere (…SH…) summer, slightly to the south of the equator. So, actually the ‘heat redistribution’ in the atmosphere is really from about 10°N to both poles in NH summer (SH winter) and from 10°S to both poles in SH summer. The other special thing about the equator is to do with the fact that the earth is a rotating sphere. So every point on the earth’s surface is rotating, once a day, around an axis that runs from the north to south pole. At the equator, this rotation requires covering a lot more distance (a circle of bigger circumference) than it does near the poles. So as air moves away from the equator (and indeed anywhere) it’s subject to forces linked to this rotation. (I’ll stop there because that’s a whole ‘nother blog post!)
2)What is a jet stream? The term ‘jet stream’ typically refers to a tight (i.e. short north-south extent) band of very fast winds, high in the atmosphere. We usually use the wind speed at 250 hPa (i.e. the level in the atmosphere where pressure is 250 hPa; about 16 km, very roughly speaking) to describe it, although in some jet streams winds are fast near the Earth’s surface too. At the simple level, each hemisphere has one, or two, jet streams. One is on the edge of the tropics, and one is in mid-latitudes (broadly about 30-60°N). Sometimes, or indeed often, the two combine. The jet streams fundamentally exist because of the two things I already mentioned above; the rotation of the earth, and the fact that the atmosphere tries to distribute heat from equator to pole. In the time-average picture, they are nice and smooth, and largely west to east, although not entirely so, and in the NH especially they are not a continuous band around the world; they are split up by, in particular, the Rocky mountains and the Eurasia continent. As time varies, however, the jets have big waves in them; they break up; they shift from north to south; they pulse in strength. They are hugely variable.
3) Is the climate system normally ‘stable and predictable’ and now becoming ‘chaotic’?
The atmosphere certainly isn’t. One of the most fundamental things about the atmosphere is that it is a chaotic system. You’ve probably heard the ‘butterfly flapping its wings causes a hurricane around the world’ saying. It’s that. Or, less dramatically; tiny changes in the atmosphere in one place can push it into a different state from the one it was heading to. It’s why we can’t do weather forecasts perfectly; because without knowing the state (by which I mean its temperature, how much water it contains, how fast it is moving and in what direction) of every point in the atmosphere, perfectly, we can’t be sure we are on the right path. The climate system on the other hand is a much trickier issue. I’m not even sure what the video is trying to explain regarding this. There are aspects of the climate system that are certainly predictable (seasons, for example. And they aren’t going to be eroding in a hurry, because they rely on changes of the earth’s orbit around the sun and the angle of its axis, which happen slowly. I.e. 10s to 100s of thousands of years slowly.) The stability of the climate system is a genuine question. Imagine a ball on a slope.
If I shove it gently, it returns to where it was. This is a ‘stable’ condition. If I shove it hard, however, it might go over the ‘hill’ to the right; it will not return to where it was. This is what is meant by unstable. A related concept is ‘tipping points’; if it goes over that hill, it might end up in another ‘bowl’; but it has been tipped into another type of behaviour (in this case, a different area to roll around in).
4)What is climatology?
Climatology is a very broad field. It’s the study of climate. For example, it’s a term that could probably cover people who study the output from climate models about what will happen in the next century; look at observational data gathered over the past 100 years; examine tree rings to try and work out what has happened to climate over the last several millennia; use mathematical techniques to explore what will happen to earth’s climate, and so on. There are better, or more detailed description for all these jobs, but climatologist is not wrong. The key thing is that a professor of climatology need not be (and in fact, probably is not) an expert on atmospheric dynamics, i.e., the movements in the atmosphere and what causes them, which is what this is all about.
5)What is this graphic?
It’s a beautiful visualisation of atmospheric conditions. It’s always worth being aware that first, the data that went into this is not as gorgeous as the end product! You can toggle the ‘grid’ button to show a point at every location where there is real data from the forecast model; the rest is interpolation between these points. And second, our eyes aren’t that good at interpreting information like this (sorry!) and are naturally drawn to colour boundaries. So, it’s easy to over-interpret patches of a different colour as indicating something very different. Now, as for some specific questions from the video, I’ll first address the idea that the jet stream should get, and is getting, ‘wavier’ as the atmosphere gets warmer. This argument is still, as far as I know, really controversial. I discussed this issue extensively during my PhD. The theory relies on a chain of about five causal relationships (i.e., A causes B, B causes C, and so on) in a chaotic system. Some of these are fairly convincing and some are not. The data providing evidence of this use information from a fairly short time series (30 years, which for finding robust results about anything other than time-average is too short). The original paper can be found here: http://onlinelibrary.wiley.com/doi/10.1029/2012GL051000/full. However, later papers examining the claim across different datasets, and for various measures of ‘waviness’, don’t find strong evidence for such a pattern (e.g. http://onlinelibrary.wiley.com/…/10…/grl.50880/abstract). Therefore, citing it as fact is very misleading. The second question is whether something remarkable just happened, and whether it can be described as ‘the jet crossing the equator’. Flow crossing the equator is not surprising; as I’ve mentioned above, the atmospheric flow is very variable. A colleague at Monash University kindly quickly checked a month of reanalysis data (essentially, this blends observations- from the surface, satellites, and upper air measurements from weather balloons- with the best available weather forecast models, to produce a historical ‘best guess’ of what the atmosphere has looked like over recent decades). She found that the wind speeds with flow across the equator of the speed seen here (about 20 metres per second) aren’t that rare. So as above; flow across the equator isn’t astonishing. Can this be described as the jet crossing the equator? Tricky; not all fast upper level winds are jet streams, and in addition the jet streams are generally a lot faster than this (about 100 metres per second). So then, is this behaviour where there appears to be some linkage between the NH and SH jet surprising (even ‘unprecedented’)? Unfortunately I can’t answer that right now! Links like this are easy to see by eye, but harder to define when you look for it in real data, so it wouldn’t be a totally trivial job to find out. However, I think that there is no reason why it would be surprising; given that the jets move around, and airflow crosses the equator. For the same reason I don’t think it’s a particularly meaningful event. Finally, it’s worth saying, the jet streams have been exhibiting weird behaviour recently. The winter of 2013/14 was really remarkable in the NH, for example, and the UK met office produced a great report about the storms and floods that occurred in the UK as a result as well as what was happening in Canada at the time. So I think it’s important to understand what the jets are doing, and why, and whether it’s new. The changing temperature of our atmosphere due to climate change is hugely worrying, and how this might impact the jet streams, particularly in the northern hemisphere, is still poorly understood. However, in conclusion, I don’t think this particular instance is panic worthy.
————————————————————————– Dr Caroline Holmes gained her PhD in climate science from the University of Reading, where her thesis investigated the effects of Arctic change (warming and sea ice loss) on mid-latitude climate, in particular the jet streams. She now works on understanding the impacts of climate change in Scotland at the University of Edinburgh.
Lying on the floodplains of the mighty Ganges, Brahmaputra and Meghna rivers Bangladesh is a rich, fertile land. These giant river systems meet in the centre of the country and flow together into the Bay of Bengal which, at over 1600km wide, is the largest delta in the world.
Rising Sea Level
Bangladesh is often cited as one of the countries that will be most negatively affected by rising sea levels from human induced climate change. Two thirds of the country lies less than 5m above of sea level. With vast regions to the south much less than a 1m above sea level. The Intergovernmental Panel on Climate Change (IPCC) claims that just 1m rise in sea level could directly expose nearly 14 million people and result in potentially 17% land loss in southern Bangladesh.
Most of the country receives on average more than 2.5m of rainfall a year, 80% of which falls in about 4 months during the peak monsoon season, resulting in large annual floods. The flood waters bring nutrient rich clays and silts from the high Himalayas and deposit them on the river floodplains. These rich soils produce bountiful harvests of rice and other crops. Unsurprisingly, farming is the most common profession.
However floods, once welcomed by farmers and their families are now harbingers of disaster. Human induced climate change has resulted in more erratic monsoon weather patterns with often larger than normal volumes of water being delivered in shorter time intervals. The resulting floods have had devastating effects on the Bangladeshi people. In 2012 three large floods hit the country in swift succession between the months of July and September directly affecting more than 5 million people. These are now a common annual occurrence.
Bangladesh is also subject to annual tropical cyclones, storm surges and tornadoes. Some of the worst natural disasters in recorded history were results of cyclonic storms in the Bengal region. Among them, the 1970 Bhola cyclone which claimed over 500,000lives! Worryingly new research into the impacts of climate change has shown that large cyclonic storms will become a more common occurrence in the years and decades to come.
The foothills of the great Himalayan mountain belt has historically been the location of many large earthquakes. Earthquakes in the continent tend to be more infrequent compared to regions such as Japan and California. However this makes them more unpredictable and often unexpected. But when one does occur it can result in significant ground shaking. The 1897 magnitude 8.1 and 1950 magnitude8.7 Assam earthquakes were two of the biggest to hit the region in recent times. The current building stock in Bangladesh is poorly built and most are not built to withstand ground shaking in an earthquake. The collapse of poorly built buildings is the greatest hazard during an earthquake.
So what can we as earth scientists do?
Bangladesh has a population of over 160 million and among the highest population density of any country in the world. With the majority of the country built on river floodplains combined with widespread corruption and ignorance a large earthquake could quite possibly result in the greatest natural calamity to have ever hit the country!
Bangladesh needs to increase its resilience if its people are to survive the multitude of natural hazards they face. Earth scientists are well placed to understand the risks involved from these hazards and can play a key role in all aspects of building a resilient infrastructure.
Climate science research is ongoing and needs to continue to better understand the affect human induced climate is having and will have on the annual monsoon. This knowledge needs to be translated into rainfall variation and flooding potentials and communicated with the people who need this information. The socio-economic issues of a rising sea level needs to be addressed and plans put in place to allow big cities to efficiently absorb and cater for migrants moving away from hazard prone coastal regions. Hydro-geologists and geochemists are helping to find sustainable clean, arsenic free water sources for drinking and farming. Seismologists and earthquake scientists are working to better understand the seismic risk in the Himalayan foothills; produce more accurate hazard maps and importantly identify the active faults within the region.
These are to name but a few of the ways earth scientists can get involved. I believe it is our moral duty to translate the practical aspects of our science into real benefits for people.
You might have heard reports recently that El Niño is expected to strengthen in the coming months to potentially become one of the strongest events since 1950. Here’s a great Met Office video explaining how they work.