Thresholds for the future of the Greenland ice-sheet
Lead Research Organisation:
University of Reading
Department Name: Meteorology
Abstract
Sea-level change is one of the mostly widely known and potentially serious consequences of anthropogenic climate change due to emissions of greenhouse gases, because of its adverse impact on the populations and ecosystems of coastal and low-lying areas. This impact is expected to increase for centuries to come. One of the contributors to global-mean sea-level rise is the Greenland ice-sheet, which is presently shrinking, with the ice which it is losing being added as water to the ocean. In a warmer climate, increased melting of the ice-sheet is projected, which will exceed the expected increase in snowfall on the ice-sheet, and hence the ice-sheet will lose mass more quickly in future.
Existing scientific information indicates that global warming exceeding a certain threshold would lead to the near-complete loss of the Greenland ice-sheet over a millennium or more, causing a global-mean sea-level rise of about 7 metres. The threshold is very uncertain, but it could be as low as 1-2degC of global warming above pre-industrial. If warming passes above the threshold, and later falls back below it, the ice-sheet might regrow, but this depends on how long and how far the warming was above the threshold. If the ice-sheet has lost too much mass, it might continue to contract and could be eliminated even if global climate returned to a state like that which existed before the industrial revolution. In that case, the sea-level rise due to the Greenland ice-sheet would be irreversible.
Irreversible global-mean sea-level rise of several metres over many centuries is a scenario which would present an extreme challenge to adaptation in the coastal zone, and avoiding it is crucial for mitigation. Thus, the long-term future of the Greenland ice-sheet is a critical uncertainty, and our project aims to provide clearer information about it. We will do this by predicting the changes in the ice-sheet in this century and for many millennia into the future using a computer model which we have developed for studying changes that occurred during the ice-ages of the last 100,000 years. There is a close relationship between these scientific interests, because what happened in the past can inform us about what could happen in the future. The model represents both the climate, on a grid covering the world, and the Greenland ice-sheet, in much greater detail. Both components are necessary because as the ice-sheet changes in shape and size it modifies the climate it experiences, and this affects the rates of melting and snowfall. We will use the model to study the consequences for the ice-sheet of various levels of global warming, maintained for various lengths of time. We will make our results available to the public, the scientific community, and policy-makers in the UK and abroad. They are relevant to international climate policy because of the global warming target of 1.5degC, which is the aspiration expressed in the Paris climate agreement signed in 2016.
Existing scientific information indicates that global warming exceeding a certain threshold would lead to the near-complete loss of the Greenland ice-sheet over a millennium or more, causing a global-mean sea-level rise of about 7 metres. The threshold is very uncertain, but it could be as low as 1-2degC of global warming above pre-industrial. If warming passes above the threshold, and later falls back below it, the ice-sheet might regrow, but this depends on how long and how far the warming was above the threshold. If the ice-sheet has lost too much mass, it might continue to contract and could be eliminated even if global climate returned to a state like that which existed before the industrial revolution. In that case, the sea-level rise due to the Greenland ice-sheet would be irreversible.
Irreversible global-mean sea-level rise of several metres over many centuries is a scenario which would present an extreme challenge to adaptation in the coastal zone, and avoiding it is crucial for mitigation. Thus, the long-term future of the Greenland ice-sheet is a critical uncertainty, and our project aims to provide clearer information about it. We will do this by predicting the changes in the ice-sheet in this century and for many millennia into the future using a computer model which we have developed for studying changes that occurred during the ice-ages of the last 100,000 years. There is a close relationship between these scientific interests, because what happened in the past can inform us about what could happen in the future. The model represents both the climate, on a grid covering the world, and the Greenland ice-sheet, in much greater detail. Both components are necessary because as the ice-sheet changes in shape and size it modifies the climate it experiences, and this affects the rates of melting and snowfall. We will use the model to study the consequences for the ice-sheet of various levels of global warming, maintained for various lengths of time. We will make our results available to the public, the scientific community, and policy-makers in the UK and abroad. They are relevant to international climate policy because of the global warming target of 1.5degC, which is the aspiration expressed in the Paris climate agreement signed in 2016.
Planned Impact
Our project aims to deliver information of four kinds:
1. The rate and amount of global-mean sea-level rise due to mass loss by the Greenland ice-sheet during the 21st century under various global temperature trajectories.
2. The probability that the Greenland ice-sheet will be partly or wholly eliminated for particular levels of global warming at 2100 if these are maintained on the long term.
3. The length of time after 2100, depending on the level of global warming, until a "tipping point" is reached, beyond which further contraction of the ice-sheet is partly or wholly irreversible, answering the question of how long the world would have to reduce the temperature enough to "save" the ice-sheet.
4. The permanent global-mean sea-level rise which would be caused if global climate returns to pre-industrial after passing a point of irreversibility.
This information can be used to assess the risks, pace and magnitude of long-term sea-level rise associated with global warming, which are among the key science areas of this call. The beneficiaries of the information will be policy-makers and the scientific community, both in the UK and internationally.
The scientific impact of our project will be to reduce the uncertainties and increase scientific understanding of the effect of long-term future climate change on the Greenland ice-sheet. We will use a new coupled ice-sheet--climate model which we have developed as part of a ongoing project funded by NERC concerned with the evolution of ice-sheets, climate and sea-level during the ice-ages of the last 100,000 years. There is a valuable link between these projects, because the past provides information about what could happen in the the future. Our work will benefit the closely related development of the new UK Earth System Model for ice-sheet projections in connection with the next assessment report of the Intergovernmental Panel on Climate Change (IPCC). We will make the results known and available to the scientific community concerned with impacts of the sea-level rise on coastal communities, ecosystems and infrastructure.
The information will be relevant to the international policy community in the context of the UN Framework Convention on Climate Change (UNFCCC), and may help to inform the UK's position. It relates especially to the IPCC Special Report in 2018 on the impacts of global warming of 1.5degC, which is the aspiration expressed in the Paris climate agreement signed in 2016. The information will benefit the UK Committee on Climate Change and the Department for Business, Energy and Industrial Strategy (which is responsible for climate policy) by adding to the body of evidence concerning warming targets for this century and beyond, and the benefits of limiting warming to lower levels.
1. The rate and amount of global-mean sea-level rise due to mass loss by the Greenland ice-sheet during the 21st century under various global temperature trajectories.
2. The probability that the Greenland ice-sheet will be partly or wholly eliminated for particular levels of global warming at 2100 if these are maintained on the long term.
3. The length of time after 2100, depending on the level of global warming, until a "tipping point" is reached, beyond which further contraction of the ice-sheet is partly or wholly irreversible, answering the question of how long the world would have to reduce the temperature enough to "save" the ice-sheet.
4. The permanent global-mean sea-level rise which would be caused if global climate returns to pre-industrial after passing a point of irreversibility.
This information can be used to assess the risks, pace and magnitude of long-term sea-level rise associated with global warming, which are among the key science areas of this call. The beneficiaries of the information will be policy-makers and the scientific community, both in the UK and internationally.
The scientific impact of our project will be to reduce the uncertainties and increase scientific understanding of the effect of long-term future climate change on the Greenland ice-sheet. We will use a new coupled ice-sheet--climate model which we have developed as part of a ongoing project funded by NERC concerned with the evolution of ice-sheets, climate and sea-level during the ice-ages of the last 100,000 years. There is a valuable link between these projects, because the past provides information about what could happen in the the future. Our work will benefit the closely related development of the new UK Earth System Model for ice-sheet projections in connection with the next assessment report of the Intergovernmental Panel on Climate Change (IPCC). We will make the results known and available to the scientific community concerned with impacts of the sea-level rise on coastal communities, ecosystems and infrastructure.
The information will be relevant to the international policy community in the context of the UN Framework Convention on Climate Change (UNFCCC), and may help to inform the UK's position. It relates especially to the IPCC Special Report in 2018 on the impacts of global warming of 1.5degC, which is the aspiration expressed in the Paris climate agreement signed in 2016. The information will benefit the UK Committee on Climate Change and the Department for Business, Energy and Industrial Strategy (which is responsible for climate policy) by adding to the body of evidence concerning warming targets for this century and beyond, and the benefits of limiting warming to lower levels.
Publications
Smith R
(2021)
FAMOUS version xotzt (FAMOUS-ice): a general circulation model (GCM) capable of energy- and water-conserving coupling to an ice sheet model
in Geoscientific Model Development
Gregory J
(2020)
Large and irreversible future decline of the Greenland ice sheet
in The Cryosphere
| Description | We have studied the future of the Greenland ice-sheet under a range of climates typical of those projected for the end of the present century, using a ice-sheet model (Glimmer) coupled to an atmosphere general circulation model (a global climate model). Over millennia, under any warmer climate, the ice-sheet reduces in size, and its final size is smaller if the climate warming is greater. If a climate like the recent one were maintained, the Greenland contribution to global-mean sea-level rise (GMSLR) would eventually reach 0.5-2.5 metres. For any global warming exceeding 3 degC, the contribution to GMSLR exceeds 5 metres. For the largest global warming considered (about 5 degC), only a small ice-cap endures, resulting in over 7 metres of GMSLR. Our analysis gives a qualitatively different impression from previous work, in that we do not find a sharp threshold warming that divides scenarios in which the ice-sheet suffers little reduction from those in which it is mostly lost. If late 20th-century climate is restored after the ice-sheet mass has fallen below a certain size (somewhat more than half its present size) it will not regrow to its present extent, because the snowfall in the northern part of the island is reduced once the ice-sheet retreats from there. In that case, about 2 metres of GMSLR would become irreversible. In order to avoid this outcome, anthropogenic climate change must be reversed before the ice-sheet has declined to this threshold mass, which would be reached in about 600 years at the highest rate of mass-loss within the likely range of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. |
| Exploitation Route | Our findings are likely to be cited in the forthcoming report of the Intergovernmental Panel on Climate Change. The coupled model is being used by another recently started NERC project. |
| Sectors | Environment |