There are a number of web and news articles around surrounding the question of whether or not we will enter another ice age. Many of these questions arise from the idea that a collapse or significant melting of the Greenland Ice Sheet will produce enough fresh water to shut down the global thermohaline circulation, dropping us into a new ice age in the next 10,000 years.
Will the Greenland Ice Sheet collapse?
The Greenland Ice Sheet is currently melting, with rapid accelerations of outlet glaciers and lowering of the ice surface. The rate of ice mass loss is currently increasing , with ice sheet mass loss currently around 273 gigatonnes of ice per year (0.75 mm sea level rise per year) . The total area with high surface melt has increased over the last two decades. If this ice sheet were to melt completely, it would raise global sea levels around 7.36 m [3, 4]. However, a total loss of the Greenland Ice Sheet is unlikely for the foreseeable future.
The recent melting and acceleration of outlet glaciers around the Greenland Ice Sheet is likely driven by changes in ocean temperatures around the ice-sheet margins . Surface melting is driven by air temperatures , but some of these losses are offset by increases in precipitation and snow accumulation with increased air temperatures [6, 7]. These two processes, surface melting and solid ice discharge across the grounding line therefore have different environmental controls . Unlike the West Antarctic Ice Sheet, the marine-grounded parts of the Greenland Ice Sheet are limited, and mass loss from iceberg calving is therefore limited. For a moderate warming, the increase in surface melt is coupled to air temperature , so a sustained large-amplitude response (i.e. an entire collapse) of the Greenland Ice Sheet is considered unlikely under these scenarios . Levermann et al. found that the sea level contribution from Greenland is 0.34 m per °C-1; a 2.2°C warming would therefore be sufficient to eventually (over multiple millennia) reduce the Greenland Ice Sheet to 10% of its current volume, resulting in around 6 m sea level rise . A recent study found that a warming of 4.5°C over Greenland (or a 3.1°C global-average temperature increase) may be sufficient to drive a prolonged negative surface mass balance of the Greenland Ice Sheet and drive the eventual total melting of the ice sheet . Together, these studies suggest that the total eventual removal of the Greenland Ice Sheet is not a significant concern over the next hundred years or so, but may be of importance over the next few hundred to thousand years.
A large uncertainty in predicting the future response of the Greenland Ice Sheet to climate change is uncertainty in how climate models represent atmospheric and ocean circulation changes and sea-ice change . However, taking possible changes in ocean circulation into account, numerical models predict that the total sea level contribution from Greenland is around 1 m for 560 ppm CO2 and around 3 m for 1120 ppm CO2  (compared with today’s atmospheric concentration of around 400 ppm CO2). Even under the highest CO2 scenarios, it still takes hundreds of years to instigate strong surface melt over the entire Greenland Ice Sheet surface. Complete loss is not inevitable, because the Greenland Ice Sheet operates over a long time scale and takes tens of millennia to respond to a temperature of around 2.1°C and a millennium or more for temperatures a few degrees above this threshold .
However, if high temperatures (around 3.5°C warming) are sustained over multiple millennia, the majority of the Greenland Ice Sheet will eventually be lost through changes in surface mass balance [4, 12]. Indeed, during the Middle Pliocene, when temperatures were 2 to 3.5°C higher than pre-industrial, the Greenland Ice Sheet was apparently near-deglaciated .
Weakening of the Atlantic Meridional Overturning Circulation (AMOC)
AMOC is a vital part of the global ocean circulation system. It is characterised by the northward flow of warm, salty water in the upper Atlantic, which helps bring warmth of several degrees centigrade to Europe and the North American seaboard . The system is sensitive to the amount of freshwater input into the North Atlantic. Most numerical models indicate that the AMOC will slow down under future warming scenarios. Indeed, it appears that this process has already started , and this may account for cold northern Hemisphere winters . This weakening has occurred alongside an ongoing freshening trend in the high-latitude North Atlantic, possibly resulting from meltwater from the Greenland Ice Sheet, sea ice and increasing river discharge into the Arctic Ocean. This dilution of the surface waters in the ocean could have weakened deep water formation, slowing down the AMOC . Further melting of the Greenland Ice Sheet over the next century could contribute to further weakening of this current.
The IPCC estimates that the AMOC has a ‘tipping point’  and suggests that there is a 10% chance that the AMOC could completely break down this century . However, predicting the future response of ocean currents to freshening and warming is complex, and the uncertainty between models is larger than the difference between simulations .
Could we have another ice age?
The Earth’s ice ages are paced by changes in the Earth’s orbit, which regulates the amount of summer solar insolation received in the Northern Hemisphere. This summer insolation is currently near its minimum, but there are no signs of a new ice age. A natural glacial inception may have been missed just prior to the Industrial Revolution, as a result of high late-Holocene (the last 11,000 years on Earth is the Holocene Epoch) CO2 and the low orbital eccentricity of the Earth . Even in the absence of human-induced climate change, it is unlikely that ice sheets would build up in the next several thousand years, and the current interglacial climate (a period of time with low ice volume) would likely continue for another 50,000 years, making this an unusually long interglacial period. However, anthropogenic CO2 emissions mean that the next glacial inception has been postponed by at least 100,000 years .
A significant point to note is that atmospheric CO2 has a long residence time in the atmosphere, and it would take a more or less permanent crash in human populations or fossil fuel consumption to move the Earth’s climate to a situation when an ice age could occur.
Would a shutdown of the global ocean thermohaline circulation (THC) trigger a new ice age? This has certainly happened before; ice core records from Greenland suggest that the global THC shut down and caused regional climate change during the last glacial-interglacial transition, around 18,000 years ago. As the North American ice sheet receded, the meltwater flowed into the North Atlantic, freshening and making the ocean surface less dense . As a result, Greenland cooled by about 7°C within several decades. However, when the meltwater ceased, the circulation pattern restarted and Greenland warmed.
Modelling experiments to investigate the likely impact of a shutdown in the AMOC suggest that a shutdown causes a cooling of the Northern Hemisphere of around -1.7°C, resulting in an increases in frost and snow cover . However, in subsequent years recovery of AMOC accelerates the rate of warming, and surface air temperatures eventually return to expected values in line with greenhouse gases within a hundred years or so . So if AMOC did shut down, perhaps in response to increased meltwater run off from Greenland, modelling experiences and data from the palaeoclimate record suggest that it would recover in about 100 years to be near the strength it has in the unperturbed experiments .
The future response of the Earth’s complex climate and ocean system to climate change is fraught with uncertainties. One potential uncertainty is, firstly, the rate and magnitude of freshwater input into the North Atlantic from a shrinking Greenland Ice Sheet. The future behaviour of the Greenland Ice Sheet is dependent on air temperatures since large-scale recession will largely be driven by changes in surface mass balance; it is thus a very different system to the West Antarctic Ice Sheet, which is prone to marine ice sheet instability. A second uncertainty is the impact of a slowdown of the global thermohaline circulation on the broader climate. This non-linear behaviour is difficult to model, but it is projected to continue to slow down over coming decades – which could contribute to colder Northern Hemisphere winters.
In summary, given the likelihood of a substantial melt of the Greenland Ice Sheet in the next few centuries, coupled with the fact that a future shutdown of the THC is likely to be temporary, it is unlikely that the total melting of the Greenland Ice Sheet could plunge us into a new ice age within the next few millennia. However, this does contribute to an uncertainty in predicting future Northern Hemisphere air temperatures and ice-sheet response, and highlight the inherent difficulty in providing a clear assessment of future climate change and sea level rise. However, we should entertain the possibility that future climate change could contain a number of ‘surprises’ [14, 20].
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