Josefino Comiso - Polar Oceans from Space

Josefino Comiso 1

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Abstract

The polar oceans are important to study because they are key components of the Earths climate engine. We have learned that many processes in the polar regions are highly linked to the thermohaline circulation of the Worlds oceans as well as their physical, chemical and biological characteristics. The region is vast, and it is only through satellite remote sensing that we have been able to examine and investigate the mesoscale and large scale properties of many parameters in the region, including surface temperature, albedo, sea ice cover, plankton concentration and primary productivity. A brief history of the polar regions and satellite remote sensing is provided, and the role of the polar regions in the climate system is discussed.

Keywords

What makes the Earth so unique in the planetary system and, perhaps, in the universe is the abundance of water. Water is the result of the bonding of one atom of oxygen and two atoms of hydrogen. In solid form, it covers about 12% of the oceans in the Northern Hemisphere and 9% of that of the Southern Hemisphere during midwinter period. In liquid form, it occupies more than 70% of the surface of the Earth. In gaseous form, water makes up more than 60% of all greenhouse gases in the Earths atmosphere.

Water is an indispensable commodity, but its importance could easily be undermined because it is so readily available. Without water, the Earth would be a lot warmer during daytime than it is today, and it would be impossible for life as we know it to exist. The lush vegetation and the very diverse forms of life that inhabit the Earth came about only because of the presence and abundance of water. Without water, the Earth, when viewed from space, wouldnt have the color and radiance associated with cloud formations, remarkable land geometries, and beautiful sceneries.

Why do we need to study the oceans? The oceans are the largest bodies of water and are integral components of the Earths climate engine. Averaging about 4 km in depth and spread out over almost three quarters of the Earths surface, the oceans are big reservoirs of heat and serve as a moderating factor that keeps fluctuations of the surface temperature of the Earth from getting into extremes. They also serve as habitats of many forms of life at all trophic levels from micro-organisms to mammals. And for the unsuspecting observer, their physical properties are constantly changing. Through evaporation and precipitation, through growth and decay of sea ice, through wind forcing and iceberg calving, and through river runoff, the physical characteristics of the oceans are always evolving. The chemical compositions of both the ocean and the atmosphere are also being altered by exchanges between these two systems through the emission and/or absorption of atoms and molecules. The oceans are also very dynamic as they are occasionally being battered by strong winds causing the formation of fronts, eddies, and a more organized movement of mass, called ocean currents. Underneath the surface, its character is slowly evolving as large masses of water are transported around the globe through the so called thermohaline circulation. Why and how the oceans are changing must be studied and analyzed to understand why and how the climate is changing.

Why do we study the Polar Oceans? By polar oceans, we usually refer to the relatively cold portions of the Worlds oceans located at high latitudes and mainly covered by sea ice in winter. In the Northern Hemisphere, they correspond to the Arctic Ocean and peripheral seas including the northern parts of the Pacific and Atlantic Oceans. In the Southern Hemisphere, they correspond to the southern portions of the Pacific, Atlantic, and Indian Oceans, where all these big oceans are united and intermixed together as huge water masses transported around the vicinity of the Antarctic continent through the Antarctic Circumpolar current. These regions have been combined into a single entity, sometimes referred to as the Southern Ocean. While generally ignored because of its remoteness and inaccessibility, many important processes that affect the global oceans, climate, weather and sea level occur in these polar oceans. Even just by virtue of its vast extent, the sea ice cover, which encompasses about 6% of the worlds oceans, is an important climate parameter. More specifically, it serves as an effective insulator that keeps the warmth of the ocean from leaking to the atmosphere during the winter while it reflects solar radiation efficiently and keeps solar heat from being absorbed by the surface during the summer. It also causes spatial redistribution of salt during ice formation and decay. Furthermore, it causes the initiation of deep ocean convection that enables the ocean to ventilate itself. And during growth stages, it causes the formation of cold and high salinity bottom water which helps drive the aforementioned thermohaline circulation of the worlds oceans.

The shelf regions of the polar oceans and peripheral seas are among the most productive regions of the Worlds oceans, as observed from space. The regions are sites of convergence zones where the cold and denser surface water sinks while the warmer and nutrient rich water upwells to take their place. At the same time, the melt of sea ice causes the formation of a stable layer of low density sea water at the surface where planktons can readily grow and multiply rapidly because of the abundance of sunlight needed for photosynthesis. In particular, sea ice causes the water to be conditioned for optimum productivity. There are other factors that affect the distribution of planktons, such as wind that induces upwelling, iron, clouds, and grazing, but these are not strictly polar parameters. Furthermore, the polar oceans are the coldest regions of the oceans where the oxygen content is relatively high. Such conditions are favorable to animal life, and with plankton being among the primary source of food, it is not a surprise that the most productive region and richest fishing grounds on Earth have been found at high latitudes. At the same time, there are large regions that are unproductive especially in the deep ocean regions of the Arctic and the southern part of the Indian Ocean where iron and/or nutrients are likely limiting factors.

A very contentious issue that human society has been dealing with in recent years is the potential change in the Earths climate associated with anthropogenic greenhouse gases released to the atmosphere since the start of the industrial revolution. At the center of the issue is the net impact of atmospheric carbon dioxide concentration which has increased by 10% alone in the last 30 years and is now at 385 ppm (parts per million), which is significantly higher than the average range of 180-290 ppm during the last 650,000 years, as inferred from Antarctic ice cores. Concurrently, other greenhouse gases like methane and nitrous oxide have also been increasing. Although a significant warming signal has been documented in several studies, it is not clear what fraction of the signal can be attributed to greenhouse gas and what fraction is due to natural climate variability. An early resolution to the problem is likely to be found in polar regions because of a much greater sensitivity of the latter to global change. Modeling studies have shown that global warming signals, such as those associated with the observed increases in greenhouse gases in the atmosphere, could be amplified by 2-5 times in the polar regions because of ice-albedo feedback associated with the snow and ice that covers much of the regions. We now have a few decades of satellite data and our preliminary analysis of that data has already revealed that large changes on the surface of the Earth are occurring, especially in polar regions. Whether or not the reasons for these changes can be resolved to the satisfaction of every concerned segment of society is still not obvious in part because of the complexity of the Earths climate system and the very demanding requirements in data and computer capabilities of physical numerical models that are needed for simulating such a system and for making sensitivity studies and forecasts for the future. Previous comparisons of results predicted by several of these models show big discrepancies if not inconsistencies. This indicates that we are still not capable of simulating the physics of such a complex system very well. But things have improved a lot in the last decade through many dedicated programs to gain insights into some of these complex processes. And as more and more global data accumulates, as our understanding of the important processes within the system improves, and as computer systems get more versatile and powerful, our assessment of the impacts of anthropogenic activities will become more and more accurate. The most recent IPCC report (2007) has indicated a 90% confidence that the observed warming is indeed mainly associated with the increasing greenhouse gases. This is already a big improvement to the previous report indicating a 50% confidence.