- Climate feedbacks are a fundamental controversy in the climate debate
- Without feedbacks, there would be very little basis for alarmist interpretations of global warming
- Many feedbacks are unknown, and the net contribution of even major known feedbacks is not yet understood
Feedbacks are processes that are themselves consequences of global warming, but which in turn influence the planet’s temperature. They are either positive feedbacks, which means they contribute more warming, or they are negative feedbacks, meaning that they cause cooling. Though this highly uncertain area of science is rarely discussed, it is one of the main scientific controversies in the climate debate.
As is discussed elsewhere on this site, whereas the claim that increased concentration of atmospheric CO2 will cause some warming is relatively uncontroversial, what has really driven debates is the uncertainty around the net balance of positive (warming) and negative (cooling) feedbacks. More alarmist and urgent interpretations of climate change depend on predictions that positive feedbacks will overwhelm the planet. More sceptical claims argue that these claims lack evidence.
Many of these processes remain unknown, meaning that the net balance of feedbacks cannot be easily quantified by science. Climate modelling (simulations of the global climate system) is in large part an attempt to settle these questions. Scientists involved in climate modelling believe that we can get an idea about how well science understands the total contribution of feedbacks to global warming by comparing models’ output with observations of the planet’s temperature. However, this has not solved the controversies, as different attempts to model the climate produce results that vary quite widely.
Positive feedbacks
A warmer atmosphere is capable of holding more water vapour, which is a greenhouse gas. Similarly, clouds can also trap heat beneath them (though their tops, being white, reflect heat). Snow, ice sheets and glaciers, are typically considered to be negative feedbacks, but in the event that they melt (due to anthropogenic global warming), this change can be considered a positive feedback. This reflection of heat is known as albedo, and it can reflect as much as 85 per cent of incoming solar radiation.
Some researchers have speculated that a warmer atmosphere will cause methane (natural gas, also a greenhouse gas) to be released from under permafrost (frozen soil) in the Arctic Circle, and also from underneath Arctic sea ice. However, recent scientific research suggests this longstanding fear now seems to be unfounded.
In the earlier days of climate politics, environmentalists and some scientists were concerned that global warming would trigger positive feedback mechanisms, which would cause more warming indefinitely, in a cataclysmic and unstoppable event called ‘runaway global warming’. Though this claim has persisted in some, more politically-motivated quarters, most climate scientists are cautious about making such statements. A 2013 study found that a ‘low-end’ runaway global warming scenario ‘would require CO2 to increase by a factor of 8–16 times’, resulting in a rise in global mean temperature by 16–24°C. This is not a plausible scenario, because we simply cannot produce the emissions required to raise CO2 concentrations by this amount.
Negative feedbacks
The tops of clouds reflect as much as a third of the heat coming from the Sun back out to space. This principle has led to suggestions that clouds could be deliberately seeded using sea water to increase their whiteness, to help mitigate global warming. According to designers of this proposal, a fleet of boats equipped with cloud-seeding funnels could travel the seas, perhaps even autonomously.
Recent research by scientists at MIT claims to have identified a mechanism which regulates atmospheric CO2 known as ‘silicate weathering’. According to the scientists involved, this stabilisation effect can only be observed over thousands of years, however — not in the timescales that could give hope to those anxious about imminent global warming. Nonetheless, the study lends evidential weight to the hypothesis that at geological timescales, the Earth acts as a self-regulating system — an idea developed by the late James Lovelock.
At shorter timescales, concerns that global warming would lead to deforestation and desertification have been allayed by the discovery, confirmed by NASA studies that the planet is in fact greening in response to increased levels of CO2. Not only does this act to regulate CO2 levels, a more recent study also found that increased vegetation from ‘global greening’ had a cooling effect on land surface temperatures.
Unknowns
The IPCC states that much remains unknown about feedback processes’ contributions to global warming:
Probably the greatest uncertainty in future projections of climate arises from clouds and their interactions with radiation. Cloud feedbacks depend upon changes in cloud height, amount, and radiative properties, including short-wave absorption. The radiative properties depend upon cloud thickness, particle size, shape, and distribution and on aerosol effects.
The IPCC admits that even recent advances in science have ‘not reduced the uncertainty attached to cloud feedbacks: even the sign of this feedback remains unknown’ — i.e. it is not known whether clouds are a positive or negative feedback. This has led to considerable debate, with sceptics pointing out that this shortcoming casts doubt on the ability of models to make reliable approximations of reality on which to base policy.
This article has discussed just a few of the known positive and negative feedback mechanisms. Some of them are newly discovered, and their individual and net contributions to global temperature and CO2 concentrations are not yet fully understood. There are also a number of known positive and negative feedback mechanisms which have not been discussed here (which are worth investigating in order to understand the debate fully, though these controversies are more technical than can be covered briefly here). And there are feedback mechanisms that are yet to be discovered. The issue is one of fundamental importance to the basic science of global warming, but which is often omitted from mainstream public discussions.