What causes ice ages – and when is the next one?

What remains of the ice age: Antarctica. Image: Getty.

Over the last 2.5m years the Earth has undergone more than 50 major ice ages, each having a profound effect on our planet’s climate. But what causes them and how do we predict when the next big ice age will hit?

About 40 years ago, scientists realised that ice ages were driven by changes in the Earth’s orbit. But, as I recently argued in Nature, it’s not that simple. Scientists are still trying to understand how such wobbles interact with the climate system, particularly greenhouse gases, to push the planet in to or out of an ice age.

During the last ice age, only 21,000 years ago, there was nearly continuous ice across North America from the Pacific to the Atlantic Ocean. At its deepest over the Hudson Bay, it was over two miles thick and reached as far south as what would now be New York and Cincinnati. In Europe, there were two major ice sheets: the British ice sheet, which reached as far south as what would now be Norfolk, and the Scandinavian ice sheet that extended all the way from Norway to the Ural mountains in Russia.

In the Southern Hemisphere there were significant ice sheets on Patagonia, South Africa, southern Australia and New Zealand. So much water was locked up in these ice sheets that the global sea level dropped by over 125 metres – around ten metres lower than the height of the London Eye. In comparison if all the ice on Antarctica and Greenland melted today it would only raise sea level by 70 metres.

So what caused these great ice ages? In 1941, Milutin Milankovitch suggested that wobbles in the Earth’s orbit changed the distribution of solar energy on the planet’s surface, driving the ice age cycles. He believed that the amount of incoming solar radiation (insolation) just south of the Arctic Circle, at a latitude of 65°N, was essential. Here, insolation can vary by as much as 25 per cent. When there was less insolation during the summer months, the average temperature would be slightly lower and some of the ice in this region could survive and build up – eventually producing an ice sheet.

But it wasn’t until 30 years later that three scientists used long-term climate records from analysing marine sediments to put this to the test. Jim Hays used fossil assemblages to estimate past sea surface temperatures. Nick Shackleton calculated changes in past global ice volume by measuring oxygen isotopes (atoms with different numbers of neutrons in the nuclues) in calcium carbon fossil in marine sediments. John Imbrie used time-series analysis to statistically compare the timing and cycles in the sea surface temperature and global ice volume records with patterns of the Earth’s orbit.

In December 1976 they published a landmark climate paper in Science, showing that climate records contained the same cycles as the three parameters that vary the Earth’s orbit: eccentricity, obliquity and precession (shown in Figure 1). Eccentricity describes the shape of the Earth’s orbit around the sun, varying from nearly a circle to an ellipse with a period of about 96,000 years. Obliquity is the tilt of the Earth’s axis of rotation with respect to the plane of its orbit, which changes with a period of about 41,000 years. Precession refers to the fact that both Earth’s rotational axis and orbital path precess (rotate) over time – the combined effects of these two components and the eccentricity produce an approximately 21,000-year cycle.

Image: author provided.

The researchers also found that these parameters have different effects at different places on our globe. Obliquity has a strong influence at high latitudes, whereas precession has a notable impact on tropical seasons. For example precession has been linked to the rise and fall of the African rift valley lakes and so may have even influenced the evolution of our ancestors. Evidence for such “orbital forcing” of climate has now been found as far back as 1.4bn years ago.

Beyond wobbles

However, the scientists realised that there were limitations and challenges of their research – many of which remain today. In particular, they recognised that variations in the Earth’s orbit did not cause the ice age cycles per se – they rather paced them. A certain orbit of the Earth can be associated with many different climates. The one we have today is in fact similar to the one we had during the most intense part of the last ice age.

Small changes in insolation driven by changes in the Earth’s orbit can push the planet into or out of an ice age through the planet’s “climate feedback” mechanisms. For example when summer solar radiation in reduced it allows some ice to remain after the winter. This white ice reflects more sunlight, which cools the area further and allows more ice to build up, which reflects even more sunlight and so forth. Therefore, the researchers’ next step was to understand the relative importance of ice sheet, ocean and atmospheric feedbacks. They discovered that greenhouse gases had an important role in controlling climate. In particular atmospheric carbon dioxide had to be low enough for the planet to start cooling before it could tip into an ice age.

So how can all this help us understand future climate? One idea is that small increases in greenhouse gases due to the expansion of agriculture that started 8,000 years ago have in fact delayed the next ice age. What’s more, if we continue emitting greenhouse gases at the same rate, we might have put off the next ice age for at least 500,000 years.

If we have merely delayed the next ice age, we will still be in the Quaternary Period – the last 2.58m years defined by the ice age cycles. But if we have stopped the ice ages, humans will have caused a much greater change and so have entered the Anthropocene period as some argue. If I had to put money on it, I’d say the Earth has experienced its last ice age for a very, very long time.The Conversation

Mark Maslin is professor of palaeoclimatology at UCL.

This article was originally published on The Conversation. Read the original article.


“Black cabs are not public transport”: on the most baffling press release we’ve seen in some time

An earlier black cab protest: this one was against congestion and pollution. I'm not making this up. Image: Getty.

You know, I sometimes think that trade unions get a raw deal in this country. Reports of industrial action almost always frame it as a matter of workers’ selfishness and public disruption, rather than one of defending vital labour rights; and when London’s tube grinds to a halt, few people will find out what the dispute is actually about before declaring that the drivers should all be replaced by robots at the earliest possible opportunity or, possibly, shot.

We should be a bit more sympathetic towards trade unions, is what I’m saying here: a bit more understanding about the role they played in improving working life for all of us, and the fact that defending their members’ interests is literally their job.

Anyway, all that said, the RMT seems to have gone completely fucking doolally.

TAXI UNION RMT says that the closure of the pivotal Bank Junction to all vehicles (other than buses and bicycles) exposes Transport for London’s (TfL) symptom-focused decision-making and unwillingness to tackle the cause of the problem.

So begins a press release the union put out on Thursday. It’s referring to a plan to place new restrictions on who can pass one of the City of London’s dirtiest and most dangerous junctions, by banning private vehicles from using it.

The junction in question: busy day. Image: Google.

If at first glance the RMT’s words seem reasonable enough, then consider two pieces of information not included in that paragraph:

1) It’s not a TfL scheme, but a City of London Corporation one (essentially, the local council); and

2) The reason for the press release is that, at 5pm on Thursday, hundreds of black cab drivers descended on Bank Junction to create gridlock, in their time-honoured way of whining about something. Blocking major roads for several hours at a time has always struck me as an odd way of trying to win friends and influence people, if I’m frank, but let’s get back to the press release, the next line of which drops a strong hint that something else is going on here:

TfL’s gutlessness in failing to stand-up to multi-national venture capital-backed raiders such as Uber, has left our streets flooded with minicabs.

That suggests that this is another barrage in the black cabs’ ongoing war against competition from Uber. This conflict is odd in its way – it’s not as if there weren’t minicabs offering a low cost alternative to the classic London taxi before Uber came along, but we’ve not had a lengthy PR war against, say, Gants Hill Cars – but it’s at least familiar territory, so it’d be easy, at this point, to assume we know where we are.

Except then it gets really weird.

With buses stuck in gridlock behind haphazardly driven Uber cars – and with the Tube dangerously overcrowded during peak hours – people are turning out of desperation to commuting by bicycle.

Despite its impracticality, there has been an explosion in the number of people commuting by bike. Astonishingly, 30% of road traffic traversing Bank Junction are now cyclists.

Soooo... the only reason anyone might want to cycle is because public transport is now bad because of Uber? Not because it’s fun or healthy or just nicer than being stuck in a metal box for 45 minutes – because of badly driven Ubers something something?

Other things the cabbies will blame Uber for in upcoming press releases: climate change, Brexit, the outbreak of the Franco-Prussian war in July 1870, the fact they couldn’t get tickets for Hamilton.

It is time that TfL refused to licence Uber, which it acknowledges is unlawfully “plying for hire”.

Okay, maybe, we can talk about that.

It is time that black cabs were recognised and supported as a mode of public transport.


It is time that cuts to the Tube were reversed.

I mean, sure, we can talk about that too, but... can you go back to that last bit, please?

RMT General Secretary, Mick Cash, said:

“RMT agrees with proposals which improve public safety, but it is clear that the driving factor behind the decision is to improve bus journey times under a buckling road network.

“Black cabs are an integral part of the public transport system and as the data shows, one of the safest.”

This is all so very mixed up, it’s hard to know where to begin. Black cabs are not public transport – as lovely as they are, they’re simply too expensive. Even in New York City, where the cabs are much, much cheaper, it’d be silly to class them as public transport. In London, where they’re so over-priced they’re basically the preserve of the rich and those who’ve had enough to drink to mistakenly consider themselves such, it’s just nonsense.

Also – if this decision has been taken for the sake of improving bus journey times, then what’s wrong with that? I haven’t run the numbers, but I’d be amazed if that wasn’t a bigger gain to the city than “improving life for the people who take cabs”. Because – as I may have mentioned – black cabs are not public transport.

Anyway, to sum the RMT’s position up: we should invest in the tube but not the buses, expensive black cabs are public transport but cheaper Ubers are the work of the devil, and the only reason anyone would ever go by bike is because they’ve been left with no choice by all those people in the wrong sort of taxi screwing everything up. Oh, and causing gridlock at peak time is a good way to win friends.

Everyone got that straight?

None of this is to say Uber is perfect – there are many things about it that are terrible, including both the way people have mistaken it for a revolutionary new form of capitalism (as opposed to, say, a minicab firm with an app), and its attitude to workers (ironically, what they could really do with is a union). The way TfL is acting towards the firm is no doubt imperfect too.

But the RMT’s attitude in this press release is just baffling. Of course it has to defends its members interests – taxi drivers just as much as tube drivers. And of course it has to be seen to be doing so, so as to attract new members.

But should it really be trying to do both in the same press release? Because the result is a statement which demands TfL do more for cab drivers, slams it for doing anything for bus users, and casually insults anyone on two wheels in the process.

A union’s job is to look after its members. I’m not sure nonsense like this will achieve anything of the sort.

Jonn Elledge is the editor of CityMetric. He is on Twitter, far too much, as @jonnelledge.

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