It is difficult these days to find an article about climate science without some mention of tipping points and the risk of abrupt climate change.
Some prominent climate scientists and policy proponents have warned ominously that we have only a decade left to change our ways to “avert catastrophe.” The clock is running.
However, details tend to be much more vague when it comes to the nature of these climate change thresholds we risk passing. Many potential abrupt climate change scenarios have been overplayed in the media to an extent not supported by the science. While some specific threats bear watching, the truly worrisome aspect of the climate system is its propensity to change from one relatively steady state to another dramatically different one in a short period of time. We are unlikely to repeat the exact changes that occurred in the past, but as we steadfastly emit our way toward temperatures not seen in the past million years we are running an experiment where the past can only be an imperfect guide.
We have historically tended to see climate as one of life’s great constants: one winter might be more severe than the next, but few lasting, systemic changes can be seen from the perspective of a single lifetime. Indeed, Earth’s climate has been remarkably stable for the last 10,000 years as we sit in an abnormally long interglacial period known as the Holocene.
Prior to the onset of the Holocene, however, Earth had been subject to dramatic swings in temperature during the last ice age. The ice age cycles are driven by small changes in Earth’s orbit, known as Milankovich Cycles that alter the distribution of incoming solar radiation. These relatively minor forcings are amplified by ice formation and melt (which changes the reflectivity – the albedo – of Earth’s surface); by greenhouse gas emissions and sequestration resulting from changes in oceanic CO2 absorption (driven in part by ocean temperature); and by vegetative growth and decay. These amplifications are known as feedbacks, and they are one of the primary reasons for concerns about the potential for abrupt climate changes today.
|Temperature record of the prior four ice ages. Image courtesy of Robert Rhode’s Global Warming Art.|
A look back at the paleoclimate record reveals a consistent pattern of small external forcings – Milankovich Cycles, fluctuations in solar output, periods of unusual volcanic activity, etc. – being magnified through feedbacks into much larger and longer-lived climate changes.
Gavin Schmidt, a climate scientist with NASA’s Goddard Institute for Space Studies, argues that the climate system exhibits a pattern characteristic of many non-linear systems:
The idea is that … a small push away from one state only has small effects at first but at some ‘tipping point’ the system can flip and go rapidly into another state. This is fundamentally tied to the existence of positive feedbacks and is sometimes related to the concept of multiple ‘attractors’ (i.e. at any time two different ‘states’ could be possible and near a transition the system can flip very quickly from one to another).
While ice ages tend to take a few thousand years to begin (and a bit less time to end), the paleoclimate record is replete with examples of much more abrupt climate changes.
During the Younger Dryas, a period roughly 12,000 years ago, the Earth abruptly cooled about 5 degrees C (9 degrees F) over the course of a decade or two. Some areas dropped as much as 15 degrees C (27 degrees F). While causes of the abrupt climate changes during the Younger Dryas are uncertain, the leading theory is that the rupture of a massive North American freshwater lake into the Northern Atlantic Ocean shut down the thermohaline circulation (THC) by reducing the prevailing salinity of water in that region.
Understanding the causes of periodic Dansgaard-Oeschger events is even more difficult. These are rapid warmings (for example, around 5 degrees C, 9 degrees F, over 30 to 40 years. There are 25 recorded Dansgaard-Oeschger events over the past glacial period, and some have argued that these are driven by changes in the solar cycle amplified by various feedbacks.
The past climate reveals that small outside forcings can lead to unexpected rapid changes in Earth’s temperature. But what mechanisms could lead to abrupt climate change today? A number of possible scenarios have been explored in the scientific literature and the media, chief among them another THC shutdown, abrupt changes in sea level as a result of ice sheet dynamics, and the abrupt release of methane cathrates from the ocean floor and permafrost.
The Risk of a THC Shutdown – Negligible
As explored in a prior article, the risk of a modern shutdown of the thermohaline circulation basically is negligible. The volume of freshwater needed to completely stop the THC in the past was orders of magnitude larger than the most pessimistic estimates of both the rate of Greenland ice melt and precipitation changes over the coming century.
The most recent IPCC report bluntly dismissed the possibility of a shutdown:
Catastrophic scenarios suggesting the beginning of an ice age triggered by a shutdown of the MOC [meridional overturning circulation - another term for the THC] are thus mere speculations, and no climate model has produced such an outcome. In fact, the processes leading to an ice age are sufficiently well understood and so completely different from those discussed here, that we can confidently exclude this scenario.
The IPCC 2007 report allows that, “Nevertheless, even if this were to occur, Europe would still experience warming, since the radiative forcing caused by increasing greenhouse gases would overwhelm the cooling associated with the MOC reduction.”
Similarly, Columbia University’s Wallace S Broecker, the scientist who first posited the idea of a THC shutdown as a driver of abrupt climate change, argues that “The notion that [a modern THC shutdown] may trigger a mini ice age is a myth.”
More Debate Surrounding Rapid Sea Level Rise
The risks of rapid sea level rise were also explored in a prior article. Here there is much wider debate over the possibility of dramatic changes within this century as a result of rapid disintegration of ice sheets. In this case, the culprit is not simply melting per se, but dynamic movements of ice sheets driven by processes such as faster flow rates resulting from lubrication of the base of the ice sheet from meltwater pouring down through holes in the ice.
Here again the paleoclimate record shows instances of rapid sea level changes. For example, at the end of the last glacial period, sea levels rose roughly 1.1 meters per century, ultimately reaching levels four to six meters higher than current sea level despite mean temperatures likely lower than the present.
Much of the current science suggests that a sea level rise of much over one meter may be unlikely this century, but there remains considerable uncertainty and a number of influential scientists argue that a sea level rise of up to 3 meters is not beyond the realm of possibility.
Methane Cathrates – ‘Significant, Not Catastrophic’?
Immense amounts of frozen methane (natural gas) lie under sediments on the ocean floor and in Arctic permafrosts. The size of these methane cathrate stocks is still largely unknown, but experts think they are much larger than all other fossil fuels combined. Some experts suggest that massive releases of methane cathrates drove the Permian extinction event 251 million years ago, wiping out the vast majority of life on Earth at the time.
Some Scientists have speculated that a warming climate could speed the release of methane cathrates, leading to a strong warming feedback as a result of the relatively high radiative forcing of methane. While the majority of cathrates are located in deep-ocean regions that will lag behind changes in surface temperatures for thousands of years, potential methane releases from permafrost and relatively shallow areas of the Atlantic have created much more concern.
Despite some recent sensationalist reporting on the subject, most scientists appear to agree with David Archer at the University of Chicago when he refers to expected methane release from cathrates over the next century as “signiﬁcant but not catastrophic”. That is to say that “The most likely response of these deposits to anthropogenic climate change is an increased background rate of chronic methane release, rather than an abrupt release.”
Scientist Gavin Schmidt divides potential abrupt climate change scenarios into “known unknowns” (such as the rate of future sea level rise, the magnitude of methane releases from cathrates, or the extent to which the THC might slow down) and “unknown unknowns” (potential changes in Earth’s climate as a result of feedbacks that cannot now be predicted). The fact that there remain many periods of inexplicable rapid climate change in the paleoclimate record suggests a healthy concern for unknown unknowns is warranted.
As Schmidt puts it:
There is also the existence of ‘unknown unknowns’ – tipping points that we are as yet unaware of. An example of this kind of surprise happened in relation to the Antarctic ozone hole, where unexpected chemistry on surfaces of ice particles led to much more efficient destruction of ozone in the polar vortex than had been expected, making an existing concern into a serious problem. By their nature, we are not able to assess how important any such surprises might be, but it is impossible to rule them out entirely.
The fact that we are soon to experience global mean temperatures possibly unprecedented in the last million years lends credence to the concern that tinkering with our non-linear climate system may yield dramatic and unexpected results.
The danger of abrupt climate change lies in the inability of human and natural systems to respond fast enough. As Columbia’s Wallace Broecker explains, “if the impact has the potential of coming at us fast – of being an abrupt change like the ones that are so abundant in the paleoclimate record – then there is far more reason to be worried, because we might not have time to adapt to it.”
How Concerned Should We Be?
The potential for abrupt climate change is certainly a serious concern, but reporters, scientists, and policy makers alike should beware a tendency to overstate the possibility of known events happening in the near future. Some of the worst reporting on climate change has taken these possible scenarios far beyond what is supported by the best science. It’s perhaps best to have a general caution about what might happen as powerful anthropogenic forcings drive an unpredictable non-linear climate system characterized by strong feedbacks, rather than to fret now over a specific scenario of impending disaster.
The most relevant abrupt changes in the short term may happen locally rather than globally. These include unexpected changes to ecosystems caused by rising temperatures, such as the devastation of forests in Alaska by the pine bark beetle. There is also reason to be concerned about severe droughts occurring in historically drought-prone regions, such as the American Southwest.
Schmidt succinctly sums up the complexities of the climate system: “… It seems more appropriate to view the system as having multiple tipping points and thresholds that range in importance and scale from the smallest ecosystem to the size of the planet.”
He cautions that the subject of abrupt climate change is an area where nuance is desperately needed, arguing that “An appreciation of that subtlety may be useful when reading some of the worst coverage on the topic.”