the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Cloud- and ice-albedo feedbacks drive greater Greenland ice sheet sensitivity to warming in CMIP6 than in CMIP5
Stefan Hofer
Trude Storelvmo
Xavier Fettweis
Abstract. The Greenland Ice Sheet (GrIS) has been losing mass since the 1990s as a direct consequence of rising temperatures and has been projected to continue to lose mass at an accelerating pace throughout the 21st century, making it one of the largest contributors to future sea-level rise. The latest Climate Model Intercomparison Project 6th phase (CMIP6) models produce a greater Arctic amplification signal and therefore also a notably larger mass loss from the GrIS when compared to the older CMIP5 projections, despite similar forcing levels from greenhouse gas emissions. However, it is also argued that the strength of regional factors such as melt-albedo feedbacks and cloud-related feedbacks will partly impact future melt and sea-level rise contribution, but little is yet known about the role of these regional factors in differences in GrIS surface melt projections between CMIP6 to CMIP5. In this study, we use high-resolution (15 km) regional climate model simulations over the GrIS performed using the Modéle Atmosphériqe Régional (MAR) to physically downscale six CMIP5 RCP8.5 and five CMIP6 SSP5-8.5 extreme high-emission scenario simulations. Here, we show a greater annual mass loss from the GrIS at the end of the 21st century, but also for a given temperature increase over the GrIS, when comparing CMIP6 to CMIP5. We find a greater sensitivity of Greenland surface mass loss in CMIP6 centred around summer and autumn, yet the difference in mass loss is largest during autumn with a reduction of 14.1 ± 4.8 mmWE for a regional warming of +6.7 °C, 12.5 mmWE more mass loss than in CMIP5 RCP8.5 simulations for the same warming. Assessment of the surface energy budget and cloud-related feedbacks suggests a reduction in high clouds during summer and autumn – in addition to enhanced cloud optical depth during autumn – to be the main drivers of the additional energy reaching the surface, subsequently leading to enhanced surface melt and mass loss in CMIP6 compared to CMIP5. Our analysis highlights that Greenland is losing more mass in CMIP6 due to two factors; 1) a (known) greater sensitivity to greenhouse gas emissions and therefore warmer temperatures, 2) previously undocumented cloud-related surface energy budget changes that enhance the GrIS sensitivity to warming.
- Preprint
(25715 KB) -
Supplement
(57822 KB) - BibTeX
- EndNote
Idunn Aamnes Mostue et al.
Status: open (until 03 May 2023)
-
RC1: 'Comment on tc-2023-24', Anonymous Referee #1, 23 Mar 2023
reply
Mostue and co-authors explore summer and autumn differences in surface mass and energy budget (SMB and SEB) components as well as cloud cover based on output of a regional climate model forced by several global climate models from two generations under the high-end baseline scenario. They start by relating anomalies in near-surface temperature with individual SMB and SEB components, showing similar behavior and magnitude to SMB, melt, runoff, longwave down and longwave up. However, the set of models used from the most recent generation of global climate models projects more warming than previously. Finally, it is shown a contrasting relationship between the anomaly of near-surface temperature and cloud cover anomaly between the two generations of global climate models. The authors hypothesize that the cloud cover decrease allows more absorption of solar radiation by the surface, generating enhanced surface melt and runoff in the ablation zone in summer extending to autumn.
This piece of work explores relevant scientific aspects, but the methods and set of variables used are not sufficient to prove the robustness of the results. The level of detail provided in the Section 3.1 and 3.2 is commendable, but presents an unnecessary detailed picture between near-surface temperature with SMB and SEB components. The most relevant part starts with the lower panels in Figure 2, serving as motivation for the rest of the manuscript. Even though the authors show no relevant changes in cloud optical thickness, it would be worthwhile to explore changes in cloud microphysics and its relationship with cloud cover. In addition, summer and autumn precipitation should be included in the analysis, given its role to surface albedo.
My comments are provided by line number (LN) or specific figure below.
The Introduction is short and does not summarize/highlight what the scientific community has recently done concerning the impact of clouds and surface albedo on SEB and SMB components over the Greenland ice sheet.
LN22: the accelerating mass loss pace since the mid-1990s is not only a consequence of increased temperatures from anthropogenic greenhouse gases, but rather a consequence of a superimposed effect with extraordinary atmospheric conditions in recent summers (Bennartz et al. 2013, Fausto et al. 2012, Tedesco et al. 2011, Tedesco et al. 2016). Consider rephrasing.
LN28: the SMB definition should not be part of the Introduction, but in the Methods section, naming individual components and explaining how do you define accumulation and ablation zones.
LN31: in addition to solar radiation, consider the role of sensible heat flux to darken the surface (Wang et al. 2021)
LN41: the authors should address the fact that as a consequence of more open waters, CMIP6 projects more precipitation and more rainfall in Greenland than CMIP5 (McCrystall et al. 2021). This point can also be later discussed as a factor contributing to decreasing albedo, as also shown by Box et al. (2022).
LN50: state that a surplus in SEB is energy available for melt and not necessarily surface melt
LN64: the last paragraph of Section 2.1 could be moved to the Introduction, where a few of these references could better distilled
LN72: it would be relevant to explain here why only RCP8.5 and SS5-8.5 is chosen for the study, as Hofer et al. (2020) made use of all the projected scenarios
LN75: it is also unclear why the period 1961-1990 is chosen. I would assume the last 3 decades (1991-2020), responsible for the accelerated mass loss, a better period for comparison with future projections
LN82: it should be indicated how the ice cover mask (more than 10\% ice cover) can influence the following results
LN83: it is unclear why a twenty-averaged period for ~4ºC is chosen for the dissemination of certain the results
LN85 how can you gain insight of changes caused by rapid Greenland warming using a twenty-year averaged period?
LN115: could you present the same charts (Figure 1 and 2) but for the differences between CMIP5 and CMIP6, making use of statistical inference to state the robustness of the mentioned differences?
Figure 1: legends and axis labels missing. Also, consider making the season as a subtitle of the subplot as in Figure 2
LN139: start the sentence with "In SON" instead of "Here". Otherwise, it is not clear to which season this sentence belongs
LN147: in LN139 you explain that more SW$_{net}$ is due to darkening and here is due to SWD. Please, rephrase it.
Figure 2: legends missing and temperature unit incomplete
LN165: why do you assume that no differences in cloud optical depth means no differences in cloud microphysics? Isn't this statement contradicting Hofer et al. (2019)? Could you elaborate your thought?
LN181: The twenty-year averaged cloud cover anomaly is compiled by a wide variety of circulation patterns. Only high frequency of a certain circulation pattern would depict the topography influence on the cloud cover composite. Thus, there is no information enough to infer the likelihood of circulation-driven cloud cover change.
Figure 4, 5, 6 and 7: use statistical inference to indicate the level of confidence in changes between CMIP5 and CMIP6.
Figure 5 and 6: consider two different color maps to stress the fact that colors shading in summer is not comparable with autumn. Perhaps, relative changes (e.g., ratio) instead of absolute changes could be here considered
LN245: precipitation has so far been discarded of the analysis, but here it would be interesting to assess if precipitation, more specifically liquid precipitation, could play a role in the snow darkening and surface runoff
Technical corrections
LN9: spell the name of the regional climate model correctly
LN11: indicate the corresponding level of uncertainty
LN27: spell the surname of the main author correctly
LN32: spell the surname of the main author correctly
LN51: downwards instead of "down towards"
LN51: LWU is defined as LWD
LN55: introduce SWD at the beginning of the sentence
LN56: suggested place to define SMB instead of doing it in the Introduction
LN59: spell the surname of the main author correctly
LN71: spell the name of the regional climate model correctly
LN154: Figure 2 c and d, instead of "a and b"
LN156 Figure 2 c instead of "a"
LN179: total instead of "toal"
References
Bennartz, Ralf, et al. "July 2012 Greenland melt extent enhanced by low-level liquid clouds." Nature 496.7443 (2013): 83-86.Box, Jason E., et al. "Greenland Ice Sheet Rainfall, Heat and Albedo Feedback Impacts From the Mid‐August 2021 Atmospheric River." Geophysical Research Letters 49.11 (2022): e2021GL097356.
Fausto, Robert S., et al. "The implication of nonradiative energy fluxes dominating Greenland ice sheet exceptional ablation area surface melt in 2012." Geophysical Research Letters 43.6 (2016): 2649-2658.
Hofer, Stefan, et al. "Cloud microphysics and circulation anomalies control differences in future Greenland melt." Nature Climate Change 9.7 (2019): 523-528.
McCrystall, Michelle R., et al. "New climate models reveal faster and larger increases in Arctic precipitation than previously projected." Nature communications 12.1 (2021): 6765.
Tedesco, Marco, et al. "The role of albedo and accumulation in the 2010 melting record in Greenland." Environmental Research Letters 6.1 (2011): 014005.Tedesco, Marco, et al. "Arctic cut-off high drives the poleward shift of a new Greenland melting record." Nature Communications 7.1 (2016): 11723.
Wang, Wenshan, et al. "Greenland surface melt dominated by solar and sensible heating." Geophysical Research Letters 48.7 (2021): e2020GL090653.
Citation: https://doi.org/10.5194/tc-2023-24-RC1
Idunn Aamnes Mostue et al.
Idunn Aamnes Mostue et al.
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 186 | 68 | 6 | 260 | 25 | 2 | 2 |
- HTML: 186
- PDF: 68
- XML: 6
- Total: 260
- Supplement: 25
- BibTeX: 2
- EndNote: 2
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1