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Over the past week we have seen two major tropical storms devastate different parts of the world. First Typhoon Hato struck Hong Kong and Southern China killing at least a dozen people. And over the weekend Hurricane Harvey made landfall from the Gulf of Mexico, bringing extremely heavy rain to southern Texas and causing devastating floods in Houston.

Tropical cyclones are, of course, a natural feature of our climate. But the extreme impacts of these recent storms, especially in Houston, has understandably led to questions over whether climate change is to blame.

How are tropical cyclones changing?

Tropical cyclones, called typhoons in the Northwest Pacific and hurricanes in the North Atlantic, are major storm systems that initiate near the Equator and can hit locations in the tropics and subtropics around the world.

When we look at the Atlantic Basin we see increases in tropical storm numbers over the past century, although there is high year-to-year variability. The year 2005, when Hurricane Katrina devastated New Orleans, marks the high point.

There is a trend towards more tropical storms and hurricanes in the North Atlantic. US National Hurricane Center

We can be confident that we’re seeing more severe tropical cyclones in the North Atlantic than we did a few decades ago. It is likely that climate change has contributed to this trend, although there is low statistical confidence associated with this statement. What that means is that this observed increase in hurricane frequency is more likely than not linked with climate change, but the increase may also be linked to decadal variability.

Has Harvey been enhanced by climate change?

Unlike other types of extreme weather such as heatwaves, the influence of climate change on tropical cyclones is hard to pin down. This is because tropical cyclones form as a result of many factors coming together, including high sea surface temperatures, and weak changes in wind strength through the depth of the atmosphere.

These storms are also difficult to simulate using climate models. To study changes in tropical cyclones we need to run our models at high resolution and with interactions between the atmosphere and the ocean being represented.

It’s much easier to study heat extremes, because we can do this by looking at a single, continuous variable: temperature. Tropical cyclones, on the other hand, are not a continuous variable; they either form or they don’t. This makes them much harder to model and study.

Tropical cyclones also have many different characteristics that might change in unpredictable ways as they develop, including their track, their overall size, and their strength. Different aspects of the cyclones are likely to change in different ways, and no two cyclones are the same. Compare that with a heatwave, which often have similar spatial features.

For all these reasons, it is very hard to say exactly how climate change has affected Hurricane Harvey.

So what can we say?

While it’s hard to pin the blame for Hurricane Harvey directly on climate change, we can say this: human-caused climate change has enhanced some of the impacts of the storm.

Fortunately, in Harvey’s case, the storm surge hasn’t been too bad, unlike for Hurricanes Katrina and Sandy, for example. This is because Harvey did not travel as far, and weakened rapidly when it made landfall.

We know that storm surges due to tropical cyclones have been enhanced by climate change. This is because the background sea level has increased, making it more likely that storm surges will inundate larger unprotected coastal regions.

Building levees and sea walls can alleviate some of these impacts, although these barriers will need to be higher (and therefore more expensive) in the future to keep out the rising seas.

Deluge danger

Harvey’s biggest effect is through its intense and prolonged rainfall. A low pressure system to the north is keeping Harvey over southern Texas, resulting in greater rainfall totals.

The rainfall totals are already remarkable and are only going to get worse.

We know that climate change is enhancing extreme rainfall. As the atmosphere is getting warmer it can hold more moisture (roughly 7% more for every 1℃ rise in temperature). This means that when we get the right circumstances for very extreme rainfall to occur, climate change is likely to make these events even worse than they would have been otherwise. Without a full analysis it is hard to put exact numbers on this effect, but on a basic level, wetter skies mean more intense rain.

Houston, we have a problem

There are other factors that are making this storm worse than others in terms of its impact. Houston is the second-fastest growing city in the US, and the fourth most populous overall.

As the region’s population grows, more and more of southern Texas is being paved with impermeable surfaces. This means that when there is extreme rainfall the water takes longer to drain away, prolonging and intensifying the floods.

Hurricane Harvey is likely to end up being one of the most costly disasters in US history. It is also likely that climate change and population growth in the region have worsened the effects of this major storm.

Disclosure

Andrew King receives funding from the ARC Centre of Excellence for Climate System Science.

The storm has hurled record rainfall at Houston, forcing thousands to flee their homes and testing flood-control systems to their limits. Parts of the city area saw more than 22in (55cm) of rain in a 24-hour period to Sunday evening; too much for the bayous to handle, too much for roads to remain passable and threatening to overwhelm emergency teams

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The arguments nearly a century ago over the use of leaded petrol.

Originally published in the Manchester Guardian on 31 August 1917

When the heavy showers had passed, the sun burst out from behind drifting clouds, and studded the dripping hedges with diamonds. For ten yards or more privet, in full flower broke the monotony of thorn and bramble, and here fifteen or twenty red admiral butterflies fanned their gorgeous wings as they sipped the sweets. The air was heavy with the scent of privet. Golden–rod, a blaze beneath the hedge, attracted other red admirals, and amongst them were small tortoiseshells and a few peacocks. True to its name, the wall butterfly was more plentiful where rugged stone walls replaced the hedgerows, but it abounded alike in all the lanes and on the rocky outcrops, covered with ragwort, scabious, and eyebright, which are so noticeable a feature of North Wales.

Privet, by the way, is troubling one of my correspondents. He finds his hedge attacked by small white grubs, which shelter in the curled and shrivelled leaves. I do not find my privets badly damaged, though a few shoots have been attacked. It is the caterpillar of one of the small leaf-mining moths, for the grubs in their earlier stages, at any rate, feed within the two layers of leaf-skin. I can only advise that he cuts off the damaged shoots and burns them so as to diminish the numbers of the moths.

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A quail chick hatching. Shutterstock

This is an article from I Have Always Wondered, a new series where readers send in questions they’d like an expert to answer. Send your question to alwayswondered@theconversation.edu.au

This question dates back to when I was a kid and no one has ever been able to answer it in a convincing way. I have always wondered: when do birds (and other egg-born creatures) take their first breath? And how do they take in oxygen before their lungs are working? Obviously since eggs squeak before they hatch, lungs are functional prior to the hatching… but when is that magical inflation-of-the-lungs moment? And how does it happen? – Gabrielle Deakin, Barcelona.

As placental mammals, our first breath of air comes after birth. But egg-born creatures like birds and reptiles don’t have an umbilical cord to feed them oxygen, so how do they breathe? And how can a chick inflate its lungs inside the egg?

First, let’s talk about the eggs themselves.

Eggs laid by birds have shells that are bumpy (at least under the microscope), made almost entirely of calcium carbonate, and have as many as 17,000 tiny pores. Because of these pores, oxygen can travel from the outside world to the embryo inside and carbon dioxide and water move out of the egg in the same way.

Lying between the eggshell and the albumen, or egg white, are two transparent membranes that prevent bacterial invasion, and also develop into a network of blood vessels. These membranes are the chorion and the allantois.

Membranes inside the egg move oxygen inside for the embryo, and pass carbon dioxide out. Pixabay, CC BY

Reptile eggs can either be hard and almost identical to bird’s eggs, as thin shelled as parchment, or soft and leathery. Most reptile eggs are porous to air and water, and tend to absorb more water from the outside world than bird eggs. Finally, the membranes of reptiles’ eggs are very similar to birds’, but don’t always entirely surround the embryo.

Regardless of these differences, the chorion and allantois have a network of blood vessels which act as a respiratory organ and is the first stage of “breathing” for bird or reptile embryos.

Birds actually go through three stages of breathing in the egg. Reptiles have a similar path, but they skip straight from step one to three.

Stage 1: embryonic

Before chicks or reptiles develop lungs, they still need to get oxygen and get rid of carbon dioxide. In placental mammals like humans (and some marsupials), all of this is accomplished by the mother through the umbilical cord and the placenta.

Some reptiles have leathery shells. Brad Chambers, Author provided

In birds, this gas exchange is done by diffusion (the movement of air from the outside to the inside of the egg) through the eggshell and a complex fusion of the chorion and the allantois called the chorioallantoic membrane. Reptiles also have a chorioallantoic area which functions as a respiratory organ.

In birds, the chorioallantoic membrane develops about three days after incubation begins and takes about two weeks to develop fully. It is highly vascularised (has lots of blood vessels), which allows for the free exchange of oxygen and carbon dioxide.

This membrane also plays a central role in the development of the embryo’s bones, because it transports calcium from the eggshell to the developing chick or embryonic reptile (excluding some reptiles which get some of their calcium from the egg yolk).

Stage 2: pre-hatching A bird hatching from the egg. Maggie J Watson, Author provided

The embryo doesn’t actually breathe via lungs for almost all of its time in the egg. When the embryo is getting close to hatching, a few differences between reptiles and birds emerge. In birds, a few days before hatching, the chick, which is now curled up tightly with its head stuck under one wing and its beak pointed towards the top of the egg, penetrates an air pocket or air cell at the top of the egg.

This air pocket began to form when the egg was laid. A freshly laid egg is the temperature of its mother’s body, but it soon begins to cool. As it cools, the inner shell membranes begin to shrink and separate from the outer shell membrane to form a pocket, which slowly fills with air and gets larger as the egg is incubated.

As soon as the chick breaks into this air pocket, it takes its first breath and the lungs begin to function. The air cell continues to be refilled with air through diffusion. Diffusion through the chorio-allantoic membrane is also still used, but is slowly replaced by lung activity as hatching nears. At the very end of this period, if you put your ear to the egg, you might hear some peeping sounds.

A chick peeps in the egg. Also visible is a distinctive ‘pipping’ pattern, as the chick hammers the inside of the egg.

In birds, this sound is made through a structure called a syrinx as birds don’t have vocal chords. But most reptile species don’t have an egg air pocket, so they go straight to stage three.

Stage 3: post-hatching

Many egg-born creatures develop a small, sharp protuberance called an “egg tooth” (technically called a caruncle) on their beak or snout. It can be made of hard skin (like in crocodiles and birds) or be an actual extra tooth (like in some lizards and snakes), but regardless, it’s used to break through the egg and falls off or is reabsorbed soon after hatching.

A baby turtle cracks through its egg. Brad Chambers, Author provided

The chick, guided by its wing placement, uses its egg tooth to hammer the inside of the egg. First the egg “stars” (when the beak begins to crack the shell), and then it “pips” (when the beak breaks through the shell). The chick uses its feet to move around in a circle and pierce the egg. The chorioallantoic membrane begins to lose function as it dries out, and the chick then relies solely on its lungs. The chick continues to peep, which tells the parent that hatching is imminent and ensures its clutch mates hatch synchronously.

Reptiles slice through their weakened eggshells (weak now because they’ve extracted most of the calcium) with an egg-tooth on their snouts and start to breath. Some reptiles (crocodiles) also produce sounds, but unlike birds they use a larynx and vocal cords, very similar to humans.

When you get right down to it, birds and reptiles do pretty much the same thing in the egg. It’s not that surprising, as birds and some reptiles are quite closely related. They’ve all evolved specific eggs to both protect growing embryos and provide them with what they need – including air.

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James Van Dyke has received funding from the National Science Foundation.

Maggie J. Watson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.

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