Recently, I have seen my self writing a lot about environmental issues related to climate changes around our globe. Maybe the reason was the past GONO cyclone or maybe my over admiration to the topic not from now, but all over my childhood years. After a couple of articles about the over all weather changes and GONO cyclone, I read a topic alos related to climate change few days ago which I find it interesting to re-write using my language to dilute its extra scientific text that any reader may not understand. But since I am good and familiar with the matter, humbly, I will share the knowledge with you in as brief as possible.
Throughout thousands of years, It has been discovered that the earth has undergone a series of climate changes or if we can say Ice Ages seperated hundreds of years apart. The discovery was the result of the extensive study for the Greenland's Ice plate that reach the hight of several miles and is considered the most precious element that helped and still helps scientists to dig out more and more data about the past climate and global changes.
In brief, Ice cores hold an amazingly detailed record of Earth's climate. Each year, snow falling on glacial areas accumulates, piling on top of thousands of years of past snow, compressing the snow into yearly layers of ice, like rings inside a tree trunk. Preserved in the ice are tiny bubbles of ancient air that tell the composition of the atmosphere at that time. The amount of dust in the snow tells how windy the climate was. The thickness of the layer tells how much precipitation fell that year. Most importantly, the amount of the "heavy" isotope of oxygen, 18O, lets scientists infer the average atmospheric temperature, since water vapor with "heavy" 18O molecules condenses out of clouds more readily at cold temperatures.
What the scientists found was surprising and unnerving. Scientists had known from previous ice core and ocean sediment core data that Earth's climate had fluctuated significantly in the past. But what astonished scientists was the rapidity with which these changes occurred. As seen in the above Figure, the ice core record showed frequent sudden warmings and coolings of 15°F (8°C) or more. Many of these changes happened in less than 10 years. And in at least one case 11,600 years ago, when Earth emerged from the final phase of the most recent ice age (an event called the Younger Dryas), the Greenland ice core data showed that a 15°F (8°C) warming occurred in less than a decade, accompanied by a doubling of snow accumulation in 3 years. Most of this doubling occurred in a single year.
Current theories on the cause of abrupt climatic change focus on sudden shut downs and start-ups of the Meridional Overturning Circulation (MOC) (also referred to as the thermohaline circulation), a complex network of ocean currents in the Atlantic. The Gulf Stream forms a portion of the MOC. The Meridional Overturning Circulation transports a tremendous amount of heat northward, keeping the North Atlantic and much of Europe up to 9°F (5°C) warmer, particularly in the winter. A sudden shut down of this current would have a ripple effect throughout the ocean-atmosphere system, forcing worldwide changes in ocean currents and in the path of the atmospheric jet stream. Studies of North Atlantic Ocean sediments have revealed that the Meridional Overturning Circulation has shut down many times in the past, and that many of these shut downs coincide with the abrupt climate change events noted in the Greenland ice cores.
How does one shut down the Meridional overturning circulation ? To answer this, we first must describe the Great Ocean Conveyor Belt (Figure 2), the system of interconnected ocean currents that girdle the planet. At the surface, ocean currents are driven by the winds, and so move parallel to the wind direction, except where continental land masses block the way. Water can also move vertically in the ocean. High density water sinks, and low density water rises. Salty water is more dense than fresh water, and cold water is more dense than warm water, so that wherever we find cold, salty water, it tends to sink.
In the tropical Atlantic, the sun's heat evaporates large amounts of water, creating relatively warm, salty ocean water. This warm, salty water flows westward toward North America, then up the East Coast of the U.S., then northeastward toward Europe, forming the mighty Gulf Stream current. As this warm, salty water reaches the ocean regions on either side of Greenland, cold winds blowing off of Canada and Greenland cool the water substantially (in the up-Figure , these regions are marked with white circles labeled, "Heat release to the atmosphere.") These cool, salty waters are now very dense compared to the surrounding waters, and sink to the bottom of the ocean. Thus, the oceanic areas by Greenland where this sinking occurs are called "deep-water formation areas". This North Atlantic deep water flows southward toward Antarctica, eventually making it all the way to the Pacific Ocean, where it rises back to the surface to complete the Great Ocean Conveyor Belt. It takes about 1000 years for the water to make a complete circuit around the globe.
Since the Great Ocean Conveyor belt is driven in part by differences in ocean water density, if one can pump enough fresh water into the ocean in the key areas on either side of Greenland where the Gulf Stream waters cool and sink, this will lower the ocean's salinity (and therefore its density) enough so that the waters there no longer sink. The Atlantic conveyor belt and Gulf Stream current will then shut down in just a few years, dramatically altering the climate.
The key question remains: how much fresh water is needed to shut down the Atlantic conveyor belt? No one knows the answer. Scientists are pretty sure that the last two abrupt coolings seen the the Greenland ice core, the "Younger Dryas" event and the "8200 years before present" event (1st Figure), both occurred when huge North American glacial melt-water lakes flooded down the St. Lawrence River into the North Atlantic when the ice dams restraining the lakes broke. The sudden addition of low-density fresh water presumably partially or totally stopped the sinking of ocean waters in the North Atlantic, slowing or completely stopping the Meridional overturning circulation. Once the fresh water got into the North Atlantic, it stayed, puddling on top of the ocean and freezing in winter. The Meridional overturning circulation stayed shut off for about 1100 years during the Younger Dryas event, then suddenly restarted, for reasons scientists don't understand. Current computer models of the climate cannot reproduce the observed sudden shut-down or start-up of the Meridional overturning circulation at the beginning and end of the Younger Dryas period.
Other sudden shut downs of the Meridional Overturning Circulation observed in ice core and ocean sediment records are not thought to be due to sudden melt-water floods into the North Atlantic. These events may have happened simply because Earth's climate system is chaotic, or perhaps because some critical threshold was crossed when increases in precipitation, river run-off, and ice melt put enough fresh water into the ocean to shut down the Meridional overturning circulation.
So the current studies are relying on the probability that global warming may result in a new shut down on the Conveyer Belt. The studies state that with the increased increase in global temperatures, heavy rains will increase river run-offs and polar snow melts which will all dump in the ocean increasing the fresh water amount in the ocean, which may decrease the density of the high salty cold water and stops it from sinking and thus shutting down the stream. But what is still vague is how much freash water shoulf be pumbed in order to shut down the stream. Apparently, in the current measures, this will not happen in the following 100 years, so we are still safe, but will our children and grand children be?
