If a northern hemisphere 鈥渋ce age鈥� is triggered, say due to the Gulf Stream failing, is it inevitable in the southern hemisphere too?
Peter Bursztyn
Barrie, Ontario, Canada
Advertisement
No. A northern hemisphere 鈥渋ce age鈥� won鈥檛 trigger widespread glaciation in the southern hemisphere.
Changing geography made glaciation possible across the Arctic, Eurasia and North America.
The Arctic Ocean was relatively ice-free until continental drift gradually enclosed it. Today, the narrow Bering Strait keeps the North Pacific current out of the Arctic Ocean. Instead, it warms the southern coast of Alaska, keeping it ice-free. Similarly, Baffin Island and Greenland deflect the Gulf Stream, preventing its warmth from reaching the Arctic Ocean. Instead, it keeps Norway鈥檚 west coast relatively clear of ice.
On the other hand, it is also virtually impossible for ocean ice to spread north from Antarctica. Various oceanic currents inject tropical warm water continuously into the Southern Ocean. This body of water swirls around Antarctica, limiting the growth of its ice shelves.
Researchers have devised indirect methods to detect past glaciations. These are based on measurements of the relative abundance of two oxygen isotopes: oxygen-18 and oxygen-16. Most oxygen is oxygen-16, but some is oxygen-18 (with two extra neutrons), which evaporates slightly more slowly. As global temperatures fall during a glacial period, the ratio of the isotopes in precipitation changes, creating a 鈥渃hemical thermometer鈥�. These isotopes can be detected in ice cores drilled out of ice sheets.
Greenland鈥檚 ice is only 180,000 to 200,000 years old. However, Antarctic ice dates back around 800,000 years! Earth is apparently due for another glacial period soon, but warming caused by human carbon dioxide emissions means this is unlikely to occur.
Mike Follows
Sutton Coldfield, West Midlands, UK
At the equator, the sun is higher in the sky, so sunlight is more intense there than at the poles. This makes the equator hotter. Heat moves from hotter to colder places, so convection currents in the atmosphere and the oceans move heat from the equator towards the poles. In the oceans, this movement is known as the thermohaline circulation. As its name suggests, it is driven by differences in the temperature and salinity of the water.
The section between the Gulf of Mexico and the North Atlantic is known as the Atlantic meridional overturning circulation (AMOC), and it keeps western Europe about 5掳C (9掳F) warmer than it would otherwise be. The surface waters, known as the Gulf Stream, flow northwards, while water on the ocean floor flows south. As it travels, evaporation from the surface of the Gulf Stream leaves the water colder and saltier and therefore dense enough to sink when it reaches latitudes around Iceland (before heading south along the ocean floor).
The worry is that the Greenland ice sheets will melt as a result of global warming. This fresh water will dilute the salty water so that it will no longer be dense enough to sink. This will weaken or halt the AMOC and western Europe will end up colder in a world that is warming. Indeed, it may lead to temporary global cooling.
Something like this has happened in the past. Earth has been experiencing the Late Cenozoic Ice Age for the past 34 million years. The latest phase of this is the Quaternary Period, during which Earth has switched between relatively cold (glacial) and relatively warm (interglacial) states. The glacial periods are sometimes confused for 鈥渋ce ages鈥�.
At the peak of the last glacial period, North America was partly covered by the massive Laurentide ice sheet. As Earth warmed, the meltwater created Lake Agassiz, a huge glacial lake in central North America. Some think that when this lake drained into the Arctic Ocean about 12,000 years ago, it caused the thermohaline circulation to stop abruptly. The temperature didn鈥檛 fall as quickly as portrayed in the 2004 film The Day After Tomorrow, but what is known as the Younger Dryas event took place over the lifetime of humans who were unfortunate enough to live through it. The average temperature of the UK fell to -5掳C (23掳F) during this time.
There is growing evidence that the AMOC is weakening, and a published in March has reported evidence of a similar thing happening in the Antarctic. Interestingly, work by a team of researchers led by Raphael Neukom at the University of Bern that the temperatures in the two hemispheres of Earth don鈥檛 always stay in lockstep. For example, the so-called Little Ice Age experienced in Europe between 1650 and 1750 took place centuries apart in different regions of Earth.
However, it seems inconceivable that the two hemispheres could develop totally independent climates. I am reminded of the novel On the Beach by Nevil Shute, where the radioactive fallout from a nuclear war in the northern hemisphere diffused towards survivors in Australia.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
麻豆传媒 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.