Possible tipping elements in the climate system.
A tipping point in the climate system
is a threshold that, when exceeded, can lead to large changes in the
state of the system. Potential tipping points have been identified in
the physical climate system, in impacted ecosystems, and sometimes in both. For instance, feedback from the global carbon cycle is a driver for the transition between glacial and interglacial periods, with orbital forcing providing the initial trigger. Earth's geologic temperature record includes many more examples of geologically rapid transitions between different climate states.
Climate tipping points are of particular interest in reference to concerns about climate change
in the modern era. Possible tipping point behaviour has been identified
for the global mean surface temperature by studying self-reinforcing
feedbacks and the past behavior of Earth's climate system.
Self-reinforcing feedbacks in the carbon cycle and planetary reflectivity could trigger a cascading set of tipping points that lead the world into a hothouse climate state.
Large-scale components of the Earth system that may pass a tipping point have been referred to as tipping elements. Tipping elements are found in the Greenland and Antarctic ice sheets, possibly causing tens of meters of sea level rise. These tipping points are not always abrupt. For example, at some level of temperature rise the melt of a large part of the Greenland ice sheet and/or West Antarctic Ice Sheet will become inevitable; but the ice sheet itself may persist for many centuries. Some tipping elements, like the collapse of ecosystems, are irreversible.
Definition
The IPCC AR5
defines a tipping point as an irreversible change in the climate
system. It states that the precise levels of climate change sufficient
to trigger a tipping point remain uncertain, but that the risk
associated with crossing multiple tipping points increases with rising
temperature. A more broad definition of tipping points is sometimes used as well, which includes abrupt but reversible tipping points.
In mathematics tipping points have been characterised into three types dependent on the underlying mechanisms:
- Bifurcation-induced tipping: System changes abruptly or qualitatively following a slow passage through a bifurcation (eg. Atlantic Meridional Overturning Circulation (AMOC) collapse)
- Noise-induced tipping: Transitions due to random fluctuations/internal variability of the system (eg. Dansgaard-Oeschger events)
- Rate-induced tipping: Failure to track a continuously (slowly) changing steady state (eg. Compost bomb instability)
In the context of climate change, an "adaptation tipping point" has
been defined as "the threshold value or specific boundary condition
where ecological, technical, economic, spatial or socially acceptable
limits are exceeded."
Tipping points for global temperature
There are many positive and negative feedbacks to global temperatures and the carbon cycle that have been identified. The IPCC reports that feedbacks to increased temperatures are net positive for the remainder of this century, with the impact of cloud cover the largest uncertainty.
IPCC carbon cycle models show higher ocean uptake of carbon
corresponding to higher concentration pathways, but land carbon uptake
is uncertain due to the combined effect of climate change and land use
changes.
The geologic record of temperature and greenhouse gas concentration allows climate scientists to gather information on climate feedbacks
that lead to different climate states, such as the Late Quaternary
(past 1.2 million years), the Pliocene period five million years ago and
the Cretaceous period, 100 million years ago. Combining this
information with the understanding of current climate change resulted in
the finding that "A 2 °C warming could activate important tipping
elements, raising the temperature further to activate other tipping
elements in a domino-like cascade that could take the Earth System to
even higher temperatures".
The speed of tipping point feedbacks is a critical concern and
the geologic record often fails to provide clarity as to whether past
temperature changes have taken only a few decades or many millennia of
time. For instance, a tipping point that was once feared to be abrupt
and overwhelming is the release of clathrate compounds buried in seabeds and seabed permafrost, but that feedback is now thought to be chronic and long term.
Some individual feedbacks may be strong enough to trigger tipping
points on their own. A 2019 study predicts that if greenhouse gases
reach three times the current level of atmospheric carbon dioxide that
stratocumulus clouds could abruptly disperse, contributing an additional
8 degrees Celsius of warming.
Runaway greenhouse effect
The runaway greenhouse effect
is used in astronomical circles to refer to a greenhouse effect that is
so extreme that oceans boil away and render a planet uninhabitable, an
irreversible climate state that happened on Venus. The IPCC Fifth Assessment Report states that "a 'runaway greenhouse effect'—analogous to Venus—appears to have virtually no chance of being induced by anthropogenic activities."
Venus-like conditions on the Earth require a large long-term forcing
that is unlikely to occur until the sun brightens by a few tens of
percents, which will take a few billion years.
While a runaway greenhouse effect on Earth is virtually impossible, there are indications that Earth could enter a moist greenhouse state that renders large parts of Earth uninhabitable if the climate forcing is large enough to make water vapour (H2O) a major atmospheric constituent.
Conceivable levels of human-made climate forcing would increase water
vapour to about 1% of the atmosphere's mass, thus increasing the rate of
hydrogen escape to space. If such a forcing were entirely due to CO2, the weathering process would remove the excess atmospheric CO2 well before the ocean was significantly depleted.
Tipping elements
Large scale tipping elements
A smooth or abrupt change in temperature can trigger global-scale tipping points. In the cryosphere these include the irreversible melting of Greenland and Antarctic ice sheets.
In Greenland, a positive feedback cycle exists between melting and
surface elevation. At lower elevations, temperatures are higher, leading
to additional melting. This feedback loop can become so strong that
irreversible melting occurs. Marine ice sheet instability could trigger a tipping point in West Antarctica. Crossing either of these tipping points leads to accelerated global sea level rise.
When fresh water gets released as a consequence of Greenland
melting, a threshold may be crossed which leads to disruption of the thermohaline circulation. The thermohaline circulation transports heat northward which is important for temperature regulation in the Atlantic region. Risks for a complete shutdown are low to moderate under the Paris agreement levels of warming.
Other examples of possible large scale tipping elements are a shift in El Niño–Southern Oscillation.
After crossing a tipping point, the warm phase (El Niño) would start to
occur more often. Lastly, the southern ocean, which now absorbs a lot
of carbon, might switch to a state where it does not do this anymore.
Regional tipping elements
Climate change can trigger regional tipping points as well. Examples are the disappearance of Arctic sea ice, the establishment of woody species in tundra, permafrost loss, the collapse of the monsoon of South Asia and a strengthening of the West African monsoon which would lead to greening of the Sahara and Sahel. Deforestation may trigger a tipping point in rainforests (i.e. Savannization in the Amazon rainforest,
...). As rain forests recycle a large part of their rainfall, when a
portion of the forest is destroyed local droughts may threaten the
remainder. Finally, boreal forests
are considered a tipping element as well. Local warming causes trees to
die at a higher rate than before, in proportion to the rise in
temperature. As more trees die, the woodland becomes more open, leading
to further warming and making forests more susceptible to fire. The
tipping point is difficult to predict, but is estimated to be between
3–4 °C of global temperature rise.
Cascading tipping points
Crossing
a threshold in one part of the climate system may trigger another
tipping element to tip into a new state. These are so-called cascading
tipping points. Ice loss in West Antarctica and Greenland will significantly alter ocean circulation.
Sustained warming of the northern high latitudes as a result of this
process could activate tipping elements in that region, such as
permafrost degradation, loss of Arctic sea ice, and Boreal forest dieback. This illustrates that even at relatively low levels of global warming, relatively stable tipping elements may be activated.
Early warning signals
For
some of the tipping points described above, it may be possible to
detect whether that part of the climate system is getting closer to a
tipping point. All parts of the climate system are sometimes disturbed
by weather events. After the disruption, the system moves back to its
equilibrium. A storm may damage sea ice, which grows back after the
storm has passed. If a system is getting closer to tipping, this
restoration to its normal state might take increasingly longer, which
can be used as a warning sign of tipping.
Changes in the Arctic
A 2019 UNEP study indicates that now at least for the Arctic and the Greenland ice sheet a tipping point has already been reached. Because of dewing of permafrost soil, more methane (in addition to other short-lived climate pollutant)
could enter the atmosphere earlier than previously predicted and the
loss of reflecting ice shields has started a powerful positive feedback
loop leading to ever higher temperatures. The resulting accelerating
climate instability in the polar region has potential to affect the
global climate, outdating previous predictions about the point in the future when global tipping will occur.
In June 2019, satellite images from around the Arctic showed
burning fires that are farther north and of greater magnitude than at
any time in the 16-year satellite record, and some of the fires appear
to have ignited peat soils. Peat is an accumulation of partially decayed vegetation and is an efficient carbon sink.
Scientists are concerned because the long-lasting peat fires release
their stored carbon back to the atmosphere, contributing to further
warming. The fires in June 2019, for example, released as much carbon
dioxide as Sweden's annual greenhouse gas emissions.
Tipping point effects
If the climate tips into a hothouse Earth scenario, some scientists warn of food and water shortages, hundreds of millions of people being displaced by rising sea levels, unhealthy and unlivable conditions, and coastal storms having larger impacts.
Runaway climate change of 4–5 °C can make swathes of the planet around
the equator uninhabitable, with sea levels up to 60 metres (197 ft)
higher than they are today.
Humans cannot survive if the air is too moist and hot, which would
happen for the majority of human populations if global temperatures rise
by 11–12 °C, as land masses warm faster than the global average. Effects like these have been popularized in books like The Uninhabitable Earth and The End of Nature.
