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.

 
 
 
 

Green roofs improve cities – so why don’t all buildings have them?

The green roof at the Kennedy Centre, Washington DC. Image: Getty.

Rooftops covered with grass, vegetable gardens and lush foliage are now a common sight in many cities around the world. More and more private companies and city authorities are investing in green roofs, drawn to their wide-ranging benefits which include savings on energy costs, mitigating the risk from floods, creating habitats for urban wildlife, tackling air pollution and urban heat and even producing food.

A recent report in the UK suggested that the green roof market there is expanding at a rate of 17 per cent each year. The world’s largest rooftop farm will open in Paris in 2020, superseding similar schemes in New York City and Chicago. Stuttgart, in Germany, is thought of as “the green roof capital of Europe”, while Singapore is even installing green roofs on buses.

These increasingly radical urban designs can help cities adapt to the monumental challenges they face, such as access to resources and a lack of green space due to development. But buy-in from city authorities, businesses and other institutions is crucial to ensuring their success – as is research investigating different options to suit the variety of rooftop spaces found in cities.

A growing trend

The UK is relatively new to developing green roofs, and governments and institutions are playing a major role in spreading the practice. London is home to much of the UK’s green roof market, mainly due to forward-thinking policies such as the 2008 London Plan, which paved the way to more than double the area of green roofs in the capital.

Although London has led the way, there are now “living labs” at the Universities of Sheffield and Salford which are helping to establish the precedent elsewhere. The IGNITION project – led by the Greater Manchester Combined Authority – involves the development of a living lab at the University of Salford, with the aim of uncovering ways to convince developers and investors to adopt green roofs.

Ongoing research is showcasing how green roofs can integrate with living walls and sustainable drainage systems on the ground, such as street trees, to better manage water and make the built environment more sustainable.

Research is also demonstrating the social value of green roofs. Doctors are increasingly prescribing time spent gardening outdoors for patients dealiong with anxiety and depression. And research has found that access to even the most basic green spaces can provide a better quality of life for dementia sufferers and help prevent obesity.

An edible roof at Fenway Park, stadium of the Boston Red Sox. Image: Michael Hardman/author provided.

In North America, green roofs have become mainstream, with a wide array of expansive, accessible and food-producing roofs installed in buildings. Again, city leaders and authorities have helped push the movement forward – only recently, San Francisco created a policy requiring new buildings to have green roofs. Toronto has policies dating from the 1990s, encouraging the development of urban farms on rooftops.

These countries also benefit from having newer buildings, which make it easier to install green roofs. Being able to store and distribute water right across the rooftop is crucial to maintaining the plants on any green roof – especially on “edible roofs” which farm fruit and vegetables. And it’s much easier to create this capacity in newer buildings, which can typically hold greater weight, than retro-fit old ones. Having a stronger roof also makes it easier to grow a greater variety of plants, since the soil can be deeper.


The new normal?

For green roofs to become the norm for new developments, there needs to be buy-in from public authorities and private actors. Those responsible for maintaining buildings may have to acquire new skills, such as landscaping, and in some cases volunteers may be needed to help out. Other considerations include installing drainage paths, meeting health and safety requirements and perhaps allowing access for the public, as well as planning restrictions and disruption from regular ativities in and around the buildings during installation.

To convince investors and developers that installing green roofs is worthwhile, economic arguments are still the most important. The term “natural capital” has been developed to explain the economic value of nature; for example, measuring the money saved by installing natural solutions to protect against flood damage, adapt to climate change or help people lead healthier and happier lives.

As the expertise about green roofs grows, official standards have been developed to ensure that they are designed, built and maintained properly, and function well. Improvements in the science and technology underpinning green roof development have also led to new variations on the concept.

For example, “blue roofs” increase the capacity of buildings to hold water over longer periods of time, rather than drain away quickly – crucial in times of heavier rainfall. There are also combinations of green roofs with solar panels, and “brown roofs” which are wilder in nature and maximise biodiversity.

If the trend continues, it could create new jobs and a more vibrant and sustainable local food economy – alongside many other benefits. There are still barriers to overcome, but the evidence so far indicates that green roofs have the potential to transform cities and help them function sustainably long into the future. The success stories need to be studied and replicated elsewhere, to make green, blue, brown and food-producing roofs the norm in cities around the world.

Michael Hardman, Senior Lecturer in Urban Geography, University of Salford and Nick Davies, Research Fellow, University of Salford.

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