The daily dance of flowers tracking the sun is more fascinating than most of us realise


Julien Christ/Unsplash

Gregory Moore, The University of MelbourneWhen I was a child, I was intrigued by the Queensland box (Lophostemon confertus) growing in our backyard. I noticed its leaves hung vertical after lunch in summer, and were more or less horizontal by the next morning.

This an example of heliotropism, which literally means moving in relation to the sun. We can see it most clearly as spring arrives and various species burst into flower — you might even get the feeling that some flowers are watching you as they move.

Many of us probably first got to know of heliotropism at home, kindergarten or primary school by watching the enormous yellow and black flowering heads of aptly name sunflowers, which moved as they grew.

These flowers track the course of the sun spectacularly on warm and sunny, spring or summer days. Sometimes they move through an arc of almost 180⁰ from morning to evening.

So with the return of sunny days and flowers in full bloom this season, let’s look at why this phenomenon is so interesting.

The mechanics of tracking the sun

A number flowering species display heliotropism, including alpine buttercups, arctic poppies, alfalfa, soybean and many of the daisy-type species. So why do they do it?

This is Heliotropium arborescens, named for its heliotropism. They were very popular in gardens a century or more ago, but have fallen from favour as they can be poisonous and weedy.
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Flowers are really in the advertising game and will do anything they can to attract a suitable pollinator, as effectively and as efficiently as they can. There are several possible reasons why tracking the sun might have evolved to achieve more successful pollination.

By tracking the sun, flowers absorb more solar radiation and so remain warmer. The warmer temperature suits or even rewards insect pollinators that are more active when they have a higher body temperature.

Optimum flower warmth may also boost pollen development and germination, leading to a higher fertilisation rate and more seeds.




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So, the flowers are clearly moving. But how?

For many heliotropic flowering species, there’s a special layer of cells called the pulvinus just under the flower heads. These cells pump water across their cell membranes in a controlled way, so that cells can be fully pumped up like a balloon or become empty and flaccid. Changes in these cells allow the flower head to move.

Venus fly trap
Fly traps have somewhat similar mechanics to heliotropism.
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When potassium from neighbouring plant cells is moved into the cells of the pulvinus, water follows and the cells inflate. When they move potassium out of the cells, they become flaccid.

These potassium pumps are involved in many other aspects of plant movement, too. This includes the opening and closing of stomata (tiny regulated leaf apertures), the rapid movement of mimosa leaves, or the closing of a fly trap.

But sunflowers dance differently

In 2016, scientists discovered that the pin-up example of heliotropism — the sunflower — had a different way of moving.

They found sunflower movement is due to significantly different growth rates on opposite sides of the flowering stem.

A sunflower facing a setting sun
Sunflowers move differently to other heliotropic flowers.
Aaron Burden/Unsplash

On the east-facing side, the cells grow and elongate quickly during the day, which slowly pushes the flower to face west as the daylight hours go by — following the sun. At night the west-side cells grow and elongate more rapidly, which pushes the flower back toward the east over night.

Everything is then set for the whole process to begin again at dawn next day, which is repeated daily until the flower stops growing and movement ceases.




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While many people are aware of heliotropism in flowers, heliotropic movement of leaves is less commonly noticed or known. Plants with heliotropic flowers don’t necessarily have heliotropic leaves, and vice versa.

Heliotropism evolves in response to highly specific environmental conditions, and factors affecting flowers can be different from those impacting leaves.

The leaves of Queensland box, Lophostemon confertus, which track the sun.
Krzysztof Ziarnek, Kenraiz/Wikimedia Commons, CC BY-SA

For example, flowers are all about pollination and seed production. For leaves, it’s for maximising photosynthesis, avoiding over-heating on a hot day or even reducing water loss in harsh and arid conditions.

Some species, such as the Queensland box, arrange their leaves so they’re somewhat horizontal in the morning, capturing the full value of the available sunlight. But there are also instances where leaves align vertically to the sun in the middle of the day to minimise the risks of heat damage.

Plants are dynamic

It’s easy to think of plants as static organisms. But of course, they are forever changing, responding to their environments and growing. They are dynamic in their own way, and we tend to assume that when they do change, it will be at a very slow and steady pace.

Heliotropism shows us this is not necessarily the case. Plants changing daily can be a little unsettling in that we sense a change but may not be aware of what is causing our unease.

As for me, I still keep a watchful eye on those Queensland boxes!




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The Conversation


Gregory Moore, Doctor of Botany, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate explained: how particles ejected from the Sun affect Earth’s climate


Earth’s magnetic field protects us from the solar wind, guiding the solar particles to the polar regions.
SOHO (ESA & NASA)

Annika Seppälä


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Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz


When the Sun ejects solar particles into space, how does this affect the Earth and climate? Are clouds affected by these particles?

When we consider the Sun’s influence on Earth and our climate, we tend to think about solar radiation. We are acutely aware of the skin-burning dangers of ultraviolet, or UV, radiation.

But the Sun is an active star. It also continuously releases what is known as “solar wind”, made up of charged particles, largely protons and electrons, that travel at speeds of hundreds of kilometres per hour.

Some of these particles that reach Earth are guided into the polar atmosphere by our magnetic field. As a result, we can see the southern lights, aurora australis, in the southern hemisphere, and the northern equivalent, aurora borealis.

Aurora Australis
Aurora australis observed above southern New Zealand.
Shutterstock/Fotos593

This visible manifestation of solar particles entering Earth’s atmosphere is a constant reminder there is more to the Sun than sunlight. But the particles have other effects as well.




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Solar particles and ozone

When solar particles enter the atmosphere, their high energies ionise neutral atmospheric nitrogen and oxygen molecules, which make up 99% of the atmosphere. This “energetic particle precipitation”, named because it’s like a rain of particles from space, is a major source of ionisation in the polar atmosphere above 30km altitude — and it sets off a chain of reactions that produces chemicals that facilitate the destruction of ozone.

The impact of solar particles on atmospheric ozone was first observed in 1969. Since the early 2000s, thanks to new kinds of satellite observations, we have seen growing evidence that solar particles play an important part in influencing polar ozone. During particularly active times, when the Sun releases large amounts of particles into space, up to 60% of ozone at altitudes above 50km can be depleted. The effect can last for weeks.

Lower down in the atmosphere, below 50km, solar particles are important contributors to the year-to-year variability in polar ozone levels, often through indirect pathways. Here, solar particles again contribute to ozone loss, but a recent discovery showed they also help curb some of the depletion in the Antarctic ozone hole.

How ozone affects the climate

Most of the ozone in the atmosphere resides in a thin layer at altitudes of 20-25km — the “ozone layer”.

But ozone is everywhere in the atmosphere, from the Earth’s surface to altitudes above 100km. It is a greenhouse gas and plays a key role in heating and cooling the atmosphere, which makes it critical for climate.

In the southern hemisphere, changes in polar ozone are known to influence regional climate conditions.

Satellite image of Earth's atmosphere
Solar particles ionise nitrogen and oxygen molecules in the atmosphere, which leads to other chemical reactions that contribute to ozone destruction.
Shutterstock/PunyaFamily

Its depletion above Antarctica had a cooling effect, which in turn pulled the westerly wind jet that circles the continent closer. As the Antarctic hole recovers, this wind belt can meander further north and affect rainfall patterns, sea-surface temperatures and ocean currents. The Southern Annular Mode describes this north-south movement of the wind belt that circles the southern polar region.

Ozone is important for future climate predictions, not only in the thin ozone layer, but throughout the atmosphere. It is crucial we understand the factors that influence ozone variability, be it man-made or natural like the Sun.

The Sun’s direct influence

The link between solar particles and ozone is reasonably well established, but what about any direct effects solar particles may have on the climate?

We have observational evidence that solar activity influences regional climate variability at both poles. Climate models also suggest such polar effects link to larger climate patterns (such as the Northern and Southern Annular Modes) and influence conditions in mid-latitudes.

The details are not yet well understood, but for the first time the influence of solar particles on the climate system will be included in climate simulations used for the upcoming Intergovernmental Panel on Climate Change (IPCC) assessment.




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Through solar radiation and particles, the Sun provides a key energy input to our climate system. While these do vary with the Sun’s 11-year cycle of magnetic activity, they can not explain the recent rapid increase in global temperatures due to climate change.

We know rising levels of greenhouse gases in the atmosphere are pushing up Earth’s surface temperature (the physics have been known since the 1800s). We also know human activities have greatly increased greenhouse gases in the atmosphere. Together these two factors explain the observed rise in global temperatures.

What about clouds?

Clouds are much lower in the atmosphere than where most solar particles penetrate. Particles know as galactic cosmic rays (coming from the centre of our galaxy rather than the Sun) may be linked to cloud formation.

It has been suggested cosmic rays could influence the formation of condensation nuclei, which act as “seeds” for clouds. But recent research at the CERN nuclear research facility suggests the effects are insignificant.

This doesn’t rule out some other mechanisms for cosmic rays to affect cloud formation, but thus far there is little supporting evidence.The Conversation

Annika Seppälä, Senior Lecturer in Geophysics

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate explained: Sunspots do affect our weather, a bit, but not as much as other things



NASA

Robert McLachlan, Massey University


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Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz


Are we headed for a period with lower Solar activity, i.e. sunspots? How long will it last? What happens to our world when global warming and the end of this period converge?

When climate change comes up in conversation, the question of a possible link with the Sun is often raised.

The Sun is a highly active and complicated body. Its behaviour does change over time and this can affect our climate. But these impacts are much smaller than those caused by our burning of fossil fuels and, crucially, they do not build up over time.

The main change in the Sun is an 11-year Solar cycle of high and low activity, which initially revealed itself in a count of sunspots.

One decade of solar activity in one hour.

Sunspots have been observed continuously since 1609, although their cyclical variation was not noticed until much later. At the peak of the cycle, about 0.1% more Solar energy reaches the Earth, which can increase global average temperatures by 0.05-0.1℃.

This is small, but it can be detected in the climate record.

It’s smaller than other known sources of temperature variation, such as volcanoes (for example, the large eruption of Mt Pinatubo, in the Philippines in 1991, cooled Earth by up to 0.4℃ for several years) and the El Niño Southern Oscillation, which causes variations of up to 0.4℃.




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And it’s small compared to human-induced global warming, which has been accumulating at 0.2℃ per decade since 1980.

Although each 11-year Solar cycle is different, and the processes underlying them are not fully understood, overall the cycle has been stable for hundreds of millions of years.

A little ice age

A famous period of low Solar activity, known as the Maunder Minimum, ran from 1645 to 1715. It happened at a similar time as the Little Ice Age in Europe.

But the fall in Solar activity was too small to account for the temperature drop, which has since been attributed to volcanic eruptions.

Solar activity picked up during the 20th century, reaching a peak in the cycle that ran from 1954 to 1964, before falling away to a very weak cycle in 2009-19.

Bear in mind, though, that the climatic difference between a strong and a weak cycle is small.

Forecasting the Solar cycle

Because changes in Solar activity are important to spacecraft and to radio communications, there is a Solar Cycle Prediction Panel who meet to pool the available evidence.

Experts there are currently predicting the next cycle, which will run to 2030, will be similar to the last one. Beyond that, they’re not saying.

If activity picks up again, and its peak happened to coincide with a strong El Niño, we could see a boost in temperatures of 0.3℃ for a year or two. That would be similar to what happened during the El Niño of 2016, which featured record air and sea temperatures, wildfires, rainfall events and bleaching of the Great Barrier Reef.

The extreme weather events of that year provided a glimpse into the future. They gave examples of what even average years will look like after another decade of steadily worsening global warming.

A journey to the Sun

Solar physics is an active area of research. Apart from its importance to us, the Sun is a playground for the high-energy physics of plasmas governed by powerful magnetic, nuclear and fluid-dynamical forces.

The Solar cycle is driven by a dynamo coupling kinetic, magnetic and electrical energy.




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That’s pretty hard to study in the lab, so research proceeds by a combination of observation, mathematical analysis and computer simulation.

Two spacecraft are currently directly observing the Sun: NASA’s Parker Solar Probe (which will eventually approach to just 5% of the Earth-Sun distance), and ESA’s Solar Orbiter, which is en route to observe the Sun’s poles.

Hopefully one day we will have a better picture of the processes involved in sunspots and the Solar cycle.The Conversation

Exploring the 11-year Solar cycle.

Robert McLachlan, Professor in Applied Mathematics, Massey University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate explained: how white roofs help to reflect the sun’s heat



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Nilesh Bakshi, Te Herenga Waka — Victoria University of Wellington and Maibritt Pedersen Zari, Te Herenga Waka — Victoria University of Wellington

Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz

Does the white roof concept really work? If so, is it suitable for New Zealand conditions?

Generally, white materials reflect more light than dark ones, and this is also true for buildings and infrastructure. The outside and roof of a building soak up the heat from the sun, but if they are made of materials and finishes in lighter or white colours, this can minimise this solar absorption.

During the warmer part of the year, this can keep the temperature inside the building cooler. This is especially important for building and construction materials such as concrete, stone and asphalt, which store and re-radiate heat.




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On a hot day, a white roof can keep the temperature cooler inside the building.
from http://www.shutterstock.com

A New Zealand study tested near-identical buildings in Auckland with either a red or white roof. It found that even in Auckland’s temperate climate, white roofs reduced the need for air conditioning during hotter periods, without reducing comfort during cooler seasons.

The study also identified several large-scale white-roof installations, including at Auckland International Airport, shopping centres and commercial buildings, but the effect was less clear.

This research suggests that there is potential for white-roof installations to significantly reduce the amount of energy needed to cool buildings. This would in turn reduce greenhouse gas emissions and also help us to adapt to rising temperatures.

It is difficult to quantify the impact for New Zealand’s housing stock because existing studies are mostly limited to larger commercial buildings. But research carried out so far suggests white roofs could be a viable approach to minimising the heat taken up by buildings during hotter parts of the year.

Cooling cities

White roofs can also help reduce the temperature of whole cities. Many city centres include large buildings made of concrete or other materials that collect and store solar heat during the day. In a phenomenon known as the “urban heat island” effect, city centres can often be several degrees warmer than the surrounding countryside.

When cities are hotter, they use more energy for cooling. This usually results in more greenhouse gas emissions, due in part to the energy consumed, and contributes further to climate change.

New Zealand is different because our land mass has a maximum width of 400 kilometres. This means that unlike many urban islands on the African, Asian or American continents, New Zealand’s city centres benefit from the cooling effects of being near the ocean.




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There are many international studies showing white roofs are effective in mitigating the urban heat island effect in densely populated cities. But there is little evidence that using white roofs in New Zealand cities could result in significant energy reductions.

A growing number of studies suggest making the surfaces of buildings and infrastructure more light reflecting could significantly lower extreme temperatures, particularly during heat waves, not just in cities but in rural areas as well. A recent study shows strategic replacement of dark surfaces with white could lower heatwave maximum temperatures by 2℃ or more, in a range of locations.

But studies have also identified some practical limitations and potential side effects, including the possibility of reduced evaporation and rainfall in urban areas in drier climates.

In conclusion, white roofs could be a good idea for New Zealand to keep homes and cities slightly cooler. As temperatures continue to rise, this could reduce the energy needed for cooling. We should consider this option more often, particularly for commercial-scale buildings made of heat-retaining materials in larger cities.The Conversation

Nilesh Bakshi, Lecturer, Te Herenga Waka — Victoria University of Wellington and Maibritt Pedersen Zari, Senior Lecturer in Sustainable Architecture, Te Herenga Waka — Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.