Ice Sheets and Sea Level

The Relationship Between Ice Sheets and Sea Level

(last updated: Feb 01, 2009)

The polar ice caps contain enough water to raise sea level by seventy meters above today's sea level. This page describes the various polar ice caps, investigates the melting of the much larger ice caps that covered the northern hemisphere during the last ice age, and attempts to estimate how quickly the ice caps will melt in response to rising temperatures from global warming.

The Water Cycle
Rising Sea Levels
The World's Major Ice Sheets
Ice Cap Melting Rates from Paleoclimate Data
Estimating Future Sea Level based on Climate Events in the Past
Arctic Climate and Sea Ice Extent
European Climate and the Thermohaline Circulation

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The Water Cycle - Where is the Water?

The chart below (from Science Aug 2006) shows the reservoirs of water on Earth, and the fluxes between them. The unit are in thousands of cubic kilometers (10³ km³). Note the large quantity of water locked up in glaciers and snow compared to the other (non-ocean) water reservoirs.

Global hydrological fluxes (1000 km³/year) and storages (1000 km³) with natural and anthropogenic cycles are synthesized from various sources (1, 3–5). Big vertical arrows show total annual precipitation and evapotranspiration over land and ocean (1000 km³/year), which include annual precipitation and evapotranspiration in major landscapes (1000 km³/year) presented by small vertical arrows; parentheses indicate area (million km²). The direct groundwater discharge, which is estimated to be about 10% of total river discharge globally (6), is included in river discharge.

The oceans cover 71% of the Earth's surface (3.5 x 10¹⁴ m²), with an average depth of 4 km. This amounts to 1.4 x 10²⁴ g, or 1.4 x 10⁹ km³ of water. Most water on Earth (97.25%) is stored in the oceans, 2.05% in ice (90% of which resides in the Antarctic ice sheet, 9% or 2.5 x 10⁶ km³ in Greenland), with the remainder stored as ground water (0.68%), while the amount of water in lakes and rivers, as soil moisture, in the biosphere and atmosphere is insignificant. The total volume of water in the atmosphere is about 1.3 x 10⁴ km³ [ref]

This chart may be missing most of the Earth's water. Estimates of how much water is stored in the mantle vary from one third of the ocean to one or two oceans [ref]. Water enters the mantle via the subduction due to plate tectonics, and returns to the surface by volcanic emissions. Of course, this happens over a very long time frame, so it has little interaction with the water cycle described above.

Distribution of Water Vapor in the Atmosphere

Rising Sea Levels

There are two sources of sea level change. Eustatic change is caused by water entering or leaving the ocean (usually from ice caps), or large scale changes to the sampe of the ocean floor. Isostatic change is caused by the local rising and falling of the coastline because of geoligical forces, and is not an indicator of global sea level.

There are three main causes of Eustatic sea level rise:

Thermal Expansion - Water expands slightly as it gets warmer. This is presently the main cause of rising sea levels. A 3 °C rise of temperature will increase the thickness of a 100 m layer of sea water in low latitudes by around 9 cm.[ref], or a 1 °C rise in the top 1 km of ocean will cause 20 cm sea level rise.
Melting Mountain Glaciers - Mountain glaciers in temperate regions are shrinking as climate warms. Their total volume is relatively small, so they are contributing a smaller portion of sea level rise. The total amount of glacier ice left is actually only enough to raise sea level by 15-37 cm. [ref]
Melting Polar Ice Sheets - The polar ice sheets are melting around the edges, but growing in their centers due to increased snowfall. These two process were roughly in balance in the 20th century, but that is not longer the case.

Averaged globally and throughout the entire water column, the temperature of the ocean has only risen by 0.04ºC since 1955. So far only the surface mixed layer with a thickness of a few hundred metres has warmed, while the average ocean depth is 3800m. [ref]

Average Sea Level Rise and Local Variations

Sea level is believed to be rising worldwide by 1.7 millimeters per year over the last century, and 2.8 ± 0.4 mm/year this decade [Science, Aug 11, 2006 p 827]. This is due to the expansion of warming water and the added outwash from melting glaciers in Greenland, Alaska, tropical highlands and elsewhere in Antarctica. The IPCC estimates that ocean levels rose by 10 to 20 cm in the 20th century. However, other factors such as wind patterns and ocean currents affect local sea level, so this rise is not uniform. For example, an El Nino can temporarily change ocean levels by up to 60 cm. in some regions. The map below shows sea level changes, measured by satellite since 1980 and calculated before that. Some parts of the ocean are experiencing falling sea levels in contrast to the general trend. Note the black triangles indicate tidal guages - most of them are located in areas with the fastest rising sea level. This has led to overestimates of sea level rise in the past.

Falling Local Sea Level near a Melting Ice Cap

An ice cap consists of a large mass of ice, which exerts a gravitational attraction on the sea water that surrounds it, causing the local sea level to rise. When some of the ice melts, the meltwater is distributed around the entire ocean, but the reduction in gravitational attraction is felt locally. Local sea level will actually fall as the ice cap melts. See [Science Mar 2002]. The map below shows the relative sea level change from the melting of the southern one-third of the Laurentide Ice Sheet that covered Canada during the last ice age. The dark blue is the region of sea level fall.

Numerical Measures Used for Ice Cap Melting and Sea Level Rise
One Gigatonne (Gt), or one billion tons of water = one cubic kilometer (1 km³) One meter of sea level rise inundates about 10¹¹ square meters of land, about 0.07% of the land area of the earth. Water flow is measured in Sverdrups, where 1 Sv = one million cubic meters per second. The Amazon River flows at 0.3 Sv, the Gulf Stream at 20 Sv.
Gigatonne, or cubic kilometer (km³)	Millimeter (mm) of Sea Level	Sverdrups (Sv)
1 Gt / year	0.0028 mm / year	3.22 x 10^-6 Sv
361 Gt / year	1 mm / year	0.011 Sv
31,000 Gt / year	85.9 mm / year	1 Sv

The World's Major Ice Sheets

There are three major ice sheets on Earth, which could raise sea level by 78 meters in the unlikely event that they all melted (as shown here). Annual snowfall on the ice sheets is equivalentto 6.5 mm of sea level [ref]. The present sea level is 120 m above that at the end of the last ice age, when the global average temperature was about 6° C colder than today, and large parts of the Earth were covered by massive ice sheets. Global sea level stood about 6 m higher during the last interglacial period about 120,000 years ago, but this event cannot be directly linked to any ice shelf collapse. The temperature of the earth was perhaps 4-5° C warmer than today 40 million years ago when the Antarctic ice sheet first made an appearance. Sea level at that time was about 70 meters higher than today.

Antarctica should be treated as three separate and distinct regions: the Antarctic Peninsula, East Antarctic and West Antarctica. Geologically these are three continental fragments that have collided together (see this PowerPoint slideshow), and they also have very different climates. Ice sheets on the Antarctic Peninsula are relatively small (enough ice to raise sea level by a third of a meter [ref]), and the climate there is warming rapidly (6° C since 1950). It is misleading to apply graphic examples of warming there to the rest of Antarctica. See this ice-free image.

East Antarctica (65 meters sea level equivalent) is by far the largest (up to 4 km thick) and most stable of the ice caps. It began forming over 40 million years ago and has been stable for at least 14 million years, when the global climate was significantly warmer than today. It would take a local temperature rise of 17° C to cause significant de-glaciation.

West Antarctica (6 meters sea level equivalent) is distinguished by the fact that most of it is resting on bedrock below sea level. It is separated from the East Antarctic ice sheet by a mountaing range. A continuous West Antarctic ice sheet first appeared approximately 9 million years ago [ref]. It has suffered at least one complete collapse in the past, and has had multiple partial collapses during the present interglacial period. Even though it is close the the South Pole, this may be the ice sheet at most risk of collapse, although no major change is expected for at least 100 years. Over the last decade it has added between 0.13 and 0.16 mm/year to global sea level.

The ice sheet covering West Antarctica is the last great marine ice sheet. Its bed lies below sea level and slopes down inland from the coast. The profile shown is based on Thwaites Glacier, West Antarctica (11). When the ice sheet is in equilibrium; influx from snowfall (q) is balanced by outflow. A small retreat will provoke changes in both the influx and the outflow. If these changes act to promote further retreat, the ice margin is unstable.

But [Science Mar 2007] reports that sedimentation at the grounding line acts to stabilize the ice sheet. At the current rate of sea-level rise, it would take several thousand years to float the ice sheet off the bed.

Greenland (6.6 meters sea level equivalent) is the furthest ice sheet from the pole, extending below the Arctic Circle. The ice cap is presently melting around the edges (the elevation-change rate is –2.0 ± 0.9 cm/year [ref]) and (maybe) growing in the center (6.4 ± 0.2 cm/year) due to increased snowfall caused by warming temperatures. Most of Greenland is above sea level, and it is almost surrounded by mountains (see this ice free image), isolating the center from the melting margins.

		change in cm / year
Antarctica	Greenland


The Ice Sheets at the Last Glacial Maximum 18,000 Years Ago During the last glacial maximum the ice sheets covered an area of 25 million square kilometers, and were up to four kilometers thick. Sea level was about 120 meters lower than today. When they melted, and area of about 20 million km³ were submerged, about 10% of the global land mass, or an area larger than South America. See this detailed map and description.

Potential and Present Sea Level Rise from the Major Ice Sheets

Ice Sheet	Volume	Potential Sea Level Rise	Present Rate of Loss	Sea Level Equivalent	References
East Antarctica	23.0 x 10⁶ km³	67.3 meters	0 ± 56 km³ / year -25 Gt / year (gain)	- 0.07 mm/yr	Science Mar 2007
West Antarctica	3.8 x 10⁶ km³	5.0 meters (partly below sea level)	148 ± 21 km³ / year 50 Gt / year	0.4 ± 0.2 mm/yr 0.14 mm/yr	[ref] Science Mar 2007
Greenland	2.46 x 10⁶ km³	6.6 meters	224 ± 41 km³ / year 239 ± 23 km³ / year 101 ± 16 km³ / year	0.62 mm/yr (my calculation) 0.54 mm/yr (their figure) 0.28 ± 0.04 mm / year	Science 2005 Science Sep 2006 Science Nov 2006
Other glaciers	0.68 x 10⁶ km³	1.9 meters	90 km³ / year	0.25 ± 0.11 mm/yr	[ref]

Although the world's glaciers only contain a fraction of the mass of ice compared to the major ice sheets, they serve as a more sensitive measure for local temperature changes. Detailed information on the changing state of glaciers can be found at the World Glacier Mmonitoring Service.

Ice Cap Melting Rates from Paleoclimate Data

Sea level is determined by the size of the polar ice caps. As the climate cools, water from the ocean is deposited as snow, and ice caps grow as ocean levels drop, and as climate warms the ice melts and the oceans rise. At any given temperature there is an equilibrium level of ice cap size and sea level.

The table below identifies sea levels (compared to today) for several time periods with different global average temperatures. It can be seen that there is roughly 6 meters of sea level rise for each degree C of temperature rise. In a time of rapid temperature change the ice does not melt all at once; it takes many hundreds or possible several thousands of years to reach equilibrium.

Date	Period Name	Relative Temperature	CO₂ ( in ppm)	Sea Level	Rise mm / year	Sea Level m / °C	Comments
next 1000 years	Anthropocene	+2 to +5° C	500-1000	3.5 m ?	3.5		Maximum rate from IPCC 2001
1993–2003	Late Holocene	0° C	380 ppm	0 m	2.6 ± 0.04	0	Measured by satellite.
past fifty years		-0.5° C			1.8		Overpeck et. al.
13,800 - 7000 years					11		The end of the last Ice Age
14.2 - 13.7 Ky	Meltwater Pulse 1a				40		Science: Mar 2002, Mar 2003
21,000	Last Glacial Maximum	-4 to 7° C	185 ppm	–130 ± 10 m		16 to 30	Science, Jan 2007
130,000	Eemian Ingerglacial (MIS-5e)	1 or 2° C	290 ppm	4-6 m	20	2 to 6	See the discussion below.
425,000 to 375,000	Four Ice Ages Ago (MIS-11)	about the same	300 ppm	up to 20 m	slow		Science, July 2008, Wiki (a)
3 million	Middle Pliocene	2 or 3° C	400-500 ppm	25 ± 10 m		5 to 10	USGS Summary
40 million	Eocene, before Antarctic glaciation	+5° C		60 m		12
100 million	Cretaceous	+8° C	1000-2000	+ 80 m		10

(a) MIS-11 was a stable interglacial period similar to the Holocene today, except that it lasted for 50 thousand years. This extra time was all that was required to melt most of the Greenland ice cap.

Sea Level Compared to Global Mean Temperature

This chart shows four measurements of sea level at representative points of time in the past. The points are roughly in a straight line, suggesting a ratio of 100 m / 10° C, or about 10 meters of sea level rise for every degree Celsius of temperature increase. This chart give no indication of the amount of time it would take for a degree of warming to increase sea level by that amount.

The red line is an estimate of the rate of sea level rise over a shorter time scale, as discussed here.

The graph below shows that the melting rate of the world's ice caps slows down as the climate becomes warmer (and CO₂ levels become higher). This is because the ice caps become smaller. The lower chart shows the rapid melting of the glaciers in the ten thousand years following the peak of the last ice age. At certain times sea level was rising about half a meter per year.

Relation between estimated atmospheric CO₂ and the ice contribution to eustatic sea level indicated by geological archives and referenced to modern (pre-Industrial Era) conditions.
(A) The most recent time when no permanent ice existed on the planet (sea level = +73 m) occurred >35 million years ago when atmospheric CO₂ was 1250 ± 250 ppmV (54).
(B) In the early Oligocene ( $~$ 32 million years ago), atmospheric CO₂ decreased to 500 ± 150 ppmV (54), which was accompanied by the first growth of permanent ice on the Antarctic continent, with an attendant eustatic sea-level lowering 45 ± 5 m (55).
(C) Today. CO₂ = 280 parts per million by volume (ppmV), eustatic sea level = 0 m. Due to global warming, we are now slowly heading toward (B).
(D) The most recent time of low atmospheric CO₂ (185 ppmV) (56) corresponds to the Last Glacial Maximum 21,000 years ago, when eustatic sea level was –130 ± 10 m.

Time series of key variables encompassing the last interval of significant global warming during the last deglaciation, starting 20,000 years ago.

(A) Atmospheric CO₂ from Antarctic ice cores.

(B) Sea surface temperature in the western equatorial Pacific based on Mg/Ca measured in planktonic foraminifera. [Is this the best proxy for the temperature actually acting on the ice sheets?]

(C) Relative sea level as derived from several sites far removed from the influence of former ice-sheet loading. Note the two intervals with rapid melting, known as a meltwater pulse (MWP). Deglacial sea-level rise averaged 10 mm/year, but with variations including two extraordinary episodes at 19,000 years before present and 14.5 kyr B.P. when peak rates potentially exceeded 50 mm/year (7–9). Each of these "meltwater pulses" added the equivalent of 1.5 to 3 Greenland Ice Sheets to the oceans over a period of one to five centuries.

Estimating Future Sea Level based on Climate Events in the Past

The table below shows sea level rise for the 20th century, and projections for the 21st century. The data suggest a sea level rising rate of 3.4 millimeters/year per °C of global average temperature increase. Observations from warmer times in the past show that in the long term there is 5 to 10 meters of sea level rise for each increase in global average temperature of one degree Celsius. Dividing this by the present melting rate gives a melting time of 3,000 to 9,000 years, assuming a constant melting rate. However, it is possible that melting rates will increase, so the ice cap melting time may be shorter than that.

Perhaps we can get a better idea from the previous large global warming event - the end of the ice age 20,000 years ago, which is shown in the table above. A warming of 5 degrees over 10,000 years caused an essentially linear sea level rise over the same time period. [But why is there no sign of the Younger Dryas cooling 12,900 – 11,500 years before present, which occured in the northern hemisphere where the major ice sheets were. The equatorial Pacific warming looks nothing like the Greenland Ice Core record.]

	Rate of Sea Level Rise in the 20th Century Rate of sea-level rise obtained from tide gauge observations (red line) and computed from global mean temperature (dark blue line). The light blue band indicates the statistical error (one SD) of the simple linear prediction (15).
	Sea Level in the 20th Century Sea level relative to 1990 obtained from observations (red line, smoothed) and computed from global mean temperature (blue line). The red squares mark the unsmoothed, annual sea-level data.
	Projections of Sea Level Rise for the 21st Century Past sea level and sea-level projections from 1990 to 2100 based on global mean temperature projections of the IPCC TAR. The gray uncertainty range spans the range of temperature rise of 1.4° to 5.8° C, having been combined with the best statistical fit. The dashed gray lines show the added uncertainty due to the statistical error of the fit. Colored dashed lines are the individual scenarios as shown in (1); the light blue line is the A1FI scenario, and the yellow line is the B1 scenario.

In the last interglacial period, about 120,000 years ago, sea level was probably 5 or 6 meters higher than today. There are suggestions that it was still higher during the interglacial 400,000 years ago (Hearty et al., Geology, 27, 375, 1999). But to find a planet 2 or 3°C warmer than now, as it will be this century in “business-as-usual” scenarios, we must go back to the middle Pliocene, about 3 million years ago. At that time sea level was 25 ± 10 m greater than today. [ref]

On the other hand, conditions during the Eemian were very different than today. As seen from this chart of temperature anomolies caused by the Milankovitch cycles, during the Eemian solar forcing in the polar regions during the critical spring period was more than 60 watts per meter above normal. Compare this with 3.7 W/m² of direct forcing from the doubling of carbon dioxide levels expected at the end of the 21st century. It is quite possible that this was the cause of the extensive melting in the polar regions, and the one or two degrees of global temperature rise was mainly a side effect.

There is also a problem of using the Pliocene to predict the effects of a similar warming today: The Pliocene was a time of cooling and ice cap formation, while the near future is a time of warming and ice cap melting. An ice cap creates its own regional climate by reducing local temperature. For example, if you removed all of the ice from Greenland, a new ice cap would not form at today's temperatures. This implies that three degrees of warming will leave a much larger ice cap than the Pliocene started with, so sea level increases may not be nearly so high.

James Hansen thinks ice cap melting rates are a lot higher:

Some ice sheet modelers believe that it requires millennia for ice sheets to respond to forcing. I’m a modeler too, but I rate data higher than models. Numerous recent studies show that when ice sheets begin to disintegrate sea level commonly rises at rates of a few meters per century. [These are not the ice sheets that exist today.] Perhaps even more important, several independent studies with independent methods, show that sea level does not change gradually with the Earth’s orbital elements. Rather it exhibits rapid changes of 10 meters or more on sub-orbital time scales. I refer, for example, to studies of Siddall et al. [also ref], Potter et al., Thompson and Goldstein, to name a few. Ice sheet models that move lethargically on millennial time scales do not produce these real-world changes, because they are missing critical physics. [ref]

Greenland Ice Cap Melting for Various Carbon Dioxide Levels

Future evolution of the Greenland Ice Sheet calculated from a 3D ice-sheet model forced by three greenhouse gas stabilization scenarios. The warming scenarios correspond to the average of seven IPCC models in which the atmospheric carbon dioxide concentration stabilizes at levels between 550 and 1000 ppm after a few centuries (4) and is kept constant after that. For a sustained average summer warming of 7.3°C (1000 ppm), the Greenland Ice Sheet is shown to disappear within 3000 years, raising sea level by about 7.5 m. For lower carbon dioxide concentrations, melting proceeds at a slower rate, but even in a world with twice as much CO₂ (550 ppm or a 3.7° C summer warming) the ice sheet will eventually melt away apart from some residual glaciation over the eastern mountains.

Other Notes:

The mid-Holocene may be a more comparable period to today, which experienced some warming, and evidence of sea level rises of a few meters.
If warming stabilises at 3 °C, the Greenland ice sheet could survive for several thousand years. But if temperatures rise by 8 °C, which several scenarios predict, then it would disappear in 1000 years. [ref]
There has been no significant change in ice cap size in Antarctica (east or west) since the 1950's [Science, Aug 11, 2006 p 827].
The simple rule of thumb is: melting Greenland during 1,000 years will provide an average freshwater inflow of 0.1 Sv (1 Sverdrup is one million cubic meters per second). Then, nobody knows how much freshwater it takes to shut down the Atlantic circulation. A recent model intercomparison concluded: "The proximity of the present-day climate to the Stommel bifurcation point, beyond which North Atlantic Deep Water formation cannot be sustained, varies from less than 0.1 Sv to over 0.5 Sv."
Over the past 30 years, Arctic surface temperatures have increased 0.5°C per decade; September Arctic sea ice extent has decreased 7.7% per decade;
We thus performed a sensitivity simulation with 0.1 sverdrup of water inserted into the present-day North Atlantic over 100 years. This freshwater forcing yields a simulated 25% slowdown of the North Atlantic thermohaline circulation and annual cooling of 1.5-C south of Greenland. [Otto-Bliesner]
The present-day obliquity and precession are not in phase as was the case for MIS-11. [ref]

Arctic Climate and Sea Ice Extent

The Dynamics of Sea Ice

As with most substances, water becomes denser with decreasing temperature. But for fresh water this process stops at 4°C, and as water cools further toward its freezing point, it expands to become less dense. And unlike many substances, frozen water is less dense than its liquid form, thus it floats.

Ocean water, with a 3.5% salinity, freezes at −1.9 °C. When it freezes, the ice is salt free. Salt water also does not have the density reversal of fresh water. The cold water below the ice tends to sink, replaced by warmer water from below. The salt expelled from the ice increases the density. also causing the water the sink. This process brings warmer water in contact with the bottom of the ice, reducing the rate at which the water freezes. For this reason, sea ice does not get more than a few meters thick. [ref]

Climate Change in the Arctic since 1920

Because sea ice is floating on the ocean, the shrinking of sea ice has little effect on sea level. Sea ice is mostly fresh water, so melting does reduce the salinity of the ocean. Perhaps more important, the very reflective ice surface is replaced with the strongly absorbing open ocean, which causes further warming to occur. The following maps show climate change in the Arctic region since 1920.

	(a-b) 1920-39, winter and summer, respectively. A strong warming trend is seen in the northern Arctc mainly during the winter season.
	(c-d) 1945-64, winter and summer, respectively. This is a period of significant cooling.
	(e-f) 1980-1999, winter and summer, respectively. Warming is more global in scope during both seasons.

Surface air temperature (SAT) trends north of 30°N in the winter (NDJFMA) and summer (MJJASO) half-years for 20-year periods representing warming, cooling and warming in the 20th century:

The following two graphs are taken from the 2004 Arctic Climate Impact Assessment paper. The temperature graph shows a linear rise from 1900 to 1940, a drop from 1940 to 1970, and a rise from 1970 to 2000. The slope of both the rise and fall is about the same. The sea ice graph shows no change between 1900 and mid century, then what looks like a linear decline. The decline in summer starts around 1950, while the winter decline takes until 1970 to begin. But,

Why does the temperature rise in the early part of the century have no effect on sea ice extent?
Why does the decline in summer sea ice extent begin in 1950, while the cooling trend does not reverse itself until 1970?
Why does the caption attached to the graph suggest the change in sea ice extent is accelerating? A regression taken from 1900 will give an exponential curve. But visually at least, the decline seems to start abruptly, and looks linear after that.

ArcticIceConcentrationAnomoly_Sept_2002-2005

These maps show the difference between “normal” sea ice extent (long-term mean), and the year indicated. The long-term average minimum extent contour (1979-2000) is in magenta. The ice extent for each year is shown by the edge of the colored region; within that extent, color bands show differing levels of sea ice concentration. Blue indicates areas where concentration is more than the long-term mean; red shows areas where concentration is less than the long-term mean. The 2005 map shows a marked reduction in extent over the past four years, all of which were also below average.

In 2002 and 2003, the ice pack also experienced much lower concentrations during the minimum, especially true north of Alaska. However, the ice cover has been much more compact during the minimums of 2004 and 2005, yielding small negative or even positive ice concentration anomalies within the ice pack.

Sea Ice extent is a measure of the area that contains at least 15 percent ice. Ice concentration is the fraction of the actual area covered by ice compared to the total area, measured in terms of percentage ice cover. The satellite does not pass directly over the North Pole; this lack of data is indicated by the gray circle in each image.

Projections of Future Arctic Sea Ice Extent

The expected time until the Arctic Ocean is ice free in summer is decreasing rapidly:

IPCC AR4: Sea ice is projected to shrink in both the Arctic and Antarctic under all SRES scenarios. In some projections, arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century.

Spatial pattern of the percent of IPCC AR4 model simulations (SRES A1B scenario) with at least 15% ice concentration for March (left) and September (right), averaged over the decade 2075 to 2084. For example, a value of 60% at a given location means that 60% of simulations predicted sea ice. Results are based on 11 models with realistic 20th-century September sea-ice extent.

European Climate and the Thermohaline Circulation

European Warmth due more to Wind than Ocean Currents

Three factors make Europe’s winters milder than those of eastern North America at the same latitude. Winds blowing toward Europe from the west pick up heat from the waters of the North Atlantic, which retain far more summer heat than the interior of the North American continent. Those balmy winds give Europe a milder, maritime climate relative to North America’s more extreme, continental climate. The Gulf Stream—the tail end of a great “conveyor belt” of currents carrying warm waters from the Southern Hemisphere—also contributes heat to the westerly winds and thus to Europe. And those westerly winds tend not to blow straight out of the west. They arrive over eastern North America from more out of the frigid north, intensifying the continentality of eastern North America’s climate. In contrast, Atlantic winds tend to blow more from the warmer climes of the south before reaching Europe.

The world’s winds carry five times more heat out of the tropics than do ocean currents. 80% of the heat that cross-Atlantic winds picked up was summer heat briefly stored in the ocean rather than heat carried in by the Gulf Stream.

The ocean heat transport of the Gulf Stream was crucial to warming Scandinavia and keeping the far northern North Atlantic free of ice. It also warmed latitudes south of Scandinavia by 3°C on both sides of the Atlantic. But the wintertime temperature contrast between Europe south of Scandinavia and eastern North America was still about 15°C. [ref]

The Thermohaline Circulation

A highly simplified cartoon of the global thermohaline circulation (sometimes called the ‘conveyor belt’) is shown in the figure above. Near-surface waters (red lines) flow towards three main deep-water formation regions (yellow ovals) — in the northern North Atlantic, the Ross Sea and the Weddell Sea — and recirculate at depth (deep currents shown in blue, bottom currents in purple; green shading indicates salinity above 36‰ [‰ means parts per mil, or in this case 3.6 percent], blue shading indicates salinity below 34‰). A recent estimate of the rate of deep-water formation is 15±2 Sv [a unit of flow expained above] in the North Atlantic and 21±6 Sv in the Southern Ocean. Northward heat transport into the northern Atlantic peaks at 1.3±0.1 PW (1 PW=10¹⁵ W) in the subtropics; this heat transport warms the northern Atlantic regional air temperatures by up to 10 °C over the ocean with the effect declining inland.

Ocean Salinity

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