About 34 million years ago, the Earth's climate transitioned from a 'greenhouse climate' to the 'icehouse climate' of today, forming a massive ice sheet on the Antarctic continent. A new study by Linda Anderson, an ocean sciences researcher at the University of California, Santa Cruz, suggests that oceanographic features in the Southern Ocean--the intensity of current flow and the amount of stratification (the formation of distinct layers at different depths)--may have played a key role in the transition.
Anderson will present her findings this week at the Fall Meeting of the American Geophysical Union in San Francisco.
Anderson analyzed the chemical properties of seafloor sediments laid down millions of years ago to piece together a picture of how Southern Ocean circulation may have looked deep in the past. The periods covered in her study include the transition from the Eocene to the Oligocene epochs about 33 million years ago and a similar transition between the Oligocene and Miocene epochs about 23 million years ago. These transitions coincided with a configuration of Earth's orbit around the Sun that facilitated ice growth. Some additional factor on Earth, however, amplified the climate response during these transitions.
The geological record suggests that this additional factor was a reduction of greenhouse warming due to a decrease in atmospheric carbon dioxide. The Southern Ocean may have played a role in the drawdown of atmospheric carbon dioxide through its influence on the global carbon cycle, Anderson said.
Her study suggests that, unlike today, the mixing of the Southern Ocean around the time of the climate shift was neither as intense nor as deep as it is now. As a result, the ocean was more stratified and regional characteristics of deep and intermediate waters were maintained. This layered structure may have had important consequences for global carbon cycling, setting the stage for the transition from greenhouse to icehouse.
Phytoplankton--tiny plants growing in the surface waters--use carbon dioxide from the atmosphere and transform it into organic carbon. When the plants (or animals that feed on them) die and decompose, most of the organic carbon is recycled but a small amount is buried within the deep ocean. In a layered ocean, dead organisms can drift into the deeper layers before they decompose, effectively burying the organic carbon in the depths.
Shifting continents have gradually changed the Southern Ocean over the past 30 million years so that water now whips around Antarctica in a strong current known as the Antarctic Circumpolar Current. The strength of the current blocks the influx of nutrient-poor surface water and, as this current squeezes through the narrow passage between South America and Antarctica, it mixes the water from top to bottom. As a result, most of the organic carbon formed within the Southern Ocean today is oxidized before it can be buried.
The impact of organic carbon burial on the atmospheric carbon dioxide depends on the relative burial of organic carbon to total carbon. The types of organisms that fix organic carbon are important, because some form shells of inorganic carbon in a process that releases carbon dioxide. Regional characteristics of the water affect which organisms are favored ecologically by controlling the nutrient content of the surface ocean. The integral relationship between biology (the organisms that fix the organic carbon) and the physical structure of the ocean (both the delivery of nutrients and removal of organic carbon for burial) ultimately control the atmospheric carbon dioxide, Anderson said.
Currents in Climate Change
But that doesn't mean that climate change isn't happening--ice core data stretching back 650,000 years show that current greenhouse gas levels of 380 parts per million (ppm) are about 80 ppm higher than any other level in that period, and still rising. And it doesn't mean that the results of such a weakening in currents couldn't have a profound impact. Between 12,800 and 11,500 years ago, the Earth experienced a temporary setback in its warming up after the last ice age. The creation of the St. Lawrence Seaway filled the North Atlantic with an excess of cold, fresh water, preventing North Atlantic salt water from achieving the density needed to sink. As a result, the conveyor belt shut down and Western Europe and North America became colder and drier. We may need that dramatic sweater mission after all.
All this just makes the international effort unfolding in Montreal this week even more important. In that freezing cold, Francophone city, diplomats from 157 countries continue to discuss climate change and what to do about it. Already last week, they ratified the Kyoto Protocol--our first, timid attempt to cut the greenhouse gas emissions associated with global warming--despite the best efforts of the U.S. and its allies to derail the global treaty. Next up for consideration is what to do after Kyoto runs out in 2012. After all we've got 80 ppm or more to shed from the atmosphere, a process that would take hundreds of years if we stopped all emissions today. The U.S. and others are adamant that such effort is not needed--or, at best, misguided, something I shall address in more detail in a future post. Indeed, some of our illustrious leaders have called climate change the 'greatest hoax ever perpetrated on the American people.'
Climate change is not a hoax, nor is it a conspiracy. It is a giant scientific experiment human beings are running on the only planet we currently have: What will happen when we bring greenhouse gas concentrations in the atmosphere to prehistoric levels and beyond? Perhaps nothing, perhaps inundation of low-lying parts of the world, perhaps a flood of environmental refugees, perhaps an increase in disease, or perhaps the loss of oceanic currents that help regulate the present conditions of our world. Maybe all of the above and more. We can't control the outcome of this experiment. We can, however, choose not to run it.
Scary thought that the likes of Bush and the Man of Steel are experimenters-in-chief.