And this has generally followed periods of large freshwater discharge into the northern N. Atlantic caused by rapid melting of glacial or multi-year ice in the Arctic Basin. It is thought that these fresh waters, which have been transported into the regions of deep water formation, have interrupted the conveyor by overcoming the high latitude cooling effect with excessive freshening. The ocean conveyor need not stop entirely when the NADW formation is curtailed.
It can continue at shallower depths in the N. Atlantic and persist in the Southern Ocean where Antarctic Bottom Water formation continues or is even accelerated. Yet a disruption of the northern limb of the overturning circulation will affect the heat balance of the northern hemisphere and could affect both the oceanic and atmospheric climate.
Model calculations indicate the potential for cooling of 3 to 5 degree Celsius in the ocean and atmosphere should a total disruption occur. This is a third to a half the temperature change experienced during major ice ages. These changes are twice as large as those experienced in the worst winters of the past century in the eastern US, and are likely to persist for decades to centuries after a climate transition occurs.
They are of a magnitude comparable to the Little Ice Age, which had profound effects on human settlements in Europe and North America during the 16th through 18th centuries. Their geographic extent is in doubt; it might be limited to regions bounding the N. Atlantic Ocean. Yet we begin to approach how the paradox mentioned above can happen: Global warming can induce a colder climate for many of us. Consider first some observations of oceanic change over the modern instrumental record going back 40 years.
During this time interval, we have observed a rise in mean global temperature. Because of its large heat capacity, the ocean has registered small but significant changes in temperature.
The largest temperature increases are in the near surface waters, but warming has been measurable to depths as great as meters in the N. Superimposed on this long-term increase are interannual and decadal changes that often obscure these trends, causing regional variability and cooling in some regions, and warming in others.
In addition, recent evidence shows that the high latitude oceans have freshened while the subtropics and tropics have become saltier. These possible changes in the hydrological cycle have not been limited to the North Atlantic, but have been seen in all major oceans. Yet it is the N. Atlantic where these changes can act to disrupt the overturning circulation and cause a rapid climate transition. A meter, high latitude buildup of fresh water over this time period has decreased water column salinities throughout the subpolar N.
Atlantic as deep as m. At the same time, subtropical and northern tropical salinities have increased. The degree to which the two effects balance out in terms of fresh water is important for climate change. If the net effect is a lowering of salinity, then fresh water must have been added from other sources: river runoff, melting of multi-year arctic ice, or glaciers. A flooding of the northern Atlantic with fresh water from these various sources has the potential to reduce or even disrupt the overturning circulation.
Whether or not the latter will happen is the nexus of the problem, and one that is hard to predict with confidence. At present we do not even have a system in place for monitoring the overturning circulation.
Models of the overturning circulation are very sensitive to how internal mixing is parameterized. Recall that internal mixing of heat and salt is an integral part of overturning circulation.
One recent study shows that for a model with constant vertical mixing, which is commonly used in coupled ocean-atmosphere climate runs, there is only one stable climate state: our present one with substantial sinking and dense water formation in the northern N. With a slightly different formulation, more consistent with some recent measurements of oceanic mixing rates that are small near the surface and become larger over rough bottom topography, a second stable state emerges with little or no deep-water production in the northern N.
The existence of a second stable state is crucial to understanding when and if abrupt climate change occurs. When it occurs in model runs and in geological data, it is invariably linked to rapid addition of fresh water at high northern latitudes. And now perhaps you begin to see the scope of the problem. In addition to incorporating a terrestrial biosphere and polar ice, which both play a large role in the reflectivity of solar radiation, one has to accurately parameterize mixing that occurs on centimeter to tens of centimeter scales in the ocean.
And one has to produce long coupled global climate runs of many centuries! This is a daunting task but is necessary before we can confidently rely on models to predict future climate change.
Besides needing believable models that can accurately predict climate change, we also need data that can properly initialize them. Errors in initial data can lead to poor atmospheric predictions in several days. So one sure pathway to better weather predictions is better initial data.
For the ocean, our data coverage is wholly inadequate. Efforts are now underway to remedy this. Global coverage of upper ocean temperature and salinity measurements with autonomous floats is well within our capability within the next decade as are surface measures of wind stress and ocean circulation from satellites. The measurement of deep flows is more difficult, but knowledge about the locations of critical avenues of dense water flows exists, and efforts are underway to measure them in some key locations with moored arrays.
Our knowledge about past climate change is limited as well. There are only a handful of high-resolution ice core climate records of the past , years, and even fewer ocean records of comparable resolution. There are long sinuous ridges where sediments got squeezed into the fractures that developed in fast-flowing ice that suddenly came to a stop. And there are even "fossil icebergs", more properly called kettle holes. These are where chunks of ice broke off the edge of a retreating sheet, got stuck in mud and then, as the blocks melted away, created voids that were later filled with a different kind of sediment to the surroundings.
We can see where it was moving quickly or where it had simply stagnated and melted away," she told BBC News. The recent state-of-the-climate report from the UN's Intergovernmental Panel on Climate Change IPCC said surface melting of Greenland as a result of increased air temperatures would dominate the territory's ice losses this century.
This will boost the flow of water to the sub-glacial "plumbing system" that produces the kinds of features recorded in the North Sea sediments. Antarctica is a little different. Ice losses in the polar south are driven largely by incursions of warm ocean waters at the ice sheet's margins. Warmer air causing melting at the surface is evident in a few places but is less of a factor. Image source, James Kirkham et al.
The tunnel valleys are now buried by North Sea bottom-muds, but their outline is seen in seismic data. More Videos Read More. In the current climate, Earth will also be looking rather different. However, today it is humankind with its emissions from burning fossil fuels that determines the future development of the planet. Scientists say climate change is slowing Earth's rotation. Wait, what exactly is an ice age? Warm and cold periods that lead to ice ages occur in regular patterns called Milankovitch cycles.
These cycles occur because the Earth's orbit around the sun is not constant. All of these changes result in varying amounts of energy i. When greenhouse gases are high, the Earth is warm, when they are low, the Earth is cool. When these changes are occurring naturally, they take thousands to tens of thousands of years to occur, whereas humans have caused this to occur in just over years.
December: It was warm at the North Pole. So, the delaying of an ice age is actually a bad sign? Global warming has reached a critical stage. If temperatures climb 2 degrees Celsius, low-lying islands would be wiped out as seas continue to rise, pushing many plants and animals toward extinction, increasing the intensity of droughts, floods, heat waves and storms.
And in a historical moment, ministers from countries at COP21, the climate change conference in Paris in December , pledged to act to keep temperatures low. John D. Sutter, columnist for CNN and creator of CNN's 2 degree project, warns against becoming complacent about global warming. It's a complicated relationship and our knowledge is always evolving. Photos: What's causing climate change? Meet the top 10 villains. Electricity and heat: Emissions from those sectors account for nearly a third of global greenhouse gas emissions, according to the World Resources Institute.
These activities are the biggest climate villains, statistically speaking. Transportation: It's the second-most important cause of dangerous global warming. Manufacturing and construction: These activities are the third-biggest contributors of heat-trapping emissions. Agriculture: Advocates say giving up meat, especially beef, would help curb greenhouse gases. Agriculture makes up
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