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.

 
 
 
 

How China's growing cities are adapting to pressures on housing and transport

Shenzhen, southern China's major financial centre. (Photo by Daniel Berehulak/Getty Images)

In the last 40 years, the world’s most populous country has urbanised at a rate unprecedented in human history. China now has over 100 cities with populations greater than a million people, easily overshadowing the combined total of such cities in North America and Europe. 

That means urban policy in China is of increasing relevance to planning professionals around the world, and for many in Western nations there’s a lot to learn about the big-picture trends happening there, especially as local and national governments grapple with the coronavirus crisis. 

Can Chinese policymakers fully incorporate the hundreds of millions of rural-to-urban migrants living semi-legally in China’s cities into the economic boom that has transformed the lives of so many of their fellow citizens? The air quality in many major cities is still extremely poor, and lung cancer and other respiratory ailments are a persistent threat to health. Relatedly, now that car ownership is normalised among the urban middle classes, where are they going to put all these newly minted private automobiles?


Yan Song is the director of the University of North Carolina, Chapel Hill’s Program on Chinese Cities and a professor in the school’s celebrated urban planning department. She’s studied Chinese, American, and European cities for almost 20 years and I spoke with her about the issues above as well as changing attitudes towards cycling and displacement caused by urban renewal. This conversation has been edited for length and clarity.

American cities face very different challenges depending on which part of the country they are in. The Rust Belt struggles with vacancy, depopulation, and loss of tax base. In coastal cities housing affordability is a huge problem. How do the challenges of Chinese cities vary by region?

Generally speaking, the cities that are richer, usually on the eastern coastal line, are facing different challenges than cities in the western "hinterland." The cities that are at a more advantaged stage, where socio-economic development is pretty good, those cities are pretty much aware of the sustainability issue. They're keen on addressing things like green cities.

But the biggest challenge they face is housing affordability. Cities like Beijing, Shanghai, Shenzhen, and Hangzhou are trying to keep or attract young talent, but the housing prices are really, really high. The second challenge is equity. How do you provide equal, or at least fair, services to both the urban residents and the migrants who are living in the city, to alleviate some of the concerns around what the government is calling “social harmony?” 

Then the cities in the hinterland, typically they are resource economies. They are shrinking cities; they're trying to keep population. At the same time, they are addressing environmental issues, because they were overly relying on the natural endowments of their resources in the past decades, and now they're facing how to make the next stage of economic transition. That's the biggest divide in terms of regional challenges.

These urban centers rely on migrant workers for a lot of essential services, food preparation, driving, cleaning. But they live tenuous lives and don't have access to a lot of public services like education, health care, social insurance. Are Chinese policymakers trying to adopt a healthier relationship with this vast workforce?

The governments are making huge efforts in providing basic services to the migrants living in the city. They're relaxing restrictions for educational enrollment for migrants in the cities. In health care as well as the social security they are reforming the system to allow the free transfer of social benefits or credits across where they live and where they work [so they can be used in their rural hometown or the cities where they live and work]. 

In terms of health care, it's tough for the urban residents as well just because of the general shortage of the public health care system. So, it's tough for the urban residents and even tougher for the migrants. But the new policy agenda's strategists are aware of those disadvantages that urban migrants are facing in the cities and they're trying to fix the problem.

What about in terms of housing?

The rental market has been relaxed a lot in recent years to allow for more affordable accommodation of rural-to-urban migrants. Welfare housing, subsidised housing, unfortunately, skews to the urban residents. It's not opened up yet for the migrants. 

The rental market wasn't that active in previous years. But recently some policies allow for more flexible rental arrangements, allowing for shared rentals, making choices more available in the rental market. Before it was adopted, it’s prohibited to have, for example, three or more people sharing an apartment unit. Now that’s been relaxed in some cities, allowing for more migrant workers to share one unit to keep the rates down for them. You see a little bit more affordable rental units available in the market now.

I just read Thomas Campanella’s The Concrete Dragon, and he talks a lot about the scale of displacement in the 1990s and 2000s. Massive urban renewal projects where over 300,000 people in Beijing lost homes to Olympics-related development. Or Shanghai and Beijing each losing more homes in the ‘90s than were lost in all of America's urban renewal projects combined. It didn't sound like those displaced people had much of a voice in the political process. But that book was published in 2008.  How has policy changed since then, especially if people are more willing to engage in activism?

First of all, I want to make a justification for urban renewal in Chinese cities, which were developed mostly in the ‘50s and ‘60s. At the time, [in the 1990s] the conditions weren’t good and allowing for better standards of construction would inevitably have to displace some of the residents in older settlements. In my personal opinion, that wasn't something that could be done in an alternative way.  

Still, in the earlier days, the way of displacing people was really arbitrary, that's true. There wasn't much feedback gathered from the public or even from the people affected. In the name of the public interest, in the name of expanding a road, or expanding an urban center, that's just directed from the top down. 

Nowadays things are changing. The State Council realized they needed more inclusive urban development, they needed to have all the stakeholders heard in the process. In terms of how to process urban development, and sometimes displacement, the way that they are dealing with it now is more delicate and more inclusive.

Can you give me an example of what that looks like?

For example, [consider] hutong in Beijing, the alleyway houses, a typical lower-density [neighbourhood] that needs to be redeveloped. In the past, a notification was sent to the neighbours: “You need to be replaced. You need to be displaced, we need to develop.” That's it. 

Nowadays, they inform all different sorts of stakeholders. They could include artists' associations, nonprofits, grassroots organisations that represent the interests of the local residents. Then they [the citizens groups] could say what they really want to preserve. “This is what we think is really valuable” and that will be part of the inputs in the planning process. Some of the key elements could possibly be preserved. They  [the authorities] also talk about the social network, because they realized that when they displace people, the biggest loss is the social network that they have built in the original location. So, it's not only conserving some of the physical environment, but also trying to conserve some of the social network that people have.  


(STR/AFP via Getty Images)

Speaking of urban renewal, there was a big emphasis in the ‘90s and 2000s on highways. A lot of auto-oriented development in Beijing, following more of a Los Angeles than New York model. There's this quote I saw from Hong Kong architect Tao Ho, during the 1990s development of Pudong in Shanghai, warning against replicating “the tall buildings and car-oriented mentality of the West." 

In the ’90s or the first decade of the 21st century, most cities in China were still making mistakes. When I was a student, in the late '90s, I was translating for the American Planning Association. At the time, Beijing was still taking out the bike lanes and the planners from APA were telling them: “No, don't do that. Don't make that mistake." 

In the past decade, that's not occurring anymore. It has been happening [adding bike lanes] for a couple of years in Beijing, Shanghai, Shenzhen. More attention has been given to improving the service quality of green transportation, upgrades to buses, the bike lane system, and so on. 

As China got richer, bikes became a symbol of poverty and, like you said, urban planners began removing bike lanes. Cities like Nanjing and Shanghai considered banning bikes from the central city entirely. 

For a long time, bike lanes were abandoned and the road surface was more devoted to the car. But in the past few years this has been changing, more road space has been given to bus rapid transit and to bike lanes. The attitude giving precedence to the private car is giving way.

Another thing they are trying to do is behavioural change, teaching younger generations that biking is cool, creating a new set of values that's more sustainable. In some major cities, you see educational campaigns, posters around the cities, [saying] bicycling is really cool. 

A recent paper you worked on looked at air quality in Chinese cities and found they are still struggling. The paper cited a study suggesting “that Chinese cities face the worst air quality across different cities around [the] world based on an extensive research of 175 countries.” Your paper recommends transit-oriented development and significant green outdoor space. Is that something you see policymakers adopting?

Yes, definitely, although with regional variations still. The eastern and southern cities are seeing more policies toward transit-oriented development. They are adapting smart technology too. For example, Hangzhou, which is the model of smart cities, the tech tycoon Alibaba installed sensors on every single traffic signal there. Then they were using technology to change the light, so when they detect a higher volume of traffic, they streamline the green lights and the red light wouldn't stop the cars, so there are less carbon emissions at the intersections. They showed that there was a reduction of up to 15% emissions. 

What about in terms of parking policy? How are policymakers trying to deal with the influx of cars in these cities? Are there parking minimums like in many American cities?

I was visiting Hangzhou in December, their “Smart City” headquarters there. They were trying to use technology to let people know where there's parking, so they don't have to drive around, which increases carbon emissions. In other cities, like Shenzhen, they were increasing the parking fee in the downtown by 50 yuan, or seven US dollars an hour. That's pretty high in the context of Chinese cities. It was 10 or 20 yuan before. So, just increasing the parking cost in the downtown area so that you discourage people from driving.

What are you working on now?

My new research is still on air quality. We had a really cool collaboration with a counterpart of Google Street Map. In China, that’s Baidu StreetMap. We asked the company to install another sensor on their cars when they take pictures. We added a sensor for air quality. So, we will know at a street level what are the current emissions by geolocation, by time. That will be really cool when we have all that data. 

Jake Blumgart is a staff writer for CityMetric.