Atmosphere, Oceans and Climate

The atmosphere plays a key role in the energy balance of the Earth. It scatters or reflects about 30% of the incoming solar radiation through molecular scattering and the presence of aerosols and clouds. Some of the incoming radiant energy at the surface of the Earth is reflected but the rest is transformed into long-wave radiative flux, and sensible and latent (evaporative) heat fluxes, the portioning depending on the nature of the surface. The atmosphere also modulates the escape of long-wave radiation to space.

While some of the long-wave radiation leaving the Earth’s surface is transmitted relatively unimpeded through the atmosphere, the bulk of the radiation is intercepted and re-emitted both up and down. The resultant emission to space occurs at much higher and colder levels than the surface, a manifestation of the greenhouse effect. Human activities have changed the carbon balance of the atmosphere to increase the greenhouse effect, creating an imbalance between absorbed and emitted radiant energy that is producing global warming of the earth’s surface and lower atmosphere.
Clouds also absorb and emit thermal radiation and have a blanketing effect similar to greenhouse gases. But clouds are also bright reflectors of solar radiation and thus act to cool the surface. The net effect for the current atmosphere, based on satellite observations, is a small cooling of the surface. However these two opposing effects on the global radiation balance bring considerable uncertainty into the feedbacks associated with cloud processes in the context of global warming.

Other feedbacks also manifest themselves as part of the atmosphere’s role in climate, for example through increased atmospheric water vapour content, itself an important greenhouse gas, in a warming atmosphere. The distribution of land and ocean, and the incoming solar radiation, also contribute to the formation of certain patterns/phenomena that are observed both in weather and climate time frames, often referred to as regimes. For example, in the northern hemisphere, the dominant circulation pattern is the North Atlantic Oscillation (NAO) and the closely related Northern Annular Mode (NAM) that reflect changes in the westerly winds across the Atlantic into Europe, while in the southern hemisphere the Southern Annular Mode (SAM) is active year round and depicts changes in the strength and latitude of the prevailing westerly winds and storm tracks.


These appear to be predominantly atmospheric phenomena, but important questions arise concerning how they contribute to the changes in the climate system, and also how they may, in turn, be affected by climate change. Understanding these regimes and representing them properly in climate models is essential. At present, the oceans absorb approximately one-third of the total amount of greenhouse gases emitted to the atmosphere. This leads to ocean acidification, which is being observed in the oceans globally. As a result of the high thermal capacity of the water and the huge mass of the ocean, the oceans have accumulated more than 90% of the surplus heat associated with increasing greenhouses gas concentrations since the 1950s. This has correspondingly reduced the associated heating effect on the atmosphere, but implies a long-term warming commitment even if the rate of emission of greenhouse gases were to be reduced to zero.

The ocean sequesters heat and moves heat, salt, and chemicals through the ocean currents, releasing them in different places and times. How ocean circulation is affected as the climate changes is a key issue. Because of the large mass (average depth 3,800m), high thermal capacity of water and areal extent of oceans, ocean-atmosphere interactions and ocean responses involve slow processes that are predictable over a variety of timescales. For the prediction of future climate on secular timescales, one of the most difficult but unavoidable science questions is how the carbon storage in the oceans will evolve in time. Most predictions expect that the future capacity of the oceans to absorb carbon will diminish.

To evaluate further the oceans’ capacity to absorb and store carbon, one needs to develop a reliable physical and biogeochemical description of the world oceans. Our understanding of the ocean is rapidly evolving due to activities of several research projects, such as CLIVAR (WCRP), SOLAS (IGBP, SCOR, WCRP, and CACGP), and IMBER (IGBP and SCOR), the Ocean Carbon Coordination Project (IOC, SOLAS, IMBER, CLIVAR) and advances in ocean observations. Also contributing is the ESSP Global Carbon Project, which develops and issues assessments of the current geographical and temporal distributions of the major components and fluxes in the global carbon cycle.
For all numerical climate predictions on time scales from several months to years and out to decades, there is a need to represent the initial observed state of the atmosphere and oceans to the optimal extent possible for any particular application. Two past WCRP experiments (the Tropical Ocean Global Atmosphere, TOGA, project and the World Ocean Circulation Experiment, WOCE) enabled better understanding of ocean circulation and its interactions with the atmosphere. TOGA, in particular, helped to improve predictions of the El Niño/Southern Oscillation and exploit this predictability in a variety of seasonal predictions. WOCE provided an unprecedented snapshot of the global ocean circulation.
Availability of satellite observations, global deployment of the autonomous floating Argo buoys, successful demonstration of the capability to assimilate ocean information under the framework of the Global Ocean Data Assimilation Experiment (GODAE), and increasing accuracy of pioneering ocean data syntheses by CLIVAR are enabling better observations and understanding of the role of the ocean in climate variability and change and the prospects for climate prediction across a variety of timescales. In order to exploit the predictability of the coupled climate system, it is imperative to correctly represent in models the fluxes of momentum, heat, moisture, gases and particles between ocean and atmosphere, to which SOLAS, CLIVAR and GEWEX contribute, and to cover gaps in global ocean observations including areas covered by sea ice. This last challenge is being addressed by the WCRP CliC project and its partners.




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