Karl Petersen/EPA-EFE/Shutterstock
Mike Follows
Sutton Coldfield, West Midlands, UK
A climate tipping point is a critical threshold at which a small additional change leads to a large, non-linear shift in the climate system. These shifts are driven by feedbacks that amplify the initial change. While tipping points can be difficult to reverse, they are not strictly irreversible. Feedbacks are the underlying processes, whereas tipping points occur when those processes push the system into a new state.
It is important to note that the planet has cooled as well as warmed in the past. For example, around 2.5 billion years ago, Earth’s atmosphere was rich in the greenhouse gas methane, which kept the planet warm despite a fainter sun. But then photosynthetic blue-green algae appeared that released oxygen, which reacted with methane to produce carbon dioxide and water, in the Great Oxygenation Event. The replacement of methane with less potent carbon dioxide caused significant cooling, triggering the Huronian glaciation, Earth’s earliest known ice age.
With lower ice cover, the same intensity of heat coming from the sun will produce greater warming of Earth
In a cooling climate, several tipping elements can reinforce further temperature decline. One of the most significant is the ice-albedo feedback: as the percentage of Earth’s surface covered by ice and snow increases, more sunlight is reflected, leading to additional cooling. If ice spreads far enough towards the equator, the system may cross a threshold into near-global glaciation, as occurred during Snowball Earth episodes. This represents a tipping point, while the reflectivity of ice is the feedback driving it.
Changes in the carbon cycle also play a key role. Colder oceans absorb more carbon dioxide, while reduced biological activity can further lower greenhouse gas concentrations. If these fall below a critical level, widespread glaciation may be triggered.
Ocean circulation is another important factor. The thermohaline circulation – driven by differences in temperature and salinity – redistributes heat around the planet. For example, Europe is kept relatively warm by a surface current carrying heat from the Caribbean. As this water cools near Iceland, it becomes denser, sinks and flows southwards at depth. However, fresh water from melting ice, such as the Greenland ice sheet, can reduce salinity and density, potentially disrupting this circulation. If it were to slow or stop, it could lead to significant regional cooling in Europe, even in a warming world.
Long-term cooling can also be driven by astronomical and geological processes. Passage through the Milky Way’s spiral arms may increase cosmic rays, enhancing cloud formation and Earth’s reflectivity. Periods of low solar activity, such as the Maunder Minimum, have likewise been linked to cooler climates. Over much longer timescales, mountain uplift accelerates chemical weathering, removing carbon dioxide from the atmosphere and promoting cooling. The redistribution of continents courtesy of plate tectonics can redirect ocean currents that can shift the climate.
Although shocks such as volcanic eruptions are usually short-lived, they can push the climate past a tipping point if it is already near a critical threshold. In such cases, internal feedbacks sustain the new state. Snowball Earth events are the most extreme examples, likely to have been caused by a combination of orbital changes, reduced carbon dioxide and reinforcing feedbacks, sometimes triggered or amplified by volcanism.
Barrie Wells
Conwy, UK
It is expected that, following a period of warming and associated ice-shelf retreat, additional cooling would be required to return to Earth’s pre-warming state. This is because ice reflects the sun’s rays, so, with lower ice cover, the same intensity of heat from the sun will produce a greater warming of Earth. The amount of ice cover will therefore exhibit a hysteresis loop, a 2D picture describing a system whose disturbance and recovery take different paths. Ren Thom derived equations capable of modelling this behaviour in 3D.
A relatively simple form of these equations is the cusp catastrophe, which has been applied to the phenomenon of ice-sheet retreat. In this formulation, the third dimension can be interpreted as a parameter governing the disparity between ice retreat and ice replacement, or the amount of hysteresis. It is difficult to determine where that parameter will place Earth in the situation described, and hence where the tipping point for ice recovery would fall, if we are ever able to reverse warming. It could vary from the human race being screwed, to the human race being completely screwed. Either way, Thom’s choice of name for his theory is apt.
To answer this question – or ask a new one – 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.
Â鶹´«Ã½ 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.
Ìý
