Supplementary material to “Identifying Crucial Issues in Climate Science”

Motoyoshi Ikeda, Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan

Ralf Greve and Toshika Hara, Institute of Low Temperature Science, Hokkaido University

Yutaka W. Watanabe, Faculty of Earth and Environmental Science, Hokkaido University

Atsumu Ohmura, Institute for Atmospheric and Climate Science, ETH-Zentrum, Zurich, Switzerland

Akihiko Ito, National Institute for Environmental Studies, Tsukuba, Japan

Michio Kawamiya, Integrated Modeling Research Program, Frontier Research System for Global Change, Yokohama, Japan

Citation:

Ikeda, M., R. Greve, T. Hara, Y. W. Watanabe, A. Ohmura, A. Ito, and M. Kawamiya (2009), Identifying Crucial Issues in Climate Science, Eos Trans. AGU, 90(2), 15. [Full Article (pdf)]

Drastic Change in the Earth System during Global Warming

By Motoyoshi Ikeda, Ralf Greve, Toshihiko Hara, Yutaka Watanabe, Atsumu Ohmura, Akihiko Ito and Michio Kawamiya

Global warming seems to be recognized as a real phenomenon crucial to human being so that Nobel Peace Prize has been awarded to IPCC and Al Gore. Although someone might think that we well know about global warming, the uncertainty is larger than 50% on the warming rate in this century. Therefore, scientific clarification is still required on important mechanisms, which potentially produce positive feedback in the earth system and have to be clarified for a more reliable prediction. The projection on distinct indicators should be made carefully by scientists to establish their credibility. Information on possible impacts of global warming is highly desirable, while uncertainty is supposed to be provided along with the projection.

We organized a symposium on June 24, 2008 at Hokkaido University, Sapporo, Japan, in association with G8 Summit. The reporters who were invited by the moderators provided the most recent update on five hot and urgent issues, which are considered to be crucial as positive feedback mechanisms, distinct indicator and severe impact on human society. Following all presentations and discussion, the symposium attendees pointed out the uncertain but crucial mechanisms for 21^st century projection.

Sea level rise: ocean warming below 2000m, Greenland and West Antarctic ice sheets
Ice sheets: basal conditions, ice shelf response to ocean
Arctic sea ice: ice thickness, feedback from ice-ocean to atmosphere
Terrestrial ecosystem: sensitivity to hydrologic cycle, CO₂ emission from soil
Oceanic CO₂ absorption: ocean circulation change, phytoplankton productivity reduction, acidification

In spite of the comprehensive review by IPCC on the past progression and future projections of global warming, there exists various skepticism, which tends to confuse citizens. Under this delicate situation, scientists themselves should carefully take their roles. An initial proposal was tabled and needs further discussion among the scientific community. The proposal includes: scientists should try to give more information beyond the non-regret policy, and pursue to clarify the crucial mechanisms. They should not be too conservative, but avoid a false alarm. They would lose credibility, once they are recognized to oversell their own research areas/projects.

This report was prepared with important input by Drs. John Church and Wieslaw Maslowski.

Five hot issues setup with (reporter, moderator)

Clarify causes and magnitude of sea level rise (J. Church, M. Ikeda)
Decay of glaciers and Greenland and Antarctic ice sheets (A. Ohmura, R. Greve)
When will summer Arctic sea ice disappear? (W. Maslowski, M. Ikeda)
Carbon uptake or emission by terrestrial ecosystem (A. Ito, T. Hara)
Marine ecosystem change resulting in carbon emission (M. Kawamiya, Y. Watanabe)

Focal points and main results in the five hot issues

1. Clarify causes and magnitude of sea level rise (impact on human society)

All scenarios in the IPCC report gave only 18-59 cm sea level rise in the 21^st century, in which both variety in scenarios and differences in model mechanism are responsible for the uncertainty: e.g., 18-38 cm in B1, and 26–59 cm in A1FI. An additional contribution of 10-20 cm (or more) may come from the potential dynamic response of the ice sheets. Oceanic thermal expansion near the sea surface has contributed nearly 50% from 1993 to 2003 and will be major in the future projection, whereas it occupied only 30% in 1961–93. Mountain glacier melting was also a significant contributor, at least comparable with the thermal expansion in last 50 years. We should ask ourselves what is missing in last 50 years. Is it warming in the deep ocean below 2000m, or ice sheet melting in Greenland and Antarctica? Then, is the future projection correct? Will the sea level rise be more than in the current set of projections?

Some of the most recent papers suggested that warming in the deep ocean and Greenland ice sheet melting have significantly contributed to the sea level rise in last 50 years, while the thermal expansion near the sea surface has occupied 30% persistently (e.g., Domingues et al., 2008). Hence, the observed rise of about 7 cm is now attributable to those four components (mountain glacier melting, near-surface and lower ocean thermal expansions, and Greenland ice sheet melting). In this century, mountain glacier contribution may be largely completed. It may be possible that Greenland ice sheet melting will contribute comparably with the oceanic thermal expansion, and also deep ocean warming might be larger than the IPCC projection. Thus, the current IPCC projection of 18–59 cm rise could be easily exceeded. The task of the scientists is to make a narrower projection of the additional components associated with Greenland ice sheet melting and deep ocean warming.

2. Decay of glaciers, ice caps and Greenland and Antarctic ice sheets (impact on human society and distinct indicator)

The sea level rise in the 21^st century is predicted to be mainly caused by thermal expansion of sea water and the melting of mountain glaciers and small ice caps. However, rapid decay of the large ice sheets in Greenland and Antarctica was detected recently using satellite measurements (Chen et al., 2006; Velicogna and Wahr, 2006), in conjunction with a general acceleration of ice streams and outlet glaciers. It is an urgent question how fast those ice sheets will decay due to dynamical processes caused by fast ice flows on ice sheet bases.

First of all, extensive observations have confirmed mountain glacier melting to be a significant component of the sea level rise during last 50 years. If the global warming proceeds at the current rate, more than 10 cm sea level rise may occur only through the melting of mountain glaciers and ice caps outside Greenland and Antarctica. In Greenland, the accelerated ice sheet decay is nearly consistent with the model results that attribute it to enhanced basal sliding caused by surface melt-water percolating to the base (e.g., Greve, 2008). However, the detailed dynamical processes are yet to be explained. If this trend continues, ice sheet decay in this century may be equivalent to several tens of cm of sea level rise. The other sensitive ice sheet exists in West Antarctica, where penetration of ocean water under the ice and consequent loss of ice shelves may cause a similar, dynamically increased decay. In-situ observations, satellite measurements and modeling should be combined to provide a more reliable projection for those two regions.

3. When will summer Arctic sea ice disappear? (distinct indicator and positive feedback)

The latest IPCC Report gave the prediction that the Arctic ice cover in summer will disappear near the end of this century. On the other hand, the recent ice cover reduction was significantly faster (Stroeve et al., 2007). Since the ice cover was anomalously small in 2007 summer, some scientists believe disappearance of summer ice cover by 2020. However, geoscientists realized significant year-to-year variability in nature, and it is an urgent task for them to examine whether such an early disappearance is realistic or exaggeration.

Some coupled ice-ocean models are capable of reproducing interannual to decadal variability in the Arctic sea ice cover, when they are forced with observed atmospheric conditions. In particular, modeling of sea ice distribution and temporal variability requires realistic sea water exchange between the Arctic and the Atlantic/Pacific (Maslowski et al., 2007, Stroeve and Maslowski, 2007). A more challenging but necessary task is to simulate sea ice thickness, which is believed to decrease faster than the areal coverage in the last 30 years based on limited data sets and models. As the ice cover decreases, sea ice experiences a new circumstance between a positive feedback (such as ice-albedo feedback) in summer and potentially a negative feedback (an increased air-sea heat flux through open water) in winter. In addition, feedback processes from the ice-ocean system to the atmosphere must be formulated accurately for more reliable future projection to confirm summer ice disappearance before the end of this century. Also, seesaw-like ice cover fluctuations occurred in last two decades over the Pacific and the Atlantic sectors in correlation with the dipole pattern over the Arctic Basin (Wu et al., 2006). Therefore, long-term projections from the ice cover data for the last decade are of limited use, and careful verification is needed between models and observed data with a focus on the crucial processes.

4. Carbon uptake or emission by terrestrial ecosystem

Terrestrial ecosystem is expected to absorb more carbon dioxide until mid-21^st century. The uptake will reduce then, but it is so variable among the future prediction models as to when the ecosystem will change into a source. As this change becomes true, more anthropogenic carbon dioxide will linger in the atmosphere and accelerate global warming.

As temperature rises moderately, terrestrial ecosystem takes a longer growing period and makes higher photosynthesis. In addition, a higher carbon dioxide content supports photosynthesis also. Observations of carbon dioxide flux over some forests have confirmed so far this trend. However, ecosystem respiration overcomes photosynthesis as temperature becomes higher. Soil warming and moisture increase enhance decomposition of soil organic matter so that more carbon dioxide may be emitted. Sensitive regions to global warming are boreal forests, in which trees have adjusted to low temperature and low light, and tropical peat land and permafrost, which respond to temperature rise and drought in addition to biomass burning, emitting carbon dioxide to the atmosphere. The further studies are needed on ecophysiological processes.

5. Marine ecosystem change resulting in carbon emission

In future prediction models, oceanic uptake of carbon dioxide is stable with a moderate feedback from the climate change. In addition to the moderate warming effects on solubility, the subpolar regions with high primary production will shrink significantly. Formation of the North Atlantic Deep Water is well predicted to reduce greatly. Therefore, it is reasonably predicted that oceanic uptake will reduce under global warming.

It is well known that formation of the North Atlantic Deep Water has been reduced in a last couple of decades. Under global warming, vertical water exchange will decrease in the Southern Ocean, equatorial regions and the northern North Pacific. Hence, primary production may significantly change in these regions. A dominant phytoplankton species may change: e.g., from diatom to coccolithophorids. If such a community shift occurs, it reduces alkalinity and may change the ocean from sink to source. The feedback magnitudes derived from these changes are still unknown factors. Uncertain mechanisms have to be clarified and quantified as soon as possible with combination of field survey, laboratory experiments and modeling.

References

Chen, J.L., C.R. Wilson and B.D. Tapley, 2006: Satellite gravity measurements confirm accelerated melting of Greenland ice sheet. Science, 313, 1958-1960, doi:10.1126/science.1129007.

Domingues, C.M., J.A. Church, N.J. White, P.J. Gleckler, S.E. Wijffels, P.M. Barker and J.R. Dunn, 2008: Improved estimates of upper-ocean warming and multi-decadal sea-level rise. Nature 453, 1090–1093, doi:10.1038/nature07080.

Greve, 2008: Increased future sea level rise due to rapid decay of the Greenland ice sheet? Proceedings of the First International Symposium on the Arctic Research (ISAR-1) (in press).

Stroeve J., M.M. Holland, W. Meier, T. Scambos and M. Serreze, 2007: Arctic sea ice decline: Faster than forecast, Geophys. Res. Lett., 34, L09501, doi:10.1029/2007GL029703.

Stroeve, J., and W. Maslowski. 2007: Arctic sea ice variability during the last half century, In Climate variability and extremes during the past 100 years, eds. S. Brönnimann, J. Luterbacher, T. Ewen, H. F. Diaz, R. S. Stolarski, and U. Neu, 143–154. Netherlands: Springer.

Velicogna, I., and J. Wahr, 2006: Measurements of time-variable gravity show mass loss in Antarctica. Sciencexpress: doi:10.1126science.1123785.

Wu, B., J. Wang and J.E. Walsh, 2006: Dipole anomaly in the winter Arctic atmosphere and its association with sea ice motion, J. Clim., 19, 210–225.