Climate sensitivity

The term ‗equilibrium climate sensitivity’ originates from the climate modeling community and is described in the IPCC 4^th assessment report (Core Writing Team For the AR4 Synthesis Report, 2007) as a measure of the climate system response to sustained radiative forcing. It is defined as the equilibrium global average surface warming following a doubling of CO₂ and greenhouse gas equivalent concentrations. The AR4 provides an assessment that ―climate

sensitivity is likely to be in the range of 2 to 4.5˚C with a best estimate of about 3˚C, and is very unlikely to be less than 1.5˚C. Values substantially higher than 4.5˚C cannot be excluded, but agreement of models with observations is not as good for those values‖. In the current context, climate sensitivity is more extensive than this narrow definition and crucially includes feedback processes that may operate on a wide range of timescales, including

phenomena such as carbon-cycle feedbacks, cloud cover, albedo, glacial processes, weathering, and acidification. Climate sensitivity may also be non-linear (feedbacks affected by feedbacks) and most likely depends on the state of the climate system (cold versus warm mean state). Climate sensitivity may also affect the characteristics of climate variability, for example the response to orbital forcing in a warm-climate compared to a cold-climate state.

AR4 states that, ―Models differ considerably in their estimates of the strength of the different feedbacks in the climate system‖ and that ―The magnitude of future carbon cycle feedbacks is still poorly determined‖. The main uncertainties highlighted in AR4 include: (1) incomplete understanding of the impacts of changes in ocean circulation on ocean CO₂ uptake; (2) the response of marine biota to ocean acidification (see section 5.2.2); (3) the role and response time of terrestrial vegetation feedbacks; and (4) our knowledge of the global methane cycle. Recent modeling studies show that uncertainty in carbon-cycle feedbacks are as significant as physical uncertainties in controlling the future increase of atmospheric CO₂ (Huntingford et al., 2009), thus allowing ocean drilling to make a major contribution to this debate.

“Recent modeling studies show that uncertainty in carbon cycle feedbacks are as significant as physical uncertainties in controlling the future increase of atmospheric CO₂.”

Huntingford et al., 2009

Increasing atmospheric CO₂ is the main driving force for future projected climate change. The rate of this increase is dependent on the addition of carbon to the atmosphere, but also on a range of feedbacks within the carbon cycle. Such feedbacks play a significant role in regional climate change through their influence on ecosystems, albedo, and water budgets. Understanding these biogeochemical feedbacks in the carbon cycle is thus fundamental to the prediction of future climate change. Important aspects of this cycle operate on timescales that make paleoclimate a powerful tool for better quantification (Henderson et al., 2009).

It is one of the major strengths of ocean drilling that it can deliver unique data necessary to quantify climate sensitivity in the past and contribute to understanding of the feedback processes that need to be

included for successful modeling of this topic of great societal relevance. This is possible because paleo proxy data can deliver estimates of both temperature and ocean chemistry, including pCO₂, from a

atmospheric CO₂ concentrations and temperature, allowing modelers to complement the knowledge gained from ice cores. Continuous, high-fidelity records from times with higher pCO₂ levels are only obtainable through drilling ocean sediments deposited during warm intervals of the past. As such, ocean drilling provides the means to answer some of the key questions related to the quantification of climate sensitivity. Ocean drilling only recently provided some of the key records of atmospheric pCO₂ concentrations throughout the Cenozoic (Palmer and Pearson, 2000; Pagani et al., 2005; Pearson et al., 2009) and temperature estimates from the deep-sea record (Zachos et al., 2001; 2008).

Early Cenozoic climate has received considerable attention because the response of climate to a broad range of high atmospheric values of pCO₂ can be examined from time periods relevant for future climate in the next few hundred years (Figs 5.3 and 5.4). The major hypothesis to be addressed by ocean drilling is: Ocean drilling can provide otherwise inaccessible high-resolution paleoclimate records on a multitude of timescales that provide information on the coupling between global temperatures and greenhouse gas concentrations, and also provide insights into the feedback processes that will allow a reduction in uncertainty of climate sensitivity and climate feedbacks for future climate predictions.

Throughout the history of ocean drilling (and paleoceanography as a consequence) there has been a co-evolution of proxy methodologies, obtaining improved and extended records, and framing new fundamental research questions. Ocean drilling has allowed us to demonstrate that paleoclimate data from Cenozoic and older time slices provide records of the global carbon cycle and global temperatures.

“The PETM either resulted from

Figure 5.3 Schematic diagram showing the potential of the geological record to constrain climate sensitivity estimates across a range of pCO2 values.

Ocean drilling will provide observations of global temperature change as a function of atmospheric pCO2 for past climates that can be compared to future predictions (Jim Zachos, INVEST meeting personal communication); red numbers indicate calendar years now and in the future, whereas the black indicates ages in the past. For progressively higher pCO2, possible geological analogues are found further and further back in time.

These data can provide answers to a number of key questions necessary to make progress with modeling future climate change, including:

 How have atmospheric CO2 levels and temperatures varied through time? What is the relationship between atmospheric CO2 levels, ocean chemistry (e.g., pH), depth of the CCD, and temperatures in various oceans and climate states?

 For past changes in the carbon cycle, what were the rates of pCO2 increase, temperature increase, and Earth system recovery, as compared to present and future rates?

 During past warming and cooling events, what was the partitioning between direct radiative forcing and feedback components to global temperature change (Zeebe et al., 2009; Pagani et al., 2006)?

 How has climate sensitivity changed in the past?

 Are there pCO2 thresholds in the climate system (cryosphere, ocean physical/chemical state)? What are the thresholds for different past time periods (deConto et al., 2008)?

 How does the meridional temperature gradient depend on pCO2? What are the implications for poleward heat transport?

 What are the properties of the hydrological cycle during warmer climate regimes? To what extent does it feedback on the climate state?

 How are transient climate states affected by climate sensitivity?

 Has Earth‘s system become more sensitive in the Neogene?

 What is the temporal evolution of pCO₂ (with higher accuracy)?

 How can we use orbital forcing to better understand climate feedbacks and variability? Why is there a change in response after 1 Ma?

Figure 5.4 Estimate of climate sensitivity for PETM (from Pagani et al., 2006). This figure demonstrates the

approach to

determining climate sensitivitiy from paleoclimate data. Note that it does not consider potential initial warming pre-dating the PETM.

 What were past methane levels?

 What role do boundary conditions play in determining Earth‘s climate (e.g., paleogeography, topography)?

New ocean drilling is required because it is the only way to obtain past records of large-amplitude climatic events that are necessary to fully understand and constrain climate sensitivity. Specific aspects of these requirements are given here.

Proxy methods to determine past pCO₂, pH, and carbonate-ion concentration have dramatically developed or improved in the last few years, allowing a more strategic reconstruction of past conditions. One exciting prospect is our ability to fully reconstruct past ocean carbon chemistry based on a new carbonate-ion proxy (B/Ca) and the dramatic improvement of our ability to measure ¹¹B as a pH proxy through new measurement techniques (see section 5.2).

In addition, very recently a new ‗clumped isotope‘ proxy has been added to the paleoclimate toolbox that will allow direct determination of sea-water temperatures and thus complete the requirements for a detailed reconstruction of past climate sensitivities.

Proportions of ¹³C–¹⁸O bonds in carbonate minerals are sensitive to their growth temperatures, independent of bulk isotopic composition. Thus, ‗clumped isotope‘ analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the ¹⁸O of waters from which carbonates grew (Eiler, 2007; Came et al., 2007). This proxy complements existing temperature, ice-volume, and carbonate-ion proxies (¹⁸O, alkenones, Mg/Ca), but currently requires a very large quantity of carbonate for measurement, thus necessitating additional coring of high-quality and

high-resolution cores. One further new temperature proxy developed during the past few years has been the TEX86 temperature proxy, based on the relative abundance of long-chain lipids from Crenarchaeota that appear to be ubiquitous in present and past oceans and have been shown to contain a paleotemperature signal (Kim et al., 2008). Although this proxy is being refined, initial applications of it have provided challenges for the climate modeling community, for example through the suggestion that during the PETM Arctic temperatures exceeded 20˚C (Sluijs et al., 2006; Bijl et al., 2009).

Previous drilling has demonstrated the need for more complete and global coverage to fully understand the coupled global carbon cycle, allowing us to more strategically select drill sites (Panchuk et al., 2008). One important requirement for additional drilling is to constrain the Pacific CCD prior to the PETM (Zeebe et al., 2009).

Existing cores provide only partial information necessary for climate sensitivity reconstructions, for many periods such as the early Pliocene. There is a need for latitudinal transects covering other warm climate intervals and continued funding is required to process existing cores for new proxies. In addition, there is an expressed need to obtain sediment cores from thermally immature, expanded sections that are stratigraphically complete. Ideally, these would be from clay-rich horizons with good carbonate, silica, and organic matter preservation (Pearson et al., 2009).

For the Miocene time interval, a much higher temporal resolution is required to address this problem. Needs include new drill sites located along latitudinal and depth gradients, as well as continent-ocean transects. Such improved geographic and depth coverage is necessary to constrain the location and latitudinal variation of the CCD and deep circulation and overturning in various oceans in detail so these data can be used in carbon-cycle models.

New drilling is needed specifically in shelf and marginal-sea regions when there is a requirement for clay-rich sediments that contain better preserved carbonates, organic fossils, and biomarkers (Pearson et al., 2009). These records will be complimentary to deep-ocean transects, as they are likely to be less complete, but will allow geochemical analyses that necessitate better preserved material.

The highest research priorities identified to address climate sensitivity and future predictions of climate are the use of chemical, isotope, and biotic proxies to reconstruct atmospheric pCO₂ levels and temperatures over the Cretaceous and Cenozoic, as well as the range, mechanism, and rates of past carbonate saturation states of the oceans.

The highest priority research objectives are then to:

 narrow the uncertainty and improve temporal resolution of paleo–pCO₂ and paleotemperatures.

 address how climate sensitivity and variability depend on the mean state and temporal variation of the climate system (Greenhouse vs. Warmhouse vs. Icehouse worlds).

 establish the spatial response of the climate system, especially temperature and changes to the hydrological cycle, requiring latitudinal and land-ocean transects.

Once long-term proxy records have been obtained it becomes possible to determine feedback systems, processes, and times necessary to switch the climate system between states. These high-priority research questions will then allow the

climate community to achieve the societally relevant goal to understand how climate feedback processes operate.

Scientific strategies required to determine and understand climate sensitivity include three main steps. As a first priority, the timescales of interest need to be identified and then continuous records need to be obtained across these timescales.

Possible target intervals include times of rapid and large-amplitude climate-state reorganizations such as the EECO (Zachos et al., 2001), the MECO (Bohaty et al., 2009), the Eocene/Oligocene boundary (Coxall et al., 2005; Liu et al., 2009; Pearson et al., 2009), the Oligocene/Miocene boundary (Pälike et al., 2006), and the Neogene (Pagani et al., 2009).

Identified target time slices should then determine drill site selection, with the requirement that the material to be cored allows the application of multiple proxies (both carbonate and organic-carbon based). This will probably require drilling near margins and in areas with high sedimentation rates. Over a ten-year period, it could be possible to conduct a latitudinal transect, following the GEOSECS approach (‗PALEOSECS‘) identified in the CHART report (2009).

Platform needs are dictated by the desirability to determine long-term pole-to-equator thermal gradients for specific time slices. One important missing piece is to recover the missing time intervals from the ACEX (IODP Expedition 302, Moran et al., 2006).

A new approach will also be required to fully exploit the opportunities offered by collaborating with international partner organizations. Interaction with ICDP is requested to connect continental processes with the oceans. Closer and more frequent interaction with the modeling community is required to develop synergies and expand observations globally. Interaction with the ice-modeling community is needed to fully appreciate glacial stream flows. More collaboration with physical oceanographers is required to comprehend the potential impact of ocean circulation pattern changes on climate sensitivity. Furthermore, ANDRILL and SHALDRIL expertise and data will form a natural link to fully exploit the breadth of data required to tackle this problem of extreme societal relevance.