Model − observation integration

Paleoclimate data obtained by ocean drilling are indispensable for reducing the uncertainty of current climate models used to predict climate over the next few centuries (IPCC Report: Solomon et al., 2007), and are thus of extreme societal relevance. The significance and importance of ocean drilling to improve models of past and future climate lies in the generation of datasets from different past boundary conditions that can challenge and improve existing climate models (see sections 2.1, 5.2.1 and 5.2.3), which can then use the data to better understand how Earth system components react to large amplitude perturbations. In addition, paleoclimate data provide estimates of underlying climate variability on multiple timescales ranging from annual through millions of years.

The INVEST meeting highlighted a number of specific recommendations for the future integration of modeling and observational components, including the necessity for integrated latitudinal and land-ocean transects, targeted time slices for data-model comparison, and the recovery of exceptionally preserved carbonate material for climate proxy application, for example from a Tanzania land-ocean transect (Pearson et al., 2009).

Climate models currently used to predict future conditions can be evaluated and tested by their ability to reconstruct past climatic conditions. One simple approach involves forward modeling of climate under relevant changes in estimated boundary conditions and comparison of the output with paleoclimatic observations, such as proxy records of past temperatures and pCO₂. INVEST discussions highlighted the difficulty of using paleoclimate data to provide quantification of past ocean circulation, windfield, and changes in the hydrological cycle, in contrast to more established temperature and sea-level reconstructions. When significant mismatches occur, this might indicate structural error in the model (missing or incorrect processes), poor parameterization in the model, errors in the data (or their interpretation), or a mis-specification of the appropriate experiment. When applied to a specific aspect of the climate system this approach can be used to assess the relative robustness of future predictions from different models. For instance, an ice-sheet model that fails to reasonably predict past Greenland ice volumes is not likely to predict future conditions on Greenland. A more statistical approach to quantify the uncertainty in future climate change predictions is to assess those

predictions in the form of probability distribution functions. This technique involves the production of an ensemble of possible model predictions of the future, often by varying uncertain parameters in the model that control the strength of physical, biological, and chemical feedbacks.

Multiple overarching themes were identified where observation-modeling integration is likely to play a major role over the next decade: (1) using ocean drilling and other paleoclimate data to reduce uncertainties for IPCC climate models and predictions; (2) using ocean drilling data to improve modeling of ocean-atmosphere circulation dynamics; (3) using ocean drilling data to provide test scenarios for Earth system models under extreme boundary conditions; and (4) using ocean drilling data to improve the understanding of the global carbon cycle.

There are a number of key uncertainties in the current modeling of climate that need urgent integration with paleoclimate data to reduce the uncertainty boundaries of future predictions (Solomon et al., 2007; Henderson et al., 2009; Huntingford et al., 2009). Henderson et al. (2009) summarized the key IPCC uncertainties where paleoclimate data are able to reduce model errors and where paleoclimate data have contributed:

 Paleoclimate archives show coherent regional changes in precipitation in different climate boundary conditions, including many regions where models disagree widely about precipitation change.

 Dated marine terraces provide clear evidence that sea level was 4-6 m higher than today at the last interglacial under conditions slightly warmer than today, and recent data have suggested the rate at which sea level can rise to this height from modern levels (Rohling et al., 2008).

 Diverse paleoclimate records have demonstrated the existence of large and rapid switches in Atlantic overturning, and the climate response to these switches on hemispheric and even global climate.

 Corals and other paleoclimate records have demonstrated the sensitivity of ENSO to modest changes in boundary conditions during orbital change (Pleistocene) and warming (Pliocene).

 Arctic temperatures have been quantified for many times in the past, demonstrating extreme polar warmth during the Cretaceous, Eocene, etc. and the conditions during the last interglacial that led to significant melting of Greenland.

 Past records of variations and perturbations of the global carbon cycle provide estimates of climate sensitivity that are quite different to those estimated by the IPCC.

Other emerging and important themes in climate model-data integration that need further verification include: an ice-free Arctic Ocean and interactions with adjacent continents, changes in meridional and zonal gradients in tropical sea-surface temperatures (SST), the unexpected amplitude of interdecadal-to-centennial scale climate variability in ‗warm‘ climates, and the sensitivity of the tropical rainbelt to high-latitude forcing. All of these topics require testing of the robustness of climate projections and the underlying assumptions. This will complement ongoing work of new proxy development, particularly for the climate and hydrological cycle, and the inclusion of ‗proxies‘ in inverse modeling approaches. Currently, the spatial and temporal

coverage of paleo datasets is often too sparse to fully constrain probabilistic inverse modeling approaches.

The following specific approaches were suggested during INVEST:

 Snapshot equilibrium simulation (atmosphere/ocean/biosphere general circulation model) is very accurate for 6 ka (mid-Holocene), 21 ka (LGM), 115 ka (last interglacial), and 3 Ma (Pliocene). Comparison between these model results and well-dated climatic records from the continent, ocean, and ice will provide critical insights into climate dynamics. Sites proposed during an ultrahigh-resolution ocean drilling workshop would contribute towards these aims (Thurow et al., 2009).

 A transient experiment of several tens of kyr with a simpler model (Earth system Model of Intermediate Complexity coupled with ice model) can be used to investigate climate variability (such as D-O/Heinrich events compared with marine temperature reconstruction). The proposed ‗Shackleton sites‘ off the Portuguese margin would contribute significant datasets to this enterprise (Hodell and Abrantes, 2009).

 A global mapping approach with the analysis of widely distributed records and their integration into an Earth system modeling study to identify teleconnections around the globe. This will foster interactions and collaborations with other research communities (ICDP, International Marine Past Global Change Study (IMAGES), etc.).

 Past ocean acidification and carbon cycle extrema provide the possibility to reduce uncertainty within the climate sensitivity parameter (see section 5.2.2). Recent work (Panchuk et al., 2008; Zeebe et al., 2009) has shown the opportunities and challenges provided by direct model-data comparison, including the lack of any pre-PETM constraints of the carbonate compensation depth (CCD) in the Pacific (Fig.

2.5).

 Past extreme glaciation and de-glaciation events offer the opportunity to link coupled climate models with observations (deConto et al., 2008; deSantis et al., 2009;

Merico et al., 2008), and require additional targeted drilling data from around high-latitude regions.

The Arctic polar regions (Stein et al., 2009) and the Antarctic Peninsula (deSantis et al., 2009) have been among the fastest warming regions of the globe in recent decades, and the IPCC projects the high northern latitudes to show the most warming in the next century. As a result, sea ice is projected to shrink in both polar regions, and an ice-free Arctic in late summer is projected by many models by the end of the 21^st Century or even sooner. Warmer polar regions may also impact the extent of permafrost and marine hydrates in these areas. This has implications not only for climate feedbacks and ecosystems, but also in the economic and geopolitical sphere. Recent studies (Bijl et al., 2009) have shown that past climates might have experienced latitudinal temperature gradients very different from those of today during episodes of warm climates. These data need to be collected to test whether climate models are able to re-create these climatic states.

Figure 2.5 Forward modeling of carbon cycle perturbations by comparison of paleo datasets that represent past evolution of the CCD vs. modeled carbonate sediment concentration from Earth system models of intermediate complexity from Panchuk et al. (2008). White circles indicate unknown CaCO3

wt%, whereas filled circles represent known CaCO3 wt% values according to the scale at right.