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ENGLAND & NAKICENOVIC: Bush, Cullenward, Frisch, Nguyen, Nkem, Robinson

Part II: The Ocean’s Role In Climate and Climate Change

07/15/09 Notes by D. Robinson.

1. The ocean's role in the Earth's climate system

Oceans play a critical role in determining the climate of the Earth. Covering roughly 70% of the Earth’s surface and extending in depths greater than 5000 meters, the capacity for the oceans to absorb and release solar radiation is profound. Unlike its terrestrial counterpart, which is typically able to store solar energy to a shallow depth of 1 meter, oceans can absorb solar radiation up to ~1000 meters in depth. While their ability to store energy is partly facilitated by surface water mixing, the density and temperature differentials in ocean surface waters, along with wind patterns, create ocean currents that create downwelling of warm water in some locations and upwelling of cool water near Antarctica. Effectively these surface and deepwater currents transport stored solar energy throughout the Earth’s oceans, and correspondingly influence local weather systems and global climate.

The degree to which we can observe climate alteration by oceans is best illustrated by the annual range of monthly mean temperatures in locations in proximity to oceans versus those found in the terrestrial interior. Continental areas have large differences between average winter daily temperatures and average summer daily temperatures (up to 20-25 degrees C), whereas more coastal locations have less variation (a range of about 8 degrees C). Oceans essentially buffer coastal climates. Oceanic currents can also shift regional climates. For example, the Gulf stream that travels up the east coast of North America transports warm equatorial water north towards the east coast of Iceland. As the warm water evaporates it is transported eastward by the prevailing Westerlies (strong winds), which leads to an increase in air temperatures across Scandinavia and parts of Western Europe. These effects influence the livelihoods and even energy consumption of the local populations, who face a milder climate than what would otherwise be present without today’s ocean currents.

Oceans themselves are greatly affected by salinity gradients. The evaporation of warm water traveling north as part of the North Atlantic current has a relatively low salinity (~34 parts per thousand). As the water travels between Iceland and the United Kingdom it begins to evaporate, which increases the salinity (~37 parts per thousand) and creates a density gradient that causes a downwelling of the newly cooled and denser ocean waters. This downwelling acts in part to pull warm and less dense surface water into that area as it begins its travel toward Antarctica. The movement of warm surface water up the eastern coast of North America is driven by Easterlies near the equator. New cool surface water travels down the west coast of Europe and Africa to replace the warm waters travelling west near the equator and complete the North Atlantic gyre. Gyres are found throughout the oceans of the Earth and typically correspond in latitude to global wind patterns (e.g. Hadley or tropical, mid-latitude, and polar cells). The duration for water to completely circulate a gyre is on the order of decades (sometimes 10-15 yrs). In contrast the vertical ocean changes based on the thermohaline as discussed above may take 1000’s of years to complete a cycle.

Several factors could contribute to altering the North Atlantic current and therefore global climate. One possibility is that increasing temperature alters the ocean’s density through thermal expansion and causes a shift in the North Atlantic current. A second possibility is the slowing or shifting of the North Atlantic current due to a change in salinity. If the Greenland ice sheets were to melt, due to global warming, the waters entering the ocean would decrease ocean salinity and therefore slow down, shift the location south, and possibly halt the North Atlantic current. Rahmstorf and Ganopolski (1999) used a climate model to show that such an alteration of the North Atlantic current would lead to cooling in northern latitudes and create a full blown ice age . Furthermore, in the case of a complete shutdown of the current, the ocean may reorganize itself and not reestablish the North Atlantic Deep Water (NADP) current.

At the other end of the earth, warm water traveling from the equator toward the South Pole is diverted east and back towards the equator before reaching the Antarctic. Blocked by the Antarctic circumpolar current, this current protects the Antarctic ice sheet from the possibility of thawing. However, this wasn’t always the case: approximately 35 million years ago the tip of South America was connected to Antarctica. This connection directed warm ocean currents further south puncturing into the protective circumpolar current, which caused a thawing of the ice sheets that subsequently reduced the albedo and altered the climate of the region significantly. Now the gap that exists between South America and Antarctica–the Drake Passage–is the narrowest point of the Southern ocean.

2. The ocean's capacity to modulate climate change

The relatively low thermal capacitance of land surfaces renders land unable to buffer the greenhouse effect. This is evidenced by a simple comparison of the temperature of sand at the top of a soil profile in the desert versus that the temperature if one was to dig down a meter. It would be much more comfortable to stand on the soil found one meter below the ground than to stand on top of the soil profile. In contrast, the ocean offers a large capacity to buffer climate change. Ocean temperatures remain relatively constant for depths of ~50 m and subsequent water temperature changes are only slight and change in smooth transitions with increasing depth up to 1000 m.

In addition to modulating climate as a heat storage and transfer mechanism, oceans act as a carbon sink. The ocean absorbs CO2 through two processes, which are termed the solubility pump and the biological pump. The solubility pump describes the oceans’ capacity to absorb CO2 through diffusion from the atmosphere into the oceans through mixing at the surface boundary layer. The rate of solubility of CO2 into the ocean has an inverse relationship with air temperature, so that with cooler temperatures there is a greater ability for CO2 to dissolve into the oceans. As atmospheric CO2 increases, more CO2 can be dissolved in the oceans, increasing the acidity of the ocean, which significantly alters ocean ecosystems. The solubility pump is further enhanced by the downwelling of CO2 rich ocean waters, which are able to store carbon for 1000s of years. A second mechanism, known as the biological pump, also facilitates the oceans ability to moderate climate change. Algae and other biotic organisms metabolize CO2 and incorporate it into the food chain of the ocean. The fecal matter of these organisms and in some cases their bodies, upon death, precipitate down to the bottom of the ocean to create a pool of carbon. It is estimated that this biological pump sequesters approximately one third of anthropogenic emissions in the ocean. While some have experimented with enhancing the biological pump through iron fertilization, it is not known if the biological pump naturally increases with increased atmospheric CO2.

3. The ocean’s capacity to bite back

While the oceans have a tremendous capacity to moderate global climate change they also have the capacity to disrupt our systems. With warmer ocean temperatures the intensity of precipitation events, hurricanes, and cyclones increase. The warming of the ocean also alters its density and creates thermal expansion of ocean waters. It has been estimated that the thermal expansion of ocean waters with a two degree centigrade warming would result in approximately a one-meter increase in global sea levels. A further and much more substantial sea level rise would occur with the melting of the Greenland and western Antarctic ice sheets. Rises in sea level would greatly alter coastal ecosystems and inundate terrestrial lands causing significant economic costs and disruption of local livelihoods

07/15/09 - Climate models

An overview was first given about the definition, classification, properties and benefits of a climate model. The process of building a model extends to verification, validation and assessment. Computer models are often preferred for their advantages in terms of expense, comprehensiveness, logic, accessibility, flexibility (capable of examining, easy for sensitivity analysis, parameter tuning etc).

When it comes to modeling climate change we talk of models of the ocean and atmosphere. There are 6 governing equations: for ocean there are 3 of momentum, conservation for heat, salt, and mass and the same is for the atmosphere with humidity instead of salt. Forcing conditions are added to represent only processes at the top of the atmosphere for full blown climate models. Starting with initial conditions we solve the governing equations over a discrete grid using observations in the forcing conditions and then integrate the solutions forward in time. Running submodels or subcomponents of the Earth requires many more forcing conditions that need to be addressed. Hence it is harder to run submodels rather than a whole model.

What kind of grid is employed determines what kind of dynamics it will represent effectively. In the late 80s, the resolution is (4 degrees by 4 degrees) 400km by 400km. Now it is about 1 degree by 1 degree. When we can't get what we want, we go down to a finer resolution by nesting a sub-grid of finer scale on top of the coarse grid. Sometimes we have to change the model or some models give us the option of where to zoom in. The option of nesting different resolutions allows for adding the affects of cities, etc that are quite small but can effect local weather/climate significantly.

The vertical vs. horizontal resolution is another issue. A spectral grid is such that the lines are parallel along the y axis but bend along the x axis. 1D e.g. that about an air column in the atmosphere and 2D models e.g. Henry Stommel's 2-boxes models still have good uses on their own. 3D models employ spectral grids which minimize numerical erros in atmospheric models.

The development of grids for use in climate modeling exemplifies only part of the progress since the 1970s. It would be interesting to track down the progress, quantitatively and qualitatively, of the scientific modeling community. What they have in common is for the most part models are run on FORTRAN90. The language is better in picking out errors in floating points. It is worth noting that the process of running a model to get its output and spatial visualization are separate. Having models is one thing, having interfaces where researchers will simply walk in and use is another option and there are some good ones. There are good models and fake ones that look like real. This is not a parade. Open access is important for refining models and sharing ideas in modeling climate changes.

Model assessment makes use of CFCs which invade the ocean in very specific way and hence can act as tracers in ocean circulation experiment. As these were introduced, the presence/level of CFCs in deep water provides an excellent sense of movement and mixing. This can be used to double-check the accuracy of ocean models, being more precise than temperature and salinity measurements. Among uncertainties in existing models, Greenland ice is a big uncertainty (1.6m is likely sea level rise, 85cm from land ice). Another is sulfite areosols–a pollutant that has helped to keep things cooler. Our effort to decarbonization will reduce their presence in the atmosphere, the impact of which is again unknown.

RUSSELL & GRÜBLER: Bamutaze, Borgeson, Engler, Ona, Pasqualini, Zellner

Media:Russel_summary.doc Here is the summary of Joellen Russel's climate modeling talks. Media:Grubler_summary.doc Here is the summary of Arnulf Grubler's talks.

RUBBIA & KUTSCHER: Gonzales, Hagerman, A. Jones, Krakauer, LaCerva, Lacroix

Media:Nir_nuclear.doc Here are comments on Carlo Rubbia's talk on nuclear energy.

Participation

Active, assertive participation by students is an intrinsic part of this event. Attendance at all program sessions is mandatory. Participants have been divided into six groups; each group designed to be as topically and geographically diverse as possible. Chairperson and rapporteur roles may rotate within each group.

Group Tasks

Groups have several tasks. One function is to discuss each day’s lectures among themselves and to be prepared to actively participate in the group discussion sessions each day. In addition, each group will be tasked to write a summary of the main points of the two lecturers, one from the first week and one from the second week. Responsibilities may be divided (that is, each of the six students might be responsible for producing a lecture transcription, editing duties could be divided, etc). The lecture summaries should include comments by the group on the broader implications of the lectures, critical analysis of the research area covered, and resonance to other presentations. Each speaker will be available to meet with the groups covering his/her lectures. This material may be edited and produced in book form.

Research Agenda

Additionally, each group will be asked to produce a research agenda as a product of the school. This would not be limited to the lectures each group covered, but rather focus on the entire two-week agenda and identify synergies, areas of disagreement, and gaps in our knowledge that can be resolved by future research. The final day of the school will include a plenary session during which each group’s rapporteur presents the research agenda. Finally, we will draw on these agendas to produce an open letter to President Obama or Science Advisor John Holdren.