BBC | 23 Degrees

Day 360: What a year it has been...

Mon, 26 Dec 2011 09:00:00 +0000

Distance travelled ~ 924'600'000 km

23 Degrees is coming to an end, but what a journey it has been and what a year.

Coming into this project we didn't have any expectations, well I didn't anyway. I think we all appreciate that it's hard to predict the weather. The most extreme tornadoes in the US mid West, huge cyclones like Yasi hitting Australia, record snows in the US and the UK. Our mission when we set out on 3rd january 2011 (the day of Perihelion) was to tell the story of Earth's annual journey around the Sun.

So what has this year's weather shown us?. That's a really difficult question.

Making 23 Degrees has had a huge impact on me, I have learnt how our climate and our weather is all generated by our orbit around the Sun. It's all interconnected. It really does feel like a huge single organism where everything is linked to create our extraordinary world. I have always liked James Lovelock's idea of Gaia, the Earth as a single organism and seeing how land , sea and atmosphere all interact powered by the Sun has made me want to look into Gaia more.

It has been a real privilege to work on this series and get a glimpse of our incredible planet's annual journey. I just hope we humans step closer to more action in limiting the affects we have on our climate.

It would be great to hear which weather events of 2011 stand out for you?

(Keep updated - The series will transmit early 2012)

Spacewatch: CME, Mercury and Venus?

Wed, 05 Oct 2011 15:30:00 +0000

Distance travelled ~ 714'327'200 km

While we have been enjoying the wonderful sunshine down on Earth (especially the recent record october warmth in the UK) - space has also been throwing up some pretty stunning weather. Yesterday, October 4th, a massive solar flare exploded from the surface on the far side of the Sun. It blasted a spectacular coronal mass ejection or CME into space. A CME is a massive burst of solar wind and plasma containing electrons and protons that blast out from the Sun's surface. They are associated with solar flares and tend to develop in areas of high solar activity such as Sun spots.

The October 4th CME was recorded by the Solar and Heliospheric Observatory

Scientists from the Goddard Space Weather Lab have plotted the course of the CME and discovered it is heading directly for the planet closest to the Sun, Mercury. The cloud of highly energised plasma and particles won't affect the planet but could disrupt the MESSENGER probe in orbit around Mercury.

To watch the moving image click here

It is possible that the CME may then hit Venus on Oct. 6th, but it is not predicted to hit Earth.

What happens when CME's hit Earth?
When CMEs do come our way the shockwave from the highly energized particles can cause a geomagnetic storm that can disrupt our magnetic shield. This can trigger dynamic auroras or Northern Lights [aurora borealis] in the northern hemisphere or Southern Lights [aurora australis] in the southern hemisphere. CMEs hitting earth can also cause disruption to radio transmissions, damage satellites and cause power cuts by knocking out electrical power cables.

They don't present a health risk for us on the surface because of our protective magnetic shield and atmosphere. But they can present problems for astronauts and even people in high altitude planes due to increased risk of exposure to radiation. But it is thought that any long-term health risk are unlikely.

Day 271: secrets beneath the sea

Wed, 28 Sep 2011 18:00:00 +0000

Distance travelled ~ 696'585'600 km

One of the things that amazes me about our planet is how it carries clues to its own past. It's a bit like a giant memory stick, the trick is find the right file. And today Helen Czerski is on the trail of one of these files, but it's not buried underground it's buried deep under water.

Helen is pushing the limits of her endurance, diving 40 metres below the waves, searching for evidence of what our world looked like 20,000 years ago. It might seem odd to be going deep under the water to unearth our climate past but you'd be surprised what you find down there.

Helen's diving the Great Blue Hole 60 miles off the coast of Belize, it's as the name suggest a great big round hole that goes down over 120 metres. It was once a cave but the roof collapsed leaving the deep blue hole. It's more than just a wonderful piece of natural architecture. It's also a window into our past. Because deep down in the hole, are clues to one of the most dramatic events in our planet's history.

It's a very tricky and technical dive as it's so deep, so Helen is accompanied by a very experienced dive team. Fortunately she's an experienced and highly qualified diver herself, so she's the prefect person for the job. Though she did have to learn how to use a special facemask designed for presenters to talk underwater. Even with all the experience on show it's still a daunting dive, but she's following in some famous footsteps. Jacques Costeau explored the Blue Hole back in 1970.

As she descends Helen must carefully monitor her buoyancy, at these depths she doesn't want to go up or down too quickly, that's not good news, plus the sheer walls of the hole will make the dive feel very enclosed.

At around 40 metres she will reach what she's looking for. Here the walls of the hole are cut away and there are some incredible rock formations several metres high. These formations are stalactites, which is kind of an odd thing to find, because if I remember my geology you shouldn't find stalactites in the ocean because they can't form underwater. Stalactites are created when mineral rich water drips from the roof of a cave over hundreds or even thousands of years, leaving behind mineral deposits. Over time these build up to create the beautiful structures. But they can only form on land so what are they doing 40 metres down the blue hole?

Here's what must have happened. At some point back in time, this cave must have been above sea level, which means that when these stalactites formed, the ocean must have been much lower than it is today. These stalactites not only show us that sea levels have changed they also can show us when.

When Cousteau explored the hole they brought up a broken stalactite and when they cut a cross-section they found a series of rings, a bit like tree rings. Each of the rings represents a period of growth when the stalactite was exposed to air. That growth would stop when it became submerged again. Cousteau's stalactite shows three growth stages, so it's a record of changing sea levels over time.

Scientists have precisely dated stalactites from the Blue Hole and by comparing stalactites from around the world with other data like Antarctic ice cores, they've built up a picture of changing sea levels dating back hundreds of thousands of years. What it reveals is that sea levels here in the Caribbean and across the world, have dramatically risen and fallen over time.

Just 20,000 years ago, the sea was an incredible 120 metres lower than it is today. That means almost the entire Blue Hole cave system would have been on dry land. But the world has a finite amount of water in it at any given time. So if that huge mass of water wasn't in the oceans just where was it? Well believe it or not it was on land, but not as water but as ice.

20,000 years ago the earth was gripped by an ice age.

How did this happen, well you'll find out when our series airs...in 2012 mind you :-)

Programme '2': clues to our Planet's spin

Mon, 26 Sep 2011 17:00:00 +0000

Distance travelled ~ 691'332'800 km

On our journey around the Sun for 23 Degrees we are focussing on three main themes that control our climate and weather, Tilt, Orbit and Spin. And at the moment we are filming the show about Spin.

The team are on the road in Ecuador. They have gone to the Ecuador rainforest to learn how solar energy powers a power circulation system in the atmosphere that dictates the climate in bands around the world. Kate is also going to drive along the equator at around 1060 miles an hour. Well not quite - her car's going 60 but the planet at the equator is spinning at 1000mph, so for a few moments she's probably the fastest person on the planet.

The next stop after the heat of South America is the Bay of Fundy in Canada. Fundy has the highest tidal range in the world and Kate and the team are going to witness it first hand.

From there it's down to Bermuda to go snorkelling to learn about our planets spin , nice work if you can get it.

One of the things I find fascinating about our planet is that it carries a record of its own history written in its rocks. Some of this history is obvious - you can't miss the impact of giant craters blasted out by asteroid strikes. But some are less obvious - unless you know where to look. Kate's studying corals as these tiny creatures hold the secret to our distant past - and how fast our planet once spun.

Every day corals lays down growth rings of limestone and these daily growth rings build up to create an annual growth ring [a bit like a tree ring]. If you count the daily rings you get 365 days a year, which is what you'd expect.

Image credit Owen Sherwood

But if you look at 400 million year old corals you get a very different picture. They have rings just like the modern coral but they are a little bit narrower. But what's really surprising is that if you count the daily growth rings you don't find 365 you find 410. That means that when this coral was alive 400 million years ago the world was a very different place.

But however you measure it - in hours or days the Earths' orbit around the Sun always takes the same amount of time. A year is always constant. The only explanation for the ancient corals having 410 daily growth rings is that millions of years ago the days were shorter. So when this coral was in the oceans there were less hours in each day - in fact a day lasted just 21 hours. And for that to happen the Earth must have been spinning faster.

If we calculate back even further in time we find that around 4 billion years ago, when the Earth was still young, a day lasted just 6 hours. Which means the planet was spinning 4 times faster than today.

That's pretty wild - and we know that because of corals. Wonderful.

Day 259: Equinox photo challenge

Fri, 16 Sep 2011 12:30:00 +0000

Astronaut photograph ISS015-E-10469, courtesy NASA/JSC

Distance travelled ~ 665'122'400 km

Autumn Equinox photo challenge:
On the journey around the Sun we are approaching another key moment in our celestial dance with our star. Next Friday September 23rd is the Autumn Equinox. Equinox means equal night, and that day Earth is in balance. There are approximately 12 hours of day and 12 hours of night. In a sense the planet is in neutral, but from that moment onward the northern hemisphere marches towards Autumn while the southern hemisphere approaches Spring. In the north days will be getting shorter and the Sun won't rise as high in the sky and it will gradually get colder.

To mark this moment we plan on featuring the best photos on our blog next week friday that reflect the Equinox. What we're looking for are shots that show the Sun and it's relationship to our planet and the journey into Autumn, or Spring if you live down-under. Let your imagination run wild. The more creative the better! Get those shutters snapping and see what you can capture.

To ensure they are considered for this special feature email them to 23degrees@bbc.co.uk, or add them to our photography pool or hashtag your photos with #bbc23degrees on twitter.

23 Degrees a global phenomenon

Thu, 25 Aug 2011 09:30:00 +0000

Distance travelled ~ 608'199'200 km

As it happens during the production of the series 23 Degrees there has been much discussion and many questions about whether this show reflects both the northern and southern hemispheres. For example, a comment left by a reader on one of my previous posts raised a fair point about the perspective we were taking:

The truth is, this is a planet-wise show. We are looking at how our cosmic dance with the Sun affects the whole globe. And to do that we are not looking at stories on a hemisphere by hemisphere basis, or for that matter continent by continent or even country by country. We are going to locations that provide us with the best opportunity to film events that illustrate our relationship with the Sun and how that affects our weather all over the planet. But if that means shooting both tornadoes and hurricanes in the US because that's where they hit, then that's where we will film.

We are going to have stories from all round the Earth, both at land and at sea, but we have to be pragmatic as well and choose locations that we can get to within our budget.

So for instance on a show like this we can't go Antarctica, it's just too expensive and difficult to get to. That doesn't mean we will ignore such places, we have access to footage taken by scientists and explorers, which we can use to tell our story.

Another interesting thing making this series that I hadn't really taken on board before I started, was just how unequal the distribution of land is between the hemispheres. The northern hemisphere has much more land compared to the southern hemisphere. In fact over 65% of the land is in the northern hemisphere.

That means that more of the observable and so filmable weather events take place in the northern hemisphere so we will inevitably do more filming there than in the south. That doesn't mean we don't go south of the equator, we've already crossed the Andes, filmed in the Atacama and sailed out onto the southern ocean off Tierra Del Fuego on the southern tip of South America.

Behind the scenes - hurricane Irene and the waiting game...

Tue, 23 Aug 2011 17:00:00 +0000

Distance travelled ~ 603'857'600 km

It's late August and the 23 degrees team are gearing up to head back on the road capturing some of the planets' most exciting weather phenomena. This time Helen Czerski and the team are planning to go south east USA to film one of the largest and most destructive weather events of them all. A hurricane.

With the help of scientists and meteorologists the team have been tracking the course of Hurricane Irene as it wreaks havoc across the Caribbean. Yesterday it hit Puerto Rico and next in line is the Dominican Republic and then the Bahamas and the Turks and Caicios Islands.

We are not heading to the Caribbean though, instead we are on standby to go to either Florida or New Orleans should Irene make landfall in the USA. The team and crew have their bags packed, flights on hold and if the experts say the storm is going to hit the US they will take the next flight out.

It's now a waiting game to see where the storm goes, it could turn west and hit Louisiana or Texas or it could veer east out into the Atlantic and miss the US altogether, Filming these destructive weather events raises a real moral dilemma for me as Series Producer. I want to film one of the monster storms for the show to explain how they work, and to do that I need it to make landfall. But I don't want it to hit anywhere where lives and livelihoods could be put at risk. It's something I struggle with, and whenever I am involved in filming this kind of event where local people could be in danger, I sit at home watching the news and hoping that no-one has been hurt or killed.

Which brings up another issue when filming wild weather; the safety of the team. We want the footage to be exciting and to get as close as possible to the storm but we also must be safe. So we spend a lot of time planning how to achieve what we want safely. To do that we talk to experts in the field and watch footage of previous events so we know what we are getting into. We also travel into these storms guided by local teams of scientists who do this for a living. On top of that we have safety briefings with safety experts at base before travel, going through all possible scenarios and making plans to make sure we avoid dangerous situations but also how we will react if something unexpected should happen.

Talking of the unexpected we have to plan for that as well. That may sound stupid planning for something you don't expect, but when we are entering any potentially dangerous situation we have to plan for what we expect and what we don't. And once we are in the storm we have to act as safely as possible, that means wearing the right gear, following the instructions of the experts and staying alert. Hurricanes can be hairy places, powerful winds gusting at over 100 km an hour and storm surges bringing giants tides of sea water rushing inland. No footage is worth anyone getting hurt.

All that preparation should keep the team safe but that doesn't stop one worrying back at base.

All being well the team will head out to the US later this week, and hopefully they will be sending back some on location blogs from inside the heart of the storm.

Skies over Canada turned green by Aurora's last night 25/07/2011

Tue, 26 Jul 2011 17:30:00 +0000

A high-speed solar wind stream has been buffeting the Earth's magnetic field for a couple of days. Last night it turned the skies over Canada green. Below are photos sent in by Bob Johnson, Saskatoon, Saskatchewan.

There's chance of more Auroras tonight over North America and possibly north Europe as the solar wind continues to be more powerful than normal. Though the best place to see them will be in the southern hemisphere. If you have no luck, there should be another solar wind stream hitting Earth on July 30th as another coronal hole has opened up on the Sun today, so there may be auroras.

Day 200: very wet in UK but it seems a dry Indian monsoon for the team on location...

Tue, 19 Jul 2011 17:00:00 +0000

Distance travelled ~ 513'809'600 km

Today the 23 Degrees team are in India to explore the greatest weather event on Earth - the Monsoon.

They start their journey in Udaipur in the northwestern state of Rajasthan. Sometimes known as the desert state it has been baking in the Sun for months and so the rains should come as a blessed salvation from the heat and dust of summer - although there has been no reports of rain today from the team out on location.

Rajasthan is an agrarian state where over two thirds of the population are reliant on agriculture. But Rajasthan only shares around 1% of the surface water in India so it is very dependent on the annual rains.

In Udaipur Kate Humble is visiting a stunning cliff top palace called the Sajjan Garh. Built at the end of the 19th century by the 72nd Maharana of Udaipur the palace was built not to view the city below but the arrival of the monsoon rains. Sajjan Garh, which means Monsoon Palace, gave the rulers of Udaipur a grandstand view of the most dramatic and most important weather event on the planet.

We in the UK often take rain for granted, and frankly moan about it, particularly with this wet July, but for the people of Rajasthan the rains are quite literally a matter of life and death. I know that's an overused term but the Indian monsoon brings water to nearly 2 billion people, and is critical for the farmers like the maize growers of Rajasthan. Should the rains not arrive their crops will fail and they will face seriously desperate times.

Kate is also visiting the famous Lake Pichola. For those who remember the James Bond film Octopussy many scenes were shot there and in the beautiful Lake Palace in the centre of the lake. It's actually a man-made reservoir, created over 600 years ago to store the monsoon rain so the local community have access to water long after the rains have gone.

While Kate Humble stays up north in Rajasthan, Helen Czerski is thousands of kilometres south on a beach in Kerala. It's no holiday; she's there to discover how the different responses of land and water to solar energy power the monsoon, generating incredibly intense rainfall. I suspect she's in for at least one soaking from the rain.

This week's space weather roundup and Saturn's northern storm

Tue, 12 Jul 2011 16:00:00 +0000

Distance travelled ~ 496'550'400 km

Day 193 on our circumnavigation of the Sun. Down on planet Earth it's been a rather boring start to July weather-wise, overcast, a bit humid a bit stormy. But in space it's been anything but dull.

July 5th NASA recorded the fiery death of an unnamed icy comet, which plunged into the Sun.

On July 6th NASA published some photos of a massive electrical storm on the planet Saturn that's been raging for months. The so-called Great White Spot is so massive it would cover half of the Earth and so violent those scientists have counted over ten lightening flashes a second. Such giant storms are relatively rare and only six have been witnessed in the last 135 years.

Image © NASA/JPL-Caltech/SSI

Then on July 7th a Coronal Hole opened up in the surface of the Sun. So what's a coronal hole. Well as we orbit the Sun we're bathed in essential warmth and energy, but we are also blasted by a blizzard of radioactive particles called the solar wind. This barrage is 24/7 but sometimes a hole opens up in the suns surface, a coronal hole, triggering the release of a really intense stream of solar wind. Normal solar wind travels at around 1,440,000 kilometres per hour but the wind blasting out of a coronal hole shoots out at up to 4,000,000kmph. These supercharged solar wind particles finally reached us two days later. Most were diverted by our magnetic shield but some hit the atmosphere and triggered auroras far south across the USA.

Today NASA released footage of some increased activity on the eastern region of the Sun. The Sun was hurling up huge amounts of material high above the stellar surface. You can see it yourself at spaceweather.com
As it happens this activity is not directed towards Earth, but NASA has detected a very active sunspot with the "oh so catchy" name 1247.

Image NASA/SDO

But don't let the unexciting name fool you, because it's anything but boring. It has a magnetic field with enough stored energy to create a M-Class solar flare, that's a pretty big explosion from the sun surface. Solar flares are classified according to their x-ray brightness. X-class flares are huge and can cause giant radiation storms and nation-wide radio blackouts. M-class flares are as the "M" suggests medium-sized and can trigger/cause small radio blackouts while C-class flares are small and have no effects on Earth.

NASA predicts there's a 10% chance of an eruption from the sunspot in the next 24 hours, and that one is directed towards Earth so watch this space.

Who said space is empty?

23 Degrees team Colorado update: thermals and paragliding

Fri, 08 Jul 2011 08:30:00 +0000

Distance travelled ~ 486'259'200 km: day 189

Back on day 161 Helen and the 23 Degrees team ventured to Colorado to study and film thermals, another complexity of our atmosphere.

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When sunlight hits the surface of the Earth it’s absorbed by the ground, and then radiated back up into the atmosphere as heat. This heat warms the air above it, which expands and becomes less dense than the surrounding cooler air. The warm less dense air rises in what’s called a thermal column. And it’s these thermal columns that support the Paraglider carrying it up into the sky.

Thermals are a bit of a free lift, and many birds have evolved to use them to travel huge distances without expending energy by having to flap their wings. Thermals tend to be stronger in the afternoon when the earth has absorbed enough solar energy to heat the air above it.

Thermals reveal on a small scale what’s happening to our climate across the Earth because they create a heat gradient between hot and cold air. While warm air is rising, cooler, denser, air flows in to replace this rising column of warm air, until it is also warms, becomes less dense and starts to rise. This action causes the atmosphere to be in a continual state of flux, with warm and cool air moving up and down and horizontally. This constant movement of air transfers energy within the atmosphere and it is this process that drives weather.

Heat gradients form the basis of all weather on planet Earth because nature hates gradients. It likes balance so if one part of the planet is getting more solar energy and heating up the atmosphere, creating a heat gradient cooler air will rush in to cancel it out. And this movement or air getting rid of heat differences in the atmosphere that generates huge weather patterns powering events like thunderstorms, Tornadoes and even snowstorms

Aphelion: shouldn't earth be at its coldest today?

Mon, 04 Jul 2011 15:30:00 +0000

Distance travelled ~ 475'968'000 km: day 185

Today July 4 is a rather special day in our annual journey around the Sun. At 15.00 GMT Earth will be at the furthest point away from the Sun it will reach all year. I know it sounds bizarre considering how warm it is today but it's true, and it's all down to our orbit.

The Earth's orbit around the Sun is not a perfect circle. It's an ellipse and the Sun does not sit at the centre, it's offset to one side. So today we are 5 million kilometres further away from the Sun than when we were closest to the Sun six months ago in January during Perihelion. And if you remember it was pretty cold in January so you might ask why isn't our proximity to the Sun in winter warming us and our increased distance away from the Sun in summer cooling us down?

The truth is our elliptical orbit and our distance from the Sun is not the primary driving force behind our climate. So what is?

Well number one is the that fact that the Earth is tilted at 23.4 degrees from vertical. During Perihelion the northern hemisphere is tilted away from the Sun, so received less solar radiation and we get winter. In our summer we are tilted towards the Sun so despite being farther away on our orbit we get increased solar radiation. Which is why today in July it's lovely and warm.

But what about the southern hemisphere? Well it's a bit more complex down there. Earth is closest to the Sun during their summer when they are tilted towards the Sun and this means they get 7% more solar radiation. Therefore you'd expect the southern hemisphere summer to be a lot warmer than the northern summer. But it's not, in fact it's actually colder.

And now at Aphelion it's winter down south, and the hemisphere is both furthest away and tilted away from the Sun. So you'd think it's winter would be a lot colder, what with the tilt pushing the hemisphere away from the Sun. Well in fact it's warmer than our winters. And that's all because of the ratio of land to oceans in the hemispheres. The northern hemisphere has a lot of land but the southern hemisphere is predominantly water with very little land and it's this that is powering the climate.

While land reacts very fast to solar heating, it warms up and cools down very quickly. The oceans react very slowly to solar energy. They take a long time to warm up and a long time to cool down. This means that at perihelion in their summer the oceans haven't absorbed enough energy to warm up the atmosphere, they are still cool from the previous winter, so they keep the temperatures cool. In the southern hemisphere winter it's the opposite. The oceans have held onto some of the heat they absorbed during the summer and are keeping the air above warm.

So even though our elliptical orbit takes us at this point of the year further away from the Sun than it will for the rest of the year, it's warm in the north because we are tilting towards the Sun and a milder winter in the south because of the actions of the oceans.

The half way point: a look back over 468'000'000 km of our planet's annual journey

Fri, 01 Jul 2011 14:45:00 +0000

Distance travelled ~ 468'249'600 km: day 182 in Earth's orbit

We've come a long way since filming the wintery and snowy mountains of Aonach Mor in Scotland on January 3rd 2011.

We are at the halfway point of our 940 million kilometre [584 million mile] annual circumnavigation of the Sun, hurtling through space at 107,200 kilometres [66,700 miles] an hour. On this odyssey we are discovering how gigantic forces like the tilt of the Earth and its spin determine our climate and influence the life cycle of every living thing on the planet. We will discover how our relationship with the sun creates the seasons, powers the most spectacular weather on the planet, and even dictates how we live our lives.

So far we have been ice road trucking in Yellowknife, the coldest place in North America, where winter temperatures routinely plunge below minus 35 degrees. We've witnessed the Sun lighting up the serpent on the side of the Temple of Kukulkan , at Chichen Itza in the Yucatan Peninsula of Mexico and the dramatic break of the ice on the waterfall on the Hay River. We have driven into the heart of a snowstorm in Upper New York State and witnessed the awesome destructive power of a Tornado on the Great Plains on Nebraska. We travelled on dog sleds to the very edge of the sea ice in Greenland and sailed deep into the southern ocean off Tierra Del Fuego on the vey southern most tip of South America.

But we're only half round our orbit. Now the northern hemisphere is tilting towards the Sun and it's mid summer. And all that warmth is about to trigger some incredible weather phenomena. Asia is about to experience its annual monsoon and Hurricane season has begun in the Atlantic - so the US can expect some of these monster storms to hit it's eastern seaboard. And the 23 Degree team will be there - witnessing the incredible power of nature unleashed and discovering how our cosmic dance with the Sun drives these stunning weather phenomena.

So stick with us for the second half of our journey - it's going to be a bumpy ride.

Will monsoons once again return to the Sahara?

Fri, 24 Jun 2011 12:50:00 +0000

Distance travelled ~ 450'240'000 km: day 175

Image and notes courtesy of NASA

Image and notes courtesy of NASA

(On August 25, 2000, the Moderate-resolution Imaging Spectroradiometer (MODIS) acquired this spectacular image of a region in Africa's Sahara Desert, including the southern part of the border between Algeria and Libya. Three large rock massifs appear to be pushing up from beneath red sand dunes: from left to right are the Tassili, Tadrart-Acacus, and Amsak massifs. Different rock types account for rock colors varying from dark brown (Acacus) to the pale tone of Amsak eastern portion (Amsak Mellet means Pale Amsak in the local Tuareg dialect) The dendritic structures of ancient riverbeds are clearly visible in the Acacus-Amsak region.)

How did the Sahara once have a monsoon? Well it's all down to the amount of sunlight hitting the region. But not quite in the way you might think. It seems obvious that a hotter Sahara would have less rain, because it's the opposite. Monsoons are not created by less solar energy, they are created by more. To get a Monsoon you need lots of solar energy that heats up the land creating a region of low pressure to suck cool moist air from the oceans towards it. Strange as it may seem 8000 years ago when the cave paintings in Wadi Sora were made the Sahara was getting more sunlight than it is now. And that extra heat helped bring the monsoon rains to this desert. But how did the Sahara get more solar energy?

It's all down to our planets orientation to the Sun and how that changes over time. This orientation, which dictates the amount of sunlight we receive, is controlled by three main factors, Tilt, Precession and Orbit. 8000 years ago when the Sahara was green, these factors were different to what they are today. The science of this is very complex but here's a simple summary of what was going on.

Currently the earth tilts at an angle of 23.4 degrees. But over a 41,000 year period it changes, wobbling between 22.1 and 24.5 degrees. Back when the Sahara was green, the tilt was close to its largest possible angle, 24.2 degrees. Which meant that 8000 years ago the Sun shone more directly, more intensely over the Northern hemisphere.

Precession is even more important. This is not a change in the degree of the tilt, but a sort of lateral wobble, which changes the direction of the tilt. The best way of explaining it is by looking at the stars. Some of you may know about the North Star or Pole Star called Polaris. While other stars move across the sky, Polaris stays fixed just above celestial north. That's certainly true now but when the people painted themselves in the cave of swimmers, Polaris wasn't close to north in fact it was over to the east. Then 8000 years ago Thubon was the North Star. And in 12,000 years, a new star, Vega, will be pointing out due north. The North Star changes because precession makes the earth wobble a bit like a spinning top slowing down and starting to wobble back and forth. This precessional wobble takes 23000 years to complete one cycle, so it will be 23000 years before Polaris will come back round to be our northern star again.

There is one final factor involved. Our annual orbit around the Sun is not a perfect circle - it's an ellipse. Also the Sun does not sit at the centre, it's offset to one side. So there are times when it is closer to the Sun than others. Just like Tilt and Precession the shape of the orbit also changes slowly over time becoming more or less elliptical moving the earth closer or further from the sun. When the Sahara was green, all these orbital factors were in alignment, so summers in the northern hemisphere were hotter than they are now, the Sahara received more sunlight which pulled the monsoon band to northward.

The changes in our orientation to the Sun change all the time and gradually the orbit, tilt and precession changed so that the amount of solar energy hitting the Sahara eventually decreased and with it the monsoons.
Around 6000 years ago, the monsoons failed completely, the rains stopped, the rivers dried up and the land began to turn to desert. But even as you read this the same orbital factors are slowly changing and at some point in the future they will align again and the monsoons should once again return to the Sahara.

But that's not the end of the story because scientists have discovered that the wet period 8000 years ago wasn't a one off, in fact it had happened many times before. Perhaps the most significant greening of the Sahara occurred 120,000 years ago at a particularly important moment in human history.

120,000 years ago homo sapiens, modern humans, emerged from Africa. For thousands of years the Sahara had been an impassable barrier, a bit like it is now. But 120000 years ago the evidence suggests the Sahara was green and criss-crossed with rivers and lakes, and scientists believe that it allowed passage for our ancestors north. They crossed the Sahara travelling along rivers and settled in North Africa, and eventually, Europe and Asia.

It really is incredible to think that such critical moment in our history could have been triggered by changes in our tilt and orbit.

A green savannah from an arid desert: 23 Degrees team heads to the Sahara

Thu, 23 Jun 2011 15:45:00 +0000

Distance travelled ~ 447'667'200 km: day 174

Today Kate Humble and the 23 Degrees team are in Egypt heading deep into the Sahara desert to one of the most fascinating most magical places on the planet. It's a place that holds clues to the extraordinary history of this part of the world.

The team are travelling from Luxor into the Sahara close to the borders of Libya and Sudan. It's a two-day trek across barren desert with temperatures hitting the high thirties and the low forties on a regular basis. The expedition has half a dozen jeeps, a military escort of Egyptian soldiers and enough provisions and water to last a week.

From Luxor their first stop is the Dahkla oasis, the last bit of civilisation for many hundreds of kilometres. From here on it's camping under the stars.

From Dahkla the team travels across endless white dunes until they reach a region of red sand, hills and ravines leading to a vast plateau rising up 300 metres. It's called the Gilf Kebir or Great Barrier. If you've read the book or seen the movie The English Patient, that might recognise the place as it features heavily in the story. In fact back in the 1930s the Gilf was explored by Hungarian adventurer Count Lazlo Almasy the inspiration for the main character in the book and film. He was looking for the legendary lost city of Zerzura. He didn't find Zerzura, but he did stumble across something that changed our understanding of the history of the Sahara. And that's what Kate and the team are heading towards.

Hidden in a gorge called Wadi Sora is one of humanities great treasures, the legendary and magical Cave of Swimmers. It is full of amazing drawings that are at least 8000 years old. There are over 300 figures; some are of hunters carrying bows. There are giraffes and deer even hippos.

There are dozens of handprints and footprints, created by blowing of coloured powder over the hands and feet. And within the many images is an extraordinary image of a child's hand inside an adult's hand, perhaps their father or mother. It's an incredible, romantic message reaching out to us from many millennia ago.

These paintings show that people once lived here, perhaps nomadic families who hunted the animals they depicted in the cave. But another image tells us something more about this region 8000 years ago. There is one painting that shows people swimming, giving this cave its name.

This particular image shows that once this region had water, lakes and rivers. So how does a desert become a lush savannah and a green savannah turn back to arid desert? To find out Kate and the team travel to another gorge about a days drive away called Wadi Bahkt. Cut into the surface is an 8-metre crack showing layer on layer of sediment. These sediment layers are created by seasonal rains. It's clear evidence that at one point this part of the Sahara had annual rains called Monsoons large enough to create rivers and lakes, with one believed to be the size of Belgium.

Like Asia, North Africa still has a summer monsoon. But now it's weak and only brings rain many thousands of kilometres to the west and south of here. But 8000 years ago, monsoon rains fell over the whole of the Sahara including the Gilf Kebir. So what happened?